10mm Earth Cable burnt

[Happyneiller]

Called out to a house this evening. Rewireable 32A fuse blown and the 10mm earth cable between the main earth terminal in the consumer unit and the TNCS connection at the cut out completely melted.

I checked ZE and PFC at supply and replaced the earth cable to the unit. Hadn't got time to investigate ring fault so left it disconnected. Never seen a fault like this where the main earth is so badly damaged.

 

[mechelec]

Sounds very much like a neutral fault, take your time and make sure you find it. Neutral faults are great for causing fires!!!!

When you carried out Ze did you disconnect main earth ? if you didnt you would get a false reading.

 

[Marvo]

Have you got a photo you can attach?

 

[Spudnik]

And TNCS Main Earth should be 16mm, which is probably the reason why it melted when the fault current flowed.

 

[Happyneiller]

Yes, main earth disconnected - to verify utility earth. Installation is in a poor condition. Changing consume unit on Friday. I will do a full PIR also. This should identify fault.

 

[GLENNSPARK]

Yes, main earth disconnected - to verify utility earth. Installation is in a poor condition. Changing consume unit on Friday. I will do a full PIR also. This should identify fault.

PIR...before you change over the c/u though mate as you might find other stuff here n all...such as the borrowed neutral and crappy IR on final circuits......earthing and bonding upgrade but i guess you would know that already..lol....:biggrin:

 

[Happyneiller]

Right - board changed on Fri, full PIR revealed a multitude of nightmares.

The ring circuit that the fault occurred on had a broken CPC ie the N L end to end was fine but the CPC end to end was open circuit.

All sockets were within acceptable levels of ZS yet the main earth still burnt. 1 leg of the ring was completely destroyed ( the earth had melted to the neutral and live phases ). Found and rectified the cpc fault but still can't explain the damage to the main earth connection or the cpc. ??????????

 

[Deleted member 26818]

And TNCS Main Earth should be 16mm, which is probably the reason why it melted when the fault current flowed.

Sorry Jason, but this is incorrect.
Where PME conditions apply, the earthing conductor is treated the same as a bonding conductor.
The minimum CSA required is 10mm².
This has occured either because the earth conductor did not satisfy the adiabatic equation, or because of a high resistance connection/joint.

 

[JD6400]

Was there a storm at the time of the fault as the only two times have seen the cpc melt the insulation down the one complete leg of a ring and damage to the main earth like what you have described , has been after a lightening strike ?
One was through the TV aerial and the other onto the building it's self .

 

[flukey]

Sorry Jason, but this is incorrect.
Where PME conditions apply, the earthing conductor is treated the same as a bonding conductor.
The minimum CSA required is 10mm².
This has occured either because the earth conductor did not satisfy the adiabatic equation, or because of a high resistance connection/joint.

where did you get that info from spin?? i also thought it was 16mm main earth tncs??

you referring to 543.4.3??

 

[tony mc]

My understanding is that where PME conditions apply its the bonding conductor which is the main concern and the earthing conductor would have to be equal too or larger in csa than the bonding conductor.

Reg 542.3.1 takes you to table 54.8

 

[IQ Electrical]

Sorry Jason, but this is incorrect.
Where PME conditions apply, the earthing conductor is treated the same as a bonding conductor.
The minimum CSA required is 10mm².
This has occured either because the earth conductor did not satisfy the adiabatic equation, or because of a high resistance connection/joint.

That's correct, the minimum is 10mm although I agree with a previous post in that it sounds like there's been a neutral fault at some stage, was there any damage to the bonding conductors?

 

[Deleted member 26818]

where did you get that info from spin?? i also thought it was 16mm main earth tncs??

you referring to 543.4.3??

Sorry, was away from my computer.
As Tony MC says, 542.31: Every earthing conductor shall comply with Section 543 and, where PME condictions apply, shall meet the requirements of Regulation 544.1.1 for the cross-sectional area of a main protective bonding conductor."....
544.1.1: ..."Except for highway power supplies and street furniture, where PME condictions apply the main protective bonding conductor shall be selected in accordance with the neutral conductor of the supply and Table 54.8."...

 

[Deleted member 26818]

That's correct, the minimum is 10mm although I agree with a previous post in that it sounds like there's been a neutral fault at some stage, was there any damage to the bonding conductors?

Whilst a neutral fault on the network, could well cause damage to an installations earth/bonding conductors, I would think it would be unlikely that it would cause a CPD to blow.
I would also expect a neutral fault in the installation, to cause damage to a neutral conductor, not an earth conductor.

 

[Deleted member 26818]

That's correct, the minimum is 10mm although I agree with a previous post in that it sounds like there's been a neutral fault at some stage, was there any damage to the bonding conductors?

Whilst a neutral fault on the network, could well cause damage to an installation's earth/bonding conductors, I would think it would be unlikely that it would cause a CPD to blow.
I would also expect a neutral fault in the installation, to cause damage to a neutral conductor, not an earth conductor.