How do you establish an electrically safe work condition when the incident arc energy exceeds 40 cal/cm²? This sometimes occurs on the line side of 480 V main breakers fed by utility transformers that have no secondary protection.

You could have the utility deenergize the transformer primary. There may or may not be a visible open disconnect when this is done. As I read 70E Article 120, you need to use a voltage detector to test each phase to verify that it is deenergized before working on the main breaker. Voltage testing is listed in Table 130.7(C)(9) as a task, so this would indicate that it is considered "interacting" with the equipment and would require PPE. How can the voltage testing be done?

We've considered requiring voltmeters permanently connected to the line side of the main breaker, but Article 120 requires testing the voltage detector before and after each test to verify that it is operating satisfactorily. Before the test is OK - if the meter show voltage before the utility deenergizes the primary, the meter works. Then the voltmeter showing zero after deergizing is the test. How do you test a permanently connected voltmeter after the test?

Would having the utility use a voltage detector on the transformer primary be satisfactory?

Is manually tripping the main breaker considered "interacting" with it, thus requiring PPE for the line side arc incident energy? If so, how is this done when the incident energy is >40 cal/cm²?

Reducing the Ei is the best way, or recalculate your Ei for the working distance the person will be at when doing the voltage testing.




Yes, this is "interacting" with the equipment, remote switching is a popular solution for this.

Your high IE comes from the long duration clearing, not from an explosive release. You can suit up to the IE level even though it exceeds 40.

This is a VERY common issue. The Danger on the label is decided on by the Arc Flash Study Engineer or the software not IEEE 1584 or NFPA 70E. They can all be WARNING labels if you so choose. I support Steve's contention above but remember verfication is the least risk method. You have to verify the absence of voltage so do it with a suit and with gusto and think of the folks who don't get to de-energize in the first place. They are the ones I'm concerned about.

Well said!

The "Danger" is on the label not because an engineer or software has determined it to be totally unsafe to perform any live work, it is there because the danger is not from the arc flash, but from the arc blast. There are 100 cal suits available, all that means is that you will be buried with a good looking corpse.

Dangerous Category

Danger on the label IS decided by the engineer doing the study or by the software.

100 cal arc flash suits have value IF you have long clearing times and low fault current or open air arc exposures where exploding parts, doors etc aren't an issue.

I have seen suits take much more than that in a few instances and workers walk away with no injuries. Rarely does a worker receive the maximum calculated energy so flash suits >40 cal are life savers.

The issue here is, IF I have deenergized, how do I do voltage testing. I THINK this circuit is dead BUT I have to test it and possibly ground it. Should be afraid because it says "DANGER"?

My contention is that you can safely test.

Thank you Hugh!
I have been trying to tell people for years that just because it comes up over 40 cal/cm2 in the calculations does not mean it cannot be safely worked on. There is NO requirement to stop all work at 40 cal/cm in the NFPA 70E. They just don't talk about anything higher than that.

Each situation needs to be assessed, and that includes times when the calculated energy is < 40 cal/cm.