Discuss High Earth current flowing in the Electrical Forum area at ElectriciansForums.net

TonyMitchell

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I have measured high currents in the CPCs of four circuits as shown in the table below.
  • This is the case both with the circuits energised and isolated.
  • Removing the CPCs for these four circuits brings the current in the MET to DNO cut-out, back to genuine L+N measured leakage levels.
  • However with the CPCs connected, the current in the MET to cut-out increases significantly under load (4.1A with a 19A load, 5.6A with a 31A load) as opposed to 19mA and 22mA respectively with the four CPCs disconnected.
  • Ze is good.
Any ideas?

High CPC current tests 110819.jpg
 
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TonyMitchell

TonyMitchell

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Bellendian

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Is it TNC-S?

I wonder if there is a slightly high resistance somewhere on the PEN conductor, and some current is finding a preferential route back via the pipework?
Cable not loaded when carrying out Ze which could be masking the problem?
 

Bellendian

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What's the physical layout of this site?

Just wondering if you have cutout at one end of building, AC units are at the other end, outside on a concrete pad, and just over the fence from the AC units sits the substation?

Out of interest, what was the original symptom that caused you to measure these leakage currents in the first place?
 

Lucien Nunes

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If it's a TN-C-S with a fair amount of drop in the CNE, any load is likely to drive current through those CPCs as well as the MEB although the majority of it should go through the MEB. Is the MEB up to scratch, or might there be something substantial, in contact with the pipes and therefore the HVAC CPCs, that is not bonded as it should be?

I have to wonder, what first drew your attention to the EC current? PME supplies specify a large CSA for the MEB exactly because these circulating currents are more or less unavoidable, so having a few amps in the EC is not automatically indicative of a problem. However, if there is a high resistance anywhere it's on the DNO's side.

Post crossed with BE's, thinking along very similar lines...
 
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TonyMitchell

TonyMitchell

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What's the physical layout of this site?

Just wondering if you have cutout at one end of building, AC units are at the other end, outside on a concrete pad, and just over the fence from the AC units sits the substation?
It is a rural residential location (period barn conversion).
A single AC outdoor condenser connected to five room airflow units over four levels, the indoor units taking their 230V from the condenser.
The DNO head is to the front of the building, the AC condenser to the rear (circa 20m apart)
There is no substation close.

Out of interest, what was the original symptom that caused you to measure these leakage currents in the first place?
I noted them whilst investigating RCCB tripping on the standby generator.
 

Bellendian

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Ah...the plot thickens! Is there a source electrode for the genny?
Presumably it's a pole mount tx, and overhead supply to somewhere relatively close to the barn, final connection maybe underground to cutout if a recent conversion?
 
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TonyMitchell

TonyMitchell

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Ah...the plot thickens! Is there a source electrode for the genny?
There will be - i have 8' of 5/8" rod on site, being buried on Tuesday, to satisfy 551.4.3.2.1. (the generator is not in standby use yet and won't be until this is done.)

Presumably it's a pole mount tx, and overhead supply to somewhere relatively close to the barn, final connection maybe underground to cutout if a recent conversion?
Exactly as you describe. The TN-S was jointed and PME'd underground 1m from the front of the property. The overhead supply is circa 30m away as the crow flies, there's probably closer to 45m of underground supply cable for the route it takes.
 

Lucien Nunes

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Bellendian said: Out of interest, what was the original symptom that caused you to measure these leakage currents in the first place?
I noted them whilst investigating RCCB tripping on the standby generator.
However, the CPC currents don't seem to be leakage and not necessarily related to any RCD tripping events as the insulation is good. It's just ordinary neutral current finding its way back to the transformer any way it can. One of the disadvantages of TN-C-S.
 

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