MCB too small for shower

[Steve__M]

I'm about to replace the heat exchanger on a friend's shower this weekend, but I noticed there is only a 30 amp MCB on a 9.5kW shower. Apparently it trips occasionally (every few months). The cable is big enough - it's the right size, and the shower switch is rated to 45 amps.

So I checked my shower (in a new house). I have a 10.5kW shower and a 40 amp MCB.

Both these MCBs seem too small - why aren't they tripping?

Are MCBs easy to change, or should I get an electrician?

(I'm guessing the one in my friend's house should be changed given that it trips occasionally)

Thanks for any help.

 

[ricicle]

The Mcb's will not trip instantaneously on an overload but will take a finite amount of time depending on how much they are overloaded.
These circuits need looking at.If your house is new then perhaps you can contact the original builders/electrician to check your concerns.

Regarding your friends, are you sure you know the cable is big enough.
In anycase changing the overcurrent device requires checks on the circuit in question for compatability.As it involves a shower and if it is in a bathroom then the work would be notifiable to the LABC under Part P of the Building Regulations

 

[davy_owen_88]

MCB's take a long time to trip during small overloads. It isn't good practice to run MCB's past their limit but unless they are causing a problem and suitably protect the cable they are meant to then I see no reason to change it.

 

[securespark]

They have made 30 mcb's Davy... Type 1, 2 & 3....

Steve

How do you know the cable is big enough? What are the circuit characteristics?

BTW, showers are only on for a brief time, so, if the overload is small & the length of time the shower is in use, the breaker will probably stay in.

To start with, an mcb does not trip at its rated. It must trip within 1. something of its rated current (can't remember what that figure is.... ). The higher the overload & longer the overload is drawing current the quicker the breaker will operate.

 

[RF Lighting]

Also they don't make 30A MCB's...

 

[davy_owen_88]

Ok, ok ok. I stand corrected

Adam pointed it out almost as soon as I posted it, then as I was in the process of changing it secure posted and now you come along to rub salt in the wound

 

[sparkyspike]

Check the circuit before changing the fuse. It could be a 30A fuse for a reason, e.g. high loop impedance on the circuit.

 

[Adam_151]

Crabtree C50 board rob? or is it one of those amercian federal electric ones? or something like that?

 

[Steve__M]

They have made 30 mcb's Davy... Type 1, 2 & 3....

Steve

How do you know the cable is big enough? What are the circuit characteristics?

It's black & red T&E with 7 strands of copper per wire - is this enough info, because I can only see the end of the cable (where it comes out of the wall into the shower casing). I assumed it was fine because its the same as the cooker cable...

Apparently someone in the house regularly has 40 minute showers, hence the heat exchanger failing I guess. I checked the shower and it does seem to be wired correctly such that it should draw its full power (I did wonder whether they'd not wired up one of the three heating elements so as to reduce the load - but it all seemed fine). Surely at full power it should be drawing about 40 amps on a 30 amp MCB.

It's not a rewirable fuse, so I guess it's one of what securespark said. I'll look more carefully when I next go.

 

[RF Lighting]

Its one of those horrid federal stablok boards that are impossible to get spares for. Yuk

 

[Steve__M]

It looks a bit more modern than the one in the picture. I'm guessing it was done in 2001 or a bit earlier. It's got 6 MCBs and an RCD. I knew I should have written down the details...

 

[ricicle]

To start with, an mcb does not trip at its rated. It must trip within 1. something of its rated current (can't remember what that figure is.... )

1.45

 

[securespark]

Funny. I had 1.3something in my head, quoted by Hager Tech the other day. Must be wrong.

The cable, if it is the same as the cooker, may well be 6 milli.

All stranded is 7 strand, it's just the thickness of those which vary. Look at 25 milli tails... 7 strand but thick with it!

 

[Adam_151]

1.13 is the lower band from bsen60698
1.45 is the upper band which also appears in BS7671 as not all protective devices meet it without extra factors (bloody semi enclosed hot wire fuses!)

 

[securespark]

Hmm. Hot Wire. You're not wrong there, matey!