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Topic Title: Tripping 100m/a rcd
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Created On: 27 November 2012 07:48 PM
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 27 November 2012 07:48 PM
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daveparry1

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Hi all,

A job today was to install a new cooker, Curry's man refused to do it as he measured Zs as 2.75 ohms. Knowing that this house was probably TT I just assumed he didn't know what Zs values to expect on TT systems as i've had this a number of times with new cooker installers. Anyway on checking I found there was a 100m/a front end rcd (current operated) which had been disconnected and replaced with a Henley block! I explained to the customer that to be able to connect the cooker i'd have to re-instate the rcd and warned her that there might be tripping issues. I re-connected the rcd and wired in the cooker and then the rcd tripped, oh bugger I thought! Disconnected various likely candidates but still the rcd tripped regularly about every ten minutes. Looking at the c/unit I found a 6a breaker that was switched off, the cables from it appeared to feed some outside lights by some indeterminate route, so I removed the cable from the c/unit and tested between N-E and got 2.7 ohms. The tripping has now stopped but i'm wondering where the 10 minutes fits in, is that the sort of time a 100m/a rcd would normally take to trip with around 2.7 ohms N-E?
Asking purely out of interest as I havent had a lot of experience with fault conditions with 100m/a rcd's, i'm sure a 30m/a one would trip immediately with only a couple of ohms N-E, (sorry for the long boring post!)

Dave.
 27 November 2012 08:37 PM
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leckie

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Hi Dave

Well neutral to earth faults trip when the load is applied so is the delay because load is applied intermittently?
 27 November 2012 08:53 PM
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daveparry1

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I don't think so Leckie, it just seemed to be time related, the mcb on the faulty circuit was already switched off and all the other stuff in the house was working all the time I was there, nothing being switched on or off,

Dave.
 27 November 2012 08:59 PM
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OMS

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With an N-E fault (and regardless of the MCB being on or off) combination of changing load in that property or even in a neighbouring property can shift the neutral voltage around and through that kind of resitance can put a 100mA into its threshold of tripping

Regards

OMS

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Failure is always an option
 27 November 2012 09:06 PM
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daveparry1

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Thanks OMS, as I said i haven't had much experience of tripping on 100m/a rcd's, I must say I was very pleased when I removed that circuit from the board and found the low reading between N-E, I guess someone in the past had a problem on that circuit, that's why the breaker was switched off,

Dave.
 27 November 2012 09:09 PM
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leckie

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I'm not as clever as oms (nobody is). So I see what your saying Dave. Mcb off so no load can be pulled through that circuit. Oms will have to explain to me how a neutral imbalance can come from elsewhere.
 27 November 2012 09:56 PM
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AJJewsbury

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OMS's theory sounds about right to me.

I guess your 2.7 Ohms is N-c.p.c. rather than N to true earth - so that's probably not the whole story in terms of calculating the leakage, as it's a TT system.

Say the customer's and supplier's electrodes together have a resistance of 20 Ohms, then the fault will have an overall resistance of 22.7 Ohms back to the supply star point. To loose 100mA through that, you'd need voltage difference, N-earth of 2.27V (or half that for 50mA). If there's no current being drawn on the faulty circuit then there's no v.d. along its N to worry about, so it would just be the v.d. along the supplier's N that would give that voltage difference N-earth. Given that the suppliers are allowed a voltage drop of over 36V, a few volts dropped along their N would be more than expected.

- Andy.
 27 November 2012 10:04 PM
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daveparry1

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Just out of interest the Zs at the cooker connection was 2.4 ohms, this must have been due to pme somewhere nearby, it wouldn't have just been from the earth electrode. I didn't bother doing an Ra test, the mec disappeared down between some paviing slabs,

Dave.
 27 November 2012 10:09 PM
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slittle

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Dave,

If you've got an N-E fault of 2.7 ohms, there's a bit of a clue there as to why your Zs was a similar value.

I've had a TT system with a Zdb of 0.2 Ohms before.... Same fault


Stu
 27 November 2012 10:14 PM
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daveparry1

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I thought it was coincidence Stu as the N-E fault I measured was just on the disconnected cable but i'm having second thoughts now, maybe I should have checked the Zs at the cooker again after disconnecting the faulty circuit cable,

Dave.
 27 November 2012 10:26 PM
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AJJewsbury

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this must have been due to pme somewhere nearby

exactly! - the 2.7 ohms fault was effectively PME'ing your installation.

- Andy.
 27 November 2012 10:47 PM
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daveparry1

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Yes Andy, i'm realising that now, at the time I was just thinking the low Zs was due to bonding to gas/water pipes common to a nearby house that was pme, I come across that quite often in my location when doing c/unit changes. I'll call back there and take a Zs reading at a socket, something under 500 ohms for a 100m/a rcd I think? (preferably much lower though)

Dave.
 27 November 2012 10:52 PM
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AJJewsbury

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something under 500 ohms for a 100m/a rcd I think? (preferably much lower though)

Yup - below 200 for stability.
- Andy.
 27 November 2012 11:02 PM
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daveparry1

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Thanks Andy.
 28 November 2012 09:43 AM
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OMS

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Originally posted by: leckie

I'm not as clever as oms (nobody is). So I see what your saying Dave. Mcb off so no load can be pulled through that circuit. Oms will have to explain to me how a neutral imbalance can come from elsewhere.


See Andy's post - thats about covered it I think - neutrals and earths rarely go through the MCB do they - so on or off the fault is still present.

It's part of the problem revolving around final circuit single pole RCBO's and upstream double (or all) pole RCD's.

If the RCBO goes on a N-E fault, the upstream RCD will follow shortly as the fault is not removed.

Regards

OMS

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Failure is always an option
 28 November 2012 10:10 AM
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daveparry1

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I have just spoken to the customer, i'll be going back later today to do a Zs test, just in case the only "earthing" was through that faulty circuit which I disconnected! (connecting the cpc to neutral)

Dave.
 28 November 2012 10:22 PM
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daveparry1

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Went back there today and took a few Zs readings, all around 160 ohms and the 100m/a rcd was tripping at 42m/s on test. Advised customer that upgrade of c/unit with 30m/a rcd's would be needed to comply with current regs if they ever wanted any new additions etc. to the installation. I was only there to connect the new cooker originally remember. So the Zs of 2.7 ohms which I was getting yesterday was definitely coming from the faulty outside lighting circuit (N-CPC) wired in 1.5mm t/e so I wonder if it's any less safe now than it was before I went there with it's pseudo pme and no rcd?

Dave.
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