Unexplained Earth Fault

[hiwj]

My 13A sockets are on a radial circuit with a 32A MCB and my cooker is on a seperate MCB (both protected with an RCD).

Last week I got a nasty shock and discovered that there was 115v between the earth on my toaster (on the radial circuit) and my cooker. There was also 115v between the neutral and earth in the socket.

I tested my sockets on the radial circuit with one of those plug-in socket testers starting from the socket closest to the CU and working out. Most of the sockets passed OK, but then there was a fault with every socket from a certain point onwards (showed up as LN REVERSE).

I have now replaced this faulty cable and all my sockets pass OK. There is now no voltage between neutral and earth.

Why didn't my MCB/RCD detect the fault and stop me getting a shock?
Would I be better converting my radial circuit into a ring circuit?

 

[EdwardCurrent]

Hello hiwj,

I think that your best bet is to get an electrician in as soon as possible to properly test the circuit that have altered.
Did you wire the circuit as a ring or radial ??
Out of interest, do you know what size cable the 'radial' circuit is cabled in ?
What made you think that the cable was at fault ?

Regards

Ed

 

[Spark123]

An MCB won't stop you getting a shock via contact with a live part, if your protective conductor has failed (which it sounds like it did) then in an earth fault it wont cause automatic disconnection and you will end up with mains potential on exposed parts connected to that part of the system. The 115v might have been due to capacitive effects where the voltage drops off rapidly when any current is drawn.
What size RCD? A 30mA max RCD wont stop you getting a shock, it should cut out before the shock level becomes fatal.

 

[rebuke]

A 32A radial sounds unusual, normally you'd expect to see a ring at that level, as to do a 32A radial would require some fairly thick cable (4mm off the top of my head, but don't quote me on that), and it would be a pain to deal with inside sockets etc - are you sure it's not a ring?

Admittedly however, a ring with a L-N reverse at certain sockets wouldn't make sense, as it would short out assuming the ends were wired into the CU correctly.... What size cabling is your radial using?

 

[hiwj]

I originally thought the circuit was a ring because there were 2 cables connected to the MCB, but it turns out that it is 2 separate radial circuits.

The cabling (which was installed in the 70s) is 4mm (has about 8 strands). When I replaced the socket faceplates with newer ones I had to cut 1 or 2 strands off the end of the cable to get it into the terminals. Does this mean that my cables don't count as 4mm any more?

Also, when I extended my circuit whilst re-doing the kitchen, I used 2.5mm cable.

I used a 32A MCB because I thought I had a ring circuit. The electrician has told me to use a 20A one if I am to keep the circuit as radial.

I am starting to think it would be best to convert it into a ring circuit (as I have 2.5mm cable and a 32A MCB in use). Any thoughts?

 

[rebuke]

Cutting strands off the cable will reduce the capacity of it, which means what's left will get hotter if it takes the full current, which is not a good thing. Personally I would probably turn it into a ring (depending on where your two radials run, it might be as simple as linking the two ends together). I'd also probably replace the 4mm cable just to avoid confusion - especially any bits that have had strands cut off, since this almost certainly doesn't comply with the regs...

Disclaimer - I'm not an electrician, just a DIYer...

 

[hiwj]

I didn't think it was a good think to cut them off at the time, but I had no choice (there was no way that 2x 4mm cables would fit into a terminal).

Replacing the old 4mm cable isn't really an option (without gutting the whole house). It would be much easier to convert the circuits into a ring (as I know where the radial circuits end - and they are easily accessible).

Thanks for the info.

 

[Adam_151]

The cabling (which was installed in the 70s) is 4mm (has about 8 strands).

It more than likely is not (I have only seen metric sized stranded twin once), what you probably have is 7/0.036 which has seven strands and has a combined CSA of approx 4.5mm²

I didn't think it was a good think to cut them off at the time, but I had no choice

Never a good road to go down...

(there was no way that 2x 4mm cables would fit into a terminal).

Oh yes there is - BS1363 requires it! (and even in light of them actually being 4.5mm², if you have trouble, then MK state their logic plus range will accept two 6mm² conductors!)

 

[rebuke]

One important thing when trying to fit large wires into a terminal, is don't twist them at all (either individually or to each other if there are multiple wires) - if you do then the overall diameter increases, and it will be much harder to fit...

 

[ban-all-sheds]

A 32A radial sounds unusual, normally you'd expect to see a ring at that level, as to do a 32A radial would require some fairly thick cable (4mm off the top of my head, but don't quote me on that)

Not saying this to get at you, but 4mm² can't be relied on to carry 32A - it's only rated at that for Method 20...

I originally thought the circuit was a ring because there were 2 cables connected to the MCB, but it turns out that it is 2 separate radial circuits.

This is why you're not supposed to do electrical installation work based on guesses.

Also, when I extended my circuit whilst re-doing the kitchen, I used 2.5mm cable.

Do you live in England or Wales?

Any thoughts?

Only one - replace the cables you damaged.

I didn't think it was a good think to cut them off at the time,

You were absolutely right - you should have stuck with your first thought.

but I had no choice (there was no way that 2x 4mm cables would fit into a terminal).

Of course you had a choice.

Replacing the old 4mm cable isn't really an option (without gutting the whole house).

It just gets worse, doesn't it...

It would be much easier to convert the circuits into a ring (as I know where the radial circuits end - and they are easily accessible).

So you've got old imperial cable, with possibly an undersized cpc, you've butchered the ends to make them fit where they didn't want to go, and you propose keeping them as part of a circuit which you extended and didn't do any testing on.

Yes - they might be alright, but you don't know that, and is betting the lives of you, the people who live with you, and those who will live there after you on the dogs dinner you've created being alright a good idea?

Plus there is no way that a valid certificate can be issued for the circuit, and IMO you have not made reasonable provision to protect persons operating, maintaining or altering the installation from fire or injury, so if you live in England or Wales you've broken the law in two different ways.

Please get someone in who knows what they are doing.