I have a question on a UPS system at a local hospital. A 100 amp breaker feeds the UPS unit from the basement to a second floor lab. The UPS then feeds a MLO 42 circuit panel in an adjacent corridor. A shorted circuit caused the UPS to shut down on overload. (2 questions) Would a main breaker on this 42 circuit panel help eliminate the same scenario from happening again? Also, is there any code issues with this panel not having a main breaker and not being in the same location of the area it is feeding (the UPS is in the room it is supplying power to)?
UPS System
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rick hart
Senior Member
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- Dallas Texas
Re: UPS System
You did not state the kVA of the UPS but assuming it is a 30 kVA @120/208 a shorted 20A circuit will only let 108A maximum current flow.
That would look like an overload to the UPS.
A panel off of the UPS output would be required to have a main disconnect or no more than six service disconnects.
You did not state the kVA of the UPS but assuming it is a 30 kVA @120/208 a shorted 20A circuit will only let 108A maximum current flow.
That would look like an overload to the UPS.
A panel off of the UPS output would be required to have a main disconnect or no more than six service disconnects.
milwaukeesteve
Senior Member
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- Milwaukee, WI
Re: UPS System
If it was a short (phase to phase), an OCPD could have helped. Unfortunately, the UPS OCP is usually very sensative do to the load(s) that are plugged into it (computers and electronics).
If it was a Fault (phase to ground), then nothing but a Ground Fault protective device, in either brach or feeder, would have saved you. Faults don't travel back on the ungrounded conductor to the panel. Faults travel through the Ground to the transformer and back to the panel. The UPS main would have been first in line before your OCPD in the 42 circuit panel.
For your second question, if the UPS system is a distribution system, and the feeder for the panel is controlled by OCPD, then the panel is just a sub panel, not OCP needed there.
If it was a short (phase to phase), an OCPD could have helped. Unfortunately, the UPS OCP is usually very sensative do to the load(s) that are plugged into it (computers and electronics).
If it was a Fault (phase to ground), then nothing but a Ground Fault protective device, in either brach or feeder, would have saved you. Faults don't travel back on the ungrounded conductor to the panel. Faults travel through the Ground to the transformer and back to the panel. The UPS main would have been first in line before your OCPD in the 42 circuit panel.
For your second question, if the UPS system is a distribution system, and the feeder for the panel is controlled by OCPD, then the panel is just a sub panel, not OCP needed there.
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- retired electrician
Re: UPS System
The UPS is a separately derived system. The panel is most likely a lighting and appliance branch circuit panel[404.14(A)] and will require line side overcurrent protection[408.16(A)]. The breaker on the line side of a UPS does not provide protection for the output side.
Don
[ May 03, 2005, 11:03 AM: Message edited by: don_resqcapt19 ]
The UPS is a separately derived system. The panel is most likely a lighting and appliance branch circuit panel[404.14(A)] and will require line side overcurrent protection[408.16(A)]. The breaker on the line side of a UPS does not provide protection for the output side.
Don
[ May 03, 2005, 11:03 AM: Message edited by: don_resqcapt19 ]
brian john
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Re: UPS System
Rick:
The curves I have for 20 A molded case circuit breakers are in the 120-200 amp range for instantaneous trip.
Rick:
The curves I have for 20 A molded case circuit breakers are in the 120-200 amp range for instantaneous trip.
Re: UPS System
If the UPS has a pretty tight current limit a short circuit will be handled by the UPS current limit, not the branch breaker.
Some UPS units have an output that IS a separately derived system, but some have a solid connection between input and output neutrals. This depends on whether the UPS rectifies all power, runs it past the battery, then then converts all power back to AC. The other type has a solid state transfer switch that normally passes line power directly to the output and kicks in the inverter if the power fails for say a 1/4 cycle or 1/2 cycle. The latter UPS is OK for loads that have a ride through time and can withstand the power bump.
If the UPS has a pretty tight current limit a short circuit will be handled by the UPS current limit, not the branch breaker.
Some UPS units have an output that IS a separately derived system, but some have a solid connection between input and output neutrals. This depends on whether the UPS rectifies all power, runs it past the battery, then then converts all power back to AC. The other type has a solid state transfer switch that normally passes line power directly to the output and kicks in the inverter if the power fails for say a 1/4 cycle or 1/2 cycle. The latter UPS is OK for loads that have a ride through time and can withstand the power bump.
Re: UPS System
The reason for the 108A fault current is because the UPS inverter can only deliver that current. Have you ever had a motor short out supplied by a VFD? Same thing- the inverter is limited by its ability to supply current. The motor breaker will not trip but the inverter will be putting out a little over 100%.
In the case of a 30KVA UPS that would be 108 A.
The reason for the 108A fault current is because the UPS inverter can only deliver that current. Have you ever had a motor short out supplied by a VFD? Same thing- the inverter is limited by its ability to supply current. The motor breaker will not trip but the inverter will be putting out a little over 100%.
In the case of a 30KVA UPS that would be 108 A.
rick hart
Senior Member
- Location
- Dallas Texas
Re: UPS System
Wdhopper is correct.
This is not a transformer with a huge utility system behind it. This is a rectifier/inverter limited by the size of the semiconductors reconstructing the AC output. A phase to phase short will look like an overload to the controller since the inverter cannot supply a similar current as the utility.
Wdhopper is correct.
This is not a transformer with a huge utility system behind it. This is a rectifier/inverter limited by the size of the semiconductors reconstructing the AC output. A phase to phase short will look like an overload to the controller since the inverter cannot supply a similar current as the utility.
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