Arc Resistant MV Switchgear

[philly]

I was curious to hear other's thoughts in general on application of MV Arc Resistant Switchgear? I see this offered as a solution or a customer request a lot of times based on the fact that its a more robust and safer solution without giving it much thought otherwise.

My experience with this is that the gear is obviously more expensive, has a larger footprint, and has the added challenge of having to deal with exhaust ducts for exhausting of gaes during an event.

Furthermore from an AF Hazard standpoint I don't believe the presence of Arc Rated Switchgear alone allows one to reduce Incident Energy levels and PPE requirements simply based on the AF Switchgear alone. In other words it does not change the outcome of the calculated Incident Energy levels and PPE requirements. It may reduce the risk due to its robust construction but I view this as more of belt and suspenders with the PPE requirements still based on the calculated IE levels?

If the IE is able to be reduced by other means (Maintenance mode's, relay settings, etc...) to an safe level does it warrant the application of Arc Rated gear in a lot of cases?

Just looking for opinions and past experiences here. Thanks!

 

[ron]

Your statements are all true, especially getting it near an exterior wall to shoot the exhaust out.

It is also only arc resistant when the doors are closed. How many arcs occur in MV SWGR when the doors are closed, not too many?

I think it is not worth it, especially since it doesn't reduce incident energy, higher cost, bigger footprint, etc, etc

 

[jim dungar]

It is also only arc resistant when the doors are closed. How many arcs occur in MV SWGR when the doors are closed, not too many?

One of the riskiest operations, for arc flash, is racking breakers on to and off of energized busbars. Arc Resistant gear racking is done with doors closed.

 

[philly]

One of the riskiest operations, for arc flash, is racking breakers on to and off of energized busbars. Arc Resistant gear racking is done with doors closed.

I believe most modern non-arc resistant switchgear has capability for racking with doors closed. Perhaps the point your making is that having the doors closed on a Arc Resistant lineup provides much more robust protection compared to closed door racking on non-arc resistant gear?

 

[paulengr]

Arc resistant gear works for operating it with the doors closed. All maintenance activities are not protected by the arc resistant stuff following the ANSI and IEC definitions. So when you need it, it’s useless! So yes 100% marketing.

That’s not to say that although there are no standards written we can’t do something far better. For instance some Square D MCCs have a disconnect mechanism. So installing and removing buckets is entirely dead. Energizing or de-energizing the bucket itself uses a non-load break switch that is mechanically interlocked with the disconnect. The stabs are so short and insulated from one another I can’t honestly see any way they can initiate an arcing fault.

Another variation from Square D that they have patents on is they put arc chutes on the bus bars at the entries to the breaker frame. So it quenches all arcs between the cell and the breaker. So it’s arc resistant in that the incident energy is clamped by the extremely short arcing times. This is on the Masterpact NW gear. So it’s arc resistant with doors in any position.

Another variation is I hate to say it but some breakers have just plain massive, long bus bars connecting to the stabs. It’s just asking for trouble. But many are not capable of touching even if the bus bars folded in half.

Another example is GIS. Since you can’t technically open it, it’s naturally not just arc resistant but arc proof. It’s pricey but there are long term benefits and as the gas pressure increases, equipment spacing is so small it saves money. Again arc proof because you can’t work on the energized mechanism.

Another example is rubber coated underground switchgear. It uses the same design as separable elbow connectors so everything is 100% shielded and insulated at all times. No practical way to initiate an arc flash. So arc proof.

Other examples are epoxy encapsulated solidly insulated switchgear developed as an alternative to GIS. Tavrida makes inexpensive very nice 35 kV class switchgear this way. Arc proof not arc resistant.

 

[wvengineer]

My comments are limited to MV switchgear. Our newest gear is arc-resistant, but the bulk is not. As others have said racking breakers is our riskiest task related to AF. That's when something is changing with the stabs and the shutters are partly open, etc. while the bus is energized. Once the breaker is out and the shutters are closed there isn't an exposed energized conductor related to the primary circuit. There's no way I'm allowing someone to perform any task with the bus energized that might open the shutters or put them near the bus. Any task like that requires a bus outage no matter what type of gear. I haven't studied up on the latest version of 1584, but a quick search of the latest 70E shows that in table 130.7( C)(15)(a) arc-resistant gear of a certain rating with the doors closed as not even having an arc-flash PPE category at all. That category would be based off of IE I would imagine. So to say it doesn't reduce IE at the face of the worker isn't necessarily true is it?. It may not reduce the way you are calculating IE. Just thinking out loud here. For example I can't guarantee the door will stay closed during a major arc fault with my gear that isn't rated for it. All else being equal my IE will be based on the distance to the face/body of the worker through open air from the point of the fault. The door should stay closed with arc-fault gear rated to contain/redirect the 'blast' at some max available fault current. That IE would have to be less outside the door. One of the types of arc-resistant gear we have the door is interlocked with the racking mechanism and the shutters. The breaker has to be disconnected and the shutters closed to open the door. You can't rack the breaker in with the door open either. This has caused some of it's own problems, until you read the manual and learn how to defeat these interlocks that is.

 

[paulengr]

My comments are limited to MV switchgear. Our newest gear is arc-resistant, but the bulk is not. As others have said racking breakers is our riskiest task related to AF. That's when something is changing with the stabs and the shutters are partly open, etc. while the bus is energized. Once the breaker is out and the shutters are closed there isn't an exposed energized conductor related to the primary circuit. There's no way I'm allowing someone to perform any task with the bus energized that might open the shutters or put them near the bus. Any task like that requires a bus outage no matter what type of gear. I haven't studied up on the latest version of 1584, but a quick search of the latest 70E shows that in table 130.7( C)(15)(a) arc-resistant gear of a certain rating with the doors closed as not even having an arc-flash PPE category at all. That category would be based off of IE I would imagine. So to say it doesn't reduce IE at the face of the worker isn't necessarily true is it?. It may not reduce the way you are calculating IE. Just thinking out loud here. For example I can't guarantee the door will stay closed during a major arc fault with my gear that isn't rated for it. All else being equal my IE will be based on the distance to the face/body of the worker through open air from the point of the fault. The door should stay closed with arc-fault gear rated to contain/redirect the 'blast' at some max available fault current. That IE would have to be less outside the door. One of the types of arc-resistant gear we have the door is interlocked with the racking mechanism and the shutters. The breaker has to be disconnected and the shutters closed to open the door. You can't rack the breaker in with the door open either. This has caused some of it's own problems, until you read the manual and learn how to defeat these interlocks that is.

1584 is just calculations.

Arc resistant gear is different. It’s like explosion proof enclosures. It allows an arc flash but contains and redirects it. The ANSI version of the requirement hangs pieces of cloth Tee shirt material around all the openings in front then initiates a worst case arc and checks if any of the cloth samples were burned. So it’s a purely performance test. No calculations involved.

CIGRE has calculations. If you can manage to close up the gear enough to contain an arc flash it will rupture ANY gear within 1-2 cycles. You can prevent door launches either with arc resistant design or open doors or doors that don’t matter. As an example AB has spring loaded latches that can vent sideways during an arc flash without ripping open the door. The only other way is to quench the arc within one cycle either by fast opening or dead shorting (arc termination). Arc flash though can be under 8 cal/cm2 quite easily for most facilities with relatively easy modifications and/or adjustments which makes it sort of a mild issue.

 

[philly]

My comments are limited to MV switchgear. Our newest gear is arc-resistant, but the bulk is not. As others have said racking breakers is our riskiest task related to AF. That's when something is changing with the stabs and the shutters are partly open, etc. while the bus is energized. Once the breaker is out and the shutters are closed there isn't an exposed energized conductor related to the primary circuit. There's no way I'm allowing someone to perform any task with the bus energized that might open the shutters or put them near the bus. Any task like that requires a bus outage no matter what type of gear. I haven't studied up on the latest version of 1584, but a quick search of the latest 70E shows that in table 130.7( C)(15)(a) arc-resistant gear of a certain rating with the doors closed as not even having an arc-flash PPE category at all. That category would be based off of IE I would imagine. So to say it doesn't reduce IE at the face of the worker isn't necessarily true is it?. It may not reduce the way you are calculating IE. Just thinking out loud here. For example I can't guarantee the door will stay closed during a major arc fault with my gear that isn't rated for it. All else being equal my IE will be based on the distance to the face/body of the worker through open air from the point of the fault. The door should stay closed with arc-fault gear rated to contain/redirect the 'blast' at some max available fault current. That IE would have to be less outside the door. One of the types of arc-resistant gear we have the door is interlocked with the racking mechanism and the shutters. The breaker has to be disconnected and the shutters closed to open the door. You can't rack the breaker in with the door open either. This has caused some of it's own problems, until you read the manual and learn how to defeat these interlocks that is.

I’ve always understood the tables in 70E to be based on risk evaluation and not specifically on IE level. So the fact that In that table it does not show a PPE category tells me that it’s based on a lower risk of exposure with doors closed and not necessarily a reduced IE level.

I guess it could be argued however that with doors closed and the blast contained there is no exposure and this the IE at the workers face is thus reduced

 

[philly]

Arc flash though can be under 8 cal/cm2 quite easily for most facilities with relatively easy modifications and/or adjustments which makes it sort of a mild issue.

That is where I am left thinking a lot of times. If I can successfully implement a mitigation technique (maint mode, differential, etc...) to reduce IE is it still worth the extra cost and complexity of Arc Rated gear?

 

[paulengr]

That is where I am left thinking a lot of times. If I can successfully implement a mitigation technique (maint mode, differential, etc...) to reduce IE is it still worth the extra cost and complexity of Arc Rated gear?

My thought is that arc rated gear is a solution looking for a problem.

Normal operation does not require arc flash PPE. Abnormal operation where the equipment is compromised does as does energized work with the doors open working inside the MAD or the “restricted approach boundary” for those who refuse to use standard terms (70E).

So when does arc resistant gear do anything? Yep...when arc flash PPE is NOT required during normal operation. In cases where it is required, it doesn’t do anything at all! No PPE reduction because either the doors are open so we bypassed the protection or the equipment is potentially compromised so we treat it accordingly.

The one potential exception is for sites that decide to suit up anyway because their equipment might not be properly maintained. OK but 70E as well as 1584 state that the incident energy could be higher under those circumstances...all bets are off. So what PPE is to be used? We don’t have a standard for improperly maintained equipment. At best we go upstream until we reach a properly maintained point and initiate LOTO there. Which is...normal operation.

So at the end of the day it is very limited in what it can do,

 

[Bwas]

I don't think arc resistant gear provides much benefit at all. I'm not sure what the cost premium is, but just by looking at the construction it must be significant. That stuff looks like it could be a safe at a major bank.

I had some 480V arc resistant MCCs from Square D on a recent project (that also had a 15kV arc resistant gear lineup). They had the little thing that requires you to remove the stabs from the bus before you can open the bucket. It also had about 1000 extra screws in the face of the gear. The electrician on that job told me they were such a pain that he was going to add $100k to his price on the next job he has to deal with arc resistant MCCs. This was a pretty big job with maybe 8 1200-2000A MCCs. Square D tech showed up to do startup and had never seen anything like this gear before. He tried to have the electrician take out all the screws but the electrician just laughed at him. Square D tech had to go to the hardware store to stock up on battery operated screw drivers just to do his checkout/startup.

I also recently had a job with a couple arc resistant 5kV MCC from eaton. First of all, the gear submittal, fabrication and delivery was a complete disaster but I digress. The front of the gear looked like the Square D MV arc resistant gear - built like a bank safe. The back side just looked like standard 480V gear so I don't have a lot of confidence that it provides any really benefit, especially with multiple MCCs lined up in a single room.

I'm convinced the gear suppliers are just adding steel and bolts without any actual analysis of what benefits are provided.

 

[paulengr]

They are using the test data for explosion proof enclosures which is entirely based on 1/4-20 fasteners, Pressure rise is 8-10 PSI, it takes a lot of bolts.

The issue is running tests is very expensive so they over design based on XP data and test just once.

Larger vents or arc absorbing material can reduce things. I’ve also seen some experimental designs using what amounts to arc chutes at the barriers which doesn’t need the bank vault door but that’s still a lab idea except some Square D Masterpact NW cassettes but it’s not common. I would say it’s more of an arc proof design since arcs can’t be sustained. Going down those roads we may eventually have arc proof designs even with doors open.

 

[Jraef]

I don't think arc resistant gear provides much benefit at all. I'm not sure what the cost premium is, but just by looking at the construction it must be significant. That stuff looks like it could be a safe at a major bank.

I had some 480V arc resistant MCCs from Square D on a recent project (that also had a 15kV arc resistant gear lineup). They had the little thing that requires you to remove the stabs from the bus before you can open the bucket. It also had about 1000 extra screws in the face of the gear. The electrician on that job told me they were such a pain that he was going to add $100k to his price on the next job he has to deal with arc resistant MCCs. This was a pretty big job with maybe 8 1200-2000A MCCs. Square D tech showed up to do startup and had never seen anything like this gear before. He tried to have the electrician take out all the screws but the electrician just laughed at him. Square D tech had to go to the hardware store to stock up on battery operated screw drivers just to do his checkout/startup.

I also recently had a job with a couple arc resistant 5kV MCC from eaton. First of all, the gear submittal, fabrication and delivery was a complete disaster but I digress. The front of the gear looked like the Square D MV arc resistant gear - built like a bank safe. The back side just looked like standard 480V gear so I don't have a lot of confidence that it provides any really benefit, especially with multiple MCCs lined up in a single room.

I'm convinced the gear suppliers are just adding steel and bolts without any actual analysis of what benefits are provided.

Everyone can interpret how they want to do LV Arc Resistant MCCs, because there are no official standards for Arc Resistant yet. But what most to, in order to show some legitimacy, is to use ANSI C37.20 standards for MV Arc Resistant gear and apply it to the LV designs. That can lead to over design to be sure, but it also depends on how you interpret it. Up to a certain amount of incident energy, typically 1200A max., you can design to pass the testing requirements; basically cotton blocks are mounted to all of the external gaps, hinges, cover plates etc., and arc blast is set off inside and if any of the cotton turns brown, you fail. So the design needs to either contain it 100%, or it can cool it sufficiently by "burping" all of the doors by using special spring loaded catches. That works for lower energy levels but above 1200A, you need blow-out vents on the top of the MCC.

Arc Resistant gear allows you to place it in areas that do not have restricted access to non-qualified personnel, like walkways and halls where people are going to be casually walking by. If you can put an MCC or switchgear into a fenced off area or locked room where only qualified workers with the right PPE and training can be near it, then you don't gain much by using Arc Resistant gear.

 

[paulengr]

Everyone can interpret how they want to do LV Arc Resistant MCCs, because there are no official standards for Arc Resistant yet. But what most to, in order to show some legitimacy, is to use ANSI C37.20 standards for MV Arc Resistant gear and apply it to the LV designs. That can lead to over design to be sure, but it also depends on how you interpret it. Up to a certain amount of incident energy, typically 1200A max., you can design to pass the testing requirements; basically cotton blocks are mounted to all of the external gaps, hinges, cover plates etc., and arc blast is set off inside and if any of the cotton turns brown, you fail. So the design needs to either contain it 100%, or it can cool it sufficiently by "burping" all of the doors by using special spring loaded catches. That works for lower energy levels but above 1200A, you need blow-out vents on the top of the MCC.

Arc Resistant gear allows you to place it in areas that do not have restricted access to non-qualified personnel, like walkways and halls where people are going to be casually walking by. If you can put an MCC or switchgear into a fenced off area or locked room where only qualified workers with the right PPE and training can be near it, then you don't gain much by using Arc Resistant gear.

To quote the 70E in multiple responses to public inputs there is NO HAZARD in just walking under normal operation. It is about the same risk as a commercial airline flight.

 

[Bwas]

Everyone can interpret how they want to do LV Arc Resistant MCCs, because there are no official standards for Arc Resistant yet. But what most to, in order to show some legitimacy, is to use ANSI C37.20 standards for MV Arc Resistant gear and apply it to the LV designs. That can lead to over design to be sure, but it also depends on how you interpret it. Up to a certain amount of incident energy, typically 1200A max., you can design to pass the testing requirements; basically cotton blocks are mounted to all of the external gaps, hinges, cover plates etc., and arc blast is set off inside and if any of the cotton turns brown, you fail. So the design needs to either contain it 100%, or it can cool it sufficiently by "burping" all of the doors by using special spring loaded catches. That works for lower energy levels but above 1200A, you need blow-out vents on the top of the MCC.

Arc Resistant gear allows you to place it in areas that do not have restricted access to non-qualified personnel, like walkways and halls where people are going to be casually walking by. If you can put an MCC or switchgear into a fenced off area or locked room where only qualified workers with the right PPE and training can be near it, then you don't gain much by using Arc Resistant gear.

You can tell this by looking at the gear. It looks to me like square d is just throwing stuff together with no real plan.

 

[paulengr]

You can tell this by looking at the gear. It looks to me like square d is just throwing stuff together with no real plan.

Square D is one of the few actually doing the testing, etc.

 

[Bwas]

Square D is one of the few actually doing the testing, etc.

I have requested some documentation of the testing or design standards for 15kV arc resistant gear as well as the arc chimney and they were not able/willing to provide any information. There is a certain clearance above the gear where they allow shutters on top of the gear and if less than that clearance is provided above the gear they require duct work to give the flash a path outside the building. I had a concrete building with a hollow core ceiling (nothing to catch fire or burn) that was about 2" short of their requirements and they insisted on the arc ductwork because "That's how we tested it." There was no analysis or any type of written standards available. That gear has been installed and operating for about a year so maybe they have standardized since then.