3 Wire fault calc LL vs. LN

[nhee2]

Bussmann uses a multiplier of 1.5 for LN faults on a single phase 3 Wire system ( relative to LL). What is the basis for this multiplier? i.e. what is the relationship of transformer % Z on a LN fault?

 

[arrehman1]

LL vs LN for 3 wire and 4 wire system

LL vs LN for 3 wire and 4 wire system

For differences between line to line voltage and phase voltage for 3 wire and 4 wire system, I recommend the following video explanations.

3 wire system (delta connected transformer): https://youtu.be/HmF20PHWyFo

4 wire system (wye connected transformer): https://youtu.be/9xsubEoKwLY

Hope this helps

 

[kwired]

Bussmann uses a multiplier of 1.5 for LN faults on a single phase 3 Wire system ( relative to LL). What is the basis for this multiplier? i.e. what is the relationship of transformer % Z on a LN fault?

For a L-N fault you have half the wire in the source coil as you have for a L-L fault - that alone will be a different impedance, but you also only have half the supply voltage. I don't know exactly where the 1.5 multiplier comes from but I think information I presented here shows it definitely will not be same source impedance as it is for a L-L fault.

Also keep in mind the further away you get from the source (long conductor run) the more the conductor impedance will rise enough to still yield a higher fault current because of the higher voltatage from line to line then it is line to neutral.

L-N available current is higher near the transformer, but L-L is higher when further away from transformer in most instances. You need to calculate both and find which one is higher for your application when selecting gear or doing arc flash calculations.