Metering

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DHkorn

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Hey all,

A customer asked me a question I couldnt answer.
They have private sub-metering in their building and their meter reader recently noted that there is an inline GE kilowatt hour meter rated 240v 3? 3W installed on a 208v 3? 4W circuit.
He believes it is probably not metering correctly, and I cant find any information one way or another.
Can anyone help?
Thanks in advance for your help.
Dan
 
It may be OK but I would need more details on the meter type and how it is connected (along with any CTs and PTs).

Here is the normal routine:
Blondel's Theorem allows us to use a two-stator meter to meter the 3-wire circuit (you can use N-1 wattmeters to meter energy delivered by N wires as long as you have one of the N wires as a common point).

To get the two-stator meter to work on a 4-wire wye circuit, we make an assumption that the voltages are balanced. Suppose we have Van and Vcn. We can then assume that Vbn ~ -Vcn + -Van (not a bad assumption for most cases). We have the watts for phase A (Van * Ia) and for phase C (Vcn * Ic).

We are missing the watts for phase B (Vbn * Ib) but we can use (Vbn * Ib) = ((-Vcn -Van) * Ib) = ((Vcn + Van) * -Ib). In other words, we can just run the phase B current backwards through the CT for the A stator and for the B stator and the meter will yield the approximate total watts.

By using the A & C voltage along with the negative B current we can approximate the watts for the third phase. The two wattmeters will measure A & C along with the B to give us a total. The accuracy falls off the more the voltages are imbalanced.
 
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