With a moon-suit I can do anything right?

[iwire]

Which one of these statements is accurate?

1) With an arc flash suit I can do any kind of hot work.

2) If the hot work is allowed I must wear the correct type of safety gear.


Just a bit bored tonight so I figured this could be a good topic.

 

[gadfly56]

I don't think commercial suits are available with a rating higher than 40 cal/cm2. Even if they were, I'm sure there's equipment out there that can generate more than that. Not to mention the simple concussive force of the blast causing blunt force trauma.

 

[mkgrady]

Number two is correct

 

[jim dungar]

I don't think commercial suits are available with a rating higher than 40 cal/cm2. Even if they were, I'm sure there's equipment out there that can generate more than that. Not to mention the simple concussive force of the blast causing blunt force trauma.

Concussive force of a blast is not directly related to arc flash incident energy.
Doors can be blown open with <4 cal/cm^2 events (e.g. maybe something like when total clearing time is <.1 sec and there is 30kA available), and doors may easily stay shut with incident energy well past 40 cal/cm^2 (e.g. 2kA fault that burns for 15 minutes).

 

[ActionDave]

Which one of these statements is accurate?

1) With an arc flash suit I can do any kind of hot work.

2) If the hot work is allowed I must wear the correct type of safety gear.


Just a bit bored tonight so I figured this could be a good topic.

If I have any say about it that is what my daughter will be wearing for her prom dress.

 

[gadfly56]

Concussive force of a blast is not directly related to arc flash incident energy.
Doors can be blown open with <4 cal/cm^2 events (e.g. maybe something like when total clearing time is <.1 sec and there is 30kA available), and doors may easily stay shut with incident energy well past 40 cal/cm^2 (e.g. 2kA fault that burns for 15 minutes).

Is the energy rating based on the total energy transferred, or is it based on a per unit time?

 

[jim dungar]

Is the energy rating based on the total energy transferred, or is it based on a per unit time?

Total energy transferred.
This is why low level faults (below the protective device's instantaneous operating point) usually result in relatively high incident energy.

 

[Microwatt]

I don't think commercial suits are available with a rating higher than 40 cal/cm2. Even if they were, I'm sure there's equipment out there that can generate more than that. Not to mention the simple concussive force of the blast causing blunt force trauma.

Here is me in a 100 cal/cm2 suit so they do exist.

 

[GoldDigger]

But although OSHA allows you to wear them, you still cannot work beyond 40, yes?

 

[jim dungar]

But although OSHA allows you to wear them, you still cannot work beyond 40, yes?

Why not?

40 cal/cm^2 was the cut off of the NFPA 70E task tables, therefore it does not provide a listing of appropriate PPE.
I don't remember ever seeing any 'official' statement prohibiting work above this point, even though it has become an industry norm.

 

[rbalex]

Why not?

40 cal/cm^2 was the cut off of the NFPA 70E task tables, therefore it does not provide a listing of appropriate PPE.
I don't remember ever seeing any 'official' statement prohibiting work above this point, even though it has become an industry norm.

I was on the (early 2003 which became 2004) 70E TC when it was considering 100 cal/cm² applications. I still have copies of the PPE subcommittee's proposed tables. (See the attachment as one example).

I wish I could give credit where credit is due. I cannot remember which TC member, a suit manufacturer I believe, said essentially, "The only difference a 100 cal/cm² suit will make is whether you can have an 'open' or 'closed' casket funeral." That halted all further discussion.

Arc flash affects are indeed cumulative over time; however once you exceed the 40 cal/cm² limit, blast still becomes the dominant effect. NFPA 70E essentially recognizes three "electrical" hazards: shock, burns (arc-flash and thermal) and blast. See the definition of Electrical Hazard in Article 100. There are other discussions throughout 70E but there's a particularly telling discussion in Annex K, especially in Section K.4. Essentially, there's no effective PPE protection from blast.

 

[jim dungar]

...however once you exceed the 40 cal/cm² limit, blast still becomes the dominant effect....

To my knowledge this is simply anecdotal, without specifying additional parameters.
The blast event is over and done with in relatively just a few cycles, so how does that relate to incident energy calculated over a 2sec protective device opening time?

Based on the many studies my team has performed, it seems most service entrance locations have high incident energy simply due to the inability of a utility protective device to interrupt the low level arcing fault that is occurring.

 

[rbalex]

To my knowledge this is simply anecdotal, without specifying additional parameters.
The blast event is over and done with in relatively just a few cycles, so how does that relate to incident energy calculated over a 2sec protective device opening time?

Based on the many studies my team has performed, it seems most service entrance locations have high incident energy simply due to the inability of a utility protective device to interrupt the low level arcing fault that is occurring.

This may all very well be true. I was never directly involved with the 70E PPE Subcommittee's development of the Tables. (I was on the multiple employer's subcommittee) I do know the quote I cited literally stopped all debate in the general TC meeting. I also note no one has attempted to reintroduce the acceptance of 40 cal/cm² + activities in the Tables in the last 10 years AND blast is still considered an electrical hazard with no recognized PPE.

 

[jim dungar]

This may all very well be true. I was never directly involved with the 70E PPE Subcommittee's development of the Tables. (I was on the multiple employer's subcommittee) I do know the quote I cited literally stopped all debate in the general TC meeting. I also note no one has attempted to reintroduce the acceptance of 40 cal/cm² + activities in the Tables in the last 10 years AND blast is still considered an electrical hazard with no recognized PPE.

Absolutely blast is an issue. I regularly try to point out that blast may also be an issue at a relatively low energy level.

There is no doubt that 100 cal/cm^2 delivered in 2 cycles is much more of an problem than 100 cal/cm^2 delivered over 2 hours, but NFPA70E PPE selection treats them the same way.
The HRC tables are based on the protective device operating within a specific short time frame (such as .033 sec) or else they are invalid, while calculated incident energy levels have no such restriction (although 2 sec seems to be a typical cutoff). This is partly why calculation results should not be backwards related to HRC.

 

[gadfly56]

Here is me in a 100 cal/cm2 suit so they do exist.

Learn something new every day. Now I'm a little less ignorant than I was yesterday. Not counting anything I may have forgotten in the last 24 hours, of course.

 

[rbalex]

Absolutely blast is an issue. I regularly try to point out that blast may also be an issue at a relatively low energy level.

There is no doubt that 100 cal/cm^2 delivered in 2 cycles is much more of an problem than 100 cal/cm^2 delivered over 2 hours, but NFPA70E PPE selection treats them the same way.
The HRC tables are based on the protective device operating within a specific short time frame (such as .033 sec) or else they are invalid, while calculated incident energy levels have no such restriction (although 2 sec seems to be a typical cutoff). This is partly why calculation results should not be backwards related to HRC.

You've hit on the arc-flash "dirty secret" - a two cycle release is effectively treated the same as a 2 hour one. From a pure worker safety point of view, of course, a worker isn't likely to hang around to wait while a low grade two-hour release is in progress or even a two - minute one to accumulate 100 cal/cm². Somewhere in the documentation there is recognition that after about 2 seconds though, dissipation phenomena begins to severely distort "pure" arc-flash calculations.

Of course, blast isn't even a two-second phenomenon; it's virtually instantaneous and one of the reasons the general cutoff is considered to be 40 cal/cm² - because the 70E TC doesn't know any other way to deal with it. This is one reason more of my previous client base has considered remote racking as well as remote operation for MV switchgear.

 

[zog]

Why not?

40 cal/cm^2 was the cut off of the NFPA 70E task tables, therefore it does not provide a listing of appropriate PPE.
I don't remember ever seeing any 'official' statement prohibiting work above this point, even though it has become an industry norm.

I don't think that "cut off" went into effect until the 2004 revision

 

[gadfly56]

You've hit on the arc-flash "dirty secret" - a two cycle release is effectively treated the same as a 2 hour one. From a pure worker safety point of view, of course, a worker isn't likely to hang around to wait while a low grade two-hour release is in progress or even a two - minute one to accumulate 100 cal/cm². Somewhere in the documentation there is recognition that after about 2 seconds though, dissipation phenomena begins to severely distort "pure" arc-flash calculations.

Of course, blast isn't even a two-second phenomenon; it's virtually instantaneous and one of the reasons the general cutoff is considered to be 40 cal/cm² - because the 70E TC doesn't know any other way to deal with it. This is one reason more of my previous client base has considered remote racking as well as remote operation for MV switchgear.

OK, so very roughly, if I have a 1,000 watt heat lamp in my bathroom (239 cal/sec) and it covers a circle 5 feet in diameter (3,435 cm/sq) and I stand under it for about 10 minutes (574 sec), I should be wearing a 40 cal/cm2 arc flash suit, right?

 

[rbalex]

OK, so very roughly, if I have a 1,000 watt heat lamp in my bathroom (239 cal/sec) and it covers a circle 5 feet in diameter (3,435 cm/sq) and I stand under it for about 10 minutes (574 sec), I should be wearing a 40 cal/cm2 arc flash suit, right?

Don't know - what are your employer's safe work practices (roughly).

 

[liquidtite]

When do guys were the moonsuits I've never seen anyone work with them.