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RCDs for both Fault & Additional Protection

PG over 5 years ago

BS7671 states for Additional Protection [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 Sections 411 to 414." - These methods being methods of protection against shock - ADS, Double Insulation, Separation & SELV/PELV.

531.3.6 states " - - - - - - -RCD - - -- - - -may provide fault protection and additional protection simultaneously."

This appears to be inconsistent. Is 531.3.6 meant to apply to only one type of neutral earthing system?

Regards

0 geoffsd over 5 years ago

Ah, with you now.

Isn't fault protection ADS?

Thanks.
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0 mapj1 over 5 years ago

There are fault types that are not L-E, such as insulation failure between phase-neutral or phase phase.

The key difference with the L-E fault is that they have potential to expose a dangerous voltage to touch, and the ADS operates.

For this reason there are strict limits on either the current or the operation time of the ADS for this.

For the other faults we may allow a longer time - almost indefinite, so long as nothing melts or burns down the building,

though 5 seconds is a good starting point for any near short condition.

Be aware that the phrase ADS often refers only to the L-E case. Obviously the MCB or fuse will do either, an RCD will not detect L-L or L-N
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0 geoffsd over 5 years ago

Yes, thanks, I know that. It is just probably down to bad wording, as you alluded.

However, if RCDs are only allowed to be additional protection for ADS and fault current/protection comes under ADS, are we not back to the original question?
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0 mapj1 over 5 years ago

415.1.2 The use of RCDs is not recognized as a sole means of protection and does not obviate the need to

apply one of the protective measures specified in Sections 411 to 4 14.

So while an RCD is clearly part of an ADS system, use of just an RCD on its own is not enough. Normally of the options permitted, the other measure you would combine would be 4.11 - the use of a CPC, but you may prefer one of the others such as an isolation transformer to make it an IT installation, or conversion to a lower voltage as FLEV PELV or SELV so a shock is unlikely to give fatal current.

Arguably actually all these measures are similar as in a sense bonding is also a measure that lowers the voltage to which a victim may be subjected.

I'm not sure how you think this contradicts that advice?.

531 .3.6 RCDs for additional protection

The use of RCDs with a rated residual operating current not exceeding 30 rnA is recognized as additional protection

in compliance with Regulation 415.1. These RCDs shall be provided to comply with the requirements of Regulation 411.3.3.

So for additional protection, against crass stupidity of users, or compromise of the intended protection mechanism, such as mechanical damage to the CPC. Or as they put it

additional protection in the event of failure of the provision for basic protection and/or the provision for fault protection or carelessness by users.
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0 AJJewsbury over 5 years ago

Perhaps to summarise:

A 30mA RCD alone only provides additional (supplementary) protection.

To provide ADS you need not only a disconnection device (which can be an RCD or OPD) but also a c.p.c. connecting all exposed-conductive-parts to suitable means of earthing and co-ordination between earth loop impedance and the disconnection device's operating characteristics (and possibly main protective bonding as well).

Using an RCD on its own is not considered sufficient protection from electric shock.

- Andy.
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RCDs for both Fault & Additional Protection

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