GE MicroVersaTrip Plus Trip Unit Sizing?

[jlr899]

Installing an air compressor with a 400Amp motor, will be powering from a GE AKR-9D-30H 800A frame breaker. The breaker currently has a 800Amp MicroVersaTrip Plus Trip unit (A208LSG) in it. My question is can I just replace the 800A (TR8B800)chip with a 400Amp chip (TR4B400)or do I also need a 400Amp Trip unit (A204LSG). Thanks

 

[Zogzog]

You can just swap rating plugs.

 

[dpc]

If this is a "supervised location" you can also just reduce the Long-Time Pickup setting.

 

[rhatcher]

You can replace the rating plug (chip) but you need to install the correct one.

The correct rating plug would be TR8B400.

Dissecting the part number helps to understand this. The 8 represents the rating of the circuit breaker current transformers, in this case 800 amps. The 400 represents the continuous current rating of the circuit breaker.

Another option, as pointed out by DPC, is to reduce the long time pickup setting to 0.5x. The pickup setting is a multiplier that is applied to the continuous current rating. In this case, 800 x 0.5 = 400 amps.

For setting the instantaneous pickup, keep in mind that this is also a multiplier of the continuous current rating. Thus, for a 2400 amp instantaneous trip (6x of motor current) the setting would be 3x. This is because 800 x 3 = 2400.

 

[jraef]

I hope from reading this that you do not intend to use this breaker as the sole motor protection scheme. Breaker trip curves are designed for conductor protection not motor overload protection. You may experience nuisance tripping on a breaker set to trip at 400A Long Time, compared to a Class 20 Thermal Overload Trip Curve of a motor protection relay or TOL.

The typical scheme would be to use the breaker as the short circuit protection only, then have a TOL or MPR on the motor controller.

"Will work for (the memory of) salami"

 

[jlr899]

Very helpfull, Thanks to all!

 

[rhatcher]

I just noticed, upon review, that your trip unit has short time protection (LSG) instead of instantaneous protection (LIG). The short time setting will be based on both a current setting and a time setting. For this size motor you should have manufacturer information on the motor's time-current curve based on your load to be able to choose the proper current and time settings for the breaker to provide proper protection.

That being said, I agree with Jraef that a combination starter with instantaneous protection and overloads will provide better protection that is easier to coordinate than a LSG breaker. However, I have seen LSG breakers often used as motor starters but this is primarily in power generation stations.