I2T IN / IT2 OUT

[mbrooke]

What does I2T IN / OUT mean? And min vs instantaneous in terms of what it does to the trip criteria?

 

[adamscb]

"I^2T OUT" and "I^2T IN" refer to the shape of the trip curve. See the attached picture.


For you question regarding what MIN, INT, and MAX refer to, you'll have to look up the manual for that trip unit. I do not believe that INT is instantaneous in this case, but "intermediate". When it comes to breaker trip settings and coordination, it's always a battle to get the OCPD coordination properly set-up, while still keeping arc flash potential low. MIN would cause the breaker to trip the fastest, but you risk unnecessarily turning off other circuits. INT is a middle ground, while MAX would cause the breaker to open in the longest time, thus increasing arc flash potential, but would be easier to coordinate properly. When you have feeder breakers set up to take longer to trip, you're relying on downstream circuit breakers to do their job and open quickly. The downfalls of poor coordination is increased downtime as well as greater difficulty in searching for the fault.

 

[mbrooke]

"I^2T OUT" and "I^2T IN" refer to the shape of the trip curve. See the attached picture.
View attachment 20690

For you question regarding what MIN, INT, and MAX refer to, you'll have to look up the manual for that trip unit. I do not believe that INT is instantaneous in this case, but "intermediate".

Thank you!


Regarding Int, I would have missed that one. Intermediate makes more sense.

Also- a bit off topic- but why does the shape of the curve's transition point matter in terms of protection?

 

[adamscb]

Thank you!


Regarding Int, I would have missed that one. Intermediate makes more sense.

Also- a bit off topic- but why does the shape of the curve's transition point matter in terms of protection?

It's sometimes easier to coordinate with other fuses or circuit breakers in the system when the line is sloped like that and not at a hard 90 degrees. Essentially what you're trying to accomplish when setting up breaker settings like your trip unit is to achieve breaker coordination in the system. For example, you don't want an entire facility to trip out because of an A/C unit going to ground. In an electrical power system, the most upstream circuit breakers have time delays programmed into them, so a scenario like I just mentioned doesn't happen. Having a time delay set up upstream means you're relying on the downstream circuit breakers to open very quickly, and isolate the fault as quickly as possible (in the case of our example, the circuit breaker feeding the A/C unit should trip first, and not the breaker feeding the entire MCC, for example).

However, the downside of achieving perfect system coordination is that the potential of an arc flash or system damage is increased. It's all a balance - arc flash potential vs system coordination.

 

[mbrooke]

It's sometimes easier to coordinate with other fuses or circuit breakers in the system when the line is sloped like that and not at a hard 90 degrees. Essentially what you're trying to accomplish when setting up breaker settings like your trip unit is to achieve breaker coordination in the system. For example, you don't want an entire facility to trip out because of an A/C unit going to ground. In an electrical power system, the most upstream circuit breakers have time delays programmed into them, so a scenario like I just mentioned doesn't happen. Having a time delay set up upstream means you're relying on the downstream circuit breakers to open very quickly, and isolate the fault as quickly as possible (in the case of our example, the circuit breaker feeding the A/C unit should trip first, and not the breaker feeding the entire MCC, for example).

However, the downside of achieving perfect system coordination is that the potential of an arc flash or system damage is increased. It's all a balance - arc flash potential vs system coordination.

Very true and well worded. Not much more that I can add.

 

[kingpb]

A good example of where I2t in or out can make a difference is when your trying to coordinate with the inrush on a transformer, the shift upward may be just the little extra to get the trip curve past that point.

 

[mbrooke]

A good example of where I2t in or out can make a difference is when your trying to coordinate with the inrush on a transformer, the shift upward may be just the little extra to get the trip curve past that point.

I will keep that in mind should this issue arise- and I know it will eventually. I have you to thank for this solution.