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Why don' we use RCD trip times for adiabatic equation

Alan B

When using adiabatic equation for calculating minimum size of CPC, every example I have seen uses 0.1 second or whatever the disconnect time of the mcb element of the RCBO  or MCB will be.

In a domestic sittuation most circuits are protected by RCD's with a trip time of 40mS with significant fault currents, in this sittuation why don't we use 40mS as T in the adiabatic equation?

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  • 0

    Most of the discussion in this thread so far is actually covered in Section 8 of the IET Electrical Installation Design Guide - the current 2022 Edition even covers the issue of why you can't (and shouldn't ) rely on the RCD element of RCBOs even in TT systems.

  • 0 in reply to gkenyon

    Does this mean that a circuit must comply with maximum ZS on the mcb portion of an rcbo? How would that work in a TT ?

  • 0 in reply to Newfutile

    That's how I read the text from the Guidance Note, but not how i read BS 7671. 

  • 0 in reply to Newfutile
    Newfutile said:
    Does this mean that a circuit must comply with maximum ZS on the mcb portion of an rcbo?

    Please don't confuse ADS (that is, meeting the Zs value for mcb) with Adiabatic for overcurrent.

    So, no, Zs for mcb as per Chapter 41 does NOT need to be met.

    Newfutile said:
    How would that work in a TT

    Guidance Note 6 (Section 6.4.3 in 2022 version) and the IET Electrical Installation Design Guide (Section 8.11 in the 2022 version) recommend that the maximum prospective fault current L-N is taken into account for earth faults in TT systems (in most cases - there may be instances this is not necessary) and that is used to calculate adiabatic using the OCPD disconnection time (which it is accepted may be far longer than the RCD).


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  • 0 in reply to Newfutile
    Newfutile said:
    Does this mean that a circuit must comply with maximum ZS on the mcb portion of an rcbo?

    Please don't confuse ADS (that is, meeting the Zs value for mcb) with Adiabatic for overcurrent.

    So, no, Zs for mcb as per Chapter 41 does NOT need to be met.

    Newfutile said:
    How would that work in a TT

    Guidance Note 6 (Section 6.4.3 in 2022 version) and the IET Electrical Installation Design Guide (Section 8.11 in the 2022 version) recommend that the maximum prospective fault current L-N is taken into account for earth faults in TT systems (in most cases - there may be instances this is not necessary) and that is used to calculate adiabatic using the OCPD disconnection time (which it is accepted may be far longer than the RCD).


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