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

by Steve on November 9, 2010

How can 0.3s RCDs comply for TT systems which require 0.2s disconnection?

Faults in cables and equipment can occur between live conductors (L-L or L-N), or from live conductors to earth (L/N-E).  The BS7671 Wiring Regulations require that breakers or fuses must disconnect a circuit within a specific time in the instance of a fault.

0.2s disconnection for TT systems
To give fault protection to electrical circuits BS 4293 RCDs are designed to operate within 0.2s where a reliable maximum earth resistance is present.  Table 45.1 indicates 1,667 ohms however Note 2 states that a value exceeding 200 ohms is not stable.  So anything less than 200 ohms will cause RCDs to trip within 0.2 seconds in the event of an earth fault.

Concerns regarding 0.3s RCDs
Concerns may arise when looking at BS EN Type RCDS and RCBOs (BS EN 61008-1 and 61009-1) which don’t appear to comply with the 0.2s disconnection time requirement.  They are designed to trip within 0.3s at x1=30mA which is obviously slower than required by Reg 411.3.2.2 .

So how do these devices do comply?
BS7671 Appendix 3 Table 3A shows us that, when tested, if a BS EN RCD operates within 0.3s at it’s operating current (x1=30mA), it will therefore operate within 0.15s at x2=60mA.  The RCD would then comply because 60mA is still a very low fault current causing the circuit to disconnect in 0.15s.

*Note that the device must also pass trip tests for x1/2=15mA and x5=150mA to comply.  Some RCD testers do also test for x2.

Table of disconnection times to comply with 411.3.2.2
BS7671: Table 41.1 gives the required disconnection times for TN (0.4s) and TT 90.2s) systems.
The note below Table 41.1 interestingly states that:
Where an Over-current Protective Device and Equipotential Bonding (411.3.1.2) is connected to all extraneous metallic parts then the maximum disconnection times for TN systems can be used for TT systems-   ie. 0.4 seconds up to 32A & 5 secs over 32A (411.3.2.3)

I would suggest, that as electricians, this is as far as we need to go with our knowledge regarding 0.3s RCDs complying with the 0.2s disconnection requirement.  Simply accept that these devices do comply.  However there are arguments for not using BS EN 61008-1 and 61009-1 RCDs in certain circumstances where a 0.2 second disconnection time is imperative, such as where livestock are present.

To find a more in-depth and heated discussion visit the IET forum.

and further discussion here.


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{ 2 comments… read them below or add one }

Adam October 6, 2016 at 10:50

Hi, you’ve explained that well. I’ve been trying to get my head around that theory for ages and couldn’t quite put all the pieces of the regulations together. I think I like your conclusion though, ‘simply accept’ it lol. I think that’s what I will be doing in future. Great Blog!!!

Andy October 16, 2012 at 18:59

Nicely written and has answered a question I have been thinking about. Around here (Surrey) we don’t have too much to do with TT systems, but I had to convert one the other day and whilst brushing up on the TT regs, this question seemed unanswered! Many Thanks!

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