EMI filters causing high current indications.

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Twoskinsoneman

Senior Member
Location
West Virginia, USA NEC: 2020
Occupation
Facility Senior Electrician
Got a situation where a 1600a CB feeds Feeds through a Corcom/Cornell Dubilier (http://www.cor.com) facility EMI filter to another 1600a CB before it goes to an ATS.

I noticed that with the second CB (one right before ATS) open, the first CB still reads 130a on its LCD panel. When I consulted with the engineer he told me that the filters don't actually consume any power but are creating a resonance that may be tricking the first breaker upstream on the filters into thinking there is 130a load on it. The only load possible would be if the filter were drawing... I haven't been able to put a meter on.

What do you think?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110516-1250 EDT

Measure the phase angle of the current. It is probably all capacitive and at least some current would be expected. Measure the power input to the filter with no load. Probably very small.

However, the current seems high. At 60 Hz and 240 V the impedance for 130 A is about 1.8 ohms. This would be a rather large capacitor in the filter.

Use other means to verify the current. Ideally use an oscilloscope. There may be substantial harmonic components.

Try to find out the circuit and values of the Corcom filter.

.
 

Twoskinsoneman

Senior Member
Location
West Virginia, USA NEC: 2020
Occupation
Facility Senior Electrician
110516-1250 EDT

Measure the phase angle of the current. It is probably all capacitive and at least some current would be expected. Measure the power input to the filter with no load. Probably very small.

However, the current seems high. At 60 Hz and 240 V the impedance for 130 A is about 1.8 ohms. This would be a rather large capacitor in the filter.

Use other means to verify the current. Ideally use an oscilloscope. There may be substantial harmonic components.

Try to find out the circuit and values of the Corcom filter.

.

Hopefully I can look at it in the future. Thanks for replying. With all the smart electricians and engineers around this forum I would have thought I'd have gotten more replies.
 

__dan

Senior Member
Without seeing it, I'm pretty sure the panel reading of 130 amps is actual current flow. I expect the reading would be confirmed with an independent instrument.

As Gar and the factory engineer are saying, it's reactive power flow through the filter's caps. Real power is close to zero but KVA is large and the current flow is large.

I would call it an impedance mismatch between the filter and the rest of the system. The caps are in circuit with net inductances in the rest of the system and under no load condition, current flow exchanges between net capacitive and net inductive elements.

The factory engineer may have a number for the expected lifetime of the caps as well as a limit number for how much reactive power flow is normal for the unit, with and without load.

As long as the caps are at their specified value, the current flow could be normal. There could also be other problems on the system that contribute to the large current flow. The caps do age and fail with time, they are a wear item.

You could try to IR scan the box for excess heat. The current flow is supposed to be going through very low impedance paths with no loss, so no large heating loss.
 
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