UPS effect on Arc Flash

[GMIEE]

I'm running an arc flash analysis and the system has a 60 kVA UPS attached. Upstream of this UPS I am getting numbers of 0.5 cal/cm^2, but I look on the side that the UPS would be powering and it jumps to 136 cal/cm^2.

Is this a common effect of a UPS in arc flash analysis?
I find it interesting that this happens.

 

[dpc]

The increase in incident energy is most likely due to big jump in arc duration since the UPS can't put out enough fault current to trip the breaker on instantaneous. You need to look at what is causing the energy to be high.

In reality, no UPS is going to be able to produce this much energy. So you need to do a reality check and figure out what is really going to happen when a short circuit occurs downstream of a UPS.

 

[wbd]

Depending on the specs of the UPS, it may be hardware and software limited on the current. For example, a UPS may be limited to 125% dc input and may shutdown within milliseconds of an overload. This would limit the output and therefore the arc flash.

 

[alehman]

Many UPS's will automatically transfer the load to a bypass source if there is a fault on the output, in order to provide sufficient current to operate the breaker and clear the fault quickly. If your UPS has a bypass, you must consider the bypass source as well.

 

[sibeen]

The answer will depend upon the UPS you are using and the way the UPS is set up. As the answers above indicate the UPS 'may' transfer to bypass (reserve) if it detects a short circuit or extreme overload on its output. The same UPS may try to current limit the output under the same conditions depending upon the 'commissioning' settings.

I have worked on UPS systems where the UPS's method of operation under an output 'short' condition had 12 different programmable parameters.