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I²t curve from Tripping curve (Circuit breaker)

CSkinner

Can I find the Max Let-through energy of a circuit breaker, by using its Tripping curve?

If I plotted the I²t of the breaker, using the Tripping curve, would this give me the Let-through energy curve?

If so, just to check, I assume the I²t would be made using the "Tripping current² × tripping time"?

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  • 0

    In theory yes, but in practice no. The big gotcha is that the sort of fault currents that you really need to know the energy let-through for are off the scale of most time/current graphs (i.e. beyond the current needed to trigger "instantaneous" tripping (which of course isn't quite zero time)).

    More usually you'd use specific energy let-through data - either from the MCB manufacturer or from BS EN 60898 (or BS EN 61009 for RCBOs) -  which will directly give you a value for each type, rating and breaking capacity of MCB. (e.g. 45,000 A²s for 6kA B-type MCBs >16A and ≤32A).

       - Andy.

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  • 0

    In theory yes, but in practice no. The big gotcha is that the sort of fault currents that you really need to know the energy let-through for are off the scale of most time/current graphs (i.e. beyond the current needed to trigger "instantaneous" tripping (which of course isn't quite zero time)).

    More usually you'd use specific energy let-through data - either from the MCB manufacturer or from BS EN 60898 (or BS EN 61009 for RCBOs) -  which will directly give you a value for each type, rating and breaking capacity of MCB. (e.g. 45,000 A²s for 6kA B-type MCBs >16A and ≤32A).

       - Andy.

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  • 0 in reply to AJJewsbury

    Thanks for the response.

    I've searched high and low, and the technical department of the manufacturer hasn't got this data to share; so guess I'll refer to the British Standard.

    Thanks again.

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