Shower Isolation Switch

[Chit]

Hi all,

I have a Creda Advantage 10.5kw electric shower which is fed by a 10.5mm t&e cable. It is RCD protected and is fed via a 40Amp MCB.

The shower was protected by a 45Amp isolator switch which I have found was bought from wickes:

http://www.wickes.co.uk/invt/204947

A few days ago, I went to switch the isolator on and got nothing from it. I checked the MCB and RCD and both were switched on.

On testing the isolator, I was getting 230V at the supply side of the switch. On switching the isolator on though, nothing was being given on the load side of the isolator.

I bought a new isolator switch and on opening the old one up again, I noticed that the plastic sheathing on the load live cable was black and crumbly. I cut and stripped this back until I got to untarred copper.

My question is why would this happen? I'm guessing the the cable had not been stripped back far enough by the electrician who fitted it and it was touching the copper internals of the isolator switch. When the current was flowing and the cable was hot, I'm guessing that this had burnt the sheathing?

I also remember that when the electrician was fitting the isolator, I did ask if it should be of a higher amperage as the max current flowing may hit 45.6 Amps but he explained that the mcb would cut the current before the isolator was affected and so it was fine. - Is it fine or should the isolator be of a higher amperage?

Many thanks.

 

[holmslaw]

..

 

[Chit]

Hi, many thanks for that. When un doing the connections, none were even slightly loose to be really honest, all were sufficiently tight?

Also, the way the electrician advised me, only a maximum of 45Amps will reach the isolator as that's the maximum allowed through the MCB. For this reason, the isolator will be protected by the MCB and therefore should be fine? Is this true?

Could it even have simply been a faulty switch ?

 

[securespark]

I would fit a 50A + isolator switch.

And a 50A MCB.

 

[sparkwright]

I would fit a 50A + isolator switch.

And a 50A MCB.

Not too sure on the cable size yet. 10.5 mm has been mentioned, so there may be some confusion in determining the cable size.

 

[kai]

Pullcord type shower switches are rubbish in my opinion, a better solution is to use a main switch type of assembly (63amp) in a two width din rail box, in the airing cupboard outside the bathroom.

The pull cord switches tend to fail on a regular basis, from reading other posts on this site over several years, and do not seem man enough for the job.

 

[EFLImpudence]

only a maximum of 45Amps will reach the isolator as that's the maximum allowed through the MCB.

Not true.

 

[Chit]

Sorry guys, have been in a rush all day and so typed a load of incorrect facts....

Firstly, the cable is 10mm t&e.
Secondly, the electrician said that only a max of 40Amps will reach the isolator as the MCB is rated at 40Amps.

 

[Chit]

Pullcord type shower switches are rubbish in my opinion, a better solution is to use a main switch type of assembly (63amp) in a two width din rail box, in the airing cupboard outside the bathroom.

The pull cord switches tend to fail on a regular basis, from reading other posts on this site over several years, and do not seem man enough for the job.

Thanks for that, do you have any links for these switches?

 

[Spark123]

Secondly, the electrician said that only a max of 40Amps will reach the isolator as the MCB is rated at 40Amps.

Rubbish, in simplistic terms the current is caused by the resistance of the load and the supply voltage.
A 10.5KW shower pulls around 44A @ 240v.
Isolators do wear out resulting in them heating up.
A 40A MCB does not trip at 40A, nor does it limit the supply current. It is a poor design to have a 40A MCB supplying a load of 40A> though.

 

[EFLImpudence]

http://www.discount-electrical.co.u...arth-terminals-ip30-w-149mm-x-h-76mm-x-d-76mm

http://www.discount-electrical.co.u...dule-dp-switch-disconnector-isolator-63a-250v

 

[Chit]

Secondly, the electrician said that only a max of 40Amps will reach the isolator as the MCB is rated at 40Amps.

Rubbish, in simplistic terms the current is caused by the resistance of the load and the supply voltage.
A 10.5KW shower pulls around 44A @ 240v.
Isolators do wear out resulting in them heating up.
A 40A MCB does not trip at 40A, nor does it limit the supply current. It is a poor design to have a 40A MCB supplying a load of 40A> though.

Really? This is exactly how the spark who installed the circuit advised me, sometimes I really wonder who to trust, you employ someone to do a job for you and they simply can't be bothered to do it properly and lie to you.... Is it dangerous? Do you get 50Amp Isolator switches?(not the pull cord type, I need a rocker switch type) - not been able to find any in local shops etc?

Also, what is the theory behind this so that I know for the future? How much current is allowed to pass through a 40Amp MCB? What would the MCB if not limit the supply ?

 

[EFLImpudence]

A 'B type' MCB will pass 1.13 times its rating (40 x 1.13 = 45.2A) 'for ever'.

Probably what he was thinking of when he told you but with the wrong conclusion.

At 1.45 times its rating (40 x 1.45 = 58A) it will take an hour to trip.

Getting progressively quicker until ...

it should trip 'instantly' with a current of 5 times its rating (40 x 5 = 200A) but that would likely be a short-circuit of much greater amperage.

 

[holmslaw]

..

 

[kai]

Thanks for that, do you have any links for these switches?

It is just a standard consumer unit format main switch, sold at B&Q and similar places, mounted in a 2 slot din-mount box or enclosure, also sold at B&Q etc.