Ia: how fast the device disconnectsHow many amps blow a 20 amp fuse in 0.4 seconds?

What it's for: using the PEFC to find how long the protective device takes to disconnect. Ia is the current that makes the device operate within a given time. It isn't a calculation as such. You read it from the time/current curves (and the tables beside them) in Appendix 3 of BS 7671.

Our example is a piece of 20 A BS 3036 fuse wire (a rewireable fuse). The wire has to get hot enough to melt, and that depends on the current through it. In normal use it mustn't blow ("melt, melt, blow, oh no, replace the fuse wire!"). But in an earth fault (a low-impedance path to earth, high current, parts live that shouldn't be) we want it to melt, and quickly.

• The circuit's maximum disconnection time is 0.4 s, so the fuse needs at least 130 A.
• Only 50 A? It takes longer than 5 s. Only 70 A? Somewhere between 1 and 5 s, probably about 3 s.
• 500 A? Well under 0.1 s. Very quick.

So the rule is: the more current, the faster it trips or blows. Every circuit has a maximum disconnection time, and if the fault current doesn't reach Ia, the device won't operate in time.

BS 7671 · Appendix 3 · time/current characteristics · BS 3036 semi-enclosed (rewireable) fuses · check your copy for the page BS 7671 · Appendix 3 · Fig 3A4 Type B circuit-breakers · p.445

PEFC (section 11 example) = 291.3 A
Table above: 260 A → 0.1 s
291.3 A > 260 A → disconnects in under 0.1 s ✓ (well inside 0.4 s)