SC Coordination study / Fault Currents

[Mike01]

When discussing short circuit coordination studies a topic of conservation came up and was looking for other opinions, when drawing a one-line in a modeling software some of my co-workers expressed that to get it done from a S.C. prospective they will indicate a feeder to a fuse and to the bus (switches and fuses are two pieces of equipment in this software) and omit the switch saying it is just another item to keep track of and adds time, however a switch has a specific x/r impedance etc. correct? To test this out I modeled the same system with the same characteristics and found that when the xfmr secondary feeder is connected to a switch and then a fuse and then the bus the fault current at the line side of the switch is approx 34ka while the fault current at the bus is approx 30Ka, when it is run with just the fuse and no switch in the circuit the approx fault current at the bus is approx 31Ka while if you omit the switch and fuse completely it is only decreased by the impedance of the cable but would be 35Ka all the way thru, is the decrease due to the impedance in the switch and fuse? It drops the fault current about 4Ka just passing thru the switch and fuse.

 

[jim dungar]

I have never seen any published data on the impedance of a 'switch'. I know that the SKM software package treats switches as 'specialty' devices and has a very limited library of them.

 

[mivey]

When discussing short circuit coordination studies a topic of conservation came up and was looking for other opinions, when drawing a one-line in a modeling software some of my co-workers expressed that to get it done from a S.C. prospective they will indicate a feeder to a fuse and to the bus (switches and fuses are two pieces of equipment in this software) and omit the switch saying it is just another item to keep track of and adds time, however a switch has a specific x/r impedance etc. correct? To test this out I modeled the same system with the same characteristics and found that when the xfmr secondary feeder is connected to a switch and then a fuse and then the bus the fault current at the line side of the switch is approx 34ka while the fault current at the bus is approx 30Ka, when it is run with just the fuse and no switch in the circuit the approx fault current at the bus is approx 31Ka while if you omit the switch and fuse completely it is only decreased by the impedance of the cable but would be 35Ka all the way thru, is the decrease due to the impedance in the switch and fuse? It drops the fault current about 4Ka just passing thru the switch and fuse.

That is what the model is telling you. The switch and fuse do have impedance. I've never thought about including them, but I guess you could if you had good impedance values.

Many times the breaker experts here talk about the voltage drop across the breaker or contact resistance.

I'm not sure what voltage you are at, but I would be interested in what the model gives as the impedance of the switch and fuse. Perhaps some of our breaker guys could tell you if they were reasonable numbers.

 

[ron]

Switches, breakers and conductor termination have impedance.
SKM (and the general IEEE color book method) does not consider the impedance of them unless you enter it as a specific impedance item.

 

[jghrist]

If your switch has enough impedance to drop the fault current from 35 kA to 31 kA, then you have a switch in desparate need of maintenance.