Switchboard Earth Bars 2

[pilts]

I have an application where I will be installing a 400V 50HZ switchboard to distribute from a 11/0.4kV 1500kVA transformer. The incomer will be a 2000A Air circuit breaker. I will need to bond the neutral bar to earth in the switchboard. How do I calculate:-
i)The size of the neutral - earth bond?
ii)The cross sectional area of the earth busbar in the switchboard?

Thanks for any help in advance.

 

[redtrumpet]

Size of neutral grounding conductor should be in your local electrical code. In Canada, that would be #3/0 AWG copper for service conductor ampacity greater than 475 A. If that sounds too small in your engineering judgement, then you can calculate your maximum neutral current under fault conditions, consider the maximum duration of the fault, and size accordingly. On a solidly grounded-wye secondary with a transformer impedance of 5.75%, let's assume you are approaching 50 kA fault current returning to the neutral on a single line-to-ground fault (125% of 3-phase fault for worst-case with unlimited source).

Based on the relationship (I/A)^2 x t = .0297 log [(T2 + 234)/(T1 + 234)] for insulated copper conductor where A is conductor area in circular mils, and assuming 250 C max temperature for T2 and 30 C for ambient temperature T1, #3/0 copper should be good for .088 seconds. If your ground trip is set for longer than this, you should upsize the neutral.

The switchboard ground bus is likely 1/4" x 1" minimum, which is usually adequate for fault currents in the 50 kA range. 5:00 and I gotta go, maybe someone else can dig up the equation for sizing the ground bus.

 

[redtrumpet]

Clarifications to my previous posting:

1. Protective device total clearing time should be less than the calculated duration of the withstand of the neutral conductor.

2. Formula for current withstand of copper conductor also has skin effect term which I forgot to add: (I/A)^2 x t x Fac = 0.0297 log10 [(Tf + 234)/(To + 234)] where

I = current flowing in cable in amperes
A = conductor cross-sectional area in circular mils
t = time in seconds
Fac = skin effect ratio or ac/dc ratio
Tf = final temperature of conductor in degrees Celsius after current change (usually taken as 250 C for copper)
To = initial temperature of conductor in degrees Celsius prior to a current change

3. If primary protective device is relied upon to clear secondary line-to-ground faults between transformer winding and secondary breaker, primary device will only see 58% of the secondary fault current of a delta-wye transformer connection. This will result in slower clearing time of primary device than would be expected based on a three-phase fault current.

 

[daveR]

OK, where you've got a dedicated transformer it is common to earth the source neutral at the first point in the LV system where the neutral is terminated - the main LV switchboard.

I agree with previous answer that sizing should be done to ensure the fault withstand current can be stood for the time taken to clear the fault (IEC 439-1 takes this time as up to 1 second) - can use the adiabatic formula (IEE) to check sizes S = Sqrt(I*I/K) (K is a constant depending on material).

Not sure if the skin effect has much of an effect at power frequencies (50/60HZ) - but I'm happy to be corrected on this.