Bolted/solid faults

[Murlinor]

Hello Friends.

I have a question regarding IEC60909 and bolted fault. Generally for a single phase to earth fault the current can be written as follows:

I''k1 = sqrt(3)*Un/(z1+z2+z0+3Zn+3Zf)

Where Zn is the impedance connected to neutral and Zf is the fault impedance. IEC omits the fault impedance and with solidy earthed system they write:

I''k1 = sqrt(3)*Un/(z1+z2+z0)

The consequence is that the current calculated will be more conservative compared to the current calculated with the fault impedance.

My question is therefore, why does IEC omit the fault impedance? Is it generally ok to omit this value?

Best Regards
Hussain

 

[Marmite]

The actual impedance of a fault is not a predictable value. Imagine an arcing overhead line conductor dancing around on the ground. How could you calculate the impedance of the fault, which would in any case vary with time significantly as the arc extinguishes and restrikes? Imagine now an operator has a switching error and closes a switch onto an earthed feeder. Pretty much a solid bolted fault with very low imedance. For these reasons we ignore fault impedance when calculating fault currents (ie assume a zero impedance bolted fault) as the calculation then results in the maximum prospective current available. If you design the system to cope with the maximum then it will be safe in real fault situations where the current will be less than the maximum.
Regards
Marmite

 

[redfurry]

Good explanation from Marmite of the importance of calculating bolted fault levels.

However, the minimum likely fault level is also important to calculate as this value determines whether protection devices will trip as expected.