Using short time protection for a low voltage motor

[MakingAComeback]

Gentlemen,

An AE firm came up with the settings for a Siemens WL breaker with an ETU trip unit. I think they used SKM software to get the numbers.

This involves a recently installed Goulds reclaim water pump driven by a Reliance 460 V, 250 hp motor. FLA=272A. LRA=1825A.

Trip Unit Settings:

Rating plug = 300A
Long Time Pickup = 1
Long Time Delay = 8s @ 6 x PU
Short Time Pickup = 8
Short Time Delay = 0.02s (fixed)
Instantaneous = 12

I think I understand the purpose of short time protection to achieve coordination between feeder breakers and upstream breakers. My question concerns the application of short time protection for a motor circuit.

What do you think it is there for?

 

[dpc]

Normally it is needed to allow selectivity with downstream breakers.

Since there is most likely nothing downstream of this breaker except the motor, you may not need it. It will have to set to allow the motor enough locked-rotor time to get up to speed. The 8X pickup probably allows for that. It will only be a factor between 8x and 12x where the instantaneous takes over. In this range it will provide faster fault clearing than the long-time element.

I would be more concerned about the long-time settings because that is what is providing your motor overload protection, unless there is some additional motor overload device.

 

[DanDel]

If the settings were provided from SKM software, there should be a time-current coordination graph showing the motor start curve plotted against the C/B settings, and the next upstream C/B settings, too, if it was done correctly.

This way you should be able to see that the C/B settings coordinate with the upstream C/B, while allowing the motor start and providing motor protection.