Zs what if

[wildgoose]

To clarify this is how I understand it:-

If the Zs meets disconnection period spec, then it will trip the CB within the alloted time period when a Live to Earth fault occurs (eg: 0.4s). However, if it can't meet it, then there must be additional protection from an RCD, to eliminate the risk of the earth becoming live because of a high ZE. This is the basic TT protection mode.

For normal ADS without RCD protection, the disconnection time is of course mandatory.

 

[Engineer54]

There's a big difference between a TN and a TT system!! The use of an RCD on a TN system is for ''additional protection'', It' shouldn't always be considered a cure all, for covering a wiring circuit that doesn't comply with required Zs when it can quite easily be made to comply. Whereas, on a TT system, especially in UK, it is the prime and only protection for earth faults

The use of RCD devices for cure all situations seems to be, becoming the norm in the UK. Get the fundamentals right first, before applying RCD additional protection