Downgrading MCB to protect cable but has fixed load greater than IZ

[gooner80]

Hi Guys

I know showers have been extensively covered on here, but this this is a slightly different question.

went to a job to replace 45amp shower switch, I noticed cable was damaged by heat, first thought maybe loose connection, so I started investigating, I noticed a 9.5kw shower on 6.00mm T&E cable enclosed in wall on 40 RCBO, I know without even looking that if it was clipped this might be ok but buried in wall no.

I explained situation to superviser and he was going to ask me to downgrade MCB. I was about to have a debate with him when I found further evidence of severe charring and at that point I said it needs rewiring, IR test showing L/E 0.14 M ohms.

My question is iz>=IN>=IB
downgrading MCB may cover IZ>IN but IB is not covered and my understanding is an MCB will aloow an overload of upto 1.45 times its rating in this case the current carrying capacity of the cable would be 34amps and the fixed load will pull more than this 46.4 for quite some time,thus damaging the cable, am I wrong? if I am in this situation again do I correct him?

thanks in advance for your help

 

[westward10]

Not doubting you but that looks like tar or something on the cable, obviously you have physically seen it.

 

[gooner80]

I've seen it smelt it and felt it, the live into switch burnt as well as the switch burnt. That is not tar that is the cable overheating

 

[Risteard]

L-E of 0.14 Meg ohms would surely trip the rcbo if the cable was as faulty as that. Was it tested with neons disconnected etc

Ohms law suggests it wouldn't with only around one-and-a-half milliamps flowing.

In answer to the OP's question though, a circuit breaker should not be used as a load-limiting device,

 

[westward10]

I've seen it smelt it and felt it, the live into switch burnt as well as the switch burnt. That is not tar that is the cable overheating

Still not doubting you, why isn't like that all the way down.

 

[Midwest]

It's interesting the number of sparks that are defensive of 9.5kw on 6.0mm. If it is designed and calculated right that's fine. Me personally I err on the side of caution, I've been to a few house fires on emergency call out.

That's probably because they've been asked to replace the existing shower, because it spits a tiny bit of hot water in the winter months with something more powerful, and just connected to an existing 32amp supply and replace the mcb with a 40amp.

 

[gooner80]

I can't tell you the complete laws of thermal dynamics or heat transference. It is like that at the switch and 30cm up from the switch. I ve seen quite a few instances of heat damaged cables the fishy smell is present

 

[hightower]

I can't tell you the complete laws of thermal dynamics or heat transference. It is like that at the switch and 30cm up from the switch. I ve seen quite a few instances of heat damaged cables the fishy smell is present

I've worked in one or two houses that had a fishy smell too. Had no cables burning though

Ohms law suggests it wouldn't with only around one-and-a-half milliamps flowing.

In answer to the OP's question though, a circuit breaker should not be used as a load-limiting device,

Yeah didn't have time to work it out this morning. You're right of course

 

[telectrix]

if Ib > Iz ( dependent on reference method of installation) then the cable is inadequate, irrespective of OCPD rating. end of.