strange fault - who can solve it?

[nivag]

hello.

just wondered if any of you have experienced this or can possibly shed some light on it.
today i went to a fault on an immersion circuit

symptons was -
upon waking up, customer noticed B16 MCB had tripped along with 1 of the split load 30ma RCDs. he reset RCD which stayed on and when he tried to reset the MCB it blew straight off again (the RCD held however)
he left it for a day and at some point the RCD had triped off again but he managed to reset it.

arrived at job with just MCB off

upon initial investigation, the load side from the Double pole switch in loft next to the cyclinder was all clear, it was just feeding the element which tested out fine. i then started to investigate the circuit between the CU and the D/P switch to which i found some serious burning of the cable, and once i opened up the switch i found some burning in there too.
the terminals of the switch were all tight and it looked to be the earth link that links the terminal screws on the cover plate that had been the source of heat, the burn of the cable also looked to be from the earth.

this is where it gets odd.

the burn to the cable was only for about a meter, and it wasnt directly next to the d/p switch either, it had a meter or so between of good cable seperating them, then the cable all the way back to the CU was ok too,

why would this be the only 2 sources of heat damage? possibly weak terminations to the link of the cover plate? and there was a very tiny corner of the duvet leaning on the cable at the point of burn so i guess this could have derated the cable CCC?

then the question is why would the earth heat up when there was no fault being inicated upon dead tests. and that is presuming the RCD was opperational and would have disconnected within the required time thus not allowing the cable to heat up to such a tempreture.

obviously it had to be repaired so with the MCB AND the corresponding sides RCD isolated, i snipped through the cable. at that very moment, the lights in the loft dimmed to at bout 25% left.
you could think possibly the lights was borrowing and earth for the neurtal path back but the RCD (presuming it was working) wouldnt have even held, and i arrived at the job with all lights fine. also, there was nothing upstream - just a meter of T&E and a disconected Double pole switch.
the other side of the split load board was totally un effected and this is the side the lights were fed from.

voltage had dropped to 151 volts

with the Mainswitch isolated, the incomming was still 150Volts.
turn the main switch and some crcuits back on and the more load the lower the voltage - got down to 30v supply.

not enough for my meter to let me carry out any live tests.

RCD test buttons opperated them at 150v but not down at 30v

it was TNS system on Econ 7

also, would a drop in voltage stop a MCB from opperating?


any ideas would be nice

thanks in advance.

 

[gazdkw82]

Could be a combination of things. Could be that the cable is faulty due to stress or other related reasons that could have rendered that part of the cable faulty

 

[AJshep]

Does sound like a damaged TNS cable.
I've had a slight tingle from the outtersheath on a TNS cable before.
If your not getting your voltages at the intake then I would be inclined to Isolate completely then CAREFULLY disconnect the suppliers Earth.

Then check voltages between each Live and the suppliers Earth to a "known Earth" Even if it means temporarily putting a stake in.

If the voltages are ok between a known earth then ring up the supply company before turning back on.

 

[Lucien Nunes]

Likely TN-C-S further upstream, with a CNE conductor failure in the road causing the entire installation current, and possibly that of other installations affected the failure, to return to the supply via CPCs and bonding that are in contact with extraneous metalwork. The immersion circuit CPC is vulnerable if there is a solid metallic pipe connection all the way to the incoming service, and the bonding is less than ideal. When you disconnected this, the installation neutral was left with a much higher resistance to the intact side of the fault, hence the voltage became much more variable.

If the supply was genuinely TN-S then there would have to be two faults to start with, before the immersion circuit cable was damaged. One N-E short, to provide a source of current into the CPCs, upstream of any RCDs, and a high-resistance neutral upstream of that.

The breaker tripping incidents were consequences of the circuit damage, not due to the abnormal conditions on the supply, since low voltage will not cause either the RCD or MCB to trip, although low voltage would have prevented the test button on the RCD from tripping it (although technically the RCD functionality would probably have still been in place).

IIRC Not long ago we had some pics on the forum of a CPC that had been cooked by return current from a CNE failure.

 

[marconi]

I have thought about what you wrote and can think of a fault situation which would produce the heating of the cpc yet not trip the rcd or mcb. It needs though a TNC-S supply and not a TNS one - are you certain it is TNS? Could you also say more about any equipotential bonding and whether the supply cable is over or under ground?

Assuming:

a. the property is TNC-S;
b. they are served by the same metal water pipe;
c. the plumbing in the property is all metal so that there is a high conductance path between this property's immersion heater cylinder and at least one other property's EP bonded pipework;
d. the immersion heater element is satisfactory;
e. your property has no or high resistance EP bonding.

then if the supply neutral is broken or high resistance, neutral currents for the whole property will flow through N-E link, the IH cpc (all or some fraction), through the shared pipework, ultimately through another property's EP bonds and either N-E link if TNCS or E if TNS.

The rcd does not trip because there is no imbalance. The mcb does not trip to begin with because there is no overcurrent but overtime the reduced csa cpc in the 2.5mm2 T/E suffers severe overheating, especially in the stretch insulated by the duvet (in summer?) which causes the insulation to deteriorate. Eventually some tracking L-cpc happens and the rcd and mcb trip - you get the idea.

When you cut through the 2.5mm2 feeding the IH you remove the alternative 'neutral' path - the property now relies on other equipments connected by their cpcs to earth and the defective supply neutral conductor which offer a higher resistance resistance - dimming the lights and resulting in a measured off load L-N of 150V which reduced as load is connected.

 

[Percyprod]

Working for dno, biggest problem was lost neutral on 3 phase system, can cause high voltage in single phase supply fed from it, and with some t&e cable insulation can break down. With a neutral fault all properties on that supply would be disconnected and each premises tested before being reconnected. Hope that makes s3nse!

 

[nivag]

thanks for the comments!

 

[Octopus]

thanks for the comments!

And your findings were????????????????????

 

[elsparko]

remove the main fuse and condemn , install candles

 

[Timbo]

remove the main fuse and condemn , install candles

Hey, you can get things called torches that run off batteries these days!

 

[Squid]

Hey, you can get things called torches that run off batteries these days!

Battery operated candles.........

 

[Strima]

How many candles?

 

[RWJ]

4 bed house, 1 candle per bedroom.....