PARTS OF ELECTRIC SERVICE ENTRANCE BASICS

How to install service entrance? You will only know it if you know its parts. The details of how the service lateral or triplex cable provided by the electrical utility is fastened and spliced to the customer’s service point are illustrated in Fig. 5-3.

The bare neutral/grounding cable that supports the triplex is fastened at one end to the utility pole and the other end is fastened at the service point with a combination insulator and anchor bolt crimped onto the cable. This attachment method leaves the end of the neutral/grounding cable and the bare ends of the two insulated “hot” conductors available for splicing.

Electrical utility employees splice these three ends to corresponding ends of the customer’s service entrance (SE) cable, which is installed by the electrical contractor. The three ends of the customer’s SE cable are pulled through the bushings in a protective metal hood called the weatherhead or service head with enough slack to permit an adequate drip loop to be formed when the three conductors are spliced.

The drip loop, which must be at least 36 in. long, prevents water from entering the weatherhead. Without a drip loop, water could drain down the conduit to the cable connections inside the meter base to the bus bars that power the watthour meter, corroding them and causing a short circuit.

The details of two different aerial feed service entrances are illustrated in Fig. 5-4. A 200-A service entrance made with three-wire service entrance (SE) cable is shown in Fig. 5-4a. The cable from the weatherhead is brought down to the meter base, where the busbar connections to the meter are made. Another length of SE cable goes from the meter bus bars to the loadcenter.

A second version of a 200-A service entrance has the SE cable protected by metal or nonmetallic conduit between the weatherhead and the meter base, as shown in Fig. 5-4b. The service entrances for 175- and 100-A service are identical except that the SE cable has either a 175- or a 100-A rating. The limits of the meter height dimensions above grade level are approved by NEC 2002.

A 200-A service with the triplex cable terminating on a metal conduit mast is shown in Fig. 5-5. The mast projects high enough to comply with NEC 2002 for the minimum distances of the triplex termination above the ground. The hollow pipe functions as both a mast and a conduit for SE cable from the weatherhead to the meter base. As in Fig. 5-4, the service entrances for 100- to 175-A service are identical except for the lower SE cable ratings. All of the dimensional limits shown are those approved by NEC 2002.

COST OF USING ELECTRICAL ENERGY BASIC INFORMATION

POWER CONSUMPTION COMPUTATION TUTORIALS

A watt-hour meter is always connected into some part of the service-entrance equipment to record the amount of energy used. Billing by the utility is generally done on a monthly basis.

The meter might be mounted on the side of the house or on a pedestal somewhere on the premise on the lot line. The utility makes this decision.

The residence discussed in this text has one meter, mounted on the back of the house near the sliding doors of the Master Bedroom. The kilowatt (kW) is a convenient unit of electrical power.

One thousand watts (w) is equal to one kilowatt. The watt-hour meter measures and records both wattage and time.

For residential metering, most utilities have rate schedules based on “cents per kilowatt-hour.” Stated another way: How much wattage is being used and for how long?

Burning a 100-watt light bulb for 10 hours is the same as using a 1000-watt electric heater for 1 hour. Both equal 1 kilowatt-hour.

kWh = WATTS X HOURS/1000 = 100X10/1000 = 1 kWh

In these examples, if the electric rate is $0.08 cents per kilowatt-hour, the cost to operate the 100-watt light bulb for 10 hours and the cost to operate the electric heater for 1 hour are the same—$0.08. Both loads use 1 kilowatt-hour of electricity.

EXAMPLE
Find the cost of operating a color television set for 8 hours. The label on the back of the television set indicates 175 watts. The electric rate is $0.10494 per kilowatt-hour.

Cost = 175 x 8 x $0.10494/ 1000
        =  $ 0. 146 9 (approx. 15¢)

THERMAL (LAGGED DEMAND) ELECTRICITY METERS

Electromechanical
In the electromechanical thermal or lagged-type demand meter, the pointer is made to move according to the temperature rise produced in elements of the meter by the passage of currents. Unlike the integrating demand meter, the lagged meter responds to load changes in accordance with the laws of heating and cooling, as does electrical equipment in general.

Because of the time lag, momentary overloading, instead of being averaged out, will have a minor effect on the lagged meter unless the overloading is held long enough or is severe enough to have some effect on the temperature of equipment.

The demand interval for the lagged meter is defined as the time required for the temperature sensing elements to achieve 90% of full response when a steady load is applied. Like the integrating meter, the lagged meter is generally designed to register kilowatt demand.

The lagged type is essentially a kind of wattmeter designed to respond more slowly than an ordinary wattmeter. An important difference in these methods of metering demand is in the demand interval. In the integrating meter, one demand interval follows another with regularity, giving rise to the term block interval.

The thermal or lagged meter measures average load with an inherent time interval and a response curve which is based on the heating effect of the load rather than on counting disk revolutions during a mechanically timed interval.

Electronic
Electronic thermal demand emulation is the logarithmic average of the power used, with a more recent load being weighted more heavily than a less recent load, (approximated exponentially). The meter will record 90% of a change in load in 15 minutes, 99% in 30 minutes, and 99.9% in 45 minutes.

Because thermal demand emulation is the logarithmic average, the demand is not set to zero on a demand reset. On a demand reset, present demand becomes the new maximum demand.

ELECTRICITY METER (KW-HR) ANTI CREEP HOLES

Without anti-creep holes, the interaction of the voltage coil and the light-load adjustment might provide enough torque to cause the disk to rotate very slowly when the meter was energized, but no current flowing.

This creep would generally be in a forward direction, because the light-load adjustment is so designed that it helps overcome the effects of friction and compensates for imperfections of the electromagnet steels. In order to prevent the disk from rotating continuously, two diametrically opposed holes are cut into the disk.

These holes add resistance to the flow of eddy currents caused by the voltage flux. Earnshaw’s Theorem explains that a conductor in a flux field tends to move to a position of least coupling between the conductor and the source of the flux field.

Because of this, the disk will tend to stop at a position in which the anti-creep hole causes the greatest reduction in the eddy currents (sometimes moving backward a portion of a revolution in order to stop in this position). A laminated disk or one of varying thickness will also tend to stop in a position of least coupling.

AUTOMATIC METER READING (AMR) TECHNOLOGIES BASIC INFORMATION AND TUTORIALS

PURPOSE AND ADVANTAGE OF AMR
To eliminate meter access problems and data entry errors while improving overall meter reading efficiency, many utilities are implementing Automatic Meter Reading (AMR) technology. The AMR technology enables utilities to collect meter data without having to visit the meter. Nationwide, an increasing number of electric, gas, and water utilities are installing these remote data collection technologies.

Electric utilities are also beginning to deploy AMR technology for reasons that go beyond metering and customer billing. As the energy marketplace deregulates and becomes more competitive, utilities are seeking ways to use advanced metering data throughout their distribution operations to achieve a variety of other objectives.

These are; improved demand forecasting accuracy, increased energy distribution system efficiency, delivery of new rates and services, and successful management of customer choice and retail competition in their service territories.

Utilities have a wide choice of AMR technologies and companies from which to choose. These technologies include wired and wireless data collection systems that use both public and private communication networks, including broadband and cellular.

Some AMR vendors offer products that use existing power lines to transmit collected data while others are working to develop systems that rely on satellites and other more exotic technologies that have yet to prove themselves as either practical or cost effective. The AMR industry also features meter data collection technologies to serve residential, commercial, and industrial customers equipped with more advanced solid-state electric meters.

From radio-equipped handheld terminals to vehicle-based systems to large fixed networks, these automatic meter-reading systems offer a wide range of data collection functionality at a wide range of costs. The suitability and cost-effectiveness of a particular AMR technology depends on a number of variables; the type of service territory (urban, suburban, or rural), the type of customer (residential, commercial, or industrial), and the data collection needs of a particular utility.

These needs can vary greatly.

Many utilities are deploying integrated AMR solutions that combine different collection technologies (e.g., network, mobile, and telephone), to deliver the desired level of data collection functionality for different service areas and customer segments in the most cost-effective manner.

Each of these technologies— handheld computers, mobile vehicle-based systems, wireless networks, telephone-based systems and powerline carrier—has its own set of strengths depending on a utility’s operational and strategic objectives.


Radio-Based Mobile Automatic Meter Reading
With a radio-based system, each meter is adapted with a compact, low-power transmitter/receiver that upon request, broadcasts a radio signal containing metering and tamper data.

These meters are polled by a mobile handheld terminal or vehicle-based transmitter/receiver that collects meter readings from many meters and carries the readings back to the central office. At the end of the day, the readings are loaded directly into the utility’s billing computer.

The mobile transceiver can be in a handheld terminal, or a higher-powered unit installed in a truck that is driven down streets, polling meters and recording replies. Depending on the service environment, meter density, memory capacity, and the type of meter readings collected, handheld terminals are typically capable of reading several hundred meters or more in a day.

More powerful vehicle-based units raise meter reading efficiency even further by reading thousands of meters in a single day.  Radio-based meter reading is fast, efficient, and generally reliable. Some meter installations inside metal buildings create difficulties for radio transmission but these site-related problems can often be solved by re-locating the
meter antenna.

Network Meter Reading
Radio-based fixed networks offer the most advanced data collection functionality of any wireless meter reading technology. There are many variations in the network data collection products available from different vendors.

Most wireless network meter reading systems involve installing a fixed communications network over a population of meters equipped with radio transmitters to send the data through the network to the host processor. Companies apply a variety of communications strategies for their network products that utilize private dedicated wireless networks, public networks, or some combination of the two.

Though more costly than handheld and mobile automatic meter reading systems, these networks provide electric utilities with state-of-the-art automatic meter reading functionality, including consumption reads, on-request reads, tamper reporting, time-of-use, demand metering, load profile/interval reads, virtual connect/disconnect capabilities, outage detection and restoration reporting, consumption monitoring, and aggregation capabilities.

Until now the handful of electric utilities that have deployed network meter reading systems have done so mostly on a large-scale, territory-wide basis to spread the cost of the network over a large number of meters—typically more than 100,000. However, recently several new more scalable and flexible network products have emerged that enable electric utilities to deploy advanced network meter reading technology on a more selective and cost-effective basis to serve specific meter populations or specific customer segments (such as commercial and industrial customers with advanced solid-state meters).

To reduce both implementation and ongoing operations costs, these new networks combine private, dedicated RF communication networks to gather data from designated populations of automated meters and then use public communications networks to back haul the data from local collection points to the host processor. While its penetration has been limited thus far, deployment of network meter reading and data collection technology will likely accelerate in the years to come as costs come down and the need for utilities to gather more advanced metering data increases.

DYNAMOMETER POWER FACTOR AND PHASE ANGLE MEASUREMENT

Measurement of Power Factor and Phase Angle

A variation of the fundamental electrodynamometer instrument is used to measure power factor or the phase angle, and is called the crossed-coil type. See Figure 6-7. In this design the moving element consists of two separate coils, instead of one which are mounted on the same shaft and set at an angle to each other.

The lead-in springs or spirals to the crossed coils are made as light or weak as possible so as to exert practically no torque. In the single-phase instrument, one of the crossed moving coils is connected in series with a resistor across the line while the other is connected in series with a reactor across the line.

The current flowing through the reactor-connected coil is approximately 90 degrees out of phase with the line voltage. The field coil is connected in series with the line as an ammeter coil.

In operation, the moving system assumes a position dependent upon the phase relationship between the line current and the line voltage. If the line current is in phase with the line voltage, the reactor-connected moving coil will exert no torque and the resistor-connected coil will align its polarities with those of the fixed-coil field.

If the line current is out of phase with the line voltage, the reactor connected moving coil will exert a restraining or counter torque and the moving element will assume a position in the field of the fixed coil where the two torques are in balance.

This instrument may be calibrated to indicate either power factor or the phase angle between the line voltage and current. In the three-phase power factor instrument, the crossed moving coils are connected to opposite legs of a three-phase system.

The fixed coils are connected in series with the line used as a common for the moving-coil connection. This instrument will give correct indication on balanced load only.

When these instruments are not energized, the pointer has no definite zero or rest position as do instruments whose restraining torque is a spring. They are therefore known as free-balance instruments.

Power factor meters may also be of the induction type. In one such type for single-phase use, the fixed element consists of three stationary coils and the moving element comprises an indicator shaft bearing an iron armature. As in the electrodynamometer type, operation is based on the interaction of a rotating and an
alternating magnetic field.

THREE-WIRE,THREE-PHASE DELTA SERVICE METERING TUTORIALS

Two-Element (Two-Stator) Meter
The three-wire, three-phase delta service is usually metered with a two-stator meter in accordance with Blondel’s Theorem. The meter used has internal components identical to those of network meters, but may differ slightly in base construction.

Typical meter connections are shown in Figure 7-4. In the top element (stator) of the meter the current sensor carries the current in line lA and the voltage sensor has load voltage AB impressed on it. The bottom element (stator) current sensor carries line current 3C and its corresponding voltage sensor has load voltage CB impressed. Line 2B is used as the common line for the common voltage-sensor connections.

The phasor diagram of Figure 7-4 is drawn for balanced load conditions. The phasors representing the load phase currents IAB, IBC, and ICA are shown in the diagram lagging their respective phase voltages by a small angle . By definition, this is the load power-factor angle. The meter current coils have line currents flowing through them, as previously stated, which differ from the phase currents. To determine line currents, Kirchhoff’s Current Law is used at junction points A and C in the circuit diagram. Applying this law, the following two equations are obtained for the required line currents:


The operations indicated in these equations have been performed in the phasor diagrams to obtain I1A and I3C. Examination of the phasor diagram shows that for balanced loads the magnitude of the line currents is equal to the magnitude of the phase currents times the sqrt of 3.

The top element (stator) in Figure 7-4 has voltage EAB impressed and carries current I1A. These two quantities have been circled in the phasor diagram and inspection of the diagram shows that for the general case the angle between them is equal to 30° . Therefore, the power measured by the top element (stator) is EABI1Acos(30° ) for any balanced-load power factor. Similarly, the bottom element (stator) uses voltage ECB and current I3C. These phasors have also been circled on the diagram and in this case the angle between them is 30° . The bottom element (stator) power is then ECBI3Ccos(30° ) for balanced loads. The sum of these two expressions is the total metered power.


Examination of the two expressions for power shows that even with a unity
power factor load the meter currents are not in phase with their respective
voltages. With a balanced unity power factor load the current lags by 30° in the top
element (stator) and leads by 30° in the bottom element (stator). However, this
is correct metering. To illustrate this more cleanly, consider an actual load of
15 amperes at the unity power factor in each phase with a 240-volt delta supply.
The total power in this load is:
3 EPhase IPhase cos 3 240 15 1 10,800 watts
Each element (stator) of the meter measures:
Top Element EABI1Acos(30° )
Since I1A 3 IPhase
Top Element 240 3 15 cos(30° 0°)
240 3 15 0.866 5,400 watts
Bottom Element ECBI3Ccos(30° °)
Since I3C 3 IPhase
Bottom Element 240 3 15 cos(30° 0°)
240 3 15 0.866 5,400 watts
Total Meter Power Top Element Bottom Element 5,400 5,400
10,800 watts Total Load Power
When the balanced load power factor lags, the phase angles in the meter vary
in accordance with the 30° expressions. When the load power factor reaches
50%, the magnitude of is 60°. The top stator phase angle becomes 30° 90°
and, since the cosine of 90° is zero, the torque from this stator becomes zero at this
load power factor. To illustrate this with an example, assume the same load current
and voltage used in the preceding example with 50% load power factor.
Total Load Power 3 240 15 0.5 5,400 watts
Top Element 240 3 15 cos(30° 60°)
240 3 15 0 0 watts
Bottom Element 240 3 15 cos(30° 60°)
240 3 15 0.866 5,400 watts
Total Meter Power 0 5,400 5,400 watts Total Load Power.
With lagging load power factors below 50%, the top element power reverses
direction and the resultant action of the two elements (stators) becomes a differential
one, such that the power direction is that of the stronger element (stator).
Since the bottom element (stator) power is always larger than that of the top
element (stator), the meter power is always in the forward direction, but with
proportionately lower power at power factors under 50%. Actually on a balanced
load, the two elements (stators) operate over the following ranges of power factor
angles when the system power factor varies from unity to zero: the leading
element (stator) from 30° lead to 60° lag, the lagging element (stator) from 30° lag
to 120° lag.


As such, the watt metering formula for any instant in time is:
Watts (Vab Ia) (Vcb Ic) Accumulating the watts over time allows the metering of watthours.

FORM (FM) 2S METERING TYPE INTERNAL CONNECTION AND DIAGRAMS

Below is a diagram of a FM 2S meter type.

Form 2S 

240 Volt, 1 Stator, 3-Wire

Self Contained Socket Meter

METER SOCKETS BASIC INFORMATION

The Type-S meter is designed so that its terminals appear as short, rigid, copper contact blades extending outward from the back of the meter. To connect this meter to line and load wires, an auxiliary mounting device is required. This device is the meter socket.

The socket comprises connectors for line and load conductors, contact jaws to receive the meter terminal blades (thus completing connections between conductors and meter coils), and an enclosure for the whole assembly.

Early sockets were round, cast or drawn shallow pans with diameters matching those of the meters. In this type socket, wiring space was limited. This limitation led to the development of the rectangular-shaped trough with a round opening the diameter of the meter.

A sealing ring, which fitted around the meter rim and socket cover rim, secured the meter in place. More recently a ringless type of socket has been developed in which the socket cover opening fits over the meter after the meter has been installed.

The socket cover is then sealed in place to provide protection. In both types the primary functions are to: (1) fix the meter firmly on the socket; (2) close the joint between the meter and socket rim against weather and tampering; and (3) provide means for sealing the meter against unauthorized removal of the meter or cover.

Meter sockets are available in continuous duty current ratings of 20, 80, 100, 120, 150, 160, 200, 320, and 400 amperes and for one, two, or three stator meters. The requirements for indoor and outdoor service differ.

Meter sockets installed on the outside of the house must not only be weatherproof but must be of a material that is highly resistant to corrosion. Under some conditions, such as leakage of pipe joints or cable assemblies, sockets will accumulate varying quantities of water. To guard against water accumulation, sockets are provided with a means for drainage.

Obviously the dimensions of both meters and sockets must be standardized and closely controlled so that meters of any of the major American manufacturers will fit all sockets. The ANSI and NEMA standards have developed standards for meter and socket sizes. The Meter and Service Committees of EEI and AEIC have agreed upon certain basic requirements applicable to meters and the associated mounting devices.

The basic requirements are:
1. Interchangeability of all manufacturers’ meter mounting devices;
2.Mounting devices to be designed for single-meter or multiple-meter mounting either indoor or outdoor;
3. One seal to serve for both meter and mounting device;
4. Terminals to be inaccessible after the meter is sealed in place;
5.Meter base not to be insulated from the mounting device;
6.Mounting device to have an uninsulated terminal for the service neutral.

The material of the socket jaws is important. A tight contact between the meter connection blade and the contact surfaces of the jaw is necessary. This requires the use of an especially high-quality resilient copper alloy, which may be bronze or beryllium.

Even with the best-quality material, it must be remembered that spreading the jaws by pushing screwdriver blades into them may spring the metal beyond its elastic limit and destroy the tight contact with the meter blades.

It is also necessary that the connection between conductors and the line a nd load terminals be secure and of low resistance. The connectors in the lowerrated sockets may be required to accept conductors as small as No. 6 while the 200- and 400-ampere sockets may be required to accept single or multiple conductors that will carry 200 or 400 amperes.

When aluminum conductors are used, the connectors must be designed for this material; that is, they must not cut the comparatively soft strands and they must not encourage cold flow when the wires are under pressure. If aluminum wire is used in the socket that has plating over the copper connectors, use an aluminum electrical joint compound (antioxidant) to prevent metallic reaction between the alumiun and copper connections.

With the growth of domestic loads and the development of self-contained Class 200 meters, the heavy-duty socket also rated at 200 amperes has been introduced. There are two types of such heavy-duty sockets. In one the jaws are made of massive material and sometimes have only one flexible member.

This may be spring loaded but will still depend on jaw resiliency for good contact. In the other type, the jaws are made of a nonflexible heavy material and the jaws are wrench tightened or lever tightened after the meter is in place. Either type of jaw can carry 200 amperes continuously without excessive heating.

FULL LOAD AND LIGHT LOAD ADJUSTMENT OF WATT HOUR METER

Full-Load Adjustment
The eddy currents in the disk caused by the permanent magnets produce a retarding force on the disk. In order to adjust the rotor speed to the proper number of revolutions per minute at a given (or “rated”) voltage and current at unity power factor, the full-load adjustment is used.

Basically, there are two methods of making the full-load adjustment. One is to change the position of the permanent magnet. When the permanent magnet is moved, two effects result.

As the magnet moves further away from the center of the disk, the “lever arm” becomes longer, which increases the retarding force. The rate at which the disk cuts the lines of flux from the permanent magnet increases and this also increases the retarding force.

The second method of making the full-load adjustment, by varying the amount of flux by means of a shunt, depends on the fact that flux tends to travel through the path of least reluctance. Reluctance in a magnetic circuit is resistance to magnetic lines of force, or flux.

By changing the reluctance of the shunt, it is possible to vary the amount of flux that cuts the disk. One way of doing this is by means of a soft iron yoke used as a flux shunt, in which there is a movable iron screw.

As the screw is moved into the yoke, the reluctance of this path decreases, more lines of flux from the permanent magnet flow through the yoke and less through the disk, so the disk is subject to less retarding force and turns faster.

In either case, the retarding force is varied by the full-load adjustment and, by means of this adjustment, the rotor speed is varied until it is correct. Normally the full-load adjustment is made at unity power factor, at the voltage and test current (TA) shown on the nameplate of the watthour meter, but the effect of adjustment is the same, in terms of percent, at all loads within the class range of the meter.

Light-Load Adjustment
With no current in the current coil, any lack of symmetry in the voltage coil flux could produce a torque that might be either forward or reverse. Because electrical steels are not perfect conductors of magnetic flux, the flux produced by the current coils is not exactly proportional to the current, so that when a meter is carrying a small portion of its rated load it tends to run slower.

A certain amount of friction is caused by the bearings and the register, which also tends to make the disk rotate at a slower speed than it should with small load currents. To compensate for these tendencies, a controlled driving torque, which is dependent upon the voltage, is added to the disk.

This is done by means of a plate (or shading pole loop) mounted close to the voltage pole in the path of the voltage flux. As this plate is moved circumferentially with respect to the disk, the net driving torque is varied and the disk rotation speed changes accordingly.

The plate is so designed that it can be adjusted to provide the necessary additional driving torque to make the disk revolve at the correct speed at 10% of the TA current marked on the nameplate of the meter. This torque is present under all conditions of loading.

Since it is constant as long as applied voltage does not change, a change in the light-load adjustment at 10% of test amperes will also change full-load registration, but will change it only one-tenth as much as light-load registration is changed.

MOTOR IN AN ELECTROMECHANICAL SINGLE-STATOR AC METER

The motor is made up of a stator sensing the phase voltage and current with electrical connections, as shown in Figure 7-14, and a rotor, which provides the function of multiplication. The stator is an electromagnet energized by the line voltage and load current.

Basic Electromagnet (for Two-Wire Meter).

The portion of the stator energized by the line voltage is known as the voltage coil and serves the function of voltage sensor. For meters built since 1960, the voltage coil consists of approximately 2,400 turns of No. 29 AWG wire for a 120 volt coil to more than 9,600 turns of No. 35 AWG wire for a 480 volt coil.

These coils are so compensated that the meter can be used within the range of 50 to 120% of nominal voltage. Because of the large number of turns, the voltage coil is highly reactive.

The portion of the stator energized by the load current is known as the current coil and serves the function of current sensor. For a Class 200 meter, the current coil usually consists of two or four turns of wire equivalent to approximately 30,000 circular mils in size. The current coils are wound in reverse directions on the two current poles for correct meter operation.


Dr. Ferraris, in 1884, proved that torque could be produced electromagnetically by two alternating-current fluxes, which have a time displacement and a space displacement in the direction of proposed motion. The voltage coil is highly inductive, as mentioned before, so the current through the voltage coil (and hence the flux from it) lags almost 90° behind the line voltage.

In modern meters, this angle is between 80° and 85°. Although the current coil has very few turns, it is wound on iron, so it is inductive. However, it is not as inductive as the voltage coil. The power factor of a modern meter current coil may be 0.5 to 0.7 or an angle of lag between 60° and 45°.

It is important to remember that the meter current coils have negligible effect on the phase angle of the current flowing through them. This is true because the current coil impedance is extremely small in comparison to the load impedance, which is connected in series. The load voltage and load impedance determine the phase position of the current through the meter.

With a unity-power-factor load, the meter current will be in phase with the meter voltage. Since current through the voltage coil lags behind current through the current coil, flux from the voltage coil reaches the rotor after flux from the current coil and a time displacement of fluxes exists.

The stator is designed so that the current and voltage windings supply fluxes that are displaced in space. These two features combine to give the time and space displacement that Dr. Ferraris showed could be used to produce torque.

In order to understand why torque is produced, certain fundamental laws must be remembered. They are:

1. Around a current-carrying conductor there exists a magnetic field;

2. Like magnetic poles repel each other; unlike poles attract each other;

3. An electromotive force (EMF) is induced in a conductor by electromagnetic action. This EMF is proportional to the rate at which the conductor cuts magnetic lines of force. The induced EMF lags 90° behind the flux that produces it;


4. If a conducting material lies in an alternating-current magnetic field, the constantly changing or alternating magnetic lines of force induce EMFs in this material. Because of these EMFs, eddy currents circulate through the material and produce magnetic fields of their own;

5. When a current is caused to flow through a conductor lying within a magnetic field, a mechanical force is set up which tends to move the current-carrying conductor out of the magnetic field.

MOVING IRON INSTRUMENT APPLICATIONS IN METERING

Measurement of Current
Since the actuating coil may be wound with a choice of many wire sizes, the instrument may be constructed to measure current from a few milliamperes up to 100 or 200 amperes in self-contained ratings. For measuring currents beyond this range, a 5-ampere instrument may be used with a current transformer.

Current Transformer Field Test Set
A special application of the moving-iron ammeter is the current transformer field test set. The circuit of this instrument is shown below.

Circuit of Current Transformer Field Test Set.

It is used to check current transformer installations in service on the secondary side, for possible defects such as short-circuited primary or secondary turns, high-resistance connections in the secondary circuit, or inadvertent grounds, any of which could cause incorrect metering.

It is essentially a multi-range, moving-iron-type ammeter with a built-in burden which is normally shunted out, but which can be put in series with the meter by the push button.


In the typical instrument illustrated here, ammeter current ranges of 1.25, 2.5, 5, and 10 amperes are obtained from the tapped primary winding of a small internal current transformer, the secondary winding of which is connected to the ammeter which has corresponding multiple scales.

It is thus possible to obtain a reading well up-scale on the ammeter for most load conditions under which the current transformer is operating. The rotary burden switch permits the addition of 0.25, 0.5, 1, 2, or 4 ohms to the secondary circuit as desired.

The imposition of an additional secondary burden on a current transformer having the defects previously mentioned will result in an abnormal decrease in the secondary current. The extent of this decrease and the ohms burden required to effect it depend on the characteristics of the transformer under test.

The check on the current transformer consists of inserting the field test set in series with the current transformer secondary circuit and comparing the ammeter readings under normal operating conditions with the readings after the additional field test set burden is added.

Measurement of Voltage
By the use of an actuating coil of many turns of fine wire in series with a resistor, the moving-iron instrument may be used to measure voltage. Such a voltmeter may have an operating current of around 15 milliamperes with a range up to 750 volts.

External multipliers may be used to extend this range. These voltmeters are used in applications where sensitivities lower than those of the rectifier d’Arsonval instrument are satisfactory.

The moving-iron voltmeter may be used on DC with some loss in accuracy. The best accuracy is obtained by using the average of the readings taken before and after reversal of the leads to the instrument terminals.

This instrument will not indicate the polarity of DC.

ELECTROMECHANICAL METER PERMANENT MAGNET OR MAGNETIC BREAK

Another essential part of the electromechanical meter is a magnetic brake. Torque on the disk caused by interaction of fluxes tends to cause constant acceleration.

Without a brake, the speed of rotation would only be limited by the supply frequency, friction, and certain counter torques at higher speeds (discussed in later paragraphs concerning overload compensations).

Therefore, some method of limiting the rotor speed and making it proportional to power is needed. A permanent magnet performs these functions. As the disk moves through the field of the permanent magnet, eddy currents result in much the same manner as though the magnetic field were changing as previously described.

These eddy currents remain fixed in space with respect to the magnet pole face as the rotor turns. Again, as in the case of eddy currents caused by fluxes from the voltage and current coils, the eddy currents are maximum when the rate of cutting flux lines is greatest.

In this case the cutting of flux lines is caused by the motion of the disk, so the eddy currents are proportional to the rotational speed of the disk. They react with the permanent-magnet flux, causing a retarding torque which is also proportional to the speed of the disk.

This balances the driving torque from the stator so that the speed of the disk is proportional to the driving torque, which in turn is proportional to the power flowing through the meter. The number of revolutions made by the disk in any given time is proportional to the total energy flowing through the meter during that time interval.

The strength of the permanent magnet is chosen so that the retarding torque will balance the driving torque at a certain speed. In this way the number of watthours represented by each revolution of the disk is established.

This is known as the watthour constant (Kh) of the meter.

D'ARSONVAL INSTRUMENT - PERMANENT-MAGNET,MOVING-COIL INSTRUMENT

Figure below represents the mechanism of a permanent-magnet, moving-coil instrument. Here, the field produced by the direct current in the moving coil reacts with the field of the permanent magnet to produce torque.

Mechanism of Permanent-Magnet, Moving-Coil Instrument.

Essentially, the permanent-magnet, moving-coil instrument, often called a d’Arsonval instrument, consists of a very lightweight, rigid coil of fine wire suspended in the field of a permanent magnet. The moving coil in most instruments consists of a very lightweight frame of aluminum, flanged for strength and to retain the windings.

The windings consist of several layers of fine enameled wire. Pivot bases are cemented to the ends of the coil frame. These bases carry the hardened steel pivots on which the coil turns as well as the inner ends of the control and current-carrying springs.

In addition, the upper pivot base mounts the pointer and the balance cross. Threaded balance weights, or their equivalents, are adjusted on the balance cross to balance the moving element in its bearing system.

The pivots ride in jewel bearings to keep friction at a minimum. A taut band suspension may be used in place of the pivot and jewel-bearing system. Here the moving coil is supported by two metal ribbons under tension sustained by springs.

Either bearing system allows a properly balanced instrument to be used in any position with little error.


Current is carried to the coil by two springs. These control and current-carrying springs oppose the torque of the moving coil and serve as the calibrating means of the instrument.

The springs are generally made of carefully selected phosphor bronze or beryllium copper specially manufactured to provide stability so that the instrument accuracy will not be affected by time and use. The torque developed by current flowing through the moving coil is a function of the field strength of the permanent magnet and of the current in the moving coil, as well as the dimensional factors of both magnet and coil.

The torque T, in dyne-centimeters, is given by this equation:

T = B x  A x I x N


where:
B flux density in lines per square centimeter in the air gap
A coil area in square centimeters
I moving-coil current in amperes
N turns of wire in moving coil

The characteristics of the moving-coil instrument are very desirable. It has a high degree of accuracy, high sensitivity, low cost, and a uniform scale. It can measure extremely small currents because of the fine wire in the moving coil.

The instrument is unique in the variety of accessories that can be used in conjunction with it. The four most commonly used are the series resistor, the shunt, the thermocouple, and the rectifier.

There are two inherent shortcomings of the permanent-magnet, moving-coil mechanism: except in a specially scaled instrument with rectifiers or thermocouples, it cannot measure AC quantities, and without auxiliary shunts or multipliers it can measure only small electrical quantities.

WHAT IS COMPENSATION METERING?

The objective of compensation metering is to determine unmetered losses which occur between the billing and metering points, and then record the losses on a loss meter or combine the losses with the metered portion of the load on a single meter. The common practice is to combine the losses with the metered portion which duplicates values which the meter would have recorded had it been located at the billing point.

Energy dissipated between the billing and metering points cannot be measured directly. The losses are calculated indirectly using transformer theory, circuit theory, and currents and voltages at the meter test switch or meter socket.

Commercially available compensation meters operate with formulas approved by meter engineers and regulatory agencies. These formulas add or subtract simulated losses to the metered load and record compensated meter readings, or uncompensated readings with simulated losses directed to a separate loss meter.

TRANSFORMER LOSSES
Losses in the transformer are caused by hysteresis, eddy currents, and load currents. Hysteresis losses are derived from energy expended as the magnetic field within the transformer continually changes intensity and direction. Hysteresis losses are a function of the metallurgical properties of the core material. Eddy current losses are caused by energy expended by current, induced by the magnetic field, and circulating within the transformer core.

Eddy current losses are minimized by building the transformer core from electrically resistive steel formed into thin, insulated laminations. Load losses, or I2R losses, are caused by current passing through the transformer windings and the resistance of those windings. Transformer losses are either no-load losses, also called core losses or iron losses, and load losses, also called copper losses.

LINE LOSS COMPENSATIONS
Lines are considered to be resistive and have I2R losses. The lengths, spacings, and configurations of lines are usually such that inductive and capacitive effects can be ignored.

Bus losses are treated the same as line losses. If line and bus losses are to be compensated, they are included as part of the transformer load losses. Most solid-state meters can compensate for both resistive and reactive losses.


Electromechanical Transformer-Loss Meters
The basic measurement requirements for transformer losses call for a meter having one voltage-squared stator and one or more current-squared stators, depending on the number of metering current circuits. All stators are combined on the same shaft which drives a register of proper ratio to record the losses in kilowatthours or kiloVARhours.

The E2 stator consists of a standard watthour meter voltage coil and a low-current (possibly 50 mA) winding, which is connected in series with an adjustable resistor that serves as a core-loss adjustment. Registration is proportional to E2. An I2 stator consists of a standard current coil and a low-voltage voltage coil connected across the current coil and a series resistor in the current circuit. Registration, therefore, is proportional to I2.

ELECTRICITY METERING FOR THREE-WIRE NETWORK SERVICE SYSTEM DIAGRAMS

Two-Stator Meter
Three-wire network service is obtained from two of the phase wires and the neutral of a three-phase, four-wire wye system, as shown in Figure 7-2. It is, in reality, two two-wire, single-phase circuits with a common return circuit and it has voltages that have a phase difference of 120 electrical degrees between them.

The voltage is commonly 120 volts line-to-neutral/208 volts line-to-line.

The normal method of metering a network service is with a two-element (two-stator) meter connected as shown in Figure 7-2. With this connection, which follows Blondel’s Theorem, each stator sees the voltage of one phase of the load. The phasors representing the load phase currents, IAN and IBN, are shown in the diagram lagging their respective phase voltages.

Figure 7-2 Two-Stator Meter on Three-Wire Network Service.

The meter current conductors carry the line currents, IAN and IBN, and inspection of the circuit shows that these currents are identical to the load phase currents. Hence, the meter correctly measures the total load power. Any loads connected line-to-line, between A and B in Figure 7-2, will also be metered properly. With this type of meter there are no metering errors with imbalanced load voltages or varying
load currents and power factors.


As such, the watt metering formula for any instance in time is:

Watts  (VAN X IA) + (VBN X IB)

Accumulating the watts over time allows the metering of watthours.


Single-element (single-stator) meters for measuring network loads have been developed and may be used with reasonable accuracy under particular load conditions.  The conventional three-wire, single-element (single-stator), single-phase meter cannot be used for network metering.

It will, of course, measure the 208-volt load correctly; but the two 120-volt loads are metered at 104-volts rather than at 120 volts and at a phase angle which is 30 degrees different from the actual. Therefore, for 120-volt balanced loads, meter registration will be close to 75 percent of the true value; but with imbalanced loads, the resulting meter error varies, rendering such metering useless.

Single-Stator Meters for Three-Wire Network Service
Single-stator meters have been developed for use on three-wire network services. These meters do not conform to Blondel’s Theorem and are subject to metering errors under certain conditions noted in the following paragraphs.

The schematic connections for the two types of meters now in use are shown in Figure 7-3. Each meter has one voltage coil and two current coils. In one case, Figure 7-3a, the meter is designed to use line-to-line voltage (208 V) on the voltage coil and the other, Figure 7-3b, uses one line-to-neutral voltage (120 V) on its voltage coil.

The currents in the meter current coils are shifted in phase to provide correct metering. Obviously, any imbalance in line-to-neutral voltages will cause metering errors and, where imbalanced voltages exist, a two-stator meter should be used for accurate results.

In electromechanical meters, the current phase shifting is accomplished by impedance networks of resistors and inductors along with the current coils to split the total line currents and shift the phase position of the meter-current-coil current the desired amount. The number of turns and the impedance of the current coil may also be varied in design to obtain a usable meter.

The current-impedance networks are shown in Figure 7-3. Electronic meters can accomplish the phase shifting using a variety of techniques. Phase sequence of voltages applied to these meters is extremely important since such meters can usually only be designed to provide the correct phase shift of meter-coil current for only one phase sequence.

If they are installed on the wrong phase sequence their energy registration is useless. All meters of this type have a built-in phase-sequence indicator.

Figure 7-3 Single-Stator, Three-Wire Network Meters.

THE MOVING-IRON INSTRUMENT - ELECTRIC METERING PARTS

The measurement of alternating current (or voltage) is the measurement of a quantity that is continuously reversing direction. The permanent-magnet, moving- coil instrument movement cannot be used since the AC field of the moving coil reacting with the unidirectional permanent-magnet field will produce a torque reversing in direction at line frequency.

Because of its inertia, the moving element will be unable to respond to this rapidly reversing torque and the pointer will only vibrate at zero. A different type of meter movement is therefore required.

The moving-iron instrument is specifically designed to operate on AC circuits. This instrument is called the moving-iron type because its moving member is a piece of soft iron in which magnetism induced from a field coil interacts with the magnetic field of a fixed piece of soft iron to produce torque.

Figure 1 - Mechanism of Moving-Iron Instrument

The mechanism of this instrument, shown in Figure 1, essentially consists of a stationary field coil, with two soft iron pieces in the magnetic field. One is fixed while the other, commonly called the moving vane, is attached to a pivoted shaft provided with a pointer which is free to rotate.

When current flows through the field coil the two pieces are magnetized with the same polarity, since they are both under the influence of the same field and, hence, repel each other, causing the pivoted member to rotate. The angular deflection of the moving unit stops at the point of equilibrium between the actuating torque and the counter torque of the spiral control spring.


The illustration shows that the operating current flows through a stationary winding. Depending upon the use for which it is designed, the coil may be wound with fine or heavy wire, giving this type of instrument a wide range of capacities. The instrument will tolerate overloads with less damage to springs and pointer than will most other types of instruments, since, with excess current, the iron vanes tend to become saturated and limit the torque.

Damping is provided by either a light aluminum vane fixed to the shaft and moving in a closed air chamber, or by a segment of an aluminum disc moving between poles of small permanent magnets.

The bearing system may consist of a pivoted shaft turning in jeweled bearings or may be of the taut-band suspension type where the moving element is supported by two metal ribbons under tension sustained by springs.

Application of Moving-Iron Instrument
Measurement of Current
Since the actuating coil may be wound with a choice of many wire sizes, the instrument may be constructed to measure current from a few milliamperes up to 100 or 200 amperes in self-contained ratings. For measuring currents beyond this range, a 5-ampere instrument may be used with a current transformer.

Current Transformer Field Test Set
A special application of the moving-iron ammeter is the current transformer field test set. The circuit of this instrument is shown in Figure 2. It is used to check current transformer installations in service on the secondary side, for possible defects such as short-circuited primary or secondary turns, high-resistance connections in the secondary circuit, or inadvertent grounds, any of which could cause incorrect metering.

 Figure 2 - Circuit of Current Transformer Field Test Set

It is essentially a multi-range, moving-iron-type ammeter with a built-in burden which is normally shunted out, but which can be put in series with the meter by the push button.


In the typical instrument illustrated here, ammeter current ranges of 1.25, 2.5, 5, and 10 amperes are obtained from the tapped primary winding of a small internal current transformer, the secondary winding of which is connected to the ammeter which has corresponding multiple scales. It is thus possible to obtain a reading well up-scale on the ammeter for most load conditions under which the current transformer is operating.

The rotary burden switch permits the addition of 0.25, 0.5, 1, 2, or 4 ohms to the secondary circuit as desired. The imposition of an additional secondary burden on a current transformer having the defects previously mentioned will result in an abnormal decrease in the secondary current. The extent of this decrease and the ohms burden required to effect it depend on the characteristics of the transformer under test.

The check on the current transformer consists of inserting the field test set in series with the current transformer secondary circuit and comparing the ammeter readings under normal operating conditions with the readings after the additional field test set burden is added.

Measurement of Voltage
By the use of an actuating coil of many turns of fine wire in series with a resistor, the moving-iron instrument may be used to measure voltage. Such a voltmeter may have an operating current of around 15 milliamperes with a range up to 750 volts.

External multipliers may be used to extend this range. These voltmeters are used in applications where sensitivities lower than those of the rectifier d’Arsonval instrument are satisfactory. The moving-iron voltmeter may be used on DC with some loss in accuracy. The best accuracy is obtained by using the average of the readings taken before and after reversal of the leads to the instrument terminals.

This instrument will not indicate the polarity of DC.

ACCURATE METER READING – REVENUE IMPORTANCE

The accurate reading of meters is an operation of major importance for any electric utility company, not only from a revenue standpoint but also in the promotion of strong customer relationships. In gaining access to meters, the meter reader makes many personal contacts with customers and is often the only utility employee seen by the customer.

With this in mind, most electrical utility companies choose, for their meter reading personnel, employees who are conscientious and have the natural attributes of friendliness, courtesy, and a

neat appearance.

A meter reader’s initial training course usually includes the fundamentals of good public relations, familiarization with the types of metering equipment likely to be encountered, and thorough explanations of the terms used in customer billing.

It is important that meter readers be familiar enough with their company’s organizational structure to be able to channel a customer’s requests and questions to the proper departments.

Meter reading is carried out under a carefully planned program, and meter routes are arranged in proper reading order. Readings are taken on a pre-arranged schedule to make the billing cycle practicable and to provide an even flow of work for other operations, such as billing, auditing, and collecting.

To prevent undue annoyance to customers and to expedite the work, meter-reading records usually contain notations showing the exact locations of meters, means of access, keys needed (if applicable), and notes of any unusual conditions, such as physical hazards and/or dangerous pets.

If it is necessary to enter a customer’s premise, it is a good practice for readers to announce themselves courteously and produce proper identification when requested.

Meter readers can be of considerable help to the company in reporting irregularities, such as changes in customers, meters without a reading record, vacant buildings, stopped meters, unsealed meters, unmetered service, and any other condition that might adversely affect customer billing, safety, or the quality of service rendered.

The principal duty of a meter reader is to obtain the meter readings and to make certain that these readings are entered on the correct reading records by verifying addresses and meter numbers. A good meter reader will enter the readings on mark sense cards or meter reading sheets in a clear, precise, and legible manner.

HOW TO READ WATT HOUR (ELECTRICITY) METER – THE IMPORTANCE OF CORRECT READING

Note: This article is specific for Electromechanical Meter Types.

There are three styles of mechanical kilowatthour meter register types in general use. One has individual dial circles as shown below:

Conventional Five-Pointer Kilowatthour Dial.

Another has interlocking dial circles as shown below:

Conventional Four-Pointer Kilowatthour Dial with Overlapping Circles.

The third style of register uses cyclometer-type dials.

Registers with dial circles have either four or five dials; five dials being provided to avoid a dial multiplier of 10 and the possibility of a register “turn-over” during the normal billing period.

Adjacent pointers rotate in opposite directions and are geared for travel so that the pointer on the right will make one complete revolution while the one next to it on the left makes one-tenth of a revolution.

When a pointer is between two figures, the smaller figure is the one to use for the reading. A watthour meter is read from right to left by reading all dials and recording the reading on a meter reading form in this same sequence.

The reason for reading the dials from right to left is that the right-hand dial governs the one to its left in each instance. With all pointers at zero and a dial multiplier of one, one clockwise revolution of the unit’s dial pointer will indicate a reading of 10 kilowatthours on the register.

A complete counterclockwise revolution of the 10’s dial pointer will indicate a reading of 100 kilowatthours on the register and so on. When reading the dials the procedure is analogous to reading 1s, 10s, 100s, and 1,000s.

Remember that each pointer must complete a revolution to advance the pointer located to its left by one division. Therefore, to correctly determine the reading of a pointer, the previous pointer (located to the right) must be consulted.

Unless this pointer has completed a revolution by reaching or passing the 0, the pointer in question has not completed the division on which it may appear to rest. For this reason, reading the meter from right to left increases both accuracy and efficiency.

A simple analogy can be made to a wristwatch. When the hour hand is near 8 and the minute hand is at 11, it is not yet 8 o’clock, but it is 7:55 and, obviously, it will not be 8 o’clock until the minute hand has advanced to 12.

Figures 17-3 and 17-4 show examples of typical watthour meter readings. To obtain the use in kilowatthours over a designated period of time, it is necessary to subtract the previous reading from the present reading.

When the dial multiplier is one, the difference will be the number of kilowatthours consumed between the two readings. When the dial multiplier is a number other than one, the difference between the readings must be multiplied by the given dial multiplier to obtain the kilowatthours consumed.

Dial multipliers of one generally are not shown, but those other than one are shown on the dial faces. Double- or two-rate registers employ two sets of dials and two complete register mechanisms that are automatically switched into gear with the moving element shaft at predetermined times. These two-rate registers are generally used in conjunction with off-peak water-heating rates.