#### AHarb

##### Member

- Location
- Atlanta, GA

I was reading through some old posts on how to calculate the neutral current on an unbalanced three phase system. Someone stated that the formula to calculate this is:

I = sqrt((Ia?+Ib?+Ic?)-(Ia*Ib)-(Ib*Ic)-(Ia*Ic))

We had a fault on our system the other day and the sequence of events showed the fault currents were Ia = 105 A, Ib = 6760 A, and Ic = 1605 A. The Cooper Form 6 relay in our breaker locked out due to the high current lockout feature. The breaker measured 8425 A on the neutral which is higher than what we show the available fault current is at the location of the fault. I decided to use the formula above to calculate the neutral current using the Ia, Ib, and Ic values. I came up with 6046.20 A which is way lower than what our relay shows.

I was just curious if the formula above is true even in a faulted situation. I'm trying to determine if the relay may have been inaccurate and should not have locked out.

Thanks in advance.