Understand how to read energy meters.

Have you ever had difficulty trying to troubleshoot incorrect readings on energy meters? This Tech Tip should make things a little clearer.

What is a normal reading?

Most power systems have a lagging (inductive) power factor such as when a current lags behind the voltage. This occurs mostly in systems where there are power factor correction capacitors which increase the chance for a leading power factor to exist.

A normal reading would be one with the following values. A power factor is lagging in the range of -0.70 to 1.00, the kW and kVA readings are positive with a negative KVAR, and the phase voltage and currents are approximately equal to one another.

A further check can be made to confirm the voltages and currents displayed on the meter match up with system values measured by a multimeter. If they don’t, then check the VT and CT ratios are correctly programmed in the meter.

Assuming all these checks out then the meter is reading correctly and no further investigation is required.

What are abnormal readings?

If the amps, kW, kVAR and KVA are reading zero then there are a couple of possibilities, one being the CT secondaries are shorted or the load on the circuit is less than 2% of the CTs primary current. For example, if there are less than 2 amps on a circuit with 100:5 ratio CTs installed.

Another scenario would be a negative KW reading and a positive KVAR with a normal lagging power factor. In this case, there are a number of possible causes, which are as follows...

All three CT polarities are mounted backward. An example may be the primary polarity mark facing towards the load instead of towards the supply or the CT secondaries are swapped. Alternatively, in the case of PTs that have not been mounted backward, it may be primarily connected to secondary and vice versa.

Another possible abnormal reading is the frequency reading is nowhere close to the supply frequency. In this case, if PTs are being used, then check that the primary and/or secondary neutral common has not been grounded or that there is not a problem with the system grounding back at the power distribution transformer.