Rats and tripping MCB

[vwes59]

Can i just check something. If a rat decides to chew a cable will it trip the MCB or RCD ?

I ask because I had one house recently that had a randomly tripping MCB and it turned out to be a chewed cable. I was surprised as i would have thought it would trip the RCD but it never did. A chewed cable would cause a short not an overload.

Ive now got to visit a house thats suddenly got a tripping MCB and they've already told me they've had a rat problem recently.

 

[Pretty Mouth]

When T+E gets nibbled, it's more likely to take out the RCD, the earth being sandwiched between the live conductors. That assumes a working RCD of course.

I guess other cable types could result in a short between L and N though, which would only take out the MCB.

 

[Dustydazzler]

Slightly on a tangent but I did and EICR on an old house recently and in the attic were some chewed cables where the animal managed to chew off inches and inches of insulation and in some places down to the copper exposing all 3 cores and at no point blew the fuse. While I admired the animals precison cable stripping , I had to give it a C1 for live exposed parts

 

[MJPD29]

L-N shirt would trip mcb, not rcd. MCBs are for overload AND short circuit protection…

 

[vwes59]

Thanks for the responses. I'll check the appliances etc first as usual then, if that doesnt show an issue ill do an IR on the circuit. If it is a damaged cable i just hope its somewhere i can get to or bypass

 

[James]

Be careful in any loft space, especially if any power is still on.
a hand or a knee on a bit of cable with no insulation can spoil your day.

 

[davesparks]

A chewed cable would cause a short not an overload.

A short circuit, being a connection of negligible impedance between live conductors, would trip an MCB.

And earth fault, being a connection between earth and live conductor would normally trip an RCD first before an MCB has reacted.

 

[vwes59]

So, an update. No continuity found on CPC, Neutral normal, Line 700 Ohms when measuring continuity. IR reading on Line 3MOhms, Neutral >2000MOhms. After quite a bit of work got IR on Line up to 600MOhms, but continuity still 700 Ohms, but both MCB and RCD now trip when only the MCB did before, so progress in a fashion! Found lots of Line insulation caught in terminals.

 

[MJPD29]

So, an update. No continuity found on CPC, Neutral normal, Line 700 Ohms when measuring continuity. IR reading on Line 3MOhms, Neutral >2000MOhms. After quite a bit of work got IR on Line up to 600MOhms, but continuity still 700 Ohms, but both MCB and RCD now trip when only the MCB did before, so progress in a fashion! Found lots of Line insulation caught in terminals.

I’m confused, when you say IR on line do you mean IR line to neutral or line to earth? 3Mohms wouldn’t be enough to trip an mcb.

 

[vwes59]

I’m confused, when you say IR on line do you mean IR line to neutral or line to earth? 3Mohms wouldn’t be enough to trip an mcb.

Sorry, my bad, I meant these were N-L values.

Moved on considerably now. Have mapped out the faulty ring, and have isolated the section with bad cabling (and it is a bad cable) - and guess what...it's in the wiring of the extension, built 2 years ago with no EIC or any other paperwork! The original wiring is all perfectly fine. So they currently have a radial, which is fine as there's no huge load like a washing machine etc on it, and I'll be going back while they are on holiday next week and try and trace exactly where the problem is. Absolutely perfect values, >1000MOhms on IR and correct continuity values. On the bad section which I've isolated - continuity 40+Ohms and IR values all over the place.

There definitely needs to be some education for customers when extensions etc are built. They must insist on the correct paperwork for any electrical work. I bet in this case it hasn't been notified either.

Having said that though, given the client has had rat issues, this is still the top suspect - a chewed cable.

 

[vwes59]

Just to end. Fault was indeed a cable, and it was the last section from the last socket in the extension back to the board...almost entirely severed, how im still unsure. Client wants me to now do a full inspection of the property.