Arc Flash -2 Second escape time

[tctctraining]

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

 

[jghrist]

What is the fault current? If the fuse clears in less than 2 seconds, then the 2 second "escape time" does not come into play. The 2 seconds is only used as a limit if the protective device takes longer than 2 seconds to clear. In your case, it seems that the fuse clears in less than 2 seconds for any fault over about 500A. Is your fault current that low?

See IEEE Std 1584 B.1.2 for discussion of the 2 seconds.

 

[tctctraining]

Fault current is around 8500 Amps. The argumenmt is what if the fault current at that location reaches that low?
Shouldn't the PPE be protected for any range of fault currents?

 

[dpc]

An Annex in IEEE 1584 suggests using a maximum fault duration for long duration faults and further suggest 2 seconds. This is not actually part of the IEEE 1584 Standard and is not mentioned in NFPA 70E as far as I can recall.

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

http://www.cvoes.com

 

[tctctraining]

The operating voltage is 28kV, so based on the last post only 100% of the fault current needs to be considered.
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.

 

[dpc]

It's reasonable to assume that the fault current for any reason might be that low,

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

http://www.cvoes.com

 

[jghrist]

Shouldn't the PPE be protected for any range of fault currents?

Any range of possible fault currents for arcing through air. This is the situation for which arc hazard incident energy is calculated. You might have a high impedance fault if an overhead conductor falls on the ground, but in this situation, you have other things to worry about than getting burned from the heat of the arc.