It is currently Tue Sep 26, 2017 1:03 pm



Post new topic Reply to topic
Author Message
ekstra   ara
 Post subject: Opening / Closing Pringle Switch - Closed Cabinet
PostPosted: Wed Apr 17, 2013 8:41 am 
Offline

Joined: Wed Apr 17, 2013 8:25 am
Posts: 2
There have been number of debates going around regarding opening or closing a pringle switch when the cabinet is closed. Is this a "safe" activity or not?

I have heard many opinions regarding arc flash and working on live electrical (cabinets open); we recognize that you MUST obey to they PPE labeled on the arc flash sticker during this type of work.

The question is, if a person is opening or closing a pringle switch with the cabinet closed, is the hazard level the same as working with the cabinet open? Does NFPA require that they wear PPE according to the arc flash study to operate the switch?


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Sat Apr 20, 2013 3:53 am 
Offline
Arc Level
User avatar

Joined: Wed May 07, 2008 5:00 pm
Posts: 668
Location: Rutland, VT
Yes, you need to wear the PPE that was determined through a study or the tables. Is this related to your other question in this forum? Seems like the same subject.

_________________
Barry Donovan, P.E.
www.workplacesafetysolutions.com


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Sun Apr 21, 2013 5:04 am 
Offline
Plasma Level
User avatar

Joined: Tue Oct 26, 2010 9:08 am
Posts: 1762
Location: North Carolina
The incident energy will be lower with the doors closed. Two caveats to this. First, there is no way to calculate how much lower. Second, there is a good possibility that the door will fly open or off. No one takes credit for decreases to incident energy because the door is closed unless it is an arc resistant switchgear design.

The likelihood of an arc flash is much less. The tables in 70E reflect a reduction in PPE right now but should reflect a yes/no decision with regards to arc flash PPE. That is what will be in the 2015 edition.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Apr 29, 2013 6:19 am 
Offline
Sparks Level

Joined: Fri May 01, 2009 9:10 am
Posts: 73
To add to the discussion... let's say the study estimated the IE to be 125cal/cm^2 at 18"... what would the appropriate arc rated PPE consist of? Is anybody else also calculating the IE at 24" or 36"?


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Apr 29, 2013 7:34 am 
Offline
Plasma Level
User avatar

Joined: Tue Oct 26, 2010 9:08 am
Posts: 1762
Location: North Carolina
viper57 wrote:
To add to the discussion... let's say the study estimated the IE to be 125cal/cm^2 at 18"... what would the appropriate arc rated PPE consist of?


There are manufacturers who are marketing 100 cal/cm^2. Haven't seen anything higher yet. This stuff existed before 70E made it popular. It is useful for guys working very near induction furnace/casting systems where the thermal energy is ridiculously high. They wear thermal coolers under the clothing. This real world example was used at Griffin Wheel (at one time had a virtual monopoly on manufacturing railroad wheels for U.S. use).

Quote:
Is anybody else also calculating the IE at 24" or 36"?


Yes. The IEEE 1584 table specifies lots of equipment that has increased distances by design. Another example used here is that we use telescoping hot sticks to change fuses and operate hook stick operated switches from the ground. Minimum distance is 10 feet from the switch so I run the calculation for this task. In a bucket truck the hot sticks are still 8 feet long so again, run the numbers. Closer in and we're into rubber glove work but also it's resistance grounded so assuming that phase-to-phase faults are nonexistant, I use the lower current that the resistance grounding provides. The tables in IEEE 1584 are basically useless once you get outside of enclosed gear, and the formula itself is only specified to be valid up to 15 kV. Good enough for MOST but not all uses. Another example would be operating equipment remotely. In this case you would frequently just use the arc flash hazard boundary but for relatively high current/low voltage equipment, the arc flash hazard boundary gets to be a silly number which means that instead, you have to do the calculation and dress appropriately or work out the likelihoods.

There is nothing magical about 18" except that for MOST 480 V equipment it is approximately the distance between the bus bars at the back of the enclosure and the position of a worker's body and face for most working positions. I think I read somewhere from the folks that originally came up with the table that the distances from the back of enclosures for bus bars were surprisingly very uniform from one manufacturer to another. Similar numbers work out for other types of equipment. The only magic here is that if you know that you are working with an atypical situation or equipment, then it is best to work out the distance for the specific situation. It is not a good idea however to vary from the standard unless you have clearly documented the information and reasons for doing so.

Some sites use 18" as a magic "worst case" and apply it uniformly to everything. Once again this leads to being overly conservative and calculating ridiculously high incident energies in cases where the real world is not like that. It would be unrealistic to directly measure working distances for EVERY task and EVERY piece of equipment depending on the size of the operation. We have around 1500 buses in our model and make a lot of generalizations to keep the model from blowing up into 10,000 buses. These assumptions led to ludicrous values when one company (Square D) did this. For instance one bus had a arc flash hazard boundary of around 4,000 feet, enough to encompass the entire plant site. Granted the "correct" value is still very high but if you start putting "silly" values on labels, nobody is going to put any credibility in the labels, knowing full well that even the "worst" cases that have happened were nothing like calculated values that high.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Apr 29, 2013 7:49 am 
Offline
Sparks Level

Joined: Thu Oct 18, 2007 7:10 pm
Posts: 248
Location: NW USA
To add complexity to this discussion, there seems to be a weakly defined difference between Risk and Hazard in arc flash discussions:

Hazard might be defined as the calculated potential energy,
Risk might include the probability of an incident occurring.

This becomes a real discussion when equipment with solid dielectric insulation, for which an arc is impossible to occur, has a high calculated potential energy, such as ABB INSUM motor controllers. I've been told they are withdraw-able, but cannot sustain an arc. Similar conclusion might be possible if door construction is special.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Apr 29, 2013 11:40 am 
Offline

Joined: Tue Sep 28, 2010 1:51 pm
Posts: 2
So say the Pringle or similar switch is a service disconnect without means for remote operation. There is a good probability the hazard level anywhere near the switch will exceed the rating of most PPE. Does the disconnect effectively become useless?


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Apr 30, 2013 8:06 am 
Offline
Plasma Level

Joined: Mon Jun 02, 2008 11:58 am
Posts: 1103
Location: Charlotte, NC
alehman wrote:
So say the Pringle or similar switch is a service disconnect without means for remote operation. There is a good probability the hazard level anywhere near the switch will exceed the rating of most PPE. Does the disconnect effectively become useless?


Well it still serves it's main purpose as a protective device but you should either mitigate the hazard level or use a remote operator.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Apr 30, 2013 6:34 pm 
Offline
Plasma Level
User avatar

Joined: Tue Oct 26, 2010 9:08 am
Posts: 1762
Location: North Carolina
Zog wrote:
Well it still serves it's main purpose as a protective device but you should either mitigate the hazard level or use a remote operator.


The likelihood of arcing failure with load break switches is much lower than a breaker if properly maintained.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu May 02, 2013 12:53 pm 
Offline
Plasma Level

Joined: Mon Jun 02, 2008 11:58 am
Posts: 1103
Location: Charlotte, NC
Thats a big "if" ;)


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri May 03, 2013 9:21 am 
Offline
Plasma Level
User avatar

Joined: Tue Oct 26, 2010 9:08 am
Posts: 1762
Location: North Carolina
Maintaining an LBS is basically doing mechanical lubrication (if ever needed, usually not) and checking the stabs for wear and pitting, and a basic check on the insulators. You may also have to check on arc chutes if it has some, and check on the puffer if it has one of those. On older overhead non-unitized switches, you really have to check the linkages carefully, especially with the ones that operate in more than one plane of motion such as the infamous Southcon's with the "roll to vertical" type movement which has to be maintained religiously every 3-5 years if it is ever going to work.

On a breaker, you have to do all the mechanical work described above, plus do a full functional test on the trip unit in whatever form it is in, plus the stabs if it is a draw out type. There are many more moving parts, sometimes as many as 60-70, and the older electromechanical or thermal mechanical trip units have horrible reliability records.

The upshot is that solidly grounded air type drawout breaker gear has an average failure rate of around 10^-4 failues/year (1 in 10,000 per year) under AVERAGE maintenance conditions as reported in IEEE Std. 493. If it's resistance grounded or ungrounded, it drops by about an order of magntiude. For molded case breakers where there is very little maintenance to be done, this drops down to 10^-6. Load break switches also have reliability rates around 10^-6 with solidly grounded gear.

For the record in the last 10 years from the data that I've been able to uncover where I work, two arc flashes were a result of attempting to remove/insert a bucket onto a live bus in a 480 V MCC. One was the result of resetting a molded case breaker multiple times without an inspection although the operator was unharmed. One was the result of a meter exploding when it was use to attempt to test the output voltage on a 2400:480 transformer that was accidentally wired up "backwards" (2400:??? much higher), but was prior to the IEC/UL 61010 standard being issued. One was the result of a pinched 4160 V cable on a mining conveyor although technically since it simply dropped hot slag down on the man below it, I'm not sure if this really counts as an "arc flash" or something else. One was the result of opening a non-LBS switch under load. Another was closing a breaker again without checking the condition, and having the arc magnetically propelled elsewhere where it caused the damage. A much older incident was the result of equipment that was known to be in poor condition and no maintenance being done. That's it with about 5,000 employees represented over 10 years.

As you can see from this list, following proper procedures and properly maintaining equipment can eliminate or drastically reduce the potential in every case.


Top
 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 11 posts ] 

All times are UTC - 7 hours


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
© 2017 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883