Arc Flash -2 Second escape time
Arc Flash -2 Second escape time
(OP)
I ran a study using SKM software,results show Cat2 for data given at 80% of the maximum fault level as a worst case senario.( Incident energy was calculated 6.2 cal/cm2)
The tcc curve of the fuse does not clear the cat#2 PPE curve after 1 second,however the same fuse clears cat#3 curve for 2 second escape time, so the adequate PPE will be cat#3 if the 2 second escape time is considered.
if a different approach without considering the 2 second escape time is taken then at 80% of the fault level cat#2 is adequate.
Does anyone considers the 2 second escape time when assignining the PPE clothing and arc flash study?
Is there any refernce in IEEE or anywhere else to this matter?
Thank you
The tcc curve of the fuse does not clear the cat#2 PPE curve after 1 second,however the same fuse clears cat#3 curve for 2 second escape time, so the adequate PPE will be cat#3 if the 2 second escape time is considered.
if a different approach without considering the 2 second escape time is taken then at 80% of the fault level cat#2 is adequate.
Does anyone considers the 2 second escape time when assignining the PPE clothing and arc flash study?
Is there any refernce in IEEE or anywhere else to this matter?
Thank you






RE: Arc Flash -2 Second escape time
See IEEE Std 1584 B.1.2 for discussion of the 2 seconds.
RE: Arc Flash -2 Second escape time
Shouldn't the PPE be protected for any range of fault currents?
RE: Arc Flash -2 Second escape time
It is common to use a maximum arc-duration of 2 seconds when calculating incident energy.
The IEEE 1584 equations take reduced fault current into account. Below a certain current level, the arc cannot be sustained anyway.
Per IEEE 1584, for faults on systems below 1000 V, a second calculation at 85% of the calculated arcing current is done to account for the variable nature of low voltage arcing. This does not apply above 1000 V. You do not have consider lower currents, at least per IEEE 1584.
David Castor
www.cvoes.com
RE: Arc Flash -2 Second escape time
However,just to clarify, how can you address the fact that the Cat#2 PPE is not protected by the fuse for currents around 800 Amps( Current axis multiplied by 10)?
It's reasonable to assume that the fault current for any reason might be that low, and if that's the case the PPE cal rating will not provide adequate protection.
RE: Arc Flash -2 Second escape time
This is not a reasonable assumption, at least based on IEEE 1584. As the voltage increases there is less and less difference between bolted fault current and arcing fault current because the arc resistance has a diminishing impact on the impedance at point of the fault. Above 15 kV, there is no difference between bolted fault current and arcing fault current using the IEEE 1584 standard.
Is this metal-clad gear or overhead equipment?
David Castor
www.cvoes.com
RE: Arc Flash -2 Second escape time