70e tables

[hawkeye23]

To use the tables in 70e it saids that we have to use the notes. In the notes it asks for two things.
1. we have to find the maximum ka scc available
2. maximum fault clearing time.
We find the ka ratings on the circuit breakers and the clearing times should be in the manufacturing company catalog that sells breakers .
Would this be a way of solving those two requirements ? Or are there calculations or other means for those two requirements ?

[don_resqcapt19]

You have to know the available fault current of the circuit, not the rating of the breaker.

[dkidd]

You have to know the available fault current of the circuit, not the rating of the breaker.

Absolutely. I have seen 10K breakers installed on systems with over 42K available.

[ron]

Absolutely. I have seen 10K breakers installed on systems with over 42K available.

And equally bad for arc flash application is 42kA breakers with 10kA available. The 10kA will take a long time to get the breaker open compared to 42kA.

[don_resqcapt19]

And equally bad for arc flash application is 42kA breakers with 10kA available. The 10kA will take a long time to get the breaker open compared to 42kA.

I thought the time to open was only a function of the fault current. Do the time trip curves change as you get breakers with a higher interrupting rating?

[zog]

I thought the time to open was only a function of the fault current. Do the time trip curves change as you get breakers with a higher interrupting rating?

I think he means if you assume a 42kA fault current based on the 42KAIC rating on the breaker and your actual fault current is only 10kA your clearing time will be longer and may exceed the clearing time limit of the tables.

[jim dungar]

Do the time trip curves change as you get breakers with a higher interrupting rating?

Sometimes.
One of the ways to get a high AIC rating is to have the breaker start opening sooner so that it finishes clearing before the fault reaches the maximum available.

[don_resqcapt19]

I think he means if you assume a 42kA fault current based on the 42KAIC rating on the breaker and your actual fault current is only 10kA your clearing time will be longer and may exceed the clearing time limit of the tables.

That makes a lot more sense than the way I was reading it.

[Jraef]

To use the tables in 70e it saids that we have to use the notes. In the notes it asks for two things.
1. we have to find the maximum ka scc available 2. maximum fault clearing time.
We find the ka ratings on the circuit breakers and the clearing times should be in the manufacturing company catalog that sells breakers .
Would this be a way of solving those two requirements ? Or are there calculations or other means for those two requirements ?

hawkeye,
These two highlighted statements from your original post are the problem. These are NOT the same thing. If you do not know how to determine the maximum short circuit current available, that's where you need to start.

The kAIC ratings of the breakers are selceted by someone who USED that information, but the breaker ratings do not tell you what the value is IN THE CIRCUIT. So for example if the breakers are rated 25kAIC, the maximum fault current in the circuit should* be something less than that, but it could be ANYTHING less than that, right down to 1000A. Worse yet under some circumstances it may even be more than that. You cannot use the numbers on devices for this purpose.

*Should, but see dkidd's comment above.

[ron]

That makes a lot more sense than the way I was reading it.

Then that must be what I meant.

Actually, that is what I meant.