mitch cochrell
Member
Can someone explain to me how engineers come up with aic ratings for gear and breakers? Any time I order switch gear this becomes an issue.
Aic rating
- Thread starter mitch cochrell
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- Massachusetts
Do you mean how do engineers figure out what rating is required?
For the service to a facility, it needs to be obtained from the power company. After that, it can be calculated based on the impedence of the system components. There are good resources on the Bussman site and many others on this issue. Also, there has been a recent article on EC&M.
mitch cochrell
Member
Thanks a lot.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Iwire alluded to it, so now I feel free to jump in.
Protective devices have AIC ratings. Gear, technically, has Short Circuit Current Ratings (SCCR), however its labeling usually says something to the effect of 'short circuit rating dependent on the value of the lowest AIC installed'. So while it is a little easier to order a panel with an AIC rating, you need to know what you are really getting.
Finally, the amount of available short circuit current is a function of the overall installed system.
Now for the OP.
The NEC requires both the AIC (110.9) and the SCCR (110.10) to be higher than the available fault current. The basic math behind fault currents is really just a variant of Ohm's Law => Current = Voltage divided by Impedance.
dkidd, provided the basic answer.
The primary impedance, when looking at the utility, is the service transformer. In most cases the utility does not want to commit to a specific transformer (they reserve the right to change whenever they want to), therefore they provide a 'design' level of fault current. This design level may bear little resemblance to what is actually available, but equipment chosen on it rarely gets 'outdated'.
After you leave the service entrance, the primary impedance becomes the conductors, until you get to another transformer. this is where the free software programs come in handy. Again to be safe, when selecting equipment, use the utility design value as your starting point.
And never-ever use a 'design' fault current as the only value when performing Arc Flash calculations.
Protective devices have AIC ratings. Gear, technically, has Short Circuit Current Ratings (SCCR), however its labeling usually says something to the effect of 'short circuit rating dependent on the value of the lowest AIC installed'. So while it is a little easier to order a panel with an AIC rating, you need to know what you are really getting.
Finally, the amount of available short circuit current is a function of the overall installed system.
Now for the OP.
The NEC requires both the AIC (110.9) and the SCCR (110.10) to be higher than the available fault current. The basic math behind fault currents is really just a variant of Ohm's Law => Current = Voltage divided by Impedance.
dkidd, provided the basic answer.
The primary impedance, when looking at the utility, is the service transformer. In most cases the utility does not want to commit to a specific transformer (they reserve the right to change whenever they want to), therefore they provide a 'design' level of fault current. This design level may bear little resemblance to what is actually available, but equipment chosen on it rarely gets 'outdated'.
After you leave the service entrance, the primary impedance becomes the conductors, until you get to another transformer. this is where the free software programs come in handy. Again to be safe, when selecting equipment, use the utility design value as your starting point.
And never-ever use a 'design' fault current as the only value when performing Arc Flash calculations.
"These are mechanical ratings that assess the device’s ability to maintain integrity if a fault condition occurs downstream of the protection device.
For example, a 10 kAIC-rated circuit breaker can safely interrupt 10,000 amps of fault current without blowing apart or internally short circuiting.
A 65 kAIC switchboard must mechanically sustain 65,000 amps of fault current flowing through the switchboard and remain undamaged."
So the first consideration seems to be that it doesn't explode or maintain an arc or make things worse by shorting. These would not be graceful failures.
Failing by opening seems like a good option.
Cascading failures is probably the worst outcome, where each upstream unit shorts and so passes the overload down to the next unit which is not designed to handle it.
It'd also be good if one of these units that had already survived several faults would give some indication of 'service life remaining.'
For example, a 10 kAIC-rated circuit breaker can safely interrupt 10,000 amps of fault current without blowing apart or internally short circuiting.
A 65 kAIC switchboard must mechanically sustain 65,000 amps of fault current flowing through the switchboard and remain undamaged."
So the first consideration seems to be that it doesn't explode or maintain an arc or make things worse by shorting. These would not be graceful failures.
Failing by opening seems like a good option.
Cascading failures is probably the worst outcome, where each upstream unit shorts and so passes the overload down to the next unit which is not designed to handle it.
It'd also be good if one of these units that had already survived several faults would give some indication of 'service life remaining.'
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