Should I change MCB for higher rating

[TonyWarrington]

Forgive me for asking what may be seen as a very basic question.

Have a double socket in the hallway of a property we have just taken over. I replaced the old plug in fuses in the CU with MCB's recently (3x5amp, 1 x 15amp, 2 x 30 amp. Tried to use it for the lawnmower this weekend which immediately caused an MCB to trip.

Checking the MCB found that it was one of the 5 amp ones!!

As yet have not checked what else may be on this circuit.

My question is would it be safe practice to upgrade the 5 amp MCB with a 10 amp unit - hopefully would be able to use the lawnmower from this socket.

 

[RF Lighting]

NO! you must never just up the size of an OCPD just because it is tripping.

You need to ensure the cable is capable carrying the load which a bigger MCB would allow.

It sounds like the socket might be fed from a lighting circuit which is not very good.

Also if you are using the socket to supply equipment outside then it really ought to be RCD protected.

 

[davy_owen_88]

No it wouldn't! Find out what cable is used for the circuit (ensuring it is consistent throughout the circuit) and also what else is fed from the circuit. Then tell us what conditions the cable/s run through (insulation, conduit, trunking, furnace etc).

EDIT: Bah!

 

[TonyWarrington]

Wow what a quick response - Thanks very much for your replies.

RF Lighting: As you say I would not just up the size without investigating further as to why tripping.

Will check the cable etc.

Lawnmower was connected to a RCD adaptor.

davy_owen_88: Will check as suggested.

I suspect that this socket must have been fed from a lighting circuit. If this is the case then I suppose that it is going to be limited for use for a plug in hall light and possibly a cordless telephone. Also I think that I should perhaps mark it as 5 amp limit!

I do really appreciate your comments - I want to be as safe as possible

 

[plugwash]

provided the wiring is not burried in insulation it should be ok to go up to 10A even on a lighting circuit with 1mm cable, this may be enough to make your mower work.

long term though I would seriously advise finding where that socket is fed from, disconnecting it and re-doing it properly.

 

[CallEdsFirst]

Twice now I have just read Plugwash come in at the 11th hour and say "Yeah thats fine but not ideal"

Are you on a mission?

 

[TonyWarrington]

The socket is connected to a lighting circuit - it is standard 1mm cable!! It looks like this could have originally been used for a burglar alarm some clues found!!.

From the quick check I did it looks like cable supplies outside flood @ 150watt plus another 3 upstairs lights @100 watt maximum (this is only a small property). Need to check a little more but I assume that maximum load on this circuit with all switched on would be 450watt (say 500watt).

If I have my sums (and correct terminology) right this means that maximum load would require 2 amp. Do not know what rating mower is but estimate 1700 watt so I believe that this would require 7 amp - so should I be surprised that the MCB tripped!

I know that I should consider disconnecting and re doing properly. The option for this are limited to either coming all the way down the stairs or under the solid floor across the hallway. These areas have just been decorated and I would be reluctant at having to re decorate again.

So for the time being at least I want to leave this alone - which leaves me with the dilema:

1. Mark socket 5 amp lighting only and limit use to lamp or similar units
2. Upgrade to 10 amp MCB - but have I considered all the safety implications?

On the face of it the easy/safe solution would be option 1.

What do you think?

 

[RF Lighting]

You might find that the inrush current needed to start the mower may still trip a 10A type 'B' MCB. (I assume it is this tripping the 5A MCB rather than a sustained overload as your MCB tripped straight away)

Normally this could be overcome by using a type 'C' MCB, but these are not produced as a plug in module for old wylex CUs.


Would it not be easier to have a proper external socket installed somewhere on the house, and just use the socket off the lights for small loads, rather that having to rewire it at the moment.?

 

[TonyWarrington]

RF Lighting Not considered the factor of inrush of current and potential to trip a 10A MCB.

External socket would be the best option normally - although do not trust one of the neighbours think he would plug his whole house in when I am not looking

Think I will just limit the socket for small loads as suggested and find another outlet - longer extension lead and of course RCD adaptor.

Thanks
Tony

 

[RF Lighting]

You could have a switch inside so the outside socket is only live when you want it to be, or IIRC smiths make a weatherproof socket with a padlock hole to prevent unauthorised use.

 

[TonyWarrington]

Thanks very much - not thought of either of those - well worth considering and of course would be RCD protected so could not forget to use the adaptor!!