MCB failed to trip mystery

[barefootandrew]

Dear all,

This is my first post on this forum, so hello!

We recently had some work done which included one ceiling light being turned into two, and the ceiling replastered.

After fitting wall switches and ceiling pendants, no 2 light wasn't working. So I went into diagnostic mode...

With the NEW cable dangling disconnected at both ends, my multimeter tells me there is continuity between live and earth. Obviously, this is suboptimal

However, this cable was at one point connected to the lighting circuit. Why didn't the live-earth fault trip the MCB? It's a Hager MT106 rated at 6A. (Our RCD covers sockets and shower only.)

I could understand this if the line/neutral polarity of the incoming cable was reversed. They are - black is line, red is neutral, black 2 is switched live. But I knew this, and would have connected new brown to old black line, and new blue to old red. The meter shows 240v across old line and old earth, and 0v across old neutral and old earth.

Any and all comments welcome.
A.

 

[JohnW2]

With the NEW cable dangling disconnected at both ends, my multimeter tells me there is continuity between live and earth. Obviously, this is suboptimal ... However, this cable was at one point connected to the lighting circuit. Why didn't the live-earth fault trip the MCB? It's a Hager MT106 rated at 6A. (Our RCD covers sockets and shower only.)

Welcome to the forum.

Was there L-E continuity at both ends of the disconnected cable? If so, if the L was connected to the MCB and the CPC ('E') to a DNO-supplied earth, then the MCB most certainly ought to have tripped. If you had a TT earthing system (i.e. an earth rod, not a DNO-supplied earth) then the MCB might not trip, but you should have had RCD protection for all circuits if it was a TT earth.

Do you have any idea what's causing this L-E connection in the cable - has someone perhaps put a nail or screw through it somewhere?

I could understand this if the line/neutral polarity of the incoming cable was reversed. They are - black is line, red is neutral, black 2 is switched live. But I knew this, and would have connected new brown to old black line, and new blue to old red. The meter shows 240v across old line and old earth, and 0v across old neutral and old earth.

You've lost me a bit there. Which conductors did you see the 'live-earth' continuity between?

Kind Regards, John

 

[deadshort]

Hi, If your cable is disconnected at each end and you are getting continuity across L+E you obviously have a nail / screw through your cable. You may not have an earth to the MET.

Also, red is line /or live .

Kind regards,

DS

 

[deadshort]

JW2 is faster at typing than me !

DS

 

[JohnW2]

JW2 is faster at typing than me !

I must be (for once!), since I typed a lot more than you, too However, as I said, if that cable were once connected, and there is a TN earth, the non-operation of the MCB is not explained!

Kind Regards, John

 

[deadshort]

I would advise you check the polarity of your lighting circuit if you have reversed polarity earth neutral short will not trip the breaker. If the red is neutral on your wiring ?

Kind regards,

DS

 

[JohnW2]

I would advise you check the polarity of your lighting circuit if you have reversed polarity earth neutral short will not trip the breaker. If the red is neutral on your wiring ?

Quite so - as I said, I was rather confused by what we were told...

You've lost me a bit there. Which conductors did you see the 'live-earth' continuity between?

Kind Regards, John

 

[barefootandrew]

Thanks everyone for the welcome and the comments thus far.

To help explain further, I have rustled up a diagram

The light grey group represents number 1 wall switch. Originally this comprised cables A/B/C/D/E; we have since added F/G/H to supply power to a new, secondary light, the wall switch for which is the light blue group (cables I/J/K).

The light green group represents the original, single ceiling pendant. A new, secondary ceiling pendant is NOT shown here, and would be driven from the new wall socket (light blue; cables I/J/K).

Cable descriptions are as follows:-

A. black switched live; connects to P.
B. black earth; has a shorter sheath and connects to R.
C. red (appears to be) loop neutral.
D. black loop line - a permanent 240V is present here.
E. black loop earth, has a shorter sheath, originally connected to B.

F. brown new loop extension line; would connect to D.
G. blue new loop extension neutral; would connect to C.
H. yellow/green new loop extension earth; would connect to B and E.

P. black switched live; connects to A.
Q. black neutral; does not have a corresponding wire in the light grey group.
R. black earth; has a shorter sheath and connects to B.

This extended system was working just fine until the ceiling was plastered. A drywall screw through the new cable (FGH -> IJK) is suspected.

Diagnostics shows up the following:-

1. Multimeter shows a permanent 240V AC between D and E. Tester screwdriver (with the illuminating filament thingy) glows when touched on D; thus D taken to be line.

2. Q and R show electrical continuity between them. We have a TN-S (* see below) system here. As I type I have not tested C and E for continuity; I shall be expecting to here a multimeter beep to confirm continuity.

3. Both F and H, and I and K, show electrical continuity.

Excluding connections A/B and P/Q/R for now, simply connecting the new cable as thus:-

C to G
D to F
E to H

should trip the MCB given that F/H short circuit, and that D appears to be line. It does not.


Earthing arrangements

We have a TN system as follows:-

* conduit from outside enters property;
* chunky line and neutral cables go into meter, and from there into CU;
* chunky earth wire goes from conduit direct into CU;
* there is evidence of a TT rod outside for each house on our street, but these have long since been disconnected;
* the CU RCD covers the sockets ring and shower circuit only.

Any and all comments welcome.
A.

 

[AndyPRK]

I think the first thing to point out is that light switches don't normally have a neutral connection. It is becoming common in modern houses and is typically denoted by having several black or blue cables connected together in the backbox behind the light switch.

If you haven't got a neutral, you can't go and connect a light fitting from there, like you have tried to.

As you have noted, Q does not go to the switch, but a neutral connection elsewhere


It would be good to try and show on your diagram the old switch connections.
e.g COM L1 L2

 

[JohnW2]

A. black switched live; connects to P.
B. black earth; has a shorter sheath and connects to R.
C. red (appears to be) loop neutral.
D. black loop line - a permanent 240V is present here.
E. black loop earth, has a shorter sheath, originally connected to B.

3. Both F and H, and I and K, show electrical continuity. ... simply connecting the new cable as thus:- C to G, D to F, E to H ... should trip the MCB given that F/H short circuit, and that D appears to be line. It does not.

I think something must be wrong with your understanding of 'what is what' in all that wiring. If F-H is a short circuit, and if (D) and (E) were as you describe above, then, given that you have a TN earthing system, the S/C you were placing between L and E (by connecting F/H to D/E) should result in the MCB immediately tripping - or, if it didn't (i.e. because the MCB was faulty), there would be all sorts of fireworks and/or melting cables and, quite probably, operation of the DNO's service fuse.

It seems unlikley that you will have mis-identified (D) (permanent live). What about (E) - how certain are you that it really is connected to earth, and how have you ascertained that?

Kind Regards, John

 

[barefootandrew]

OK, let me do some more testing, to see if I have C/E the wrong way round, or have otherwise misinterpreted their function.

I shall return...
A.

 

[JohnW2]

OK, let me do some more testing, to see if I have C/E the wrong way round, or have otherwise misinterpreted their function. I shall return...

Fair enough - I look forward with interest! However, even if you had got C/E the wrong way around, you would have been putting a short-circuit between L and N - which, again, ought to result in MCB tripping and/or 'fireworks'.

Have you considered the possibility that "E" (perhaps intended to be an earth) is, for some reason, not connected to anything at all? ... maybe more testing?!

Kind Regards, John

 

[barefootandrew]

Sorry for the delay getting back to this thread - busy few days.

You were right John! Finally traced it back to an earth parking terminal that should have two earth wires going in, and one had come out and was just "dangling". Presumably in the wrestle of getting the wall switch into position with several cables needing moulding into position.

A.

 

[JohnW2]

Have you considered the possibility that "E" (perhaps intended to be an earth) is, for some reason, not connected to anything at all? ... maybe more testing?!

You were right John! Finally traced it back to an earth parking terminal that should have two earth wires going in, and one had come out and was just "dangling". Presumably in the wrestle of getting the wall switch into position with several cables needing moulding into position.

That explains the lack of MCB tripping, then! I presume you now know what needs to be done to get this all sorted out.

Kind Regards, John

 

[barefootandrew]

I do. Earth wires connected up. Damaged cable replaced. All is well

Thanks John and everyone else for your comments on this.
A.