Recommended 40A MCB trips

[technick]

I wired in a new CU last night for my 9.5kW shower. It has a 63A 30mA RCD and 50A MCB supplied. I bought a 40A MCB and fitted that in place of the 50A, as that is what was recommended by the shower manual. However, the 40A MCB trips after about 1 minute of shower use on a normal heat setting. With the 50A MCB fitted, there is no problem.

The cable used is 10mm2 and runs for about 10m.

Unfortunately the company does not seem to make 45A MCBs, will I be OK to continue with the 50A MCB?

 

[IanDB]

This //www.diynot.com/forums/viewtopic.php?t=10936 will give you some guidance. Cable is fine but on the link in the topic they recommend 45A mcb. They are available. 50A is too high.

 

[plugwash]

50A should be ok

45A exist but not in all brands?
what brand is this shower CU?

 

[technick]

It is a Volex, I could not find any Volex 45A MCB on a google search, though I have found other makes, would any other make be compatible with the CU?

 

[plugwash]

i belive wylex and volex breakers are compatible but i would check the conenctions match before fitting one

but wylex don't make 45A either

just put back the 50A breaker and don't let it worry you

 

[technick]

OK, thanks for your help.

 

[pipme]

Excuse me gents, are the right questions being asked here ?
Could there not be a fault of some kind present ?
It looks from the time / current graphs in Whitfields Guide that a 40A MCB (B) would be pulling a fair bit over 40A to trip in 1 min ... Shower heat at 'normal setting' suggests not using full power anyway.

Interesting to note how long before the 50A MCB trips, if at all.
Surmising correct connections.
Could the 40A MCB be faulty ?

P

 

[BR]

Excuse me gents, are the right questions being asked here ?
Could there not be a fault of some kind present ?
It looks from the time / current graphs in Whitfields Guide that a 40A MCB (B) would be pulling a fair bit over 40A to trip in 1 min ... Shower heat at 'normal setting' suggests not using full power anyway.

Interesting to note how long before the 50A MCB trips, if at all.
Surmising correct connections.
Could the 40A MCB be faulty ?

P

These are all very relevant points. There is a variation between manufactures in terms of overload characteristics of mcbs, however not to this extent. In theory a thermostatically controlled shower will draw it's full rated current when on full, but in practice the actual load placed is often slightly less. The problem could simply be a case of high voltage on the premises, or maybe some fool has spurred something else off the shower circuit. In any case an inspection of the installation would be in order.

 

[technick]

No 'fool' has spurred off this circuit. This is brand new circuit direct from the supply, via a new dedicated CU.

The shower has two heat settings, this was on the higher setting, then to vary the heat on that setting the flow is varied.

The 50A MCB has not tripped as you would expect, however a 45A would be preferable, I will see if the Wylex are compatible with the CU.

 

[ban-all-sheds]

The non-tripping current for a 40A Type B is 45.2A.

If your shower is rated 9.5kW at 230V, and your supply is actually at the maximum tolerance of 252V it will be drawing 45.26A - nowhere near enough to trip the breaker in 1 minute - needs around 100A to do that.

 

[BR]

- needs around 100A to do that.

not true, not even for rewirable fuse.

 

[securespark]

BS3036 fuses (r/w's to most) generally need twice the rated to blow instantaneously.

 

[pipme]

So gents, are the aforesaid graphs as shown in Whitefields Guide, incorrect ? They definitely show the kind of figures quoted and since rejected.
Also graphed in a different manner here Click
In particular Para 1.4 Fuse and MCB characteristics

If there are differences of opinion at this level - doesn't give much hope to us customers ..... I bet lots of jobs are carried out to calcs either off the back of a fag packet or 'because that's how we've always done it' , ok, so when I get work done in future I expect more than I have recieved so far ... I want the written up calcs too, otherwise 'on yer bike', if I am paying for a pro job then brother you better start providing one !!

I wonder if there is anyone accessing this site who has been present at Lab tests on MCBs and witnessed construction of 'time to trip' against 'current throughput graph's etc ?

Troubled times are approaching I think !!


P

 

[Lectrician]

Agree, BS7671 charts show a touch over 100amp to trip a B40 trip in 60seconds. BUT in reality it would happen ALOT quicker.

 

[ban-all-sheds]

But no absolute guarantee, hence all the Ib In Iz I1 I2 stuff...