Disabling instantaneous protection possible while protecting switchboard?

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Magic Gorge

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Lexington, KY
For a selective coordination scheme, I am considering turning off the instantaneous function for both main and feeder breakers in a switchboard. Switchboards in question are designed per UL 891, meaning a 3 cycle withstand rating. The breakers have a minimum short-time delay setting of 0.05 seconds, which corresponds to 3 cycles. So, under these conditions is it possible to disable the instantaneous while also protecting the switchboard?
 
Magic Gorge said:
For a selective coordination scheme, I am considering turning off the instantaneous function for both main and feeder breakers in a switchboard. Switchboards in question are designed per UL 891, meaning a 3 cycle withstand rating. The breakers have a minimum short-time delay setting of 0.05 seconds, which corresponds to 3 cycles. So, under these conditions is it possible to disable the instantaneous while also protecting the switchboard?
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A breaker listed to UL 489 will have what is equivalent to an instantaneous override point which means the Inst function is never completely removed. So, a properly sized UL489 breaker will always protect a UL891 switchboard.

However, you should consider the Arc Flash implications of removing the Inst function. At the least you may want to include an Arc Flash Incident Energy reduction switch.
 
jim dungar said:
A breaker listed to UL 489 will have what is equivalent to an instantaneous override point which means the Inst function is never completely removed. So, a properly sized UL489 breaker will always protect a UL891 switchboard.

However, you should consider the Arc Flash implications of removing the Inst function. At the least you may want to include an Arc Flash Incident Energy reduction switch.
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Correct.
Besides, I don’t think you can fully disable the instantaneous trip of an MCCB listed to UL 489, it’s part of the listing of the breaker because that also protects the breaker itself from excessive current (up to the IC rating). In fact when you buy a UL 489 listed Molded Case Switch, which supposedly has no trips in it at all, it STILL has an instantaneous trip that is factory set for 10x the rated current of the device and 0.05sec. You can’t see it or adjust it on most brands, but it’s in there because UL requires it.
 
jim dungar said:
A breaker listed to UL 489 will have what is equivalent to an instantaneous override point which means the Inst function is never completely removed. So, a properly sized UL489 breaker will always protect a UL891 switchboard.

However, you should consider the Arc Flash implications of removing the Inst function. At the least you may want to include an Arc Flash Incident Energy reduction switch.
Click to expand...
What if the available fault current is less than the override value? Say 50kA and override at 65kA. So the curve would look like short time pickup at 0.05 sec but the override never picks up. There are ARMS settings tabulated separately for maintenance mode.
 
If you can dial the Inst pickup to that level you must be using an Insulated Case or Power Breaker, as a MCCB can't be dialed that high. When you turn OFF Inst the breaker will default to its max pickup setting, so its never OFF. If the pickup is greater than the AFC then Inst is out of the picture and STPU is the only protection for fault current. You mentioned the STPU can be set for .05s but that the lowest pickup delay time, the band is much higher than that for the max time it may take it to trip/clear. So that may exceed your board rating. You should be able to adjust Inst on the MCB higher than the Feeders and accomplish SC. Your AFR switch can be used on the MCB to lower it when needed. Not sure why you can achieve SC with the ETUs on your breakers.
 
MCCB can't be dialed that high
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This won't always be true. It depends on the available fault current. On a weak system, it's possible to set the instantaneous trip of a MCCB high enough that it will never trip on instantaneous. Maybe not common, but it's definitely possible.

For low voltage power circuit breakers, it is possible to not have any instantaneous trip at all, not even an override. This is why they are superior to MCCBs for coordination in most cases. These breakers have a 30 cycle withstand rating, while MCCBs are not required to have that. In older trip units, they had to be specifically ordered without IT. Many of the newer units have switchable IT units. But again, this is only for LVPCBs.
 
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