Undersized Breakers

[The_Scotty]

Just out of interest.

Whats the consequence of running an appliance that is in theory too big for the breaker protecting the circuit.

Reason I ask is that I was asked if I could fix an electric shower that wasn't operating properly. It wouldn't switch on because it had been poorly fitted and the button on the front panel wouldn't line up with the back.

Thing is it was a new shower that had been 'fitted' as the property was previously leased and was recently sold.

The shower was 10.5Kw, the cable was 6mm and it was protected by a 32aMCB.

As the folk had just bought the house they don't know how long the shower had been in for therefore if it had been used.

I have passed on their details to an electrician I know and as far as I know its been replaced.

Is this set up dangerous to the unsuspecting householder?

 

[RF Lighting]

Possibly.

unfortunatly MCBs and fuses do not operate if they are slightly overloaded. The bigger the overload, the quicker the OCPD will operate.

It may be that the MCB will not trip as showers are used for relatively short periods, which will heat the thermal trip part of an MCB, and as it is getting ready to trip the shower is turned off, which allows it to cool again.

This is not good for the MCB, and as the cable may be being overloaded, there is also a risk of the insulation on the cable failing, and causing a permanant short circuit, or even a fire.

Oh and it also breaches BS7671. (The wiring regs)

 

[Cremeegg]

To put a few numbers on it from BS7671 & OSG - 6mm cable will take at best 46A but depending on the way the cable is run - through insulation etc - it may be as low as 32A. A 32A breaker will run for 15 minutes at 50A or a good 20 seconds at 150A.

Far better with 10mm cable and a 45A breaker.

 

[securespark]

Or a 50A?