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Time delay RCD - BS7671

Apostolos
Good morning.

We have a feeder to a consumer unit that for the last 30cm of its route it goes into a metal stud wall partition.

This was not caught early so that we could use an earthed metal conduit for this length and the walls are all up. Done and dusted.

This practically means that the feeder needs an RCD protection in the beginning.

End of story since we cannot interfere with the wall now.

The problem is that the CU has an 30mA RCD incomer itself.

So the RCD we are going to install upstream will be in series with the RCD incomer of the CU.

Does BS7671 accept a time delay 30mA RCD for the feeder that goes within the wall partition?

If it would I could still have some kind of "discrimination" with the RCD downstream in the CU.

I have been looking through BS7671 to find a respective regulation but I could not find something.

My gut says that its unlikely that I can use a time delay RCD but I cannot substansiate this with a regulation.

Any help is more than welcome!

Cheers

Parents
  • 0
    At least in principle, you could, so long as with the delay it was still inside the 0,4 or 0,2 seconds required for ADS to operate depending if TN or TT.

    50- 100mS maybe OK.

    Now

    Some earth fault relays can be set up to a threshold of 30mA and small delays -

    BUT I caution that is not the normal problem they are used to solve, and once you have seen the price of that method I fear that not only will cutting the wall be back on the table, so will most of a complete rebuild.  Or deciding it is an acceptable non-compliance for reasons that may be hard to justify, or trying to slip some armour-flex over the top of it from the free end, or covering that tricky 30cm hop with an access panel ...

    this is just the earth fault detecting part. Sensing transformer and suitable contactor or aux coils to fire an MCB are extra !   


    Another example 


Reply
  • 0
    At least in principle, you could, so long as with the delay it was still inside the 0,4 or 0,2 seconds required for ADS to operate depending if TN or TT.

    50- 100mS maybe OK.

    Now

    Some earth fault relays can be set up to a threshold of 30mA and small delays -

    BUT I caution that is not the normal problem they are used to solve, and once you have seen the price of that method I fear that not only will cutting the wall be back on the table, so will most of a complete rebuild.  Or deciding it is an acceptable non-compliance for reasons that may be hard to justify, or trying to slip some armour-flex over the top of it from the free end, or covering that tricky 30cm hop with an access panel ...

    this is just the earth fault detecting part. Sensing transformer and suitable contactor or aux coils to fire an MCB are extra !   


    Another example 


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