(1) Undersized cables and (2) metal DB

[EastAnglian]

I've just completed an EICR on a 5 year old re-wire in a village hall (not mine!).

Before committing to paper I'd like to check 2 of my (unrelated) observations here:

(1) Heaters

3 x 3kW heaters installed on a single 32A MCB, switched via a contactor located on the board.

Cable from MCB to contactor = 4mm

Individual cables to heaters from the output of the contactor = 2.5mm T&E (all run in trunking, several metres length)

I contend that the 2.5mm cable is undersized for the 32A protective device and that 433.3.1 can only be applied if the adiabatic equation in 434.5.2 is satisfied.

Would you agree?


(2) Metal DB on TT system

Its a TT system. A metal DB has been used with an RCD main switch on the DB . Short tails to DB. I feel on safe ground here that this is incorrect but is 531.4.1 the correct reference to quote to justify that the RCD must be located at the origin as the external conductive part which is the case of the DB is not protected?

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I'm keen to get this right as they may want to refer this back to the original installer if there are deficiencies in the design & installation.

Thanks

Malcolm

 

[Richard Burns]

3 x 3kW on 32A MCB Total load switched by contactor is 39A this is a small overload of long duration 433.1

Cable from MCB to contactor = 4mm ​current carrying capacity (if clipped direct) 37A, cable overloaded at 39A. 523.1 and .2

Individual cables to heaters from the output of the contactor = 2.5mm T&E (all run in trunking, several metres length)

I contend that the 2.5mm cable is undersized for the 32A protective device and that 433.3.1 can only be applied if the adiabatic equation in 434.5.2 is satisfied. 433.3.1 would probably apply and the adiabatic will be satisfied, but I would not like it.

Would you agree?


(2) Metal DB on TT system

Its a TT system. A metal DB has been used with an RCD main switch on the DB . Short tails to DB. I feel on safe ground here that this is incorrect but is 531.4.1 the correct reference to quote to justify that the RCD must be located at the origin as the external conductive part which is the case of the DB is not protected?
411.3.2.1 states that a fault from a line conductor to an exposed conductive part must be disconnected in the required time, a fault from the supply line conductor to the exposed conductive part (the DB body) will not be disconnected as the fault current will not operate the supply fuse.
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I'm keen to get this right as they may want to refer this back to the original installer if there are deficiencies in the design & installation.

Thanks

Malcolm

Just my thoughts on this when I should be asleep!
Apologies if I have missed anything.

 

[telectrix]

all i'll add to that is the 2.5mm is supplying fixed loads which i assume have FCUs for each. in that case, thew 32A MCB will provide short-circuit protection, while the FCU 1362 will deal with overload.

 

[EastAnglian]

Thanks guys, very useful indeed.

FCUs replacing the existing 2-pole isolation switches will be the most likely solution to overcome the problem on the heaters.

Hopefully I managed to help you overcome your insomnia :21: