Meter Set Up Question

[hbendillo]

I already have this on another forum here but think this is where it should be.

One of my clients has an Amprobe DM II Pro meter. This meter will measure the current and voltage of each phase and the reports will show phase voltages and currents; real, reactive and apparent power; and power factor.

The amperage clamps are rated for up to 1000 amps and there is one for all three phases of a three-phase system and one for the neutral. The feeder were measuring is rated 1200-amps, 208-volt three-phase, four-wire. The feeder has three conductors per phase.

Because of the way the conductors were attached to the lugs at which were taking the measurements my client could not get the amperage clamp around all three conductors of each phase. They only clamped around one conductor of each phase and neutral. The results were interesting. Where I expected to see probably 200 to 300 amps of phase current the report showed 2000 to 3300 amps. This is roughly three time what I would have expected if we would have been able to clamp around all three conductors. Is there a correlation here? What is the effect of only clamping around one conductor of each phase? Can I extrapolate the actual current on each phase from this?

 

[jghrist]

If the current doesn't vary quickly with time, then you can approximate the total by adding the measurements from each conductor.

I would suspect a measurement settings error if you get 2000 to 3300 amps on a 1200 amp rated feeder.

 

[LionelHutz]

No, I would not consider that a valid test method.

The current on each conductor may not be equal so you can not measure 1 conductor and multiply by 3.

The current transformer used may have loaded or modified the resistance of the conductor slightly and changed the current in that conductor skewing the results.

You should be able to a current transfomer around each conductor of a phase, ie use all 3 current transformers for one phase instead of one per phase and then add up the 3 currents you measure. However, you can get a current measurement but lose the ability to use the power features of the meter.

The results may be caused by a set-up error with the instrument and an error caused by only using 1 of the parallel conductors.

 

[rbulsara]

Flexible current sensors (Flex CTS) are easily available that will go around all three or more phase conductors. Just invest in or rent right equipment.

I agree with both of above comments.

 

[hbendillo]

Thanks for the replies but Iwas hoping someone could come up with a logical explanation for the readings.

 

[rbulsara]

Most probably reason: You have a multiplier or scaling error. If you know the total load is not more than 1000A, you could be reading 2000-3000A per phase. It could only be an error some place. There is no logic for that.