MCB problem

[reluctantbutskint]

Hi guys.
My mates bathroom wasn't protected apart from an old fusebox (has LV downlights and a fan fitted by previous owners), so he introduced a 32mA MCB, in line with a double pole switch fused at 3A in between the main and the light transformers and fan.

You would think this would be straightforward enough however the damn MCB trips. It doesn't go immediately but between 5 and 60 seconds from resetting.

trying to help find the problem I have turned off the switch which is downstream of the MCB, isolating the lights and fan, and it still trips. Interestingly it also trips when the earth connections are removed from the back of the MCB, so I assume there must be some problem upstream in the ring between live and neutral??? Have treble-checked all connections from the ring though the MCB and switch...

any help or guidance welcomed. Note that this is on a spur off a 30A ring main not the lighting circuit. Maybe he would be better getting a sparks in to rewire it off a light circuit, as reading other similar problems sounds like some appliances on a ring can break down over time and cause problems ref insulation live to neutral, which I assume is the probable cause here?

 

[bernardgreen]

32mA MCB

Do you mean a 32 AMP MCB or a 30mA RCD ?

when the earth connections are removed from the back of the MCB

An MCB has a Live in and a protected Live out. No earth connections. Or do you mean an RCBO

Either way you appear to have a serious problem as the device is not tripping immediately. and a serious lack of knowledge between you a nd your mate.

If it is a 32 AMP MCB taking a few seconds to operate you have 8 kilowatts of heat being created somewhere until the device trips.

If it is a 30mA RCD tripping then there is an earth fault and possibly without adequate earthing which could create a hazard of electric shock.

Bite the bullet and get an electrician to sort out the mess your mate seems to have made.

 

[reluctantbutskint]

Thanks Bernard
It is a 32mA ''MCB'' (Wickes call it an MCB...probably its an RCBO?) bought from wickes fits into a standard single socket, trips at 32mA earth leakage, trips both the live and neutral on fault but leaves earth intact. There is live and neutral supply in, and protected live/neutral out, and two earth connectors at the back (not visibly connected by a plate etc) although the instructions say put all earths into one of them, with the other one having a small wire leading inside the unit. Does that make sense to you?

Having now disconnected everything, there is literally the main connection, the MCB, and then a switch. With the switch turned off it still trips. with the switch turned off and the earth removed from the 'mcb' it still trips. It doesn't seem right.

 

[riveralt]

It would be helpful if you could correctly identify what your friend has actually fitted.
Is it - a Consumer Unit Based RCBO?
Is it a Consumer Unit Based MCB
Is it a RCD socket
Is it a RCD plug in socket
is it a RCD/FCU?

Photograph or pointer to Wickes site would also be helpful.

 

[reluctantbutskint]

Sorry it wasn't clear. Couldn't find it in wickes website but here is a link to the same thing in B+Q

http://www.diy.com/diy/jsp/bq/nav.j...33}&fh_eds=ß&fh_refview=lister&isSearch=false

they call it a volex RCD fused connecting unit

 

[riveralt]

Assuming he has wired everything correctly got the feed and load in their correct terminals and the line across the fuses.
These come with 13Amp fuse(s) fitted. He has changed the fuse to 3Amps - If I remember rightly there are two fuses in these units and both should be 3Amps.

 

[reluctantbutskint]

also a link to a discussion about the unit. my mate did link the two earth terminals by the way.

http://www.talk.electricianforum.co...bs-en-61008-fused-connection-unit-wiring.html

 

[reluctantbutskint]

yep hes fused it down correctly, replacing the 13a with a 3a.
what I find odd is that it doesn't trip out right away, but after a short period, and whether or not there is a load running and whether or not there is an earth connection.

 

[riveralt]

yep hes fused it down correctly, replacing the 13a with a 3a.

That will be TWO 3Amp fuses - Yes?

If this is the case and he is sure he has wired it as per manufacturers instructions then he needs to test the continuity and insulation resistance (with load removed) of the cable. If that is okay then he should conduct a ramp test on the RCD.

 

[reluctantbutskint]

there are two fuses in there but looks lijke only one (the back one) active the other is simply a holder for a spare as far as I know but yes both of them are 3A and both work as I just checked them.

 

[EFLImpudence]

Why does it have TWO fuses?

 

[reluctantbutskint]

i think it just has a holder at the front of the fuse for a spare fuse. don't think thats the point here.

 

[EFLImpudence]

I was just interested as I have not come across such things with two fuses.

To your problem - you will have to get it tested as riveralt says but IF you have connected everything as it should be and it trips when the switch downstream is off then this would indicate a fault in the cable between the unit and the switch.

However, you say it also trips when the earths are disconnected but not instantly. To me this would indicate a faulty unit.

 

[reluctantbutskint]

interestingly it trips even when the main board fuse is pulled out (ie no power at the RCD)

any RCD experts out there?

 

[EFLImpudence]

Further - what about disconnecting the cable to the switch at the unit and see if it still trips?

I wrote this before your last reply.