Let through Energy - Cable Sizing

AJJewsbury:

To be clear, I wanted to ask, When the cables have been already verified using adiabatic equation, Do I still need to check the cables for let through energy verification?

In principle they're both the same thing (S = √(I²t)/k is the same a S²k² = I²t, just re-arranged (divide through by k and square both sides)) - but in practice people often refer to different conditions - e.g. the adiabatic is often only considered when sizing c.p.c.s - i.e. for Earth Faults whereas L-N, L-L and L-L-L faults are more commonly thought of when thinking of energy let-though.  Of course faults between live conductors can produce quite different levels fault currents than L-PE faults - so a calculation based on an earth fault current might not be valid for the other kinds of faults.


As a first approximation if a conductor is "reduced" - i.e. it has a rating below that of its protective device (Iz < In) - then some check will likely be necessary.


Also keep in mind that the worst case isn't necessarily a fault at the far end of the circuit - that gives the lowest fault current and likely the longest disconnection time - and for fuses probably the highest energy let-through but for MCBs faults with higher currents (e.g. a fault next to the MCB) often don't disconnect significantly faster than a far-off fault - so I²t can often be higher. Unfortunatley some text book examples seem to have been written with fuses in mind and miss that point.


    - Andy.

That's not quite true, based on the question.


For short disconnection times and circuit breakers (mcb's and RCBO's), BS 7671 refers to manufacturer's data ... so you will have to use the device data because BS 7671 doesn't have it.


It's not possible to simply assume that if the adiabatic is met for the  highest value of current shown in BS 7671, it will be met for all higher values of current, because circuit breakers don't behave like fuses.


Also, whilst in reality Ipsc should be measured, current guidance for small (100 A single-phase) installations, is to assume 16 kA for prospective fault current at the origin.

AJJewsbury:

To be clear, I wanted to ask, When the cables have been already verified using adiabatic equation, Do I still need to check the cables for let through energy verification?

In principle they're both the same thing (S = √(I²t)/k is the same a S²k² = I²t, just re-arranged (divide through by k and square both sides)) - but in practice people often refer to different conditions - e.g. the adiabatic is often only considered when sizing c.p.c.s - i.e. for Earth Faults whereas L-N, L-L and L-L-L faults are more commonly thought of when thinking of energy let-though.  Of course faults between live conductors can produce quite different levels fault currents than L-PE faults - so a calculation based on an earth fault current might not be valid for the other kinds of faults.


As a first approximation if a conductor is "reduced" - i.e. it has a rating below that of its protective device (Iz < In) - then some check will likely be necessary.


Also keep in mind that the worst case isn't necessarily a fault at the far end of the circuit - that gives the lowest fault current and likely the longest disconnection time - and for fuses probably the highest energy let-through but for MCBs faults with higher currents (e.g. a fault next to the MCB) often don't disconnect significantly faster than a far-off fault - so I²t can often be higher. Unfortunatley some text book examples seem to have been written with fuses in mind and miss that point.


    - Andy.

That's not quite true, based on the question.


For short disconnection times and circuit breakers (mcb's and RCBO's), BS 7671 refers to manufacturer's data ... so you will have to use the device data because BS 7671 doesn't have it.


It's not possible to simply assume that if the adiabatic is met for the  highest value of current shown in BS 7671, it will be met for all higher values of current, because circuit breakers don't behave like fuses.


Also, whilst in reality Ipsc should be measured, current guidance for small (100 A single-phase) installations, is to assume 16 kA for prospective fault current at the origin.