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 Post subject: Refer to Table 130.7(C)(15)(A)(a) + Incident Energy Method?Posted: Fri Aug 10, 2018 11:38 am

Joined: Thu Jul 06, 2017 9:19 am
Posts: 12
I have a situation where a 12.47kV / 600V transformer's secondary side has a long duration low arcing current. Very common occurrence on secondaries that results in high incident energies.

My question is, in my arc-flash study where I'm using the IEEE method to calculate incident energies am I okay to:

1. Provide all the incident energies as they are (and they are high), ignore the 2 second rule, and recommend a blanket rule that all work on switchgear+breaker on transformer secondary should be done in a de-energized state.

2. If the answer to above is yes, do I need to provide Arc Flash labels still despite the blanket rule?

and

2. When it comes to closing the secondary side breaker, obviously, the line side has to be energized. Can I then refer to Table 130.7(C)(15)(A)(a) and state that PPE is not required for simply closing a 600V breaker if its properly maintained and cover doors are closed, etc.?

I ask because I know mixing of methods is not allowed.....

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 Post subject: Re: Refer to Table 130.7(C)(15)(A)(a) + Incident Energy MethPosted: Fri Aug 10, 2018 1:43 pm
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2173
Location: North Carolina
mbk.2k3 wrote:
I have a situation where a 12.47kV / 600V transformer's secondary side has a long duration low arcing current. Very common occurrence on secondaries that results in high incident energies.

My question is, in my arc-flash study where I'm using the IEEE method to calculate incident energies am I okay to:

1. Provide all the incident energies as they are (and they are high), ignore the 2 second rule, and recommend a blanket rule that all work on switchgear+breaker on transformer secondary should be done in a de-energized state.

The two second rule is based on a sort of "get away" factor. If your breaker/fuse operation time on the high side exceeds 2 seconds, you cut it off at 2 seconds. This rule was somewhat arbitrary but has stood the test of time unless there is some reason that you believe that escape is not possible such as in a manhole.

The issue with your approach is...how are you going to test for absence of voltage? Is it going to be acceptable to test on the high side to check for low side for instance? Is everything within visible range? No chance of back feeding? Layouts are such that nobody can get confused? Yours truly once found myself in a 2400 VAC energized transformer large enough to take me out because at some point the plant swapped transformers and rerouted conduit to do it where they criss-crossed at the ceiling, then wired the part of the plant that was dead during emergency operations back up to the second switch so that now everything was swapped. HOWEVER they didn't move the labels so I powered down and checked for power on both high and low side disconnects and everything was "dead". Then commenced to opening the transformer and when I had my hands around a live tap that had corroded so badly it would not stay on anymore is when I realized something had gone terribly wrong as far as labels go. Testing at the dry transformer one more time after testing at the high and low side disconnects would have prevented this near miss from occurring. So I believe that you still have incidents where some energized work might be necessary so think it through VERY carefully but the idea is sound.

You have to address what to do in both NORMAL and ABNORMAL conditions. By way of example let's say that I'm just doing a routine LOTO and there is no reason to suspect the equipment is going to malfunction. This is very different from say right after the breaker or fuse tripped on the high side and you come up to the transformer and there's smoke rolling out of the termination enclosures. That is pretty much the definition of an ABNORMAL condition, one in which an arcing fault could be very likely. Similarly as I described above the rule is to test directly at the work site. Sometimes that is not practical particularly because of very high incident energy but then we have to do the work somehow...just putting on PPE is NOT always the best solution. In fact according to the hierarchy of controls that should be the last option.

Quote:
2. If the answer to above is yes, do I need to provide Arc Flash labels still despite the blanket rule?

This comes from the NEC rule. Are you going to be doing work "frequently" in that area? As in say once a year or more often? NEC 110 is pretty clear on this...they have a suggested list of locations where labels are needed. You might need more or less depending on frequency of maintenance. I can name plenty of disconnect switches that might not get even so much as used that often, for better or worse (mostly worse) even though the examples given in NEC include disconnects. A lot of plant main incoming gear often fails to meet this criteria.

Quote:
2. When it comes to closing the secondary side breaker, obviously, the line side has to be energized. Can I then refer to Table 130.7(C)(15)(A)(a) and state that PPE is not required for simply closing a 600V breaker if its properly maintained and cover doors are closed, etc.?

I ask because I know mixing of methods is not allowed.....

[/quote]

It is stated also in an informational note about "normal operation". Most sites simply label everything where it needs to be labelled and then provide a set of work rules such as normal operation of equipment, or applying maintenance switches to reduce the hazard. Thus the sticker is worst case and it directs workers to look for ways around the limitations for performing the task. So they get different instructions if they take extra steps or depending on the situation where for instance the sticker should apply if the breaker/fuse upstream of the transformer just tripped.

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