Let through Energy - Cable Sizing

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.

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.