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.

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.

To save you a few minutes, on a single phase 230V AC supply,

when you get there the numbers you need will be

And for B types, similar, but  perhaps unsurprisingly a bit quicker on the breaking so I²t is a bit less though maybe not as much as you'd think.

Real breakers are sometimes quite a bit quicker, but if you do not know the make of the breaker, you have to assume it may be changed for one right on the limit at some point....

Mike.

And often the official 60898 let-through is too high for standard circuits. For example, a 6kA B6 protecting a l.5/1.0 T&E lighting circuit, or a B16 protecting a 2.5/1.5 T&E power circuit will both let through more energy than the cable can theoretically withstand.

Even a small length of 1.5/1 will attenuate the fault current dramatically. With 6KA at the board, 5m of 1.5 T/E reduces fault current to around 1.8KA. So would we not be better looking at A2s at various fault currents rather than reference to the Icn values?

For a traditional  fuse  the I²t changes slowly  with fault current and it is safe to assume for large faults that  it is more or less constant and so is the energy let through and the downstream damage done while it blows.

However  an MCB with moving parts becomes almost a constant time device at the really fast end of the curve, as the speed limits of what can be done with mechanical contact separation are reached- so the damage, the let through energy and the I²t all rise as the square of the current.  A modest increase in PSSC can have a devastating consequence for the downstream wiring, and how chunky it needs to be to not have its insulation cooked, and as Lyle notes, the reverse is also true, a few tens of milliohms down the wire, and no amount of vampire hunters hammering silver stakes into the wiring is going to kill it in any more serious way than firing the trip.

However to keep the burden of testing on makers of MCBs and so on to a reasonable level, this is generally only tested at the worst case - the full 6000 A or whatever, and a few other points, so again you are in a rather grey area  and guessing what the performance will be in between.

It is maybe helpful  to work out how fast the breaking is at 6000 A to get say 100,000 J/ohm  of the 32A C type breaker

I² = 36million A², so if I² t is 0.1 million we are looking at  1/360 of a second, or about 3 milliseconds . Now that is a sort of rectangular equivalent time, as if the full fault current flows and then stops dead. It is important to realize that it does not.

Really as an arc forms and stretches, there  is a low and variable resistance in the path that rises and then becomes infinite, so the current waveform has something of a tadpole shape, by which I mean a large head and a thin tail, probably only dying out totally at the next zero crossing of the AC cycle. (and we sort of know it relies on this this as we can set fire to an ordinary AC only MCB if we ask it to interrupt a DC of the same voltage.)

lyledunn said:

Even a small length of 1.5/1 will attenuate the fault current dramatically. With 6KA at the board, 5m of 1.5 T/E reduces fault current to around 1.8KA. So would we not be better looking at A2s at various fault currents rather than reference to the Icn values?

Agreed - protection against fault current is required at all points along the length of the cable unless Reg 434.3 applies so some or all of the cable.

For fuses, the recommended approach is to plot the conductor adiabatic line against the fuse time/current curve between the minimum to maximum considered prospective fault current range. For a circuit-breaker, the conductor limit should be plotted on the I²t curve for the protective device between the minimum to maximum considered prospective fault current range.