Reading manufactures time current curves

[mmuel]

Hello,
I am trying to decipher an Eaton/Cutler-Hammer TCC. The drawing number is 70C1034 and it is for a Digitrip 1150 trip unit.

The problem I have is a heater shorted out and instead of tripping the 30A MCCB in the unit or the 50A panel MCCB, it tripped the 800A distribution LVPCB. It tripped at 1707A on an inst trip. I have plotted the TCC s but do not understand the inst portion of the curve.

I was told that the Inst. region actually goes all the way down to the current axis with the understanding that there is a “minimum time” that it will take for the breaker to trip. That being said, what is the purpose of what appears to be the adjustable Minimum Clearing Time on the lower side of the inst. region? Am I better off adjusting the short pick-up or the short time? I guess I always thought you could adjust the short time up and raise the inst portion up so the inst portions of the downstream breakers don’t overlap.

Just for clarification the short slope is flat.

 

[rbulsara]

Firstly, you should check the INST setting of the 800A CB. Typically set at 8 to 12 times the LTPU. If so 1700A should not trip the 800A CB.

Also verify if the 800A CB has ground fault element, if so that could be the culprit. GF of a breaker generally would not coordinate with the downstream breakers without GF element.

MCCBs do not always coordinate in INST region. If the overcurrent is more than INST setting of the breakers, they all will have a race and the most sensitive one would trip first.

You need to verify the settings and models of the breakers and compare the curves of both the 30A CB and the 800A CB on a common TCC graph.

If all looks OK then, test the breakers.

Rafiq Bulsara

http://www.srengineersct.com

 

[jghrist]

There is a Short Time Setting which has a pickup and a time delay. The instantaneous setting has a pickup only and has no intentional time delay. Once the current exceeds the pickup, it will trip, even if the current is gone by the time the contacts open.

 

[Zogzog]

and to add to jghrist post, the ST may also have an I2t in/out which would explain the "flat short slope" with it out, more of a curve with it in.

 

[rbulsara]

If you post your TCCs with settings displayed, more useful responses may be received.

I am not sure what do you mean by:

That being said, what is the purpose of what appears to be the adjustable Minimum Clearing Time on the lower side of the inst. region?

Rafiq Bulsara

http://www.srengineersct.com

 

[alehman]

What is the inst pickup setting? Some will go as low as 2x rating plug. Has the breaker/trip unit been tested?

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[mmuel]

Thanks for all of the replies.

I was able to have a colleague try different settings using SKM software and apparently the short time setting makes no difference using a flat short slope with the instantaneous turned on.

 

[alehman]

Yes, if the instantaneous pickup is set lower than the short-time pickup.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.