Rcd disconnection time on TT

[newfutile]

On a TT system the rcd must operate withing 0.2 seconds for many circuits. Why when we test rcd's does it not say 300ms for TN systems or 200 ms for TT systems?

 

[timhoward]

Why when we test rcd's does it not say 300ms for TN systems or 200 ms for TT systems?

Well, it does!
As you say, the trip time depends what the purpose of the RCD is.
For fault protection, see 643.7.1 which has a bit in the middle saying that chapter 41 stuff must be met.
For additional protection see 643.8

 

[cliffed]

How many times you have to rethink because of unreadable regulations over think the situation..I do it regularly… do belief the 0.2 secs is the maximum 200ms….most RCD“S trip before that‘

 

[pc1966]

The TT 200ms final circuit is an odd one because that is for fault conditions. Since the Ra is chosen to keep voltage below 50V at 'In' then under typical fault conditions you will see almost 5*In through the RCD, leading to much faster disconnection.

That is why the usual "selective" RCD as an incomer is able to meet final circuit disconnection of 200ms (even though they typically measure 300-400ms at In), as well as achieving selectivity with 30mA "instant" ones used for additional protection.

I agreed the wording is pretty opaque