Fault Current Rating
- Thread starter tim
- Start date
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- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Fault Current Rating
You might try doing a word search for previous, related questions that have been addressed on this Forum.
But in quick and simple terms, if you have a short circuit "fault" next to (i.e., immediately downstream) of the service equipment (or any other component, for that matter), the amount of current that will pass through that fault will depend on many things. They include the power available from the source, the resistance of the circuit (including the internal resistance of the source and the resistance of the wires from the source to the fault point), and any extra current that will be fed back from any large motors that happened to be running at the time. In order to make sure the service equipment is not destroyed by this fault, it has to have a "rated fault current" that is at least as high as the worst possible fault to which it might be subjected.
You might try doing a word search for previous, related questions that have been addressed on this Forum.
But in quick and simple terms, if you have a short circuit "fault" next to (i.e., immediately downstream) of the service equipment (or any other component, for that matter), the amount of current that will pass through that fault will depend on many things. They include the power available from the source, the resistance of the circuit (including the internal resistance of the source and the resistance of the wires from the source to the fault point), and any extra current that will be fed back from any large motors that happened to be running at the time. In order to make sure the service equipment is not destroyed by this fault, it has to have a "rated fault current" that is at least as high as the worst possible fault to which it might be subjected.
friebel
Senior Member
- Location
- Pennsville, N.J.
Re: Fault Current Rating
FAULT CURRENT: If you would take a metal bar and short out the Secondary of a transformer that would feed a sub-station, that would be the available Maximum fault current at that location.
As you proceed down-stream from the sub-station with wiring, you will have resistance in the system and the "FAULT CURRENT" will be less due to the resistance of the wiring.
For Example: On a fuse that has a rating of 15, 30, 100 Amperes, etc. there will be a AIC rating of 250 K Amps shown on the barrel of the fuse, or 300 K Amps. This rating will keep the fuse from blowing apart when a fault to ground happens on your system. Many years ago we would experience fuses that would blow apart, because they were not rated for the Fault-Current like they are today.
The same with your Circuit-Breakers in your Panel Board. If you do not see a Fault rating the Circuit Breaker is rated for 5000 Amps Interrupting rating, (AIC). Above that you will see the CB rating of 10K AIC.
At our Industry Site, we have a documented Fault Current of 50,000 AIC, and as we continue downstream with wiring and equipment the AIC will be Lower, and we have this documented on a print.
FAULT CURRENT: If you would take a metal bar and short out the Secondary of a transformer that would feed a sub-station, that would be the available Maximum fault current at that location.
As you proceed down-stream from the sub-station with wiring, you will have resistance in the system and the "FAULT CURRENT" will be less due to the resistance of the wiring.
For Example: On a fuse that has a rating of 15, 30, 100 Amperes, etc. there will be a AIC rating of 250 K Amps shown on the barrel of the fuse, or 300 K Amps. This rating will keep the fuse from blowing apart when a fault to ground happens on your system. Many years ago we would experience fuses that would blow apart, because they were not rated for the Fault-Current like they are today.
The same with your Circuit-Breakers in your Panel Board. If you do not see a Fault rating the Circuit Breaker is rated for 5000 Amps Interrupting rating, (AIC). Above that you will see the CB rating of 10K AIC.
At our Industry Site, we have a documented Fault Current of 50,000 AIC, and as we continue downstream with wiring and equipment the AIC will be Lower, and we have this documented on a print.
sceepe
Senior Member
Re: Fault Current Rating
You can get the same equipment with different fault current ratings. The higher the rating the higher the cost.
Lets say you are going to install a panel to supply a new additon. You need to know the available fault current (aka short circuit current) at the point where you are going to tie in the new panel so that you can buy equipment with a high enough rating but not so high that you are wasting $
You can get the same equipment with different fault current ratings. The higher the rating the higher the cost.
Lets say you are going to install a panel to supply a new additon. You need to know the available fault current (aka short circuit current) at the point where you are going to tie in the new panel so that you can buy equipment with a high enough rating but not so high that you are wasting $
tx2step
Senior Member
- Location
- North Texas, DFW area
Re: Fault Current Rating
If you have, for instance, circuit breakers installed that don't have an AIC rating as high or higher than the available fault current. Then if a maximum (bolted) fault happens on the load side of that circuit breaker, it may literally explode. It may very well not clear (open) the fault, and the fault may keep going until another overcurrent device further up the line opens or until something in the circuit path burns in two enough to open the fault. (Think fire
If you have, for instance, circuit breakers installed that don't have an AIC rating as high or higher than the available fault current. Then if a maximum (bolted) fault happens on the load side of that circuit breaker, it may literally explode. It may very well not clear (open) the fault, and the fault may keep going until another overcurrent device further up the line opens or until something in the circuit path burns in two enough to open the fault. (Think fire
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