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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. 

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  • 0 in reply to Newfutile

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

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