Chapmeister
Electrical
- Jun 22, 2006
- 33
I'm having some trouble!
Can someone tell me if this is right? or point out the flaw in my calculations?
If I have a bolted fault current value on the secondary of a transformer, the closest upsptream protective device will be called on to interrupt the arc. If the closest device is on the primary (upstream in a piece of switchgear). In order to determine the trip time of the device, the arc current has to be reflected across the transformer using the LV/HV ratio, and then read from the TCC curve for the fuse. Using the fuse melt time, the NFPA arc flash calculations can be done. Check this out:
Here are the numbers I'm having trouble with:
Transformer Specs:
112.5kVA
600/220V (delta-wye)
5.0 %Z
Available Fault Current (from custom spreadsheet):
24.5kA at the switch gear
16.4kA at the primary (due to feeder impedance)
4.34kA on the secondary (due to transformer impedance)
Fuse Data:
C200HR English Electric (bought out by GEC)
My arcing current on the secondary is then 2.45kA. (using equation D.8.2(a) from the NFPA70E)
Do I then take the 2.45 * 220/600 = 0.898kA to get the primary side current. Reading from the TCC I get ~24 second melt time.
This gives a normalized incident engery of 0.6J/cm^2 and incident energy of 652 J/cm^2 at 24 inches (460mm). This number is WAAAAYYY bigger than I would have expected.
Where is the flaw in my procedure????
Can someone tell me if this is right? or point out the flaw in my calculations?
If I have a bolted fault current value on the secondary of a transformer, the closest upsptream protective device will be called on to interrupt the arc. If the closest device is on the primary (upstream in a piece of switchgear). In order to determine the trip time of the device, the arc current has to be reflected across the transformer using the LV/HV ratio, and then read from the TCC curve for the fuse. Using the fuse melt time, the NFPA arc flash calculations can be done. Check this out:
Here are the numbers I'm having trouble with:
Transformer Specs:
112.5kVA
600/220V (delta-wye)
5.0 %Z
Available Fault Current (from custom spreadsheet):
24.5kA at the switch gear
16.4kA at the primary (due to feeder impedance)
4.34kA on the secondary (due to transformer impedance)
Fuse Data:
C200HR English Electric (bought out by GEC)
My arcing current on the secondary is then 2.45kA. (using equation D.8.2(a) from the NFPA70E)
Do I then take the 2.45 * 220/600 = 0.898kA to get the primary side current. Reading from the TCC I get ~24 second melt time.
This gives a normalized incident engery of 0.6J/cm^2 and incident energy of 652 J/cm^2 at 24 inches (460mm). This number is WAAAAYYY bigger than I would have expected.
Where is the flaw in my procedure????