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?

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.

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.