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Ia: the current the device needs to operate, and the time/current graphsDoes more fault current mean a faster trip?

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Ia: the current the device needs to operate

This is the one people get backwards. For a Type B circuit breaker, the current required to operate it in the required time is five times its rating:

Ia = 5 × In  (Type B)

Reading it off the table

6 A Type B   ->  5 × 6  = 30 A
20 A Type B  ->  5 × 20 = 100 A
32 A Type B  ->  5 × 32 = 160 A
✋It is a threshold, not a race. It's tempting to think more fault current means a faster trip. In fact the device needs a minimum of 160 A to operate in time. 198 A passes because it clears the threshold, not because it is bigger.
Ia and breaking capacity are different numbers entirely. A B6 needs 30 A to trip in time, but can carry a fault of several thousand amps before it is destroyed. One is the floor, the other is the ceiling.

The time/current graphs

The table above is not something to memorise — it is printed. Known as "the graphs of doom": BS 7671 · Appendix 3, time/current characteristics for Type B circuit-breakers to BS EN 60898 and RCBOs to BS EN 61009-1.

Beside the curves is a table giving the current required for 0.1 s to 5 s:

RatingCurrentRatingCurrent
6 A30 A40 A200 A
10 A50 A50 A250 A
16 A80 A63 A315 A
20 A100 A80 A400 A
25 A125 A100 A500 A
32 A160 A125 A625 A
Worth tabbing. The Type B page in Appendix 3 is one you will want to find instantly in the exam.

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