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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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 24 July 2014 06:11 PM
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Avatar for leckie.
leckie

Posts: 1872
Joined: 21 November 2008

Hi all.

Got a call to friends, the immersion heater keeps tripping the mcb.

So of I went for a look. It's on a off-peak tariff that cones on three times a day. The supply is TN-C-S. A 16a MCB, no rcd. Wired in 4.00 T&E. A plumber had fitted a new immersion heart and thermostat as that is what he presumed the fault to be.

So I disconnected the L and N at the heater. I carried out an insulation test, all greater than 299 between L-N and lives to earth. Carried out an IR test between the heater element to earth, about 6Meg. Measured the resistance of the 3kW element it was 20ohms approx, so OK.
Reconnected and switched on. Measured the current drawn, 12.5A.

I left it working but did not identify a fault.

Got a call today, "it's tripped again".

I returned.

Went through the same tests plus:-

I connected a ammeter around both the line and neutral conductor in the milliamp range of my ammeter. It went over the range. I swapped it to the amp setting and it measured 25A!

I have checked the continuity of the supply to the heater R1+R2 and also Rn+R2; all as to be expected.

So it's draws 12.5A down the line, 12.5A down the neutral, but measures 25A across both! Any ideas?
 24 July 2014 06:13 PM
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leckie

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Oh, and I tested it with two different ammeters to check the first one worked!
 24 July 2014 06:23 PM
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slittle

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Have you measured what's being carried by the neutral alone ?? or just assuming as I would that the 12.5A going in the live is coming out the neutral ?

Stu
 24 July 2014 06:25 PM
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potential

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Have you measured the current being carried by the CPC?
 24 July 2014 06:25 PM
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OMS

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The supply is TN-C-S


Turn the main switch off and try the ammeter again - that will be informative

Then turn the main switch back on and see what happens

Then test for volts between the immersion earth terminal and a screwdriver pushed in the lawn

Then test a few other circuits to find the low resistance fault problem depending on what the previous shows

Regards

OMS

-------------------------
Failure is always an option
 24 July 2014 06:50 PM
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phantom9

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Seems like the switch (spur or 20A dp) could be at fault?
 24 July 2014 07:05 PM
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geoffsd

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I don't suppose there are two elements.
 24 July 2014 07:22 PM
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UKPN

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"test for volts between the earth terminal and a screwdriver pushed in the lawn"

could be a problem if its a flat 200 ft at the top of a block!

stick a test lamp across L/N

Regards
 24 July 2014 07:59 PM
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slittle

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I'm thinking OMS and I are looking in the same pot at the moment.

UKPN no probs with a 200ft high flat, that's why singles is on 100m drums


Stu
 24 July 2014 08:08 PM
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Zoomup

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Originally posted by: geoffsd

I don't suppose there are two elements.


That is a sensible observation. Either a twin element immersion heater connected onto one circuit OR two single elements one top and one bottom of the copper cylinder both running through one 16 Amp MCB.
 24 July 2014 08:14 PM
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Zoomup

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Originally posted by: leckie

Hi all.



Got a call to friends, the immersion heater keeps tripping the mcb.



So of I went for a look. It's on a off-peak tariff that cones on three times a day. The supply is TN-C-S. A 16a MCB, no rcd. Wired in 4.00 T&E. A plumber had fitted a new immersion heart and thermostat as that is what he presumed the fault to be.



So I disconnected the L and N at the heater. I carried out an insulation test, all greater than 299 between L-N and lives to earth. Carried out an IR test between the heater element to earth, about 6Meg. Measured the resistance of the 3kW element it was 20ohms approx, so OK.

Reconnected and switched on. Measured the current drawn, 12.5A.



I left it working but did not identify a fault.



Got a call today, "it's tripped again".



I returned.



Went through the same tests plus:-



I connected a ammeter around both the line and neutral conductor in the milliamp range of my ammeter. It went over the range. I swapped it to the amp setting and it measured 25A!



I have checked the continuity of the supply to the heater R1+R2 and also Rn+R2; all as to be expected.



So it's draws 12.5A down the line, 12.5A down the neutral, but measures 25A across both! Any ideas?


The L is one supply to an element. The N is a second L supply to a second element. Both have a common neutral. Both elements have been connected via one 16 Amp MCB. Somehow when the timer brings in the second "off peak" element it overloads the 16 Amp MCB. The system is wrongly wired up. In the past there was a purpose made switch that brought in either sink or bath elements, but not the two together. This prevented this type of problem.

Bye,

Z.
 24 July 2014 08:18 PM
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OMS

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Originally posted by: UKPN

"test for volts between the earth terminal and a screwdriver pushed in the lawn"

could be a problem if its a flat 200 ft at the top of a block!

But is that likley - and we can still buy long cables


stick a test lamp across L/N

So what's that telling you in a PME system then ?


Regards


You DNO boys just kill me

Regards

OMS

-------------------------
Failure is always an option
 24 July 2014 08:25 PM
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daveparry1

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This reminds me of a job I had a year or so ago where a plumber had replaced a double element cylinder with a single element one and had wired the one single element to both the normal and the off-peak supplies. All was ok until the off-peak supply kicked in then the rcd tripped!
 24 July 2014 08:55 PM
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AJJewsbury

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A 16a MCB, no rcd.

So it's draws 12.5A down the line, 12.5A down the neutral, but measures 25A across both! Any ideas?

Have you checked the CU end? Ignoring PME diverted N currents for a moment, it sounds like the L and "N" are both L (i.e. in phase, so meter is seeing the sum not difference). I could imagine someone using brown/red to feed one element, blue/n to feed L to another and use the bare wire as N (perhaps still connected to earth). Both wires into the MCB at the CU. At least it should be an easy thing to check.
- Andy.
 24 July 2014 09:06 PM
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leckie

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Joined: 21 November 2008

Right, I am just back from the pub, but I will try my best to explain a bit better.

There are two elements, the other standard tariff is supplied from the ring-main via a switch fused spur from another DB.
 24 July 2014 09:08 PM
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leckie

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I have swapped the double pole switch, but did not check the current down the CPC.
 24 July 2014 09:09 PM
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daveparry1

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I suppose both supplies are on the same phase?!
 24 July 2014 09:16 PM
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leckie

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Sorry, I'm catching up with the replies, so I am answering each as I read it!

The off-peak cu incoming terminals measure approx 240v L-N and L-E. 0V N-E.

The heaters are two 3kW standard immersion heaters, one high and one low. So all standard.

The supply to the off-peak I'm heater is twin and earth and I have checked R1+R2 and Rn+R2 as per my earlier comment. They are both approx 0.15 ohms
 24 July 2014 09:20 PM
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leckie

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The building is a standard dwelling so I suppose the supplies are on the same phase. I haven't checked that but I would be amazed if they weren't . I ran out of time today as the off-peak tariff shut down as I was trying to sort out what was going on. I'm back there Saturday afternoon for another go.
 24 July 2014 09:46 PM
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leckie

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So, if I do a Ze to each board, a polarity check to each board and each circuit, a continuity check to each individual core to each immersion heater circuit this should resolve all connection issues and also confirm there is not a loss of earth connection? I haven't IR tested the on-peak supply yet as it is connected to the ring main and the lady of the house wanted to watch Deal or No Deal.
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