ish1284
Member
- Location
- Arlington Heights, IL, USA
Hey all, so I have a system in which one bus (let's call it Bus 1) feeds one UPS, which feeds another bus (let's call it Bus 2). I have developed settings for the feeder breaker from Bus 1 to the UPS (Input Breaker 1) and the breaker from the UPS to Bus 2 (Output Breaker 1). For the sake of this discussion, let's assume those settings are correct.
The customer has indicated that they would like to install a second UPS which will be operating in parallel with the first UPS (it will have it's own set of external input and output breakers). They requested an alternative set of circuit breaker settings for two UPS's in parallel.
The UPS's are equipped with a static bypass and for the sake this topic, we're assuming that they have switched to bypass mode due to a fault on Bus 2.
The purpose of the UPS's in parallel is more for redundancy rather than load addition. Therefore the LTPU of the input/output breakers is not going to change.
However, my thoughts are that the STPU of the breaker should be reduced by half because for a fault on Bus 2, the input/output breakers will see half of the through fault current when two UPS's are in parallel. If it is not changed, then coordination with the upstream equipment could be impacted because the breaker's TCC is effectively shifted to the right by a current multiplier of two.
The instantaneous is disabled for reasons not relevant for this conversation.
Does my take on reducing the STPU by half sound reasonable? My peers have been scratching their heads but it seems pretty straight forward to me. Thoughts?
P.S. I drew out a very simplified one line to give you a visual of the layout. Also, these are 1600A breakers with an electronic trip unit. The UPS's are 1000kW to give an idea of the size.
The customer has indicated that they would like to install a second UPS which will be operating in parallel with the first UPS (it will have it's own set of external input and output breakers). They requested an alternative set of circuit breaker settings for two UPS's in parallel.
The UPS's are equipped with a static bypass and for the sake this topic, we're assuming that they have switched to bypass mode due to a fault on Bus 2.
The purpose of the UPS's in parallel is more for redundancy rather than load addition. Therefore the LTPU of the input/output breakers is not going to change.
However, my thoughts are that the STPU of the breaker should be reduced by half because for a fault on Bus 2, the input/output breakers will see half of the through fault current when two UPS's are in parallel. If it is not changed, then coordination with the upstream equipment could be impacted because the breaker's TCC is effectively shifted to the right by a current multiplier of two.
The instantaneous is disabled for reasons not relevant for this conversation.
Does my take on reducing the STPU by half sound reasonable? My peers have been scratching their heads but it seems pretty straight forward to me. Thoughts?
P.S. I drew out a very simplified one line to give you a visual of the layout. Also, these are 1600A breakers with an electronic trip unit. The UPS's are 1000kW to give an idea of the size.
