AC PRO Trip units.

[Thedroid]

After combing through the manual I still can't determing how this unit differentiates a Ground Fault from a Long Time, Short Time, or Instantaneous Fault. They are installed on a 480 Wye three wire secondary.

After playing around with the tripping time calculator on the Utility Relay Company website, It seems like there is no difference. Just different names for different curves.

If this is the case, it seems that the GF setting would have to be set higher than all others, or the unit would always trip on GF.

 

[davidbeach]

Ground would be the residual current, the vector sum of the phase currents. Generally it can be set well be low load as load doesn't have any residual current. In there is a current carrying neutral in the system, then there needs to be a CT on the neutral also so that current in the neutral gets included in the vector sum. Anything other than zero is a fault.

 

[Thedroid]

No neutral is brough out for any of the MCC's. fI'm assuming that the solid state unit does all the calculations then and can diffentiate between a GF and and other fault.

Another problem with the coordination is that the main breaker uses a GE IAC instantaneous relay hooked up to a GF CT. This causes it to pick up before any of the other downstream breakers. We can't really change this setup, because some of these breakers feed large MV motors and also 480V load centers. If we were to raise the pick up current, of increase the delay, we would be losing some of our protection for the motors.

I'm trying to get a understanding of all of our protection schemes since I'm new to this plant.

 

[davidbeach]

Ground fault relays only trip on ground fault, they do not trip on phase-phase or 3-phase faults. Ground relays can be, and usually are, set well below maximum load as they don't see load. Phase relays have to be set well above load as they do respond to load and you don't want them tripping on load. Just because you have a much lower pickup on ground than you do on phase it doesn't mean that your ground unit will be the first to trip, it just depends on the fault.

 

[Thedroid]

The 4160 feeders all have GE IAC relays set up for ground faults. They are set up for tap 5 and .5 delay. They seem to be far more sensive to ground faults than any of the other breakers. I'm trying to keep them from tripping on a small GF, and taking down the plant, but still provide protection to the MV motors associated with them.

As for the sum of the vectors explanation, I found it on the first page of the insruction manual. I had went through that manual several times, but neglected to read the first paragraph, oh well. Thanks for the explanation though.