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Topic Title: A head scratcher!
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Created On: 24 July 2014 06:11 PM
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 29 July 2014 09:10 PM
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leckie

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It is the party wall between two timber frame buildings. So in fact at the time of the build, it would have been two stud walls with a void in between them, and the walls would be double boarded to maintain the fire rating. Pretty standard for timber frame at the time.

I am going to try to visit the property tomorrow if I can and I am hoping all will be revealed.
 29 July 2014 09:52 PM
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timothyarnold

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Its like Christmas Eve...
 29 July 2014 10:16 PM
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leckie

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What, you mean one more sleep!

It's not that interesting
 02 August 2014 04:22 PM
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leckie

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Well, an update.

I have been back, I cannot get the meter to read 25A again around the L & N. I got this reading with different meters previously. I cannot get any readings between any of the conductors of the two immersion heater circuits, and the MCB hadn't tripped again. So I just swapped the faulty main switches to both the consumer units and have had to leave it as it is. I have no idea what has happened here.
 02 August 2014 07:59 PM
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mapj1

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Oh well, come back if it happens again.
I was rather hoping for a spectacular Agatha Christie style solution,
probably better for your nerves this way though.

-------------------------
regards Mike
 05 August 2014 10:36 PM
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leckie

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The problem of the phantom fault had been solved!

I explained the scenario of the 25A test reading with a clamp meter around L and N to a 3 kW load to a friend of mine, a genius. Embarrassingly, he thought for fully 15 seconds about the problem, before demonstrating on a DB I was testing at the time, the reason for the "fault".

He grabbed a neutral cable for a circuit and pulled a loop into it. He then took my clamp meter and clamped the L and N of the circuit. It read double the load of the circuit. He then explained that as the neutral core was bent back 180 degrees, instead of the current flow of each core balancing the circuit load and reading zero, because the current was flowing in the same direction in both the L and N, the meter was reading as if it were two single phase line conductors.

So simple! Yet I didn't work it out, and I don't think any of us did, unless I misunderstood any of the replies. 15 seconds!
 05 August 2014 10:52 PM
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mapj1

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However this does not fit you reported 2 lots of 12.5A in phase & neutral adding to 25A - I'd have expected the resultant to be smaller, near zero unless something is happening that means the wire you have in your hand actually loops in a way that puts the neutral current thought the clamp part of the meter in the opposite direction


july 25th, 5.38

Sorry, unable to resist showing off.
Of course it wasn't in bold then, and there were lots of extra words above and below to confuse the unwary reader and lull him to sleep..
Actually on your behalf I'm really glad there is nothing too sinister. So what do we now think?
Perhaps just a failing spring on the breaker with overheating when the immersion originally failed or something. Relief all round.

-------------------------
regards Mike
 05 August 2014 11:06 PM
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leckie

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Apologies Mike, you are also a genius!

I think that the MCB had become faulty due to repeated tripping prior to the heater element failing completely. I'm not sure why the main switches to both boards failed, but nothing to do with me being an idiot and not realising I was measuring the neutral 180degrees out of kilter!

A combination do red herrings and me being a silly billy. But now I know, so all is well!
 05 August 2014 11:17 PM
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mapj1

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course I am

No, not really but as the owner of a clamp meter that's a bit deaf, I have been known on occasions to wind the wire to be measured through it a number of times and then mentally divide the reading rather than run down a set of ladders to get a more sensitive instrument.
(another one for the as yet unwritten Mike's book of dodges)
edit,
Consider 5 amp.turns, it may be the same magnetic field in the core of the current clamp, if it is the same 1A going round the outside and then through again 5 times, or 5 amps going through the hole just once - the instrument can't tell.
(so long as the electrons all pass through the hole the same way, so they reinforce of course, cough !)

-------------------------
regards Mike

Edited: 05 August 2014 at 11:27 PM by mapj1
 05 August 2014 11:20 PM
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leckie

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Ah, now that's very clever, I will be trying that out!
 05 August 2014 11:36 PM
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AJJewsbury

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The problem of the phantom fault had been solved!

I explained the scenario of the 25A test reading with a clamp meter around L and N to a 3 kW load to a friend of mine, a genius. Embarrassingly, he thought for fully 15 seconds about the problem, before demonstrating on a DB I was testing at the time, the reason for the "fault".

He grabbed a neutral cable for a circuit and pulled a loop into it. He then took my clamp meter and clamped the L and N of the circuit. It read double the load of the circuit.


Doh. I thought we'd eliminated that - your post of 28 July 2014 09:01 PM .

I'm glad you got to the bottom of it anyway!

- Andy.
 06 August 2014 09:21 PM
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leckie

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Apologies Andy. You took the time to answer my problem but I didn't read your answer, or Mikes, carefully enough. Both of you fully answered my "non problem".

I was getting distracted by the blowing mcb (faulty) and the duff main switches. Two red herrings.

However, now I have learnt something, so thanks again.
 06 August 2014 10:10 PM
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Zs

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's OK Leckie, I've been looking out for this thread and I'd not seen it either. On top of that I am so going to run this through its paces at the first opportunity. Will of course report back. How can anyone not have a play board? I cannot resist mine. I blame Alancapon for this...whe I met this forum he always used to be running tests at the weekend.

For now though, the one I am glued to is JP's cowshed conundrum. I'm watching that like a hawk because I'm hoping that our friend JP isn't taking the flack for giving advice in his report and being blamed for the fact that a required RCD won't hold. I've also had one of these things on the go lately in the form of a second hand fantasia ceiling fan that worked perfectly. But 'in the old house the light worked on the switch regardless of the position of the remote and the fan only worked on the remote'. How I wished I'd not agreed to go to that job. Sorted, several calls to fantasia and no, it didn't work like that. But for £50, what a pain.

I'm glad your client is happy now and I hope they appreciate you. I do.

Zs
 06 August 2014 10:30 PM
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leckie

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You would love my play bench Zs. It's got TPN, 110v reduced voltage, and other fun bits. I just like messing about!

Oh the fun I have
IET » Wiring and the regulations » A head scratcher!

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