Arc Resistant MV Switchgear

philly

Senior Member
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

Senior Member
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

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
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

Senior Member
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

Senior Member
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

Member
Location
WV
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. :)
 

Attachments

paulengr

Senior Member
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

Senior Member
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

Senior Member
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

Senior Member
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,
 
Top