I'm installing a new 300kVA UPS to replace an existing UPS of the same size. The manufacturer is proposing a unit that does not have an overcurrent device on the output circuit. The unit has two inputs--one to the rectifier, one to a static bypass switch. His argument is that the output cables are protected because the UPS electronically senses overloads and faults, and switches to the bypass circuit within a half cycle. The bypass circuit then clears the fault by way of the upstream feeder breaker. I can't find anything in the NEC that touches on this issue. Keeping in mind that the UPS is a separately derived system when in normal operation, does anyone have any thoughts regarding overcurrent protection on the output circuit?
UPS output OCPD
- Thread starter msteiner
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You basically follow the manufactures instruction and design guidelines. You can certainly add a OCPD, but it wouldn't serve any purpose other than a point of failure, something a UPS is not made for.
Look at it this way, A UPS is a soft source, and could not produce enough fault current to operate your output OCPD.
Typically on a large scale UPS, the output is delta. From the output you then run to devices called PDU's for branch circuit distribution. Inside the PDU is the delta/wye transformer (SDS) where you have secondary branch circuit OCPD.
Look at it this way, A UPS is a soft source, and could not produce enough fault current to operate your output OCPD.
Typically on a large scale UPS, the output is delta. From the output you then run to devices called PDU's for branch circuit distribution. Inside the PDU is the delta/wye transformer (SDS) where you have secondary branch circuit OCPD.
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