how to detect easily damaged cable under floorboards

[nk1255]

hello

theres a damaged cable under floor boards in either the living room, hallway or dining room.

the 'downstairs sockets' mcb type b in consumer unit trips as soon as i flick it back up, so the problem is in one of the rooms above.

how do i detect which cable under the floorboards it is without having to remove carpets and dining room tiles, as that's a big job and costly.

are there any qualified electricians on this forum that can help with how to identify the problem from the consumer unit?

i was told that you can check from the consumer unit ( from the mcb type b that has tripped) or from the ring main.
apparantly you can use a multimeter(?) to check which cable from the ring main (tripped mcb type b) has dropped voltage/amps.
also can check the cables in each socket which is affected with a multimeter.

how can a megger ( insulation meter) be used to check which cable is damaged?

thanks

​

 

[pc1966]

As already mentioned, start by ruling out the accessible stuff by checking all of the socket outlets fed from that MCB.

Obviously you have to make absolutely sure the power is off to the sockets being checked, simplest DIY option is a socket tester (having proved it works in a working socket, e.g. upstairs) on each and every downstairs socket before you open it up. Never assume they are all on the same supply, someone might just have done something really dumb... Or if you can (daytime, enough light, etc) just put off the main CU switch.

If you can rule out a fault at the socket backs (e.g. cable cut on back box as no grommet, etc) then it could well be a cable fault under the floor, or some hidden junction box, but identifying that location is much harder. Generally UK sockets tend to be wired in a "ring final circuit" (though they might be radial) and if you have a suitable insulation tester you can disconnect the CU end, split the ring at a known location such as furthest socket, and then see which of the halves has the fault. Then proceed to divide and conquer until you have found the faulty link.

However, messing around with the CU is really not to be suggested as a DIY activity as there is a real and serious risk of if going badly wrong. Also you need to be aware that a multimeter is not very good at fining cable faults as they only test at a couple of volts max, and if you ever do plan on using a multimeter on mains cable always make sure it is CAT-III or CAT-IV rated! Some cheap or old meters have little protection against a high energy fault and sadly folk have died from the resulting explosion/burns before, so it is not something to ignore.

Measurement category - Wikipedia

en.wikipedia.org

Typically an electrician would have a Multi Function Tester (MFT) that includes insulation resistance at 250V or 500V, though some prefer a stand alone IR tester (often known as "megger" in the same way we refer to hoovering the floor, first big brand name). Not only are they ideal for cable fault testing, they are also designed to be safe if accidentally testing a live circuit.

Which we all know should not happen as we all follow safe isolation procedures, don't we? But sometimes in life sh*t happens and you really don't want your day to get any worse due to inadequate instrument safety.

 

[littlespark]

We’re not actually allowed on the forum to give “step by step” instructions…. Even for educational purposes.

 

[Petej999]

london, wembley

BIT to far for me was going to try and help out by a visit just a thought ,sorry no can do, good luck with it anyway.

 

[Dustydazzler]

Give Delroy a call , he loves fault diagnostics

Home - Eastway Electrical

www.eastwayelectrical.com

 

[IAmSparkytus!]

Give Delroy a call , he loves fault diagnostics

Home - Eastway Electrical

www.eastwayelectrical.com

Delroy, is that you??

 

[nk1255]

All plugs from the sockets in affected room are out of socket, i tried to push mcb up thats tripped, and the mcb stayed up but mains socket rcd tripped. So cant be socket problem!
Maybe cable in the mcb??
Maybe the mcb??

 

[Mikegh]

All plugs from the sockets in affected room are out of socket, i tried to push mcb up thats tripped, and the mcb stayed up but mains socket rcd tripped. So cant be socket problem!
Maybe cable in the mcb??
Maybe the mcb??

Haven't read the whole thread but you said "room " ?

You.need to get everything unplugged and switched off connected to a tripping rcd

That's your starting point, that or isolating circuits to see where the problem is

 

[nk1255]

Everything unplugged in affected rooms which are connected to the tripped mcb

 

[DPG]

I would definitely get an electrician in to so some testing. It's not good to repeatedly close a circuit breaker on a fault.

 

[nk1255]

As already mentioned, start by ruling out the accessible stuff by checking all of the socket outlets fed from that MCB.

Obviously you have to make absolutely sure the power is off to the sockets being checked, simplest DIY option is a socket tester (having proved it works in a working socket, e.g. upstairs) on each and every downstairs socket before you open it up. Never assume they are all on the same supply, someone might just have done something really dumb... Or if you can (daytime, enough light, etc) just put off the main CU switch.

If you can rule out a fault at the socket backs (e.g. cable cut on back box as no grommet, etc) then it could well be a cable fault under the floor, or some hidden junction box, but identifying that location is much harder. Generally UK sockets tend to be wired in a "ring final circuit" (though they might be radial) and if you have a suitable insulation tester you can disconnect the CU end, split the ring at a known location such as furthest socket, and then see which of the halves has the fault. Then proceed to divide and conquer until you have found the faulty link.

However, messing around with the CU is really not to be suggested as a DIY activity as there is a real and serious risk of if going badly wrong. Also you need to be aware that a multimeter is not very good at fining cable faults as they only test at a couple of volts max, and if you ever do plan on using a multimeter on mains cable always make sure it is CAT-III or CAT-IV rated! Some cheap or old meters have little protection against a high energy fault and sadly folk have died from the resulting explosion/burns before, so it is not something to ignore.

Measurement category - Wikipedia

en.wikipedia.org

Typically an electrician would have a Multi Function Tester (MFT) that includes insulation resistance at 250V or 500V, though some prefer a stand alone IR tester (often known as "megger" in the same way we refer to hoovering the floor, first big brand name). Not only are they ideal for cable fault testing, they are also designed to be safe if accidentally testing a live circuit.

Which we all know should not happen as we all follow safe isolation procedures, don't we? But sometimes in life sh*t happens and you really don't want your day to get any worse due to inadequate instrument safety.

Thank you for your detailed explanation. I have called a few electricians to come and check. Ill make sure they are qualified and have safe meters as per your info. Are you in london? If so you could have come!!

 

[nk1255]

Give Delroy a call , he loves fault diagnostics

Home - Eastway Electrical

www.eastwayelectrical.com

Thanks

 

[Lister1987]

Delroy, is that you??

Is that you John Wayne or is this me? ?

 

[Dustydazzler]

Is that you John Wayne or is this me? ?

 

[nk1255]

Can someone please recommend a reasonably priced electrician that can find the fault and fix near wembley.

First instance id rather get recommendations from here

Second google search

Thank you

 

[Diddy]

I think reasonably priced would be subjective as anybody visiting your property would have to pay nearly £30 if they don't have a nearly new van. That doesn't include parking nor fuel lol. Plus what you are looking at is a call out charge which usually includes first hour and then an hourly rate to diagnose and find the fault. Repair the fault and issue any certs if applicable.
I don't know what your locals charge but down here in sunny/rainy east kent im £75 plus vat for the call out and £45 per hour after that. I would be adding the above London charges onto any customers bill though if I lived and worked up there.
Anyway Delroy is local and has a great reputation and has already been recommended.