time delay rcd on tt system

[wannabesparky]

can someone please tell me why when you change an old rewireable fuseboard to a circuit breaker consumer unit does it have to consist of a time delay rcd on a tt system? and also what would happen if you didn't put a time delay rcd in?

 

[WarrenG]

Onsite guide - P24 circuits in a TT system need to be protected by RCD/ RCBO's to protect the circuits.

A 100ma 'S' Type is used as a main switch to protect the whole installation and is delayed for discrimination betwenn the other the RCD's/ RCBO's in the installation.

Hope that makes some sense?

Warren

 

[slocm3105]

If all circuits are rcd/rcbo protected by a 30mA device, why is there a need to have a 100mA as main switch if others will trip before it?

 

[wannabesparky]

what will happen though if a time delay rcd isnt put in?

 

[WarrenG]

If all circuits are rcd/rcbo protected by a 30mA device, why is there a need to have a 100mA as main switch if others will trip before it?

This ones been debated before and I think that you are right in saying that if all the circuits are protected by an RCD/RCBO to the 17th Ed then it could be just a main switch. Can't remember now what the conclusion was for it without getting the regs book out.

what will happen though if a time delay rcd isnt put in?

If one of the RCBO's trip then you could have what they call on upstream effect tripping the main RCD out and losing power to the whole installation. Don't forget that the main RCD is a higher rating but looking after the whole installation and not just one circuit.

 

[Spudnik]

A standard main switch with RCBO's or dual RCD,s protecting all circuits is permitted on a TT system, providing the CU is either class II or of an all insulated construction.

Happy days.