Most electric metering people know what a test board is – the equipment used to calibrate the watt-hour meter. The technical name for a test board is a watt-hour meter calibrator.
The watt-hour meter calibrator is the standard against which all watt-hour meters are tested/adjusted. If your calibrator is out of tolerance, all the meters tested/adjusted against it may also be inaccurate. This could be devastating to a company’s bottom line, without the company even knowing that there is a problem. If the calibration is running too fast, it’s possible that the meters tested are running too fast for the customer as well. This could result in refunds to consumers and – in extreme cases – law suits. If the meters are running too slow, this will lower the income of the company substantially. As an example, if a utility has 100,000 meters and the meters are running 1% low for a kilowatt hour, and the utility charges 10 cents per kilowatt hour, the utility loses $1,000 per hour for each hour the meters run under full load. Add this up over the course of a year and the amounts are staggering.
A three-phase watt-hour calibrator being compared to 3 watt-hour standards
REGULAR CALIBRATION NEEDED
As watt-hour meters have become highly developed and more accurate, so has the calibrator, which is a very accurate, highly sophisticated piece of test equipment. Over the years there have been many manufacturers and models of watt hour meter calibrators, and the documentation supplied by all manufacturers of test boards (or any electronic standard) states that the equipment should be calibrated on a yearly basis. In order to verify the calibrator’s accuracy, a primary watt-hour standard has to be compared to it, which must be calibrated to the National Institute of Standards and Technology (NIST) which holds and maintains electronic standards of all types. The watt-hour is a product of voltage current and phase angle. Once the primary standard is traceable to NIST, it can be compared to the test board. The accuracy of newer test boards is usually around .05% error or better – a very high level indeed, and one reason why the calibrator must be verified annually. The manufacturers of newer test boards have software specially designed to compare the NIST traceable standard to the test board. Depending on your calibrator, special adapters and wiring will be required. Some calibrators need internal adjustments, some calibrators need resistors and capacitors changed to achieve the manufacturer’s specifications for calibration, and some rely on computer offsets. If your test board relies on computer offsets and the computer fails, the calibrator will require re-calibration.
In each state in the US there is a governing body overlooking the electricity industry, called the Public Utility Commission (PUC). The PUC has different rules for public, private, municipals, co-ops, towns and cities, and their requirements vary from state to state. Each state PUC has a website and additional information can be found there. In some states, utilities are required to send proof of calibration to the PUC every year.
The PUC conducts on-site inspections of testing facilities for watt-hour meters. The PUC will monitor the handling and testing of the meters, as well as inspection of the calibrator’s verification history. If there is a customer complaint about high energy usage, the PUC can become involved and ask to verify the utility’s handling and testing procedures.
The watt-hour meter will be traceable to NIST in an unbroken chain. This chain would be the primary watt-hour standard compared to the watt-hour meter calibrator and then down to the watt-hour meter. The accuracy of the watt-hour meter is specified by the manufacturer of the meter, and ultimately the utility will decide on an acceptable accuracy level for the meters it has in the field.
SINGLE VERSUS POLYPHASE
There are single phase meters and polyphase meters, and there are also single phase test boards and polyphase test boards. The single phase test board means there is no phase shift between the elements, while a polyphase test board has the capability to induce a phase shift between the elements.
Any meter can be tested on either test board. For example, if a 9 S meter is being tested in a single phase test board there would be no phase shift between A to B, B to C, C to A, but if a polyphase test board is used, the 9 S can be tested in a 4-wire WYE configuration, which would induce a 120° phase shift between A-B-C phases. When running an external standard against a polyphase test board, you will in some cases need three traceable primary watt-hour standards.
Many utilities purchase primary standards and have them calibrated yearly so their test board can be compared against the standards. Others contract out the test board calibration, because the cost is often less than the cost of maintaining their own standards, and in addition they prefer to have an outside source to ensure that traceability is maintained. If an outside source is used, it is important to make sure it is traceable to NIST, uses the correct number of standards, and uses the correct adapters and wiring for your unit.
What is clear is that if instruments such as watt-hour meters are to be tested and if necessary adjusted, the equipment that is used to perform these tests must be correct in its measurements.