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RyanC83
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Join Date: Jul 2011
Posts: 43

Residual Current Devices- Particular Scenario

12/01/2011 12:32 AM

Dear All

I have another question worth of a discussion regarding RCDs obviously within the context of its application in an electrical system. As it is the BS/IEE standard specifies a 30mA RCD for the saftey and protection of of human beings. One thing which is not yet specified is what if you have a combination/cascade of RCDs; simply put what happens if we install two RCDs at the end of the line? What happens is that upon the given conditions both RCDs will trip. This is a common scenario if we have apartments and the cable feeding the respective consumer unit is protected with an RCD (apart from the consumer unit having its own RCD).

There is an option that the cable feeding the consumer will be of armoured type thus avoiding the RCD at the cable origin or there is some people saying to combin an RCD with a time delayed one. (the latter is not recognized in the standards)

I would like to ask you whether there are any further options which one can employ.

I thank you in advance

Ryan
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SolarEagle
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Join Date: Mar 2007
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#1

Re: Residual Current Devices- Particular Scenario

12/01/2011 12:50 AM

I'm not familiar with your local codes but generally speaking more than one RCD feeding another is unnecessary, provided they have been wired properly. One exception is the case of a TT earthing system where the earth loop impedance may be high, meaning that a ground fault might not cause sufficient current to trip an ordinary circuit breaker or fuse. In this case a special 100 mA (or greater) trip current time-delayed RCD is installed covering the whole installation and then more sensitive RCDs should be installed downstream of it for sockets and other circuits which are considered high risk.

http://en.wikipedia.org/wiki/Residual-current_device

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RyanC83
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Posts: 43
#3
In reply to #1

Re: Residual Current Devices- Particular Scenario

12/01/2011 1:58 AM

Yes i am referring to a TT system.
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dbman
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Join Date: Dec 2011
Posts: 13
#8
In reply to #3

Re: Residual Current Devices- Particular Scenario

12/07/2011 10:17 PM

I dont think you will have a TT system on apartments?
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kvsridhar
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#2

Re: Residual Current Devices- Particular Scenario

12/01/2011 1:09 AM

Schneider Electric's paper ECT 114 is a very good one on this subject.

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PWSlack
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United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
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#4

Re: Residual Current Devices- Particular Scenario

12/01/2011 3:09 AM

An installation to the latest issue of British Standard 7671 will have only 1 30mA RCD in circuit. For a TT installation, there will be a 100mA one upstream of it. Clearly, the 30mA one will trip first.

Legacy installations may differ, however there is no absolute requirement to upgrade until the installation is altered.

There is no safety implication in having two trips in circuit with the same rating, however, "nuisance tripping" may be a consequence.

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electricalexpert65
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#5

Re: Residual Current Devices- Particular Scenario

12/01/2011 3:40 AM

I think the IEC recognises a time delayed RCD (with an 'S' Curve) for discrimination purposes and it has got an inverse time-current charectersitics.
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kvsridhar
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#6
In reply to #5

Re: Residual Current Devices- Particular Scenario

12/01/2011 3:46 AM

ECT 114 describes the G and S types.

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Wal
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Join Date: Jun 2011
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#7

Re: Residual Current Devices- Particular Scenario

12/02/2011 12:24 AM

With proper discrimination the nuisance tripping of cascaded RCD protection can be reduced.

Reduced not eliminated. It ultimately comes down to the size of the crowbar.

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Users who posted comments:

dbman (1); electricalexpert65 (1); kvsridhar (2); PWSlack (1); RyanC83 (1); SolarEagle (1); Wal (1)

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