Main Breaker Tripping? or Destroyed?
Main Breaker Tripping? or Destroyed?
(OP)
I am having a strange case here. Ona 480VAC system, the arrangement goes like this. There is an MCC with the following feeders: 2 motor loads rated for 3kW, and the rest are transformer feeders rated not greater than 45kVA. The total connected load is 433A and this MCC is connected to an ATS connected to normal and emergency supplies with main breakers rated 400A each. Assuming the ATS is at emergency supply, when the normal comes back, the ATS re-transfers in an open transition scheme. At this point, the 400A main breaker according to the report fails (not trip). In their report, the breaker is damaged and claimed that the 400A breaker is having a rating of 3200A and the in-rush is 11000A (quite strange). I told them to deactivate the instantaneous setting of the main breaker and check settings to allow for in-rush to dissipate. However, they claim that the breaker fails to trip and get damaged. For all I know, there is no such thing as a 3.2kAIC breakerat 480V at 400A. What do you think is the problem? I feel estranged by the way they made the report.
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata






RE: Main Breaker Tripping? or Destroyed?
If the transformers are driven into saturation the current may easily damage the breaker.
Depending on the impedance the asymmetric fault current may be 10,000% or more of rated full load current. If the transformers are being saturated the inrush current may be higher.
I would seriously consider installing closed transition or a synchronized very fast transition.
respectfully
RE: Main Breaker Tripping? or Destroyed?
But wait, you have a 433A connected load on 400A breakers? Start with that problem my friend. You should have 600A breakers, or at the very least, 500A 100% rated breakers. Better yet, get some good electronic trip circuit breakers with adjustable instantaneous trip settings that will allow for the transformer inrush.
RE: Main Breaker Tripping? or Destroyed?
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata
RE: Main Breaker Tripping? or Destroyed?
RE: Main Breaker Tripping? or Destroyed?
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata
RE: Main Breaker Tripping? or Destroyed?
Not all breakers have adjustable instantaneous trips, in fact most of the standard low cost versions you find in panelboards do not. You may need to change it out. That's why I said to use a good electronic trip breaker, it will accommodate that.
RE: Main Breaker Tripping? or Destroyed?
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata
RE: Main Breaker Tripping? or Destroyed?
respectfully
RE: Main Breaker Tripping? or Destroyed?
RE: Main Breaker Tripping? or Destroyed?
What is the momentary rating of the breaker? According to the OP, the rating is 3.2 kA, but I'd agree that this is unusually low for a 480 volt (or any voltage) breaker. The OP says the breaker failed but did not trip. It didn't say if it failed while trying to trip. I don't know the relationship between momentary and interrupting ratings of low voltage breakers.
RE: Main Breaker Tripping? or Destroyed?
RE: Main Breaker Tripping? or Destroyed?
You say there are 2 each 3KW motor loads, but the remaining ~355KVA load is small transformers (8 or more 45KVA or smaller)? Are they 3 phase, 1 phase, or a combination of both?
How exactly is the breaker failing? Are the contacts welding? If so, your problem is DC offset due to remnant flux in the transformers. See this paper for more detail:
http://p
The simple solution is to disable the instantaneous function. If this is not an option, you need to implement one of the mitigation steps mentioned in the paper (harmonic restraint, etc.).
Additional comment for statements made by other posters: The connected load may or may not be an issue depending on what loads are energized at any given time. There is not enough information in the post to make this determination. I would seriously doubt that all those transformers would be fully loaded. Many industrial applications have connected loads greater than 200% of incoming breaker ratings due to sparing or batch operations. It all depends.
RE: Main Breaker Tripping? or Destroyed?
I have read an old journal that it is recommended to provide synchro check relay or a phase monitor on the ATS switch to ensure that the emergency generator is in phase with the normal supply before initiating an open transition transfer to relieve the in-rush currents...I don't kinda understand this part...maybe some folks can clear some picture here.
Lastly, is there a electronic trip molded case circuit breakers available with a defeated instantaneous setting to allow a time delay. I am always under the impression that molded case circuit breakers (adjustable or not) will have a fixed built in instantaneous element that actuates at high faults to protect themselves.
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata
RE: Main Breaker Tripping? or Destroyed?
Series C KDC400amp, 3 Pole, 600Vac/250Vdc, Cat KDC3400F
Digitrip Trip 310, Cat 4KES400T
In=Ir=400amps
STpk=8 x Ir
Stdelay=300ms
If the in-rush is supposed to last for only a few cycles and the breaker is already set at 300ms, what is causing the breaker trip? is there a possibility that the in-rush may last for more than 300ms? Although the breaker is set at 300ms time delay...is the breaker having an undefeated instantaneous element? requesting for your opinions.
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata
RE: Main Breaker Tripping? or Destroyed?
It is not always so.
Any abrupt change in applied voltage and or phase angle may trigger a transient current spike.
There are several ways that an ATS may transfer.
Slow open transition; This has enough delay between opening and closing that the residual voltages may decay. The inrush can be expected to be the same as the normal energization inrush.
Closed transition: The supplies are synchronized and then paralleled, similar to bringing a generator online with other generators. There is no inrush if done properly.
Fast open transition; The transition is completed in less time than it takes the residual voltages to decay.
the instantaneous currents may be much worse than normal as the applied voltage and the decaying voltage may be 180 deg. out of phase. The voltage may be 200% and if the transformers saturate the current will be much higher than expected.
Fast open transition with phase check or synchronization: This is a compromise between closed and open transition.
The fast transition is delayed until the voltages are in phase. As the voltages are in phase there is very little "bump" or inrush. The faster the mechanical action of the transfer switch the better.
Fast closed transition; Once you go to closed transition the duration of transfer is of low importance.
Open versus closed transition:
A closed transition raises issues with the utility, and with protective relay settings. These are avoided with the fast synchronized transition and the inrush currents are reduced to much less than normal energization inrush currents.
RE: Main Breaker Tripping? or Destroyed?
waross, I am just curious though, if there is a large current spike due to out of phase switching in addition to transformer in-rush...this should normally last for a few cycles right and should not trip a time-delayed circuit breaker set at 300ms (0.3 seconds). This defeats the notion of coordination. Does this transient last 300ms and above which causes the time delayed circuit breaker to trip?
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata
RE: Main Breaker Tripping? or Destroyed?
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata
RE: Main Breaker Tripping? or Destroyed?