It is currently Fri Nov 17, 2017 11:05 pm



Post new topic Reply to topic
Author Message
ekstra   ara
 Post subject: Questions - PLC/Control Cabinets
PostPosted: Thu Sep 27, 2012 1:49 pm 
Offline

Joined: Thu Sep 27, 2012 1:18 pm
Posts: 2
I work at a Systems Integrator where we design and install "Industrial Control Panels". Typically these contain a PLC with a small amount of 120VAC circuits to power devices with the majority of the panel using 24VDC control. Occaisionally these panels incorporate 3 Phase 480VAC Variable Frequency Drives or standard starters for motor control.

These panels use a standard Rotary Style disconnect mounted on the panel with a shaft to the actuator on the door (compliant with NFPA 79).

We are getting more formalized with compliance to NFPA 70E due to one of our customers stringent safety rules. We also want to do a better job of compliance in general for our own worker's safety. I have a few questions that I am hoping some of the folks more Experienced with this Standard can answer for me.

Question #1:
If I install a plexiglass barrier over the 120V/480V circuit portion of the panel and I open the cabinet without de-energizing anything in order to do "Voltage Testing" on the 24VDC controls, do I still need to worry about Shock and Arc Flash protection?

Questions #2
If I have the panel open (no plexiglass barrier) and the panel disconnect is properly locked out (this can be done with the panel open on NFPA 79 compliant disconnects), can I work on the panel without worrying about Arc Flash/Shock Protection? I ask this because theoretically the Line side of the disconnect is still Live. I add the caveat that the disconnect does come with a cover over the Line side terminals.

Question #3
If I have the panel open (no plexiglass barrier), the panel is Live, I am doing "Voltage Testing" on the 24VDC circuits in one section of the panel and not working within the Restricted Boundary for the higher voltage (120 or 480 VAC) circuits, can I do so without using Insulated Gloves? Also, if I am outside the Arc Flash Boundary for those higher voltage conductors/components, can I not worry about Arc Flash protection as well?

Question #4
When figuring out the Prohibited Approach boundary, is this only counted from an "Exposed" (I assume they mean bare) component? What about handling an Insulated Conductor? What about a component that theoretically has "Exposed" terminals, but they are very small (i.e. IP20 "Finger Safe")?

Thanks in advance for your thoughts and opinions on this.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Sep 28, 2012 10:39 am 
Offline
Sparks Level
User avatar

Joined: Tue Apr 17, 2012 8:19 am
Posts: 240
Location: Charlotte, NC
I have been in the industrial control field for 25 years. Before that I was an Avionic Systems Specialist in the Air Force for 8 years. I think I can shed a bit of light on your questions.

Question #1:
The plexiglass barriers I have seen are primarily designed to make certain areas of the panel touch-resistant against shock. The question of Arc Flash protection depends on the incident energy, the barrier proximity to the arc flash, and the design of the barrier. At best the barrier will divert the hot gasses somewhat - or the barrier may just melt depending on how close it is to the arc flash source. Worse case is the barrier becomes a projectile. You really need to know what the incident energy is.

Questions #2
If you have live terminals exposed, yes. In the time I have been working on and around industrial equipment (probably in excess of 75,000 hours) I have seen accidents and been involved in a few. The liklihood of an arc flash incident may be low, but if you look at the data in an accident occurance pyramid, electrical incidents are 3 times more likely to be fatal than other industrial accidents. If the accident involves arc flash the fatality rate increases to 12 times that of an electrical accident not involving arc flash. In addition, an arc flash incident is 7 times more likely to cause disabling injuries. It isn't really a question of getting by or even what OSHA dictates for safe working practices. It is a question of the practical potential dangers to the worker.

Look at NFPA 70E table 130.7(C)(15)(a) - formerly table 130.7(C)(9). For the panels you are talking about, you are looking at an HRC of 1 or 2. This level of protection is relatively non-cumbersome.

Question #3
Also addressed by NFPA 70E table 130.7(C)(15)(a) - formerly table 130.7(C)(9). Insulated Gloves are required for work on energized electrical conductors, including voltage measurement. You are allowed to break the Prohibited Approach boundary with test probes.

Question #4
Approach boundaries are measured from bare live components. You should use caution handling an Insulated Conductor as there is a chance the wire can come loose from the terminal (even finger-safe terminals) - and that is never a good thing. I am unaware of anything in NFPA 70E regarding "theoretical Exposed" terminals. Personally, I would always err on the side of caution (See Question #2's discussion of dangerous)

I hope this helps


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Sep 28, 2012 11:37 am 
Offline

Joined: Thu Sep 27, 2012 1:18 pm
Posts: 2
Larry Stutts wrote:
I have been in the industrial control field for 25 years. Before that I was an Avionic Systems Specialist in the Air Force for 8 years. I think I can shed a bit of light on your questions.

Question #1:
The plexiglass barriers I have seen are primarily designed to make certain areas of the panel touch-resistant against shock. The question of Arc Flash protection depends on the incident energy, the barrier proximity to the arc flash, and the design of the barrier. At best the barrier will divert the hot gasses somewhat - or the barrier may just melt depending on how close it is to the arc flash source. Worse case is the barrier becomes a projectile. You really need to know what the incident energy is.

Questions #2
If you have live terminals exposed, yes. In the time I have been working on and around industrial equipment (probably in excess of 75,000 hours) I have seen accidents and been involved in a few. The liklihood of an arc flash incident may be low, but if you look at the data in an accident occurance pyramid, electrical incidents are 3 times more likely to be fatal than other industrial accidents. If the accident involves arc flash the fatality rate increases to 12 times that of an electrical accident not involving arc flash. In addition, an arc flash incident is 7 times more likely to cause disabling injuries. It isn't really a question of getting by or even what OSHA dictates for safe working practices. It is a question of the practical potential dangers to the worker.

Look at NFPA 70E table 130.7(C)(15)(a) - formerly table 130.7(C)(9). For the panels you are talking about, you are looking at an HRC of 1 or 2. This level of protection is relatively non-cumbersome.

Question #3
Also addressed by NFPA 70E table 130.7(C)(15)(a) - formerly table 130.7(C)(9). Insulated Gloves are required for work on energized electrical conductors, including voltage measurement. You are allowed to break the Prohibited Approach boundary with test probes.

Question #4
Approach boundaries are measured from bare live components. You should use caution handling an Insulated Conductor as there is a chance the wire can come loose from the terminal (even finger-safe terminals) - and that is never a good thing. I am unaware of anything in NFPA 70E regarding "theoretical Exposed" terminals. Personally, I would always err on the side of caution (See Question #2's discussion of dangerous)

I hope this helps


Larry,

Thank you for your insight. I was trying to use the Insulating Barrier over higher voltage circuits to avoid needing to use gloves when working on the low voltage part of the panel. However, I see your point, that in the event of an Arc Flash event, that panel could quite easily become a hazard. This has since changed my mind and I think I need to abandon that idea.

The main issue we have with PPE requirements are the insulating gloves and protective covers. We are getting a couple of pair of 00 rated gloves with goatskin covers next week, but from thier appearance, it looks like it will be difficult to use small terminal block screwdrivers and possibly plug in connectors/cables on PLCs and other devices.

Unfortunately, I don't see any easy way around this, as there is still the Arc Flash protection issue which is almost always going to require protective gloves if you have your hands close enough to the panel to use a screwdriver or are plugging in a cable.

I guess we will just need to get used to working this way to be compliant.

If anyone else has any insights into these issues or has any panel design pointers to help make the panel safer/easier to work on, I'd love to hear it.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Sep 28, 2012 6:55 pm 
Offline
Plasma Level
User avatar

Joined: Tue Oct 26, 2010 9:08 am
Posts: 1796
Location: North Carolina
Charles Casalie wrote:
These panels use a standard Rotary Style disconnect mounted on the panel with a shaft to the actuator on the door (compliant with NFPA 79).{/quote]

I hate those things. I have banned them. The linkages wear out over time and get very sloppy and even when new, it's hard to get them to line up quite often. Once they get sloppy you can no longer tell when it is open or closed, a violation of NFPA 79. Better to use a proper cable operator and traditional handles than this junk.

Second more generally the design practice these days at the major automotive plants (GM, Ford at least) is to build all PLC panels with nothing but 24 VDC components inside. A second side panel contains the power supplies, drives, etc. That way you can safely ignore shock and arc flash rules altogether while working on the PLC panel. Once you get into drives or MCC's it's no longer practical to do this anymore. I am aware of at least one vendor that suggested going to 48 VAC coils and starters but it got very troublesome very quickly and everything ends up being oddball.

Quote:
If I install a plexiglass barrier over the 120V/480V circuit portion of the panel and I open the cabinet without de-energizing anything in order to do "Voltage Testing" on the 24VDC controls, do I still need to worry about Shock and Arc Flash protection?


Shock protection becomes a nonissue. Arc flash is a different question. If you are not doing something that would cause the 480 V side to readily change states (as per the definition of an arc flash hazard), then no, there is no arc flash hazard. If there is since you've already eliminated the shock hazard then look at the arc flash hazard. If it's 2 or less you can wear all leather gloves. I suggest looking at TIG gloves. They are very light weight and dexterity stays pretty good compared to typical work gloves.

Quote:
I add the caveat that the disconnect does come with a cover over the Line side terminals.


Again see above...shock is not a problem even if it has recessed terminals as long as you cannot accidentally make contact. See the definitions of "exposed". Touch safe terminals are not insulated but they are guarded so they are not considered exposed, so the shock approach boundaries disappear altogether. BUT the arc flash boundary is still an issue if again you may cause something to change state. Since you are working on dead equipment then no, no hazard.

Quote:
If I have the panel open (no plexiglass barrier), the panel is Live, I am doing "Voltage Testing" on the 24VDC circuits in one section of the panel and not working within the Restricted Boundary for the higher voltage (120 or 480 VAC) circuits, can I do so without using Insulated Gloves?


That's the idea. You will have to restrict unqualified personnel from entering the area (limited approach boundary was crossed). Voltage testing is an acceptable reason for crossing the limited approach boundary and does not invoke an EEWP (it's an exception).

Quote:
Also, if I am outside the Arc Flash Boundary for those higher voltage conductors/components, can I not worry about Arc Flash protection as well?


No. The arc flash hazard is <1.2 cal/cm^2 but not 0.0 cal/cm^2. Do not wear meltable fabrics. Of course this is much easier to achieve than working within the arc flash hazard boundary.

Quote:
When figuring out the Prohibited Approach boundary, is this only counted from an "Exposed" (I assume they mean bare) component? What about handling an Insulated Conductor? What about a component that theoretically has "Exposed" terminals, but they are very small (i.e. IP20 "Finger Safe")?


Exposed means NOT insulated, guarded, or isolated. Bare and visible are not the only criteria. No blind reaching or poking wires into known live compartments that you can't see either obviously. Insulated wires are not exposed. The general rule is no ACCIDENTAL contact. So even a molded case breaker with recessed terminals does not count until you start taking a screw driver and poking around those holes.

Also note that 70E does a crummy job of explaining shock protection with energized equipment. IEEE 516 does a much better job. There are 4 work methods:
1. De energize it.
2. Rubber glove method. That's the only thing most people seem to catch on to.
3. Insulated tool methods. If you use a properly insulated and tested screwdriver where only a small section of the tip is exposed so that it cannot cause a phase to phase or phase to ground fault and has guards to prevent fingers from slipping and making contact, it is just as good or better than insulated gloves.
4. Live line, bare hands.

Under 300 V, the insulated tool method is usually vastly preferred and safer than rubber glove work methods due to the small size of the terminals. 00 gloves are also a waste of time. With the limitations you may as well use class 0 which isn't significantly thicker or harder to work in than 00. This lets you dispense with the leather protectors in limited cases (where there is no danger of damaging them).

Honestly, you need to read 70E much more carefully and think about what the rules really mean. Shock protection is required when there is a shock hazard present, not all the time. Same with arc flash hazards. Don't be afraid to split your cabinets up either. On a certain large servo drive system that I have to work with (10 MW DC system), the auxiliary 480 V system had breakers attached to the transformer enclosure itself making them >40 cal/cm^2. Simply upgrading the main lugs on the 480 VAC MCC to a main breaker gave us a simple, usable lock out point for the ventilation system without having to kill all the lights, cranes, HVAC, etc.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Oct 01, 2012 6:53 am 
Offline
Sparks Level

Joined: Fri Apr 15, 2011 7:43 am
Posts: 151
Location: Colorado
As a consulting engineer with over 10 years of arc flash experience, I would still consider any panel with 480V to have an arc flash hazzard based on the upstream (outside the panel). The plexiglas (as you indicated) could melt or fragment. My clients generally do not like it when I do this but it is the conservative approach. I try to educate them do design future panels with any 480V in a separate panel.
The plexiglas may reduce the shock hazzard but not the arc flash.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Oct 01, 2012 8:43 am 
Offline

Joined: Wed Aug 25, 2010 7:15 am
Posts: 24
Location: St. Paul, MN
I agree that putting the 480V equipment in a separate enclosure is the ideal and good design. Having said that, I don't think it is the only possible way of having a safe control panel that can be worked on with minimal PPE.

NFPA 70E 130.7 Information Note No. 2 says, "It is the collective experience of the Technical Committee on Electrical Safety in the Workplace that normal operation of enclosed electrical equipment, operating at 600 volts or less, that has been properly installed and maintained by qualified persons is not likely to expose the employee to an electrical hazard." (emphasis added).

The definition of enclosed is, "Surrounded by a case, housing, fence, or wall(s) that prevents persons from accidentally contacting energized parts.

If the 480V circuits in a control panel are protected by plexiglass then they are "enclosed electrical equipment", and as long as they are properly installed and maintained they are not likely to expose the employee to an electrical hazard. I understand 130.7 & Information Note 2 to mean that you don't need to use PPE for the potential arc flash hazard associated with the enclosed equipment in this situation.

In regards to question 3, if you are not within the restricted approach boundary of exposed energized conductors or circuit parts operating at 50 volts or more, you don't need to wear voltage rated gloves while working on 24VDC circuits.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Oct 02, 2012 10:32 am 
Offline
User avatar

Joined: Wed Sep 01, 2010 1:11 pm
Posts: 11
bvadams wrote:
I agree that putting the 480V equipment in a separate enclosure is the ideal and good design. Having said that, I don't think it is the only possible way of having a safe control panel that can be worked on with minimal PPE.

NFPA 70E 130.7 Information Note No. 2 says, "It is the collective experience of the Technical Committee on Electrical Safety in the Workplace that normal operation of enclosed electrical equipment, operating at 600 volts or less, that has been properly installed and maintained by qualified persons is not likely to expose the employee to an electrical hazard." (emphasis added).

The definition of enclosed is, "Surrounded by a case, housing, fence, or wall(s) that prevents persons from accidentally contacting energized parts.

If the 480V circuits in a control panel are protected by plexiglass then they are "enclosed electrical equipment", and as long as they are properly installed and maintained they are not likely to expose the employee to an electrical hazard. I understand 130.7 & Information Note 2 to mean that you don't need to use PPE for the potential arc flash hazard associated with the enclosed equipment in this situation.

In regards to question 3, if you are not within the restricted approach boundary of exposed energized conductors or circuit parts operating at 50 volts or more, you don't need to wear voltage rated gloves while working on 24VDC circuits.


That was going to be my question. If the live 480V terminals are effectively placed behind a barrier such that the shock hazard is eliminated, then how is an arc flash incident going to occur as a result of an operators presence and work on the lower voltage equipment? How could someone cause a "change of state" on the 480V lines as mentioned in an earlier reply to occur when there is a barrier preventing the precurser of an arc flash event to occur?

I'm not saying I can't imagine a scenario. For instance someone could shake/move the enclosure enough where if the lugs weren't tight they could then slip out and possibly cause an event. But now we're going down a what-if road, so where does this end?

I know the priority is safety first, and since HRC 1 and 2 aren't cumbersome I think that even with a barrier it is wise to require their use, and in my opinion it should be mandatory . But l would say the real reason to do so is simple prudence and risk management in the context of preventing human suffering. I find that an operational rationale is harder to justify.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Oct 02, 2012 12:17 pm 
Offline
Sparks Level
User avatar

Joined: Tue Apr 17, 2012 8:19 am
Posts: 240
Location: Charlotte, NC
I can't help but think of one particular enclosure I happenned on during a field service call in the 1990s. It was certainly safe by period standards. The enclosure housed a 3-axis servo control. I drove probably 3 hours to the customer's facility. When I got there I opened the panel for the machine they were reporting a fault and found the entire insides coated in a reddish-orange substance. Come to find out, the operator had set his pimento cheese sandwich wrapped in tin foil on the servo heatsinks to warm it up for lunch.

It just goes to show you that it doesn't matter how safe you try to make something, a determined person can find some way of wedging something into the enclosure to create an arc fault.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Oct 02, 2012 1:08 pm 
Offline

Joined: Wed Aug 25, 2010 7:15 am
Posts: 24
Location: St. Paul, MN
Larry Stutts wrote:
I can't help but think of one particular enclosure I happenned on during a field service call in the 1990s. It was certainly safe by period standards. The enclosure housed a 3-axis servo control. I drove probably 3 hours to the customer's facility. When I got there I opened the panel for the machine they were reporting a fault and found the entire insides coated in a reddish-orange substance. Come to find out, the operator had set his pimento cheese sandwich wrapped in tin foil on the servo heatsinks to warm it up for lunch.

It just goes to show you that it doesn't matter how safe you try to make something, a determined person can find some way of wedging something into the enclosure to create an arc fault.



I think good judgement is required for every situation, but one thing I like about 70E is that it allows for some judgement to be used. That is why they include risk as one of the factors in an arc flash hazard evaluation.

While working on the 24VDC portion of a control panel you can start and stop the motor, or change inputs to a drive, so you can definitely interact with and cause a change of state on the 480V equipment. But, if the equipment was properly installed and has been well maintained, it is not likely to expose the employee to an electrical hazard. On the other hand, if it is old and has not been maintained, or if there is a pimento cheese sandwich melted into it then I would wear the PPE whether the enclosure was plexiglass or metal.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed Oct 03, 2012 8:24 am 
Offline
User avatar

Joined: Wed Sep 01, 2010 1:11 pm
Posts: 11
Larry Stutts wrote:
I can't help but think of one particular enclosure I happenned on during a field service call in the 1990s. It was certainly safe by period standards. The enclosure housed a 3-axis servo control. I drove probably 3 hours to the customer's facility. When I got there I opened the panel for the machine they were reporting a fault and found the entire insides coated in a reddish-orange substance. Come to find out, the operator had set his pimento cheese sandwich wrapped in tin foil on the servo heatsinks to warm it up for lunch.

It just goes to show you that it doesn't matter how safe you try to make something, a determined person can find some way of wedging something into the enclosure to create an arc fault.


There's always going to be employees where the only effective PPE is a pink slip.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Oct 08, 2012 10:16 am 
Offline
Sparks Level

Joined: Fri Apr 15, 2011 7:43 am
Posts: 151
Location: Colorado
In general having the 480v behind a protective shield will prevent arc flash from occuring but will not eliminate it. A couple of times it could happen are: 1. cycling the power with the doors open, 2. foreign objects or rodents being jarred when the doors are opened. 3. fish tape being pushed into the barriered section. I am sure there are other scenarios - however unlikely - that people can find ways of getting into trouble.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Oct 08, 2012 1:15 pm 
Offline
Sparks Level
User avatar

Joined: Tue Apr 17, 2012 8:19 am
Posts: 240
Location: Charlotte, NC
engrick wrote:
In general having the 480v behind a protective shield will prevent arc flash from occuring but will not eliminate it.


In general having the 480v behind a protective shield MAY prevent arc flash from occuring but will not eliminate it.

A bit of semantics - if it will prevent it, then it does eliminate it.

thus it is more precise to say that a protective shield may prevent some sources of arc flash, but cannot eliminate all possible sources.


Top
 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 12 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:  
cron
© 2017 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883