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Discuss disconnection times and RCD in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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sambotc

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On the 2391 training course for all of this week and something popped up that I would like to what other people made of it.


So we are talking disconnection times, and the question of RCD's pop up. More to the point, circuits not meeting the required disconnection times. I thought I would use the example of a TT arrangement where an RCD is used regularly as disconnection times cannot be achieved, and the lecturer quickly informed me that this does not actually comply with BS7671. (415.1.2)

"the use of RCDs is not recognised as a sole means of protection and does not obviate the need to apply one of the protective measures specified in section 411 to 414"


What are people views on this?!
 
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SirKit Breaker

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  • #2
Have a look at GN3 page 51, it explains things better than i can write it. Its all about selecting the correct RCD that will disconnect before the voltage rises above the safe touch value of 50v.

Cheers............Howard
 
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MarkieSparkie

  • Thread Starter Thread Starter
  • #3
Say you have a simple TT installation that has an Ra <200 Ohms and the installation (or final circuit) is fault protected by a 30mA rated BS EN 61009 RCD and has a maximum earth fault loop impedance of 1667 Ohms in accordance with BS 7671:2008+A1:2011 Table 41.5. Then if my mental arithmetic serves me correctly, 230V divided by 1667 Ohms would have a prospective earth fault current flow of about 138mA. This is significantly greater than 30mA rated residual operating current and certainly exceeds 2xIΔn. BS 7671:2008+A1:2011 Appendix 3 Table 3A tells us that the device will operate in 150mS or less at this current (ie. >60mA or 2xIΔn). When I check back to BS 7671:2008+A1:2011 Table 41.1 it tells me that the RCD needs to disconnect in 200ms (0.2s).
BS 7671:2008+A1:2011 411.5.2 allows a TT system to use an RCD or an overcurrent device (providing Zs is suitably low) or both for fault protection. In the special case where a 30mA RCD provides the fault (shock) protection of a TT installation (or final circuit) under the previous regulation and Overcurrent (thermal/short-circuit) protection is provided by overcurrent device(s) then the installation complies with BS 7671:2008+A1:2011 415.1.2 by implication and RCD testing at 5xIΔn would determine compliance with BS 7671:2008+A1:2011 415.1.1
I personally do like relying on a single RCD for fault protection and certainly would not advocate designing a TT installation that way, even though the regulations allow it.
 
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sambotc

  • Thread Starter Thread Starter
  • #5
Said it was a bit of a grey area but technically not allowed as an RCD is only able to provide additional protection and shouldn't replace fault protection through ADS as outlined in 411 -411.3.2.6 i think, long day might be wrong!
 
yes according to the regs it would require supp bonding and the RCD
 
Might be wrong here but the reg hes quoting is correct RCDS wont operate on line line faults, hence the need for MCBS/suitable bonding to provide ADS ie you can't just have a rcd as fault protection. From what im reading this is missed your point i would clairify with tomorrow
 

Risteard

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Arms
Esteemed
RCDs can be used to provide fault protection as well as additional protection.

Also, to clarify SirKit Breaker's post an RCD will not limit the touch voltage (which is limited by circuit impedance) but merely the duration it is present.
 
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Deleted member 26818

  • Thread Starter Thread Starter
  • #9
On the 2391 training course for all of this week and something popped up that I would like to what other people made of it.


So we are talking disconnection times, and the question of RCD's pop up. More to the point, circuits not meeting the required disconnection times. I thought I would use the example of a TT arrangement where an RCD is used regularly as disconnection times cannot be achieved, and the lecturer quickly informed me that this does not actually comply with BS7671. (415.1.2)

"the use of RCDs is not recognised as a sole means of protection and does not obviate the need to apply one of the protective measures specified in section 411 to 414"


What are people views on this?!
It's not a grey area, use of an RCD as sole means of protection is not acceptable, overload protection is also required
 

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