MCB Tripping on unrelated circuit

[dmh]

Good morning to everyone reading this. My first post so please be gentle with me

I'm reasonably new to this business and have been asked to investigate a fault which I will describe below. I haven't yet seem the installation so some info is a little sketchy still.

An electric shower (don't know power yet or fuse rating) when used is frequently, although not always, causing a downstairs ring to trip. I am told that it is the MCB for the ring that is tripping not an RCD, the shower itself continues to work fine.

In theory, although not yet confirmed as I haven't been on site yet, these two circuits are totally unconnected.

Has anyone any pointers as to where the fault might lie?

Thanks

Darren

 

[nickblake]

Not having much info its hard to asses , if the circuits are RCD protected you could have an intermitant earth leakage fault IE neutral to earth ,it could also be caused by a fault on the board loose connection could be a faulty or sensitive MCB that when the shower is switched on the surge causes the mcb to fail ,firstly check make sure its not the RCD thats tripping so many people get confused about this

 

[sparkylad]

Yep my first impressions of the problem ,granted with little info to go on, is check the consumer unit for loose connections ;especially the ring circuit---L, & N

 

[Deleted member 9648]

If the mcb's are adjacent I have known heat generated in a heavily loaded mcb to trip an adjacent one.....if you cant find any other cause and the mcb's are adjacent try separating them to allow some air space.

 

[cbw]

Same problem here, when shower used water leaks from the waste and down the conduit into the downstairs socket, causing the MCB to trip.

 

[dmh]

Thank you to everyone who posted some suggestions. I'm looking at this on Thursday so I will post what I find.

 

[ian]

Had a similar problem a few years ago, eventually tracked the fault to the steam from the shower getting into the light, shorting out L&N. I installed a IP rated light, a fan then low and behold the problem vanished. Now that’s magic.

Ian

 

[dmh]

I have now had the opportunity to look at this site and can report the actual sitution is as follows:

When the shower is on and for example you switch on the kettle in the kitchen the RCD that protects the sockets trips. With the shower off you can switch kettle on and nothing trips.

There are, too say the least, a few issues with the installation, it is a TT system with a Ze of 278 ohms and no overall RCD proction. The shower has no RCD protection, there are lighting circuits with 4 cables coming from the MCB, rings with 3 cables from MCB, a 1.5mm cable coming out of the 50amp MCB in additon to the shower cable etc.

Under testing the 30 mA RCD that is protecting the socket outlets will not trip at all and on a ramp test still hasnt tripped at 51mA. Replacing the RCD gives the same results so not looking at a faulty RCD.

In the house there is the main consumer unit with two MCBs, one feeding a secondary consumer unit and the other feeding the shower circuit. The secondary consumer unit has the normal cicuits you would expect for lights, sockets etc. The socket outlet circuits are RCB protected.

I appreciate that there is a lot to be sorted with this installation but I'm still puzzled at to why the shower being on casues the RCD to trip. The shower cable runs direct from the main unit to the shower switch and to the shower and there does not seem to be any interconnections with other circuits. In the shower room there are lights and a shaver socket niether of which are on RCD protected circuits. There is no sign of any water leakage and the symptons don't support a water leak situation (trips if shower on, as soon as shower off will not trip even though water is still present).

I've only had a brief opportunity to investigate so far as the customer could only give me a couple of hours access.

 

[colinv]

Check to see if there is a borrowed neutral. ie has a neutral been taken from the RCD protected board to feed a circuit on the non rcd protected board area. If so it will cause the RCD to trip. Also strongly encourage the house owner to have an RCD fitted to protect all the circuits as it is a TT supply.