# Shunt Trip Circuit Breaker Application



## DMB5mil (Aug 21, 2008)

Hi All,

I'm having trouble understanding the following scenario involving a shunt trip circuit breaker and a fire pump controller I'm mapping out. I've attached a schematic diagram. Here's the story:

A fire pump controller is connected to an automatic transfer switch which is fed from either utility power or by a standby generator as shown in the attachment.

When turned on, the fire pump controller opens the 200/3 breaker in Panel Gen in order to lighten the generator load (Panel N no longer is powered) Both the fire pump controller and generator are at 277/480V.

However, the shunt trip does not happen directly between pump controller and generator. The 20/1, 120V circuit breaker in Panel E is the mix and is somehow providing control. Here is what happens:

If the 20/1 breaker is open, when the fire pump controller is on it does NOT open the 200/3 breaker.

If the 20/1 breaker is closed, when the fire pump controller is on it DOES open the 200/3 breaker.

I can see how the fire pump controller could trip the 200/3 breaker directly (the controller acting as a switch energizes the shunt trip coil of the 200/3 breaker, causing it to open) but how does the 20/1 120V breaker fit in? Why do you suppose it is required? I can't visualize what's going on here.

Is it required because the control voltage within the pump controller is 120V, not 480V?

Any advice is appreciated. If you want to mark up the attached schematic feel free!

Thanks,

Brian

Any guidance, websites, schematics, etc. would be helpful. Maybe I need a good resource for understanding shunt trip circuit breakers...

Shunt_Trip_Question.pdf


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## Twofrogs (Aug 22, 2008)

DMB5mil said:


> Hi All,
> I'm having trouble understanding the following scenario involving a shunt trip circuit breaker and a fire pump controller I'm mapping out. I've attached a schematic diagram. Here's the story:
> 
> A fire pump controller is connected to an automatic transfer switch which is fed from either utility power or by a standby generator as shown in the attachment.
> ...


It appears that the 120V circuit may be routed through a "normally open" contact on the controller. It does seem odd, since a controller can have a lower voltage control circuit built-in via transformer, with a lock-out circuit ("normally open") available, so tracking down that situation would be warranted. If the 120V circuit off the E panel is used for that purpose, I'd recommend using a lock on the breaker to keep it in the closed position and have it marked accordingly.... though I'd check with the NEC first..... it has been quite some time since I've done design. Other than that, a conversation with the manufacturer could shed more light on the arrangement as well as a getting/creating a relay logic diagram for a better understanding of the existing connections. Hope this helps!


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## Art (Aug 22, 2008)

without a schematic (vs power single line) it anybody's guess

mine: it's external power for the shunt trip

they don't want to use the FP controller control power (perhaps code prevents it) because an external fault would disable the pump when you most needed it...


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## mudpuppy (Aug 23, 2008)

I think Art is right in that we're taking guesses in the dark, but here is my best guess:

If the utility feed fails and the ATS/E fails to transfer Panel E to the generator, then the 200/3 may not need to be opened to drop Panel N because the load on Panel E has already been dropped. If I am understanding things correctly, this would be accomplished by this scheme because the 20/1 circuit would be deengergized in this scenario (even though the 20/1 breaker is closed) since Panel E would still be connected to the dead utility feed.


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## Art (Aug 24, 2008)

the way I see it:

normal power available

sufficient oomph to carry N, E and the FP

emergency power-genset/no FP

the genset can carry N &amp; E

emergency power-genset/FP called on

the genset is not large enough to carry N, E and the FP...so N is shunted off

E = Emergency loads

N = normal or non-critical loads, so they are shed...to keep the genset size smaller in the rare event of a power outage and fire

just looked in NFPA 20-6 &amp; NFPA 70-695 (NEC)...any control power source (power for shunt trip CB) must be independent and failure of such shall not prevent the pump from running

btw: the control power source (xfmr) in a FP controller is not protected (fused, etc.) it and the pump/wiring are considered sacrificial...

allow it to run or attempt to until it fails/burns up


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## DMB5mil (Aug 25, 2008)

Art said:


> the way I see it:
> normal power available
> 
> sufficient oomph to carry N, E and the FP
> ...


Thanks for all the replies! I spoke with my manager and he is confident that the 120V, 20/1 breaker is supplying coil voltage to the 200/3 shunt trip as suggested by your replies. The coil is energized by the 20/1 circuit when the contacts from the fire pump controller close.

Per NFPA 20 Section 9.6.2.2 the generator must provide a minimum of 8 hours run time for the fire pump, apart from the other generator loads. My guess is that Panel N is dropped to ensure the generator can run long enough to meet code.

Thanks for your help everyone.


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