residual current rating for an RCD where the Zs exceeds OCPD limit.

[e4electrical]

Hi, quick question....long answer!!

An RCD has been installed in a single phase circuit as the measured Zs exceeds mcb. Circuit's wired in thermoplastic T&E 2.5/1.5, length of 58m.
Ze is 0.33 ohms on TNCS system.The OCPD is C20 60898.
How do I determine what the residual current rating of an RCD is in order to comply with reg 411.4.9??

PS. It's an exam Q that I have been a set!!

Cheers
Guys

 

[Darkwood]

Hi e4electrical

Are you a trainee in an active course, if so then you would benefit greatly from applying for our Trainee section where we have selected mentors and like minded trainees to help you out.

When it come to homework, exam questions etc.. we do not give direct answers as this does not help you learn, having said that you may give your thoughts on the matter and hopefully generous members can guide you without giving you a direct answer.


I would suggest you look up 'RCD - fault protection' in the back of the BS7671 to help you with your question.

 

[Richard Burns]

You are looking to ensure the circuit will disconnect in the required time of 0.4s (for a final circuit <=32A on a TN system) , but at the moment the Zs of the circuit will not permit this with a C20 BS60898 as the resistance is too high.
You are now being asked to consider an RCD for Fault protection rather than additional protection.
You need to know at what maximum Zs various ratings of RCD will trip, then select the most suitable one with a maximum Zs higher than the Zs you calculate for the final circuit.

Oh yes and read reg 411.4.9

 

[stevethesparks]

An RCD may well help meet the required disconnection time in the event of a fault as in live - earth fault on an higher than required impedance circuit. What worries me is that conducting a line impedance test, a circuit may not necessarily meet the required disconnection time in the event of a short circuit or heavy overload. In this case however, it should.
Anyone here believe that a line impermanence test should be incorporated into future test sheets as the RCD is not a cure all?

 

[Richard Burns]

There is no required disconnection time for a line to neutral fault except in the design standards of protective devices.

 

[Darkwood]

You are looking to ensure the circuit will disconnect in the required time of 0.4s (for a final circuit <=32A on a TN system) , but at the moment the Zs of the circuit will not permit this with a C20 BS60898 as the resistance is too high.
You are now being asked to consider an RCD for Fault protection rather than additional protection.
You need to know at what maximum Zs various ratings of RCD will trip, then select the most suitable one with a maximum Zs higher than the Zs you calculate for the final circuit.

Oh yes and read reg 411.4.9

Just realised I put additional instead of fault protection ... oops!..


Also good to look up is 'RCD - earth fault loop impedance for' ... I forgot how much the BS7671 jumped you around from reg' to reg' - A to B via C F G :willy_nilly:

 

[Richard Burns]

Just realised I put additional instead of fault protection ... oops!..


Also good to look up is 'RCD - earth fault loop impedance for' ... I forgot how much the BS7671 jumped you around from reg' to reg' - A to B via C F G :willy_nilly:

But do you not know the regs off by heart? :shocked3:, I am shocked:sunny:

 

[Darkwood]

But do you not know the regs off by heart? :shocked3:, I am shocked:sunny:

I know enough to get by without refering, but I'm usually working to the BS60204 anyway so my work if I do slip back into the BS7671 is key area's I know off by heart, although I do have the new copies because no-one can remember everything..excluding the gifted, photographic memories and some of those on the autistic spectrum. :yesnod: