Correct me if I'm wrong.

Say you have a long interrupting time on a feeder to switchgear that creates a 30 cal/cm² incident energy. Maybe the source breaker is on the primary of a transformer. If you open the source breaker, you still need Category 4 PPE to test the voltage at the switchgear to confirm that the breaker opened and that the switchgear is de-energized.

If I add a maintenance switch at the source breaker so that an instantaneous relay trips for a fault in the switchgear and lowers the incident energy to 3 cal/cm², then I can work on the switchgear energized with Category 1 PPE. I don't have to test every time to see if the maintenance switch will work properly.

Seems like I'd be safer opening the breaker than closing a maintenance switch.

You still need to justify working on the system energized, regardless of the arc flash hazard. But lets assume you are talking about voltage measurements or troubleshooting.

By source breaker are you talking about the main breaker in the switchgear? Or are you talking about a switch or breaker on the primary side of the transformer? Usually the primary switch or breaker will have a lower arc flash hazard, the main breaker in the switchgear is where you usually have your high Ei's and can run ito problems.

The protective devices are required to be tested per NFPA 70B or NETA as mentioned in the 70E so you can assume that it will work properly.

I guess I am not sure what your question is and am not 100% clear on how your system is set up, more etails please.

I'm talking mainly about the source breaker being on the primary side of a transformer and the switchgear of concern being on the secondary side.

For instance, at one plant that we are studying, there is a 4 kV breaker feeding a 480 volt MCC through a transformer. The incident energy calculated at the main 480 volt breaker is 64 cal/cm² because of the long relay trip time on the 4 kV breaker. Without mitigation, even opening the 4 kV breaker will not allow working on the 480 volt MCC main breaker section because you cannot test to verify that it is de-energized.

The 4 kV breaker relay is a microprocessor type with the capacity to have multiple setting groups. I'm considering adding a switch or programming a pushbutton for maintenance to lower the instantaneous setting to see a fault on the 480 volt MCC. This could reduce the arc hazard level to Category 1. Now you can work on the main section without de-energizing it, if energized work is justified. Or, if you open the 4 kV breaker, you can test that the MCC is de-energized.

It just doesn't seem right that you can work energized with the maintenance switch when you couldn't work de-energized without it. I just want to make sure that I'm interpreting things correctly before recommending adding the maintenance switch as a mitigating measure.

Ok, I see clearly now. Seems you are also. Your best bet for mitigation is to add a CT (Or light sensing) on the 480V side of the transformer with inputs to the 4kV breaker, that will give you the fastest trip and best mitigation. The maintenence switch (E.G. Quick Trip) would be used on the 480V Main breaker to reduce the Ei's on the switchgear (Minus between the transformer secondary and up to and including the main breaker). If you dont have a 480V main breaker it gets harder.I have a great IEEE white paper on this but it is too big to attach here, PM me with your email address if you want and I will send it.

Actually, the only place where we have a problem is on the 480V main section where clearing is from the transformer primary. A 480V bus fault is cleared by the short-time delay portion of the main breaker trip unit. We may have a problem when we check operation with backup generation instead of with the utility source.

Yes but ST isnt that short, a Quick Trip system would reduce the Ei on this sub dramatically.

I see the paradox. Live work assumes that the breaker trips instantaneously within it's specified clearing time when needed, and thereby snuffs the arc.

De-energized work assumes (initially until proven otherwise) that the breaker has already failed to open, or that an overlooked path is present.

Of course the second path could just as easily exist during the live work, and never be identified.

The safest course overall, is the de-energized route.

You state that the 64 cal/cm^2 IE calculated is due to the long clearing time. Long clearing times mean blast pressure is reduced. You can suit up for the 64 cal/cm^2.

Is there an approved way of calculating when the blast pressure is too high to use a higher incident energy PPE? Say a limit on the incident heat flux (cal/cm2/s)?

No but the NFPA, IEEE, NETA, and some other groups are working on developing those calculations as we speak. i think there a few guys on this forum that are involved in that project, maybe they can update us all.

As everyone figured out, the duration has a big impact on whether the incident energy is a blast or not. The committee / testing is addressing this and I'll know more next week after the next IEEE 1584 meeting. I am hoping we go the route of looking at energy per time in addition to just total energy like we do now because that way I believe we could more closely correlate it to whether it is a blast or not and what to do about it.

The present calculation methods are the way they are because this originally all started with burn injury which is total exposure but we see the minor (?) problem when we translate this to device clearing time.

Energy per time could also be used for determining hearing protection. The 70E hearing protection for HRC 0 is overly conservative because of the few HRC 0 exposures that are short enough to be loud.