Pete Fisher asks a question about an 800A ABB ACB, and associated Zs value calculation. It has been answered by ABB (theme - Testing, Tools & Equipment):
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Question: Could you please advise what the maximum permitted Zs values for an 800A ABB air circuit breaker (ACB) are, or where I can find them? Note that this ACB has been set to 560A.
The only details I have, other than those above, are that it is one of the old BS4752 type - which I believe is now the BS EN 60947-2.
If you can't provide a Zs, could you instead provide the tripping current and type, if applicable?
Answer: The Zs value of an ACB is actually dependant on the short circuit current setting (I3), rather than the over current setting (I1). It is calculated based on the I3 setting multiplied by the nominal current of the breaker - in this case 800A.
For an I3 value of 10 times, the Zs value would be 0.024 ohms. For an I3 value of five times, the Zs value would be 0.048 ohms.
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