Melted on earth sleeving on a ring circuit

[sparkwright]

A ring circuit for sockets is tripping the 32 amp circuit breaker, and even the 63 amp submain circuit breaker on occasion.

On many sockets, towards the centre of the ring perhaps, the earth sleeving has melted, and left an indent on the live and neutral cores.

Ring circuit seems very large, covering over 150 square metres, with cables run from above, running down the walls, so a fair bit of cable used.

Anyone ever encountered such a thing?

 

[JohnW2]

A ring circuit for sockets is tripping the 32 amp circuit breaker, and even the 63 amp submain circuit breaker on occasion. On many sockets, towards the centre of the ring perhaps, the earth sleeving has melted, and left an indent on the live and neutral cores.....

There presumably must be a major L-E fault somewhere? In the absence of faults, there should be virtually no current in the CPC, let alone enough to melt its sleeving.

Kind Regards, John

 

[AndyPRK]

You could break the ring in half using 2 20A breakers.

That may help locate which half has the issue.

 

[sparkwright]

Yes, will need to check inside every socket anyway to repair any damage, and look for obvious faults - obviously no point tidying it up too much if fault still present as earth sleeving may melt again.

Not seen this happen before.

 

[JohnW2]

Yes, will need to check inside every socket anyway to repair any damage, and look for obvious faults - obviously no point tidying it up too much if fault still present as earth sleeving may melt again. .... Not seen this happen before.

I can't say that I have, either, but that doesn't prove much!

I suppose this happening could stimulate a discussion about the wisdom of having cables with 'undersized' CPCs (smaller CSA than L & N conductors). Provided that the OPD is appropriately rated for the cable, it should guarantee that the L & N conductors will not come to harm (and I presume that includes getting hot enough to melt insulation) as a result of overcurrent. However, since (other than for 1mm² cable) CPCs of T+E have a smaller CSA than the L & N conductors, the CPC in these cables is presumably not as 'adequately protected' - hence may get hot enough to be harmed (before OPD operates), even though the OPD operates before the L conductor comes to harm.

I suppose what you have seen is very rare since 'protracted' high L-E currents (which is what presumably would be needed to melt insulation) are themselves very rare - L-E faults usually result very high fault currents which result in the fault being cleared by the OPD 'instantly' (before there has been time for much thermal damage to occur).

However, since you've seen it happen, I suppose it is a good selling point for RCD protection!

Kind Regards, John

 

[333rocky333]

I once see a T and E lighting cable where the earth had burnt right through the outer sheaving, because the old wylex fuse blew the woman had cut a bit of flex of an old hoover to use, she said within seconds there was smoke coming through the carpet upstairs, i quess there was a dead short between live and earth

If its been regularly tripping it may be the client has struggled trying to reset it under fault and caused the issue.
Insulation test maybe the best thing.

 

[securespark]

How is the ring final connected at the board?

We need pictures!

 

[JohnW2]

How is the ring final connected at the board? ... We need pictures!

We certainly like pictures, but I have to say that I'm rather intrigued to know what you have in mind in this case!

Kind Regards, John

 

[securespark]

I'm not sure, but mention of current running through the cpc makes me think that maybe there is a fault somewhere which is insufficient to trip but is heating up the conductors.

Just a look-see at some sockets and the inside of the board would be interesting.

 

[JohnW2]

I'm not sure, but mention of current running through the cpc makes me think that maybe there is a fault somewhere which is insufficient to trip but is heating up the conductors.

... except that we are told that it is tripping! Are you perhaps thinking that, even when it doesn't trip, there might be high L-E fault currents (but not quite high enough to cause a trip) still flowing?

If that were what's happening, as I said, it's only really possible because of the 'undersized' CPCs in our T+E cables. If the circuit is designed properly (OPD appropriate for cable), then the OPD will trip before L or N conductors become unacceptably hot. However, a current not high enough to trip the OPD (quickly enough), hence the L and N connectors deemed to be 'safe, might be enough to overheat the (smaller CSA) CPC if the current was also going through that.

Kind Regards, John

 

[EFLImpudence]

It's difficult to imagine how a steady fault of 4, 5 or 6Ω could occur for long enough to trip the 32A MCB and which sometimes was lower resistance to trip the 63A.

Is there a large cooker wired wrongly on the circuit? Even that wouldn't trip the MCBs, would it?

 

[AndyPRK]

I was thinking the 63A was an rcd.
Pictures help.

 

[securespark]

A ring circuit for sockets is tripping .............. the 63 amp submain circuit breaker

What kind of circuit breaker is this? To what standard?

 

[bernardgreen]

and even the 63 amp submain circuit breaker

Is this a very large building, residential or commercial and is there any Earth leakage protection ( RCD etc ). ? ? If there are Earth leakage devices are they tripping ?

CPCs can over heat due to high currents created by the CPC connecting two different "Earths" together when those two "Earths" are at different potentials. A cable bonding a metallic supply pipe to the MET of a PME system can carry very high currents when the PME Neutral is pulled a few volts above local ground potential by un-balanced loading on the three phases in the local netwrork.

Connecting the CPC of a PME system to a boiler, electric shower or other device that has metallic connection to a low impedance Ground will result in current in the the CPC when the Neutral is not at local Ground potential.

 

[RF Lighting]

I once went to a double fault years ago. Fault one was a N>E short circuit on a final circuit, and fault two was a neutral bar in the CU which had been installed on the wrong side of the cage clamp in the main switch and had eventually failed open circuit. All the current for this board (4 OP heaters) was flowing down the 1.5mm2 CPC of the faulty final circuit until it eventually failed, completely destroying the cable in the process.