Power CB feeding UL 891 MLO Switchboard

[elec_eng]

I am reviewing a project and noticed ANSI Power CB (2000A) is feeding a UL 891 MLO switchboard(2000A) downstream. There is a short time delay of 12 cycles on the Power CB for the coordination with the MCCBs in the switchboard.

Since UL 891 switchboard is only rated for 3-cycle, I don't think The upstream ANSI PCB (12-cycles) can protect the the downstream switchboard, if there is a fault at the bus in the switchboard. Is that right? I think the solution is they should have a MCB at the switchboard for the bus fault and the upstream PCB will protect the feeder fault only. Any thoughts?

 

[petersonra]

That would seem to make coordination difficult. The main on the switchboard would just trip instead of one of the branch circuit breakers. It seems likely the reason they left the main off the switchboard is to avoid that problem all together. But maybe they created a different problem.

Just wondering here. With a main lug only on a switchboard a fault on a branch would still be cleared by the branch breaker. A fault above the main lugs would be cleared by the Upstream breaker without having any effect on the switchboard at all. The only kind of fault that might create a problem would be one that was directly on the bus itself in the switchboard. I wonder what the chance of that happening is. It would seem to be fairly low. Although not 0. It seems to me that adding in a Main on the switchboard doesn't really substantially effect coordination at all. If you have a bus to bus fault in the switchboard you're probably going to have a much bigger problem than what circuit breaker trips to clear the fault.

 

[jim dungar]

I am reviewing a project and noticed ANSI Power CB (2000A) is feeding a UL 891 MLO switchboard(2000A) downstream. There is a short time delay of 12 cycles on the Power CB for the coordination with the MCCBs in the switchboard.

Since UL 891 switchboard is only rated for 3-cycle, I don't think The upstream ANSI PCB (12-cycles) can protect the the downstream switchboard, if there is a fault at the bus in the switchboard. Is that right? I think the solution is they should have a MCB at the switchboard for the bus fault and the upstream PCB will protect the feeder fault only. Any thoughts?

Why not add an INST function to the existing Power breaker Or, just replace that breaker, if it is mechanically too slow.

 

[elec_eng]

That would seem to make coordination difficult. The main on the switchboard would just trip instead of one of the branch circuit breakers. It seems likely the reason they left the main off the switchboard is to avoid that problem all together. But maybe they created a different problem.

Just wondering here. With a main lug only on a switchboard a fault on a branch would still be cleared by the branch breaker. A fault above the main lugs would be cleared by the Upstream breaker without having any effect on the switchboard at all. The only kind of fault that might create a problem would be one that was directly on the bus itself in the switchboard. I wonder what the chance of that happening is. It would seem to be fairly low. Although not 0. It seems to me that adding in a Main on the switchboard doesn't really substantially effect coordination at all. If you have a bus to bus fault in the switchboard you're probably going to have a much bigger problem than what circuit breaker trips to clear the fault.

I thought the same thing but the fact that the switchboard is not protected from the bus fault isn't that a code violation???...although I don't seem to find the code section to cite...

 

[elec_eng]

Why not add an INST function to the existing Power breaker Or, just replace that breaker, if it is mechanically too slow.

Not quite sure what you meant...did you mean to turn on the INST without any time delay? I think that would make the coordination impossible with down stream MCCBs. I think that was a team they put a time delay on the PCB. And What would they replace the PCB with??

 

[petersonra]

I thought the same thing but the fact that the switchboard is not protected from the bus fault isn't that a code violation???...although I don't seem to find the code section to cite...

one would think so.

 

[Mike01]

From my experience, I have often posed the same question…Here goes…As long as the upstream CB contains an instantaneous trip response then the downstream UL-891 switchboard is protected. Only when the instantaneous is turned off would the switchboard not be protected because now you are relying on the short time to protect the bus that is only rated for a three cycle withstand. With the instantaneous on the instantaneous would be protecting the UL-891 switchboard. If the switchboard is rated at 65Ka depending on the ampacity you will find most LVPCB with instantaneous have a max. of 12-14X the sensor / plug combination so even a 1600A LVPCB with instantaneous will trip within its instantaneous range at 22.4Ka of fault current as long as the instantaneous is provided in the trip unit. Even a 3200A LVPCB @ 14x is clearing at 44.8Ka in the instantaneous range. My understanding is as long as the LVPCB upstream has an instantaneous function activated and the downstream switchboard and breakers are suitable rated for the fault current you are properly protected. There would be more of an issue with a lower fault current that may exist into the short time / long time delay where the lower fault current may exist for an extended period of time even beyond 10 sec. However you will not find a bus “Thermal damage curve” at least not to my knowledge to compare fault current to bus bracing like you would a transformer damage curve. One of the bigger issues is transfer switches since these are withstand ratings when using a time based ratings UL states that the upstream device must have an instantaneous response without a short time delay for non short time rated switches, so this makes it tough when selective coordination is required and using circuit breakers with electronic trip units unless you utilize short time rated transfer switches….anywho just my experiences…

 

[jim dungar]

I thought the same thing but the fact that the switchboard is not protected from the bus fault isn't that a code violation???...although I don't seem to find the code section to cite...

The switchboard bussing is protected, just like the feeder conductors are.
A main device can be located anywhere upstream, it does not need to be integral to the equipment.

Not quite sure what you meant...did you mean to turn on the INST without any time delay? I think that would make the coordination impossible with down stream MCCBs. I think that was a team they put a time delay on the PCB. And What would they replace the PCB with??

Yes, I meant to get the breaker to operate within the 3 cycle rating of the switchboard.
Molded case circuit breakers are routinely coordinated with power circuit breakers. I can't think of any large hospital or data center that doesn't have selectively coordinated breakers.

 

[petersonra]

Yes, I meant to get the breaker to operate within the 3 cycle rating of the switchboard.
Molded case circuit breakers are routinely coordinated with power circuit breakers. I can't think of any large hospital or data center that doesn't have selectively coordinated breakers.

I think you are saying that the expected level of a switchboard bus fault would trip the upstream OCPD based on its instantaneous setting, while still allowing a branch circuit to trip the branch breaker first. that seems reasonable.

 

[elec_eng]

Thanks for all good comments. This is a new hospital but they are still under NEC 2011. That means they are required to have 100% selectivity in Emergency system. I understand that was relaxed in a newer code to .1 sec for the hospital but that's not applicable for this project.

I will make a note of this in my review comments and the design engineer will have to verify this for me.