AIC ratings for relays

[micase]

Some manufacturers show AIC ratings for relays in lighting control panels. What is the relavance of an AIC rating for a relay? Is bigger necessarily always better?

 

[jim dungar]

Some manufacturers show AIC ratings for relays in lighting control panels. What is the relavance of an AIC rating for a relay? Is bigger necessarily always better?

Are they really calling it AIC?

Amps Interrupting Capacity (AIC) is usually associated with devices which are intended to break fault currents (i.e. circuit breakers and fuses). See 110.9

Short Circuit Current Rating (SCCR) is now the term for the amount of short circuit amps that an item can withstand while it is waiting for a protective device to clear a fault. See 110.10, especially the commentary in the NECH.

 

[LarryFine]

Are they really calling it AIC?

Amps Interrupting Capacity (AIC) is usually associated with devices which are intended to break fault currents (i.e. circuit breakers and fuses). See 110.9

If this number is in the Ka ranges, it's a fault-current rating. Yes, bigger is better; what's required is that the rating meets or exceeds the available fault current at the terminals.

However, contactors open under load all the time, and this rating would be in the under-1Ka range. What is the number, and how does it compare to the relay's contact rating?

 

[jim dungar]

If this number is in the Ka ranges, it's a fault-current rating. Yes, bigger is better; what's required is that the rating meets or exceeds the available fault current at the terminals.

However, contactors open under load all the time, and this rating would be in the under-1Ka range. What is the number, and how does it compare to the relay's contact rating?

Larry,

The contactor needs to have a fault current rating at least equal to the amount that is available at its line side terminals. I have never seen a relay or contactor with an AIC rating as they are not intended to interrupt fault currents. In the old days relays/contactors were supplied with "withstand" ratings, but now the term is SCCR.

While an SCCR and an AIC rating look and sound alike (so many kA at such and such a voltage) they are two different animals and are not interchangeable. NEC 110.9 deals with AIC and 110.10 with SCCR.

 

[micase]

AIC ratings for relays

In my experience, a typical application is a 20 Amp rated relay that usually divides branches of a circuit from a 20 Amp breaker to lighting loads so that they can be controlled by a time clock or individual switches or by a photocell. Some manufacturers show a 10,000 AIC rating and some show a 5000 AIC rating. The manufacturer with the 10,000 rating tells EEs that the 5000 AIC product is inferior. I don't have a copy of the NEC and don't really want to spend the money to buy one to research the answer. Is there anywhere I can download just the sections about AIC?

 

[George Stolz]

110.9 Interrupting Rating. Equipment intended to interrupt
current at fault levels shall have an interrupting rating
sufficient for the nominal circuit voltage and the current
that is available at the line terminals of the equipment.
Equipment intended to interrupt current at other than fault
levels shall have an interrupting rating at nominal circuit voltage
sufficient for the current that must be interrupted.

110.10 Circuit Impedance and Other Characteristics.
The overcurrent protective devices, the total impedance, the
component short-circuit current ratings, and other characteristics
of the circuit to be protected shall be selected and
coordinated to permit the circuit-protective devices used to
clear a fault to do so without extensive damage to the electrical
components of the circuit. This fault shall be assumed
to be either between two or more of the circuit conductors
or between any circuit conductor and the grounding conductor
or enclosing metal raceway. Listed products applied
in accordance with their listing shall be considered to meet
the requirements of this section.

Here's about all the NEC has on it, I believe. Not of much use.

I'd say that 110.9 isn't of much importance, because the relay is not to interrupt current at fault levels: it's unlikely a fault would occur at the same instant that the control wiring breaks contacts on the relay, if you think about it.

 

[jim dungar]

... it's unlikely a fault would occur at the same instant that the control wiring breaks contacts on the relay, if you think about it.

That is true George, but the relay does need to be rated to close onto a fault and wait for a protective device to interrupt it.

As long as 110.10 is complied with, a relay with a 10KA fault current rating is not "better" than one with a 5kA simply because the fault rating is higher.

 

[micase]

Thanks

Thanks

Thanks for the info. I am a little less confused and a lot more informed.

 

[BLUEDEVIL]

you need meet or exceed the available fault current at the terminals....in arizona you have to show fault current calculation up to the lighting control panel....just to be sure that the rating of the installed lighting control panel meet or exceed the fault current at the terminals....

 

[George Stolz]

That is true George, but the relay does need to be rated to close onto a fault and wait for a protective device to interrupt it.

So that would fall under the second sentence of 110.9, correct?

As long as the relay was rated as normal, say 20A @ 120V or what have you, and that's the circuit it was connected to, then all would be well, right?

Edit to add: You did catch me there, I didn't think about it that way.

 

[jim dungar]

So that would fall under the second sentence of 110.9, correct?

As long as the relay was rated as normal, say 20A @ 120V or what have you, and that's the circuit it was connected to, then all would be well, right?

Edit to add: You did catch me there, I didn't think about it that way.

Don't forget 110.10, which requires the available fault current and the timing of the protective devices must be coordinated not to damage the relay contacts.

...circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit.

 

[George Stolz]

I'll buy that - but would a damaged ice cube constitute "extensive damage"?

 

[jim dungar]

I'll buy that - but would a damaged ice cube constitute "extensive damage"?

Maybe if the relay was part of the EPO for a computer room.
Imagine a situation where the relay closes onto a miswired circuit. The relay contacts vaporize at the same time as the protective device opens. The mis-wiring is corrected and the protective device is reset, because the contacts have "disappeared" it appears the problem has been solved. Next time the relay is expected to operate it won't.

 

[George Stolz]

Been watching Apollo 13 again, Jim?

 

[jim dungar]

Been watching Apollo 13 again, Jim?

Great movie:smile:
I remember trying to watch the news back then, lots of facts and hardly any fluff. I wonder what it would have been like with one of the 24/7 shows of today?:-?

 

[ELA]

Maybe if the relay was part of the EPO for a computer room.
Imagine a situation where the relay closes onto a miswired circuit. The relay contacts vaporize at the same time as the protective device opens. The mis-wiring is corrected and the protective device is reset, because the contacts have "disappeared" it appears the problem has been solved. Next time the relay is expected to operate it won't.

Would vaporized contacts that had "disappeared" imply normally open :smile:

I have seen problems where EMO relay contacts have been "welded shut" do to excessive current. That is a real problem in that the EMO circuit will no longer shut the power off. That becomes a big safety issue.
When I used to work on these types of circuits we used redundant relays with cross checking for a higher level of safety against welded contacts.

 

[LarryFine]

I'll buy that - but would a damaged ice cube constitute "extensive damage"?

I'd bet it would from the relay's point of view. :grin: