help with TN-C-S earthing system

[TommyS]

Just a quick question, on the above topic,

TN-C-S earthing system has now a prospective fault current to 9ka, what would you guys recommand the appropriate protective device I could use?

Cheers

 

[plugwash]

Assuming the installation has a service fuse of 100A or less don't worry about it. The service fuse will provide backup protection in the event that a MCB fails to break an extremely high current short.

 

[TommyS]

forgot to add that the cables are all PVC/PVC imperial sized, and that they are semi enclosed fuses, (yes it needs a rewire)

cheers

 

[TommyS]

BTT

 

[TommyS]

Would these be ok the use, the BS 1361 yes these are cartridge fuses with a fault capacity of 30kA the next one down which is BS 1361 and they have a fault capacity of 6kA which is no good.

HELP

 

[TommyS]

no worrys, Ive found that the best protective will be to use the MCB's to BS 3871

 

[kai]

... As long as they are M9/M10 breaking capacity rated or above.

 

[TommyS]

any more info on the M9/M10 breaking capacity.

 

[Adam_151]

You will find the consumer unit and all associated devices should have a proviosnal rateing of 16ka provided the upstream device is a bs1361 II rated at 100A or less, this might at first seem a cavilair attitude to take, it assumes a fault won't occur within 3m of the CU* (if it did, the cable may be damaged if the breaker welded shut and the service fuse had to operate however the adiabatic indicates that anything over 4mm will survive the ordeal!, but lets face it, if you put a nail through a cable within 3m of the CU, you are going to be replaceing the segment back to the CU anyway...) Also your real fault current wont be more than 7ka... (thats the 'cut off current' for a 100A bs1361 at 9ka- I belive the effect is due to the increased resistance due to the fuse element heating up)

*the resistance of the circuit cables causes the fault current to drop off sharpish the further away from the cu you get

Service fuse data: http://www.bussmann.co.uk/images/Data Sheets/BS1361/LR85.pdf

 

[TommyS]

nice one m8,

*looking at that site you posted*