GOOD PRACTICES FOR METERING PERSONNEL

Efficient metering personnel will make good installations. In doing so, they will observe certain practices that will be helpful to both their company and the customers. These good practices have many benefits. They insure good service by preventing unnecessary outages.

They insure good customer relations by preventing damage to the customer’s equipment. Also, the meter employee will not have to return to the customer’s premises for things forgotten or left undone and thereby undermine the customer’s confidence in electric metering.

All these benefits, in turn, help the company and the employee. Following are some of the good metering practices, not necessarily in order of importance. Competent metering employees will:

Recognize their responsibilities while on the customer’s premises.
• Take the nearest and safest route to accomplish the work.
• Take care not to damage any of the customer’s property.
• Leave the area clean upon completion of the job.
• Report any hazards to the meter department supervisor.

Work in the safest possible manner.
• Keep in mind that no job is so important that it cannot be done safely.
• Inspect all meter wiring connections for correctness
• Check connections to prevent outages, damage to meter installation, damage to customer’s property, and personal injury.

Inspect for loose connections.
• A loose connection can cause intermittent service or it can cause a complete outage. Loose connections generally arc, causing a fire hazard. Even if there is no fire damage, heating around the connection occurs.
Inspect for good grounding.
• Check for equipment ground at the installation. Realize that no ground at the installation is a potential hazard and report it to the supervisor.

Pay attention to details.
• Check meter voltage links. Inspect connections between two dissimilar metals.
• Connection between two dissimilar metals often causes corrosion.
• Corrosion can be prevented by using the proper connector and by protecting the connector and conductors against oxidation.
• Wires corroded at a joint have the same effect as a loose connection since corrosion has a high resistance and causes heating, which, in turn, assists the corrosive action.

Check for proper voltages.
• Voltage should be checked before installing the meter.
• A reversal of the power and lighting leg on a four-wire delta system causes excess voltage on customer’s equipment. Also, a reversal of the hot leg and ground has serious consequences.
• Grounded conductors and the power leg of four-wire delta services should be permanently identified.

Check phase rotation.
• Phase rotation on installations which have been disconnected temporarily for service work should be checked. If a reverse phase rotation is connected to the customer’s motors, they will reverse, possibly causing extensive damage. This could also mean personal injury.

Check for single phasing.
• It is possible to prevent damage to the customer’s property by disconnecting or warning the customer to disconnect the load on a three-phase service when one phase is out. A running three-phase motor may continue to run on single phase but will overheat. A stopped motor may attempt to start but cannot, which causes overheating.

Observe direction of disk rotation.
• Whenever possible, try to get a load applied to the meter in order to check for correct disk rotation.

Check for diversion.
• Always check for circuits tapped ahead of the meter or current transformers.

Check for correct installation information.
• Check for correct phase, amperes, volts, and frequency.
• Check for such details as multipliers, full scales, readings, and similar data.
• Check all written records against actual nameplate data.

Check to see if meter is level.
• An out-of-plumb meter may be inaccurate. Besides being inaccurate, it presents an unsightly appearance to the customer and may undermine his confidence in the electric metering.

Give the entire job a good once over before leaving it.
• Check the job in general for good workmanship and safety before leaving. Be sure the area surrounding the meter is left clean and neat.

Meter employees are the company in the eyes of many customers. They can make a good impression on the customer by being neat in dress, accurate in work, and courteous at all times.

Having equipment and tools in good, clean condition will build the customer’s confidence in the company and assure the customer of the employee’s skill. Sloppy dress, actions, and equipment leave a poor impression.

There are probably many other practices which are followed on local levels throughout the country, but if employees observe those listed, they will turn out a good job.

Failure to follow any one of these practices may result in extensive property damage, personal injury, outage of service, or a loss of revenue. And, last but by no means least, it may impair that valuable asset to a public utility—good customer relations.

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.

DIGITAL KILOWATT HOUR METER RESOLUTION AND ACCURACY

Resolution
The following example illustrates the resolution and accuracy of the digital-ramp A/D converter. Assume the following values for the A/D converter of Figure 6-1: D/A converter has a 10-bit input and a full scale analog output of 10.23 volts; the comparator can detect a voltage difference of 1 millivolt or greater; Vin is 3.728 volts.

Since the D/A converter has a 10-bit input, the maximum number of steps possible is (210 1)  1023. With a full-scale output of 10.23 volts reached in 1023 steps, the step size is 10 millivolts. This means Vout increases in steps of 10 mV as the counter counts up from zero.

Since Vin  3.728 volts and the comparator threshold is 1 mV, then Vout has to reach 3.729 volts or greater before the comparator switches Low. At 10 mV per step, this requires 373 steps.

At the end of the conversion, the counter holds the binary equivalent of 373, which is 0101110101. This is the digital equivalent of the analog input of Vin  3.728 volts. The resolution of this A/D converter is equal to the step size of the D/A converter which is 10 mV, or approximately 0.1% (.010/10.23  100  0.1%).

The resolution of an A/D converter is equal to the resolution of the D/A converter that it contains. The D/A output voltage Vout is a staircase waveform (digital ramp) that goes up in discrete steps until it exceeds Vin. Thus, Vout approximates Vin.

When the resolution (step size) is 10 mV, the accuracy we can expect is that Vout is within 10 mV of Vin. The resolution of the D/A converter is an inherent error, often referred to as a quantizing error. This quantizing error can be reduced by increasing the number of bits in the counter and in the D/A converter. It is specified as an error ± 1 least significant bit (LSB), indicating that the result can vary by that much due to the step size.

From another point of view, the input voltage Vin can take on an infinite number of values, from 0 to full scale. However, the output voltage Vout has only a finite number of discrete values. This means that similar values of Vin within a small range could have the same digital representation.

For example, if the counter goes through 1,000 steps from zero to full scale, any value of Vin from 3.720 to 3.729 will require 373 steps, thus resulting in the same digital representation. In other words, Vin must change by 10 millivolts (the resolution) to produce a change in the digital output.


Accuracy
The D/A converter accuracy is not related to the resolution. It is related to the accuracy of the components in its circuit such as the resistors in the D/A network, comparator, level amplifiers, and the reference power supply. If a D/A has an accuracy of 0.01% full scale, the A/D converter may be off by 0.01% full scale owing to non-perfect components.

This error is in addition to the quantizing error due to resolution. These two sources of error are usually specified separately, and for a given A/D converter are usually of the same order of magnitude.


In addition to the inherent errors noted above, the accuracy of an electronic instrument depends on proper selection of the meter range. Normally, the uncertainty of measurements is expressed as a percent of the reading plus the number of counts of the least significant digit (LSD) displayed for that range.

If the 1,000 volt DC range is selected to measure a 2 volt signal for a three-and-a-half digit digital multimeter with a nameplate accuracy of 0.5% of input voltage 1 LSD, this setup would result in a meter accuracy of 50.5%, as shown below.

Given: Meter Range Accuracy (MRA) is 0.5% of input voltage 1 LSD
Meter range set to 1,000 volts DC
Input voltage is 2 volts DC
Then: Meter Accuracy  [(MRA  input V  LSD)/Input Voltage]  100
 [(0.5%  2  1)/2]  100
 50.5%

However, selecting a meter range of 2 volts DC on the same digital multimeter would result in an accuracy of 0.60%, nearly 100 times better, as shown below.

Given: Meter Range Accuracy is 0.5% of input 1 LSD
Meter range set to 2 volts DC
Input voltage is 2 volts DC
Then: Meter Accuracy  [(MRA  input V  LSD)/Input Voltage]  100
 [(0.5%  2  0.002)/2]  100
 0.6%


Digital Display Resolution and Accuracy
Typical handheld digital instruments display from 3 to 5 digits. Laboratory digital instruments often offer 7 or 8 digits. The number of digits directly affects the available resolution of the reading.

For example, a full 4-digit display is capable of presenting numbers from 0 to 9999 (with a decimal point somewhere in the display depending on the range setting of the instrument). This display can provide 10000 different readings for a particular range setting, so its resolution is limited to 1 part in 10000, or 0.01%. You may see this display referred to as a 10000-count display.

A 6-digit display can present numbers from 0 to 999999. This display resolution would be 1 part in 1,000,000 or 0.0001%. It may be called a 1,000,000-count display.

Examples in the previous section used LSD, Least Significant Digit, to adjust accuracy calculations to the characteristics of the display.

The design of a digital instrument often further limits the display. A 4-digit display, by design, may display numbers from 0 to 3999, rather than to 9999. That is, the left-most digit is programmed such that it only displays the numbers 0 to 3.

This display is described as a 31/2-digit display or as a 4000-count display. This design does not further affect the accuracy of calculations. The value of the LSD is the same for a 31/2-digit display as for a 4-digit display.