CPC over heating

[PhilB]

I was called to a farm today as the RCBO protecting several 13amp sockets on a work bench kept tripping intermittantly.
Upon removing the first socket (all metal clad twins fed in PVC conduit/egatube) I found that the 1.5mm CPC insulation had melted and welded itself to the neutral & live conductors along most of its length which is about a 15/20mtrs run from the consumer unit. The cabling is all in singles on a radial circuit with not a lot of load, a pillar drill, small grinder and a car battery charger.

I didn't find or notice any over heating coming from the L-N conductors. The incoming supply is a TT system and the Zs/Ze readings are perfectly fine. Running in with this radial circuit are two red 1.5mm singles (feed & switch wire) supplying two fluorescent fittings but other than that the conduit isn't jam packed. The lighting is fed from a different breaker but on the same phase as the sockets. Any ideas why?

 

[Billynm]

Ask him where he plugs his welder in !!!

 

[ashrow]

Strange
so there was no damage to the other existing 2 red cables inside the conduit ?

 

[PhilB]

Ask him where he plugs his welder in !!!

No I didn't but then a welder would damage the conductors and not the CPC surely.

 

[ashrow]

philB

How new was the socket install ? Could it be Fake cable (as seen recently on tv)
Can you share the readings you took on site for the circuit and supply etc ?

 

[PhilB]

philB

How new was the socket install ? Could it be Fake cable (as seen recently on tv)
Can you share the readings you took on site for the circuit and supply etc ?

The RCBO in question is a 16amp type B to BS EN 61009 MEMERA 2000 style (maximum Zs 2.3 Ohms) the Zs test reading came out at 0.3 ohms and the Ze (without removing the earth lead) was 0.18 ohms.

I am aware that some of this Ze reading would be "going down" the metal structure of the barn. The socket wiring is a red & blue single but doesn't look "fake" ,and is I would say in excess of 15 years old with signs of changes/additions/maintenance but not amateur. The CPC appears to be a bare 1.5mm copper wire covered in the old green sleeving for the entire length, that must've took some patience pulling that on.

 

[Badged01]

Thats an intersting thought ashrow, was just taxing my little brain as to how a 'CPC' could overheat without there being a fault current down it that didnt 'blow' the RCBO first!

 

[Deleted member 26818]

I take it that the CPC and the circuit conductors are the same CSA?

 

[Guest55]

maybe his workshop hobbies have got a bit out of hand , power wise ..........










:-D

 

[Darkwood]

I also believe the welder is the culprit..... if faulty or not clamped correctly it can use alternative paths usually earth wires to complete it circuit, ive seen so many issues similar to this and not only do they pose a big fire hazard its has a risk of fatality.
Remember the current on the output end of a welder can be several hundred amps, ive seen welders melt 16mm earths before, your issue here if it is a welder problem is it may have compromised and damaged any or all the earht conductors of other none related circuits, a full inspection of all electrics and not just a part inspection and an engineer who knows about welders would be recommended advice.

Poor clamps, connections etc on the welder grounded return cable are a big danger to both user and installation so dont try resolve it if you have no experience in this area.

To clear up confusion of the damaged circuit with overheated earthing this can happen irrespective of the circuit been energised or not as the high current is not related to the circuit to start with.

 

[PhilB]

The CPC is 1.5mm and the conductors are both 2.5mm but that doesn't explain the CPC over heating. :/

 

[PhilB]

I also believe the welder is the culprit..... if faulty or not clamped correctly it can use alternative paths usually earth wires to complete it circuit, ive seen so many issues similar to this and not only do they pose a big fire hazard its has a risk of fatality.
Remember the current on the output end of a welder can be several hundred amps, ive seen welders melt 16mm earths before, your issue here if it is a welder problem is it may have compromised and damaged any or all the earht conductors of other none related circuits, a full inspection of all electrics and not just a part inspection and an engineer who knows about welders would be recommended advice.

Poor clamps, connections etc on the welder grounded return cable are a big danger to both user and installation so dont try resolve it if you have no experience in this area.

I have just spoken to the farmer five minutes ago and he says he sometimes plugs a welder in that would explain it improper clamping etc. When I was there earlier today the farm hand didn't mention a welder and I didn't see one anywhere around. Many thanks guys

 

[Darkwood]

The CPC is 1.5mm and the conductors are both 2.5mm but that doesn't explain the CPC over heating. :/

Read my post no10#

 

[Darkwood]

You need to discuss with farmer about the damage this may have done, the installation needs a 100% inspection to confirm its integrity is sound, he also needs to get a welder engineer out to check the welder out and it clamps etc, failure to do so puts both the property, electrics and staff at high risk..... i cannot express how dangerous this situe can be.

 

[PhilB]

Read my post no10#

Have also just been informed that the welder is situated in a completely different location on its own supply/circuit. I have taken your advice on board darkwood, many thanks.

 

[Darkwood]

Remember its not the incoming supply to the welder that caused the damage... if this was undersized wiring then as you rightly pointed out it would overheat the line wires and/or Neutral if it had one; the damage is caused by the lower voltage output circuit and high impedance or open circuit issues may see the currents upto 800amps -( welder and settings dependant) hijack any earth system if it can find a return circuit to its negative terminal hence any unrelated circuit could have been damaged regardless of them been enrgised or not. 10yrs ago we had to rewire a full trunking system in a factory because the stray est' 600amp current used the 10mm earth in a trunking which turned white hot and melted into all the other circuits, 24 X 3ph circuit melted and shorted into a clump with the earth, it was only when it blew the welder supply circuit that the current stopped but the damage had already been done by then.

 

[ashrow]

darkwood
I learnt something new today, having nothing to do with welders in my job (i am off to google and learn more)

 

[Darkwood]

PS its a shot in the dark but ive seen what you found so many times because of my field of work and it would be my no'1 suspect but the only other suspect is a loose neutral usually supply end where circuit may take advantage of a N/E fault that may have existed in the circuit you found damaged, id be checking all N connections from the incomming to the local DB.

 

[Darkwood]

darkwood
I learnt something new today, having nothing to do with welders in my job (i am off to google and learn more)

Easy way to understand it is to look at the current path of the low voltage/high current secondary circuit of the welder, it has to go from positive through the weld back to negative, now because most welders benches and items they weld are metal and sat on the ground its can give rise to a potential dangerous situe, if you clamp the return cable to a girder instead of the job it will try find an alternative path to get to that clamp on the girder and because its sat on the ground the many parallel paths will exist from the motor on the turning bed if the job is sat on one to going through the local trunking/earthing system to get to the clamp on the girder and because of the high currents it does alot of damage, it is designed to melt steel and weld it after all, copper has a much lower melting point too :sick: than steel.

 

[Knobhead]

The fact the welder is powered from elsewhere really makes no difference. Welders (person) have a lovely habit of striking an arc whether they have a good earth return path to the set or not.
Electricity will take the path of least resistance, it may be longer but that’s the way it’s going to go. Go in to engineering place and you will see welders happily working away with the return clamp just resting on the bench. He’s got his sparkler going and couldn’t give a ---- there’s smoke coming out the light fitting, socket or anything else. (Tell them the kettles on fire you may get a response).