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| SKM-UPS Modeling For Arc Flash https://brainfiller.com/arcflashforum/viewtopic.php?f=23&t=4389 |
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| Author: | ewbengineering [ Wed Jul 12, 2017 9:35 am ] |
| Post subject: | SKM-UPS Modeling For Arc Flash |
When modeling UPS's in SKM, what is the correct data to enter for three phase short circuit info? Is 100% the correct thing to do even though it reduces the bolted fault currents downstream? Or are you putting them in Bypass mode and making a "worse" case? |
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| Author: | bbaumer [ Wed Jul 12, 2017 11:50 am ] |
| Post subject: | Re: SKM-UPS Modeling For Arc Flash |
If equipped with a bypass I just show the UPS as a bus. |
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| Author: | ewbengineering [ Wed Jul 12, 2017 11:57 am ] |
| Post subject: | Re: SKM-UPS Modeling For Arc Flash |
bbaumer wrote: If equipped with a bypass I just show the UPS as a bus. Thanks for the response. Yes it does have a bypass. |
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| Author: | PaulEngr [ Fri Jul 14, 2017 4:13 am ] |
| Post subject: | Re: SKM-UPS Modeling For Arc Flash |
If you select bypass then it becomes just a bus. Thus the inductive loads will elevate the available fault current due to asymmetrical current which SKM accounts for. It will also generally reflect a much higher available fault current to downstream loads. This may be what you want for instance at a disconnect switch located on the load and line side of the UPS. However the effect on the overcurrent protection in most cases is to reflect a much lower incident energy than would exist with the UPS in service. It needs to be checked both ways...you can do this with additional scenarios but due to the coarse grained nature of that method often it is to test locally both ways. Due to the way that most breakers and fuses respond to reduced fault current (incident energy increases in the inverse time region and drastically increases if breakers go from instantaneous to inverse time), the assumption that lower fault current = lower incident energy is simply not true in many typical cases. Mind you the idea that incident energy is approximately proportional to roughly I*t does not change (V plays a role but a minor one). The issue is that when these variables interact with each other rather than in isolation as they do with overcurrent protection devices where t varies as a function of I, the result is not the same as when they are analyzed in isolation. |
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| Author: | Robertefuhr [ Mon Jul 31, 2017 10:32 am ] |
| Post subject: | Re: SKM-UPS Modeling For Arc Flash |
Be aware that you should run two scenarios for the equipment downstream from the UPS. One scenario should be when the UPS is in By-Pass. This will produce the maximum short circuit current but not necessary the highest arc flash energy. The second scenario is when the UPS itself is feeding the downstream equipment. This lower fault current may cause the output breaker to take longer to trip. The result sometimes is higher arc flash energy. Typical short circuit from a UPS is 2.5 to 5 X Ifl. However, some UPS units shut down very quickly for a fault external of the UPS. You may have to override the protective trip time and enter this UPS shut down time. |
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