Swgr Doors open/closed?

[blaker]

thread279-155022
The above thread provides a good preface to my following comments:
Our arc hazard analysis is complete and we have the labels affixed to the equipment indicating the Hazard Risk Category and appropriate PPE etc. Realizing that NFPA-70A (soon to be a CSA document)and IEEE-1584 are not perfect documents and deal more with the radiant heat from an arcing fault and not the arc blast components, I am attempting to push the idea to the field maintenance personnel that the PPE dictated by the HRC on the label should be worn weather the door is open or closed for tasks performed on gear without arc resistant doors. I have heard numerous comments at conferences reminding us that non arc resistant doors are not designed to withstand an internal arc blast and could greatly contribute to the effects of the flash/blast. This idea is especially being contested when opening/closing 480V branch circuit breakers, using the outside handle with door closed. I'd appreciate your thoughts and comments on this.

 

[dpc]

NFPA 70E is inconsistent on this issue, in my view. Even though there are no exposed live parts, the Table 130.7(C)(9)(a) shows a Hazard/Risk category of #0 for this operation, which does imply some level of risk.

High energy level arcing faults in an MCC are certainly capable of blowing off a closed and latched door. I have worked with several companies who required PPE just to enter an electrical room. Others required PPE to operate 480 V breakers in MCC and switchgear, but not panelboards.

Anywhere the arcing fault would be downstream of a CLF or molded-case breaker, the energy will **usually** be quite low.

If it were my plant, I'd require PPE for operation of any large 480 V circuit breakers in switchgear or switchboards and MCCs.

But I'd also have anyone doing switching wearing 8 cal/cm2 shirt and pants at all times regardless.

 

[blaker]

Thanks for your comments. The thing that irks me about the NFPA-70E "0" rating for operating these 480V branch circuit breakers is that the bucket, housing the branch circuit bkr, contains two levels of HRC and must be labeled as the higher of the two. The line side, that for us can be as high as HRC 3 and the load side, which is typically negligible. So if it's the HRC 3 line side that decides to fault while someone is operating the breaker (with door closed), they'd better be wearing the appropriate PPE. We also have to remember that the PPE is designed to protect against receiving a curable 2nd degree burn and offers little if any protection against the blast elements (which could include the door) of the arcing fault.

 

[dpc]

The logic behind the NFPA 70E task-based table is questionable. While there is probably a lower of risk of a fault occurring while operating a breaker as opposed to other maintenance activities, if a fault does occur, the energy released is likely to be the same, since the fault won't "know" what you were doing to initiate it.

But the concept of the task-based Hazard-Risk Category is popular and I expect to see it more widely incorporated in the next NFPA 70E update, tied with the use of Energized Work Permits.

 

[rovineye]

My take has been to apply PPE and boundaries for breaker operations when commissioning the system/circuit, testing, troubleshooting, reclosing after trips etc. I treat it the same as if the doors were open. Not during routine operations (automated breaker operations). Since most of my switchgear is inside an operating station it would not be possible to enforce Limited Approach Boundaries let alone the PPE requirements at my high energy levels.

I struggle like everyone else seems to. Some of the PPE in 70E lists the purpose as protection from flying debris, which would be the arc blast not just flash. There is very little discussion in 70E about arc blast, possibly because it is so difficult to protect from, as opposed to arc flash which is easily quantified.

 

[dpc]

The previous version of 70E had a Hazard/Risk Category #5 that extended up to 100 cal/cm2, because PPE is available that can withstand that much heat. Cat #5 was removed in the current version because of concerns about the blast pressure problem.

 

[Zogzog]

The HRC 5 was proposed but never made it into the actual standard, your reasoning is correct. The human body cannot survive the pressure from an arc >40 cal/cm2 (Or so they believe)

I have dozens of photos from emergency repairs we have done after an arc flash, I have never seen the doors on 480V MCC's and switchgear hold, you can almost count on the door flying open or at least bowing enough for the arc to "Squirt" out. Plus sometimes the swgr is missing retention hardware or have vent openings. To simplify ESWP's I would recommend the same PPE to be worn regardless of the doors being open or closed.

There are over 200 ROP's for the tables for the next 70E cycle, thats just on the tables folks.

 

[dpc]

What I continually have to stress to plant engineers and safety departments is that this is not an exact science and the NFPA-70E and IEEE 1584 folks are basically making this up as we go along. People see energy calcs to two decimal places and they assume the numbers are accurate. Some the test data used (and extrapolated) for IEEE 1584 equations was done under very unrealistic conditions, so as more testing is done, I'd expect a lot of changes in the calculation methods.

I agree that counting on a door for protection is risky business. I'd rather have it closed, but with full knowledge that it could come flying off.

 

[alehman]

So this begs the question - Should it be required to wear PPE to enter the arc flash boundary if there is no intention of "interacting" with the equipment?

An example would be a motor control center. Here you have motor starters that may operate automatically and sometimes frequently. Who's to say a starter won't fail and blow the door off of a bucket while someone is standing nearby? Does the guy servicing the adjacent air handler need PPE?

I could envision a situation with where a 480V service panel for a small office building is located in a break room or maintenance shop that might normally be occupied. Should such areas be restricted? Should the codes be revised to prohibit such installations?

We're getting in to a very gray area trying to quantify risk. The question is how much belt-and-suspenders is enough? In many cases, more safety precautions are better - but at what burden? Tough to say. It might be safer if everyone on a commercial airliner wore a parachute, but that doesn't seem very practical. I don't envy the 70E committee.

 

[Zogzog]

"It might be safer if everyone on a commercial airliner wore a parachute"

Thats great, mind if I use that quote?

 

[rovineye]

"Should it be required to wear PPE to enter the arc flash boundary if there is no intention of "interacting" with the equipment?"

My 480 v switchgear (shipboard) is located in the operating station. Nice, clean, quiet, air conditioned. I can imagine what would happen if I required the crew to wear a Class 4 multi layer flash suit while in there for their coffee break or standing watch. Operating the HMI trackball or keypads with those gloves on would be a sight to see. An arc blast would be nothing compared the ass beating I would get first time I came aboard!

I second the opinion on doors. I have seen them vaporized along with the top drip shield.

 

[dpc]

Existing installations are one matter, new designs are another. We need to provide electrical equipment layouts where 480 V and higher gear is NOT located in areas normally occupied by operators, visitors, etc.

I don't think anyone is saying that the current version of NFPA 70E requires PPE just to be in the vicinity of electrical equipment (if no work is being done). Some companies have adopted this approach when dealing with dedicated electrical rooms.

 

[emailtheG]

Zogzog, rovineye, and any others ...

Could you please elaborate on the nature of the MCC failures you have seen? Specifically, did these failures occur while an operator was switching a breaker on/off or performing maintenance or when the contactor pulled in remotely or some other condition altogether? What scenario is most likely to cause a failure in this type of equipment (in your experience)?

 

[dpc]

Most faults occur when something is moving. Either the breaker is being operated, the contactor is being operated, the door is being opened, or the electrician's hands are moving inside the bucket.

Few fault occur when a starter is sitting there - either off or on.

I know of one case where a large 480 V breaker blew up when it was being manually closed with the door closed. Fortunately, the operator was wearing a 100 cal/cm2 flash suit and was not injured. Estimate energy level was about 40 cal/cm2.

Anybody can have a bad day or be unlucky, but faults normally occur when something is being operated or worked on.

 

[rovineye]

My equipment is shipboard, with poor access and limited downtime for maintenance. Vibration is my enemy. Catastrophic failures are usually from a connection that vibrates loose, the worst being bussing bolted faults if a joint comes loose.

I protect myself the most when initially energizing, manually operating and testing, by wearing upgraded PPE and standing behind my biggest electricians. But in 30 some years I have yet to actually experience a serious fault at those times.

 

[Zogzog]

I agree with DPC, soemthing has to start the fault. However, moisture, animals, conductive dust, etc. can cause an arc flash to occur at any time. Routine PM can help in reducing the chances of these types of failures.