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Topic Title: RCD's in series
Topic Summary: Discrimination
Created On: 16 April 2014 10:13 PM
Status: Post and Reply
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 16 April 2014 10:13 PM
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Thripster

Posts: 646
Joined: 22 August 2006

Anyone tried obtaining satisfactory discrimination for identical RCD's in series both configured for TT installations by tripling the earth rods at the peripheral RCD? Do you think this approach would work?

Regards
 17 April 2014 06:56 AM
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GeoffBlackwell

Posts: 3538
Joined: 18 January 2003

Why should it make any difference?

RCDs respond to an imbalance between line and neutral current flow causing a resultant magnetic flux in the magnetic circuit. This flux produces a potential across a detecting coil, and this is used to trigger the device. RCDs used to use a precision snap action relay that activated as soon as a threshold driving current through its coil was reached.

Electronic versions probably use a Schmitt Trigger or similar threshold detector.

So operation is dependant on a threshold being reached, and not on a particular value of resistance to earth. All that is required is that the fault current magnitude is sufficient to allow the trigger threshold to be reached.

Electrode sets interconnected - A fault to earth occurring downstream of the the downstream RCD will flow to earth by all available earth paths - so if these paths are continuous between the RCDs - both sets of electrodes would carry fault current.

Electrode sets not interconnected - Assuming the fault is at the down stream side of the downstream device the far set of electrodes will carry all of the fault current.

However in both cases - since the RCDs are in series the line neutral current differential causing the imbalance is common to both devices so each receives the same stimulus at the same time.

You cannot achieve selectivity between devices based on fault current - all you can do is delay operation of the upstream device.

Regards

Geoff Blackwell
 17 April 2014 07:25 AM
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Thripster

Posts: 646
Joined: 22 August 2006

Thank you Geoff. Now you've pointed this out, the errors in my proposal become, laughably, clear. Oh well.

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