RCD Main switch on TT system

[Cooper_day]

Hello everyone,

Does an RCD as main switch in a TT system a must or not? My question is can you use an isolator and then RCDs for the rest of the distribution board?

Thank you

 

[pc1966]

Strictly speaking you must meed the disconnection times on all final circuits, how you achieve that is open.

But given the high Zs for TT systems the only practical solution is going to be RCD protection for all circuits. So you do not have to have an RCD incomer if every final circuit has RCD protection.

Having said that, if you have a fault to earth your only means of disconnection is the RCD, so it had better work! That is one reason I would always prefer on TT to have the likes of a 100mA S-type up-front and 30mA RCBOs for the final circuits providing per-circuit additional protection. If one RCBO fails to trip the incomer still should open. But that is going to cost a lot more than some folk can afford, and is not strictly necessary as a single working RCD is sufficient.

 

[Strima]

The main switch does not have to be an RCD. However the incoming tails must be suitably support and protected so that under fault conditions the cahssis of the board will not become live and present danger.

i.e. Use of suitable cable entry gland, cable grip etc.

 

[pc1966]

As above cable glands!

Everyone should be using cable glands for tails & earth, not just for protection against the edge of the hole, but to provide strain relief as well. Also maybe the Doncaster 19-strand easy fit tails, makes life easier for you fitting it, and less strain passed on to the terminals to risk any wires coming out.

Cable glands, did I mention them?