Motor Contribution in Calculation

Motor contribution adds to the calculated bolted fault current. The calculated bolted fault current is then used to calculate the arcing fault current, which is then used to identify the clearing time of the protective device. Here's my comment on this and I would greatly appreciate others to voice theirs:

Motor contribution to the calculated bolted fault current is great for selecting hardware that does not blow apart during a worse case (and rare) bolted fault, but seems terrible for personnel safety. It is common for technicians to work on the power system without all -very possibly none - of the motors running. Therefore, the calculated arcing fault current is - for all practical purposes - too high and can result in a clearing time that is too fast. With time being directly proportional to incident energy, the result can be a falsely low incident energy and corresponding PPE category.

How are you folks addressing this? Does the available software allow this factor to be considered?

I am doing the calculations long-hand. So, I am using bolted fault current that includes motor contribution to calculate the arcing fault current and non-adjusted incident energy. However, I then remove motor contribution and recalculate the arcing fault current and use this value to determine the protective device clearing time that is used to calculate the adjusted incident energy in cal/cm2. I believe this provides me a safe, conservative value for the incident energy and risk category. It should result in a PPE number that is safe if all or none of the motors operating.

Thoughts?? (please excuse the length)

Thanks...

You are correct, motors not running can cause incident energies to go up. When doing my studies, I create several scenarios. Max fault current from utility, 110% of fault current, 70% of fault current, 40% of fault current. I run each of these with motors on and with motors off. Then I use the worst case scenario for each device. This is not a problem when using software, but could be very tedious long-hand.

The motor contribution current does not flow through the main protective device, so it needs to be excluded when determing the clearing time of the main protective device. This is true both in the adjusted and non-adjusted analyses.

The motor contribution does contribute to arc energy, but the time can be limited to a few cycles. You can calculate arc energy in two different time periods. First with the motor contribution for a few cycles (say 5). Then without the motor contribution for the time after 5 cycles until the protective device clears the fault. Then add the two energies from the two time periods.

Motors

Software would help a great deal on motor contribution. Easypower software makes this very easy to calculate. It usually takes a large motor >100HP or a group of motors to contribute significant amounts of SSC. As others have mentioned the decay of maybe 5 cycles the SSC contribution is almost null. Really depends on some specific motor factors and regent characteristics.