TT Systems. Time-delay RCD main switch

by Steve on March 22, 2009


Is a time-delayed RCD main switch required in the consumer unit of a new domestic installation supplied by a TT system?



Provided that the consumer unit is of all insulated construction and all the final circuits are RCD-protected, a time-delayed RCD main switch is not required.

How is 0.2s Achieved?
A 0.2s disconnection time can be acieved by the use of RCDs.
An external Earth Impedance (Ze) below 200 Ohms is considered an acceptable and stable source of TT earth.
When a Ze of less than 200 Ohms exists then BS4293 RCDs are designed to trip within 0.2s but newer BS EN Types such as RCBOs may raise some concern as they are specified as having 0.3s disconnection times.  But that’s a great subject for another article!  

Read more… –  Why use 0.3s RCDs for TT when they don’t comply with 0.2 disconnection?



{ 2 comments… read them below or add one }

Somerset Electrician August 24, 2011 at 13:09

I have always installed time delay rcd on the TT system, but I have never had any regulations making us do so

Luke September 29, 2010 at 22:23

I have been told by the eca man The Reason people use time delay rcd on tt system is to avoid both rcd going off . But you have the same with any split load board !
Remember on tt systems to take out the old earth leakage trip (ottermill) as they are illegal now !
100ohm’s or 200ohm’s

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