Electric shower Switch intermittent behaviour

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The electric shower in my house has been intermittently tripping the circuit breaker in the main consumer unit. First time it happened, a bang was heard in the wall mounted 45A 1 Gang neon switch and the breaker in the main consumer unit cut off. I open the switch and to my surprise I found the live feed (from the mains) was slack. Tightened it with other connection screws pushed back into the box and it worked ok for few hours then tripped again. Did the same thing again and was good until the following morning, the shower was used without and problem then sometime in the afternoon it was tripped again. I then replaced the switch with a new one, put it back again but it tripped after 20 minutes. At this point I was rather puzzled. Called a qualified electrician who kindly came straight away and he initially suspected there must be short connection between the L and N or E somewhere in the 10mm wire. However, he tested the wires at all ends (from the mains to the switch box and from switch box to the shower) and also the shower and found nothing. But he was surprised when he tried to undo the wires from the switch, to find them slack again despite the fact I did tight them again. He tighten them again pushed them back to the box as far as they go then pulled them out slightly, tighten them again then screwed the switch to the box. Since last night and up to now the shower is behaving but I am not holding my breathe. The explanation he gave (which I tend to agree as I trust the guy) is that because the wire is so thick (10mm) when I push the switch back into the deep box the get slack again which create spark inside the switch and this in turn makes the wire hot, increases the resistance and that make the circuit breaker trip in the consumer unit to prevent fire. He also suggested that of this happen again we may need to renew the wire.

I need help with the following:

  1. Has anyone have experience same in the past, if so how it was sorted out?

  2. Has anyone have another opinion on why this is happening

  3. Is there another type of switch I can use to secure the connection

  4. If I need to lay a new wire, can I get away with 6mm instead of 10mm for a shower of 9.5K. 10mm is very hard to lay and connect.
 
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What is tripping?

Is it an MCB
upload_2016-11-17_17-48-37.jpeg


or an RCCB or RCBO? - both of which have test buttons
upload_2016-11-17_17-50-49.jpeg
upload_2016-11-17_17-50-13.jpeg
 

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this one is not connected to RCCB Or RCB, it is connected to a normal Square D MBC B40 and has been working perfectly since we bought the house 1997.

Also to add, we came back from work this evening and found the MBC was tripped. put it up and as I walked to the kitchen where the isolation switch is I heard a bang and flash from inside one of the the thick 10 mm wires. I think it is the wire that is going from the isolation switch to the shower unit. However, visual inspection of the wire reveals no sign of burns. I believe there is minor damage of the live wire inside which makes in contact with the earth next to it. I have now disconnected the wire that is going to the shower from the switch and only left the wire that is coming from the consumer unit. so far nothing has tripped yet. however, any help, idea to resolve this issue would be highly appreciated.
 
If damage to cable has occured, it likely may be at a point close to terminations, i.e at the switch or inside the shower casing........happens often as people sometimes nick the cores while stripping back the sheath etc.

Can you do an insulation resistance test on the cabling to see if there is a short?
 
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Then, I would say clearly, the Line(live) is exposed and near either the Neutral or earth when the cables are squeezed back in.

If you've had more than one 'bang' I would think there must be some burn marks on the live wire.
 
the first thing the electrician did last night was the resistance test looking for short. He disconnected all the terminals from the consumer unit, the switch and the shower unit and test all in sequence but no short was found. the irony is the MBC does not trip from one bang, we believe from serval. the wife was in the kitchen last night and heard few bangs but the MBC did not trip until after we went out to work this morning. I believe several of those bangs generated enough heat over a period of time which in turn change the resistance of the wire and hence the drawn current at which point the MBC trips
 
it is scary, that is way we need to understand what is happening . The bang and the flash happened not close to the switch, it is further down the wire. I believe the problem is somewhere inside the wire. if I can be sure this is the case then I have no problem changing the wire.
 
The IR has been done.
Probably - if "resistance test looking for short" represents an IR test - but the point was that those tests (whatever they were) were not undertaken by the OP, and that the OP (the "you" I quoted) is not going to be able to undertake IR tests.

Indeed, anyone able to do an IR test would hopefully not be asking these questions in a forum.

Kind Regards, John
 
Is that another prediction ? :D I would predict the cable is foocked:)

DS
 

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