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Topic Title: Supply fault - any suggestions as to cause of readings
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Created On: 20 January 2014 10:28 PM
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 20 January 2014 10:28 PM
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Banners

Posts: 94
Joined: 06 October 2013

Investigated tripping RCD today. IR live to earth circuit by circuit all good measurements, likewise IR neutral to earth.

Clamp meter on main earth with all fuses in and all earths at MET connected wandering around low to mid twenties (mA)

Removed fuses (3036s) one by one and still getting high mA earth leakage

Measured Ze = 1.44? PFC = <1A

Presumed supply cable fault. Called DNO who said they had had a few reports of issues in the area (RCD first started to trip last Friday)


Spoke to customer after DNO engineer had attended. Engineer had told him it had been a problem with one of their fuses.

Anybody able to elaborate what the exact nature of the fault might have been.

Edit? Engineer meant fuse their end - nothing inside the property was touched.

Edit 2 Voltage measured 213V Live to earth and Neutral to Earth. Triple checked this.

Just prior to leaving I performed tests again Voltage 244V Checked again and 239V
Ze and PFC remained the same

Edit 3 VERY VERY SORRY - I did not mean 213V Neutral to Earth I meant 213V Neutral to Live!!!!!

Edited: 20 January 2014 at 10:50 PM by Banners
 20 January 2014 10:34 PM
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leckie

Posts: 1605
Joined: 21 November 2008

When you clamp your ammeter to the main earth, bear in mind there may be currents going down other parallel paths. So to reads the total leakage current you want to clamp across all the live conductors, L and N for single phase.

Not sure what they mean describing the problem as a fault with one of their fuses.
 20 January 2014 10:41 PM
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Banners

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Leckie - i am aware of that but I quickly surmised that the current must be coming from outside as even with the main switch off their was still up to 27mA in the main earth cable. On my initial measurement it was in the low thirties but I assumed I must have connected something wrongly as I thought the RCD would have tripped out at such a level.

I am assuming some kind of earth neutral fault because PFC read as less than one Amp.A reading I have never come across before and was wandering what might cause that. System had a solid TNS clamp around the sheath.
 21 January 2014 08:24 AM
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phantom9

Posts: 363
Joined: 16 December 2002

Banners, the PFC reading is low because the Ze reading is high. Ohms Law. Impedance will be the same as it is fixed and increased voltage will increase current. As it's TN-S you need to report the high Ze. RCD tripping may have been caused by erratic voltages imbalancing the supply or as Leckie said higher than usual earth leakage currents due to the fault.

If, as you checked, the voltages increased from 213V to around 240V PFC will have increased.

213/1.44=148A

240/1.44=167A

The high Ze is affecting the PFC.

Edited: 21 January 2014 at 08:50 AM by phantom9
 21 January 2014 01:37 PM
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Delbot321

Posts: 77
Joined: 06 November 2012

Hi Banners

Does your RCD that is tripping have a small tail that goes to the earth connection? If it does then the problem is likely to be that the voltage between the neutral and earth is rising which will cause the RCD to operate. It provides protection against the loss of neutrals out on the network and the over voltage faults that then go with it. Not sure why the DNO service engineer is blaming their fuse though - sounds more like he's been told to say that to avoid the possibility of claims from overvoltage to equipment.

The maximum external loop for most DNO's should be 0.8 for TNS or 0.35 for TNCS although some DNO's accept 0.8 for TNCS in some areas. As you're getting 1.44 ohms it's either too high and they should be called to repair it as phantom9 said, OR it's a TT earth arrangement in which case its fine as the RCD is providing the protection.

Hope this helps.
 21 January 2014 03:17 PM
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AJJewsbury

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Measured Ze = 1.44? PFC = <1A

There's something not right there - if Ze is 1.44 Ohms then PFC should be at least 159A, if PFC is below 1A then Ze would be over 230 Ohms. Are you sure the meter isn't saying kA rather than A?

Anybody able to elaborate what the exact nature of the fault might have been.

Only guessing (but based on what we had once) - fuse could have had loose contacts (e.g. due to repeated overheating) - causing occasional arcing, which puts a lot of EMI down the line, which can upset electronics, including that inside RCDs.

- Andy.
 21 January 2014 05:27 PM
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phantom9

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Originally posted by: Delbot321


The maximum external loop for most DNO's should be 0.8 for TNS or 0.35 for TNCS although some DNO's accept 0.8 for TNCS in some areas. As you're getting 1.44 ohms it's either too high and they should be called to repair it as phantom9 said, OR it's a TT earth arrangement in which case its fine as the RCD is providing the protection.



Hope this helps.


These are typical values, not absolute maximums. Common misinterpretation.

Edited: 22 January 2014 at 08:05 AM by phantom9
 21 January 2014 06:54 PM
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tillie

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Hi Banners , was your PFC reading the PEFC or the PSSC ?

If the latter then it could be a high resistance joint on the incoming neutral which could also cause the rcd to trip.

Just a thought.

Regards
 21 January 2014 07:01 PM
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Banners

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Thanks for the replies, folks. The readiings are exactly as I described them. For PFC my machine (Megger MFT) displayed "<1 A " This really surprised me as it seemed to be saying tthat there was effectively no fault current. Higher than permitted values of Ze readings I have come across. Voltage was also 213V. Just before leaving I decided to check again in case engineer arrived and could find no fault. Voltage readings L-N was 244V I took prongs off then applied them again and got reading of 239V. However, Ze stubbornly remained at 1.44 ohms and PFC read <1 A.

Because chap on the phone from DNO (he knew what I was talking about in terms of figures so I assumed I had been put through to an engineer) said they had been having "reports of problems in that area" I assumed they must have known about some kind of existing fault on the network. I do not know what the engineer who came out to the client's property (I did not meet him) meant by saying that the fault was with a fuse. I have not been back to the property and have not received any further call from the client so I assume he has had no more trips in the meantime. The RCD had tripped three times yesterday morning prior to my arrival and once when I was on site.
 21 January 2014 07:05 PM
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Banners

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Joined: 06 October 2013

Tillie, PFC was PSCC i.e. L-N
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