UL891 Motorized Breaker Power Question

[JuliaTruchsess]

I'm (re)designing a double-ended UL891 switchboard containing a number of motorized breakers, having a total power requirement of around 2000VA. All breakers are required to operate in the absence of either source. The previous designers used a pair of large control transformers, each with its primary across one of the sources, respectively, ahead of the main breakers, i.e. at the service entrance. They then paralleled the two secondaries to run the breaker motors. Apart from potential issues with paralleled secondaries, I don't like this solution due to the size, weight, heat, and cost of the transformers, and would like to replace them with a simple 30A relay to arbitrate between the two sources, with the relays and breaker control wiring protected by a couple of 25A fuses or breakers.

UL891 sections 8.6.6.7 and 8.6.15, however, as well as Table 20, seem to state that 15A or 20A is the maximum allowable current in a controls branch circuit in a switchboard, which seems odd to me; maybe I'm reading it wrong.

Any UL891 experts here who can clarify?

Thanks!

 

[Jraef]

Unless you are going to use the (un-described) line voltage for the motor breakers, you need transformers anyway. Using the line voltage for the motors is problematic from an arc -flash potential and SCCR standpoint, so it may not be as simple as you think. So that means having the transformers anyway.

If your thought was to have one transformer and a “relay” for the primary feeds from each source, the SCCR issue remains; you would need a relay /fuse combination suited for the available short circuit current of the primary. Again, it may not be as easy as you think.

 

[JuliaTruchsess]

Unless you are going to use the (un-described) line voltage for the motor breakers, you need transformers anyway. Using the line voltage for the motors is problematic from an arc -flash potential and SCCR standpoint, so it may not be as simple as you think. So that means having the transformers anyway.

If your thought was to have one transformer and a “relay” for the primary feeds from each source, the SCCR issue remains; you would need a relay /fuse combination suited for the available short circuit current of the primary. Again, it may not be as easy as you think.

The system is 208Y/120; my thinking is to use 120VAC motor operators and 20 or 30A current-limiting fuses or breakers upstream of the relays. My understanding of the standard is that control circuits don't need to have an SCCR, but the definition of "control circuit" is a bit elusive for me - if it's everything outside the main power circuitry then it would seem this qualifies.

I'm not a PE and this will all be reviewed by a PE eventually; I'm just trying to understand the parameters and come up with a viable starting point.

 

[ron]

UL891 sections 8.6.6.7 and 8.6.15, however, as well as Table 20, seem to state that 15A or 20A is the maximum allowable current in a controls branch circuit in a switchboard, which seems odd to me; maybe I'm reading it wrong.

Any UL891 experts here who can clarify?

Thanks!

I'm no 891 expert, but if it holds true, you can feed the 30A circuit to a panelboard wit 15A circuits to the control circuits.

 

[ATSman]

Why re-invent the wheel?

Why re-invent the wheel?

Julia,
The double ended substations I have seen use 2 CPT's with an 83 throwover relay to switch between sources
depending which side (or both) is energized.
I tried attaching a dwg but the size was too big.
Maybe I will try to PM you.

Tony

 

[JuliaTruchsess]

Julia,
The double ended substations I have seen use 2 CPT's with an 83 throwover relay to switch between sources
depending which side (or both) is energized.
I tried attaching a dwg but the size was too big.
Maybe I will try to PM you.

Tony

Thanks, Tony. I am indeed implementing an 83 transfer relay, but need to eliminate the transformers. There is a second, 3kVA system, powering IT equipment, which arbitrates similarly between four sources - so altogether I'd need six transformers and that's just too much space, weight, heat, and cost. My current plan is to treat these as feeders and protect all relay inputs with class CC fused disconnects which will lower the AFC to 5kA, same as the assumed SCCR of "auxiliary devices", under which I presume general-purpose relays fall.