Arc-Resistant Switchgear

[WastefulMiser]

Have you worked with it, i.e spec'd or installed?

It is typically built more ruggedly incurring additional costs --- do you think it is worth it?

What's the typical price difference?

What is the largest size CPT you would feel comfortable with spec'ing inside a vertical section?

It seems to have some limitations v. non-Arc-Resistant which may be a great check on management from cramming so much in such a limited space putting the operator more at risk.

What are your general thoughts on it?

 

[SG-1]

I have the pleasure of working with it. I get to play with all the bells & whistles, & lights before you get it.

It is built from heaver gauge steel. How much is additional safety worth ?

I am told the price difference is small, but I do not price it. They tell me that we nearly give it away.

Our standard CPT in a drawer is 15KVA. We can put larger transformers in the rear.

You would not believe what we can cram inside now. The spaces that the relays meters, etc occupy can be accessed with out sacrificing the arc rating. You are correct there is less room.

If you are gun-ho safety then it is the way to go. It is also a royal pain, because you will spend more time gaining access to the arc compartments.

If I knew it was going blow & I was going to be near, give me arc.

 

[zog]

If you compare the added costs of arc resistant switchgear to the costs of one arc flash accident the solution becomes clear. IMO arc resistant gear will not even be an option in the future, it will be standard. There was a time you could get the seat belt option on a car.

 

[WastefulMiser]

I have the pleasure of working with it. I get to play with all the bells & whistles, & lights before you get it.

It is built from heaver gauge steel. How much is additional safety worth ?

I am told the price difference is small, but I do not price it. They tell me that we nearly give it away.

It seems more laborous due to the all the weldments.

Our standard CPT in a drawer is 15KVA. We can put larger transformers in the rear.

How would you feel about a 75kVA CPT in the rear?

You would not believe what we can cram inside now. The spaces that the relays meters, etc occupy can be accessed with out sacrificing the arc rating. You are correct there is less room.

The instrument compartments never seem large enough for two-high MV switchgear. I've never been a fan of two-high switchgear, though it isn't my money.

If you are gun-ho safety then it is the way to go. It is also a royal pain, because you will spend more time gaining access to the arc compartments.

You are right. Operators need to be well trained.

If I knew it was going blow & I was going to be near, give me arc.

Me too!

It says my message is too short.

 

[ron]

Installations need to have good headroom for the blast to go upward. Some allow you to duct the blast out, but you need to be near an exterior wall and have heavy duct.
They only work when the doors are closed. Although it happens the other way too, most accidents that I see have a door open with someone working on it......

 

[nollij]

Properly designed systems (protective relaying) are more beneficial than venting on top of the gear to focus the blast/flash it outside.

There are no installations of the Arc Resistant gear in our plant.

I can not rationalize the additional cost of the gear + installation (will it fit in the building?) when the Arc Flash numbers on a new section of distribution system has numbers in the single digits at every location. Bus differential protection is cheaper and much more effective. Granted, if money and room for the equipment are not an issue, then go hog wild.

 

[SG-1]

I think I have seen a 75KVA in the rear before. The vertical sections are 36 inches wide (standard).

If I am mistaken,
the worse case is the transformer takes up the entire vertical section.

The biggest problem is ventilation.

 

[jdsmith]

Have you worked with it, i.e spec'd or installed?

I have installed 38 kV arc resistant gear and I am somewhat involved in operating and maintaining it.

It is typically built more ruggedly incurring additional costs --- do you think it is worth it?

It is clearly more rugged than standard metal clad gear, but what is that worth aside form the arc resistance capabilities?
1. The extra ruggedness makes it a little easier to install - there aren't any issues with having a section out of square.
2. The reason we bought the arc resistant gear wasn't for operator protection - it was to lower the risk that a fault in one cubicle would take an adjacent cubicle out of service. The extra ruggedness and the arc venting plenum provide this additional reliability. From this perspective it is worth the cost.

What's the typical price difference?

The adder to go from metal clad to arc resistant is typically 10-15% - perhaps $40-50k on a typical MV lineup. This isn't much when the entire project of installing a new MV substation is $3-5 million. Our 38 kV arc res gear was in the range of $1.5 million for a lineup 80 feet long with 15 breakers plus fused rollouts for PTs and CPTs.

What is the largest size CPT you would feel comfortable with spec'ing inside a vertical section?

We had a 75 kVA 34.5/480 CPT in our first arc resistant lineup explode after 3-1/2 years in service. In retrospect it seems foolish to install such a heat producing device in a nonventilated compartment, and our practice now is to install all CPTs in metal clad or arc resistant gear outside of the switchgear and outside of the substation.

It seems to have some limitations v. non-Arc-Resistant which may be a great check on management from cramming so much in such a limited space putting the operator more at risk.

What limitations have you noticed? My experience is with 38 kV arc resistant which is a 1 high configuration with a large instrument compartment above the breaker. I have not inspected any 5 kV or 15 kV arc res gear. Based on our 2 high metal clad 5 kV gear, I agree that space would be very tight - perhaps 1 high construction would be required.

What are your general thoughts on it?

I agree with nollij:

Properly designed systems (protective relaying) are more beneficial than venting on top of the gear to focus the blast/flash it outside.

As I mentioned above, we installed 38 kV arc resistant gear to decrease the chance that a failure in one cubicle would take an adjacent cubicle out of service. I firmly believe that relaying schemes should be designed with arc flash considerations in mind and that good relaying schemes are the first line of defense for workers.

We operate the arc resistant gear the same as we operate the rest of our MV switchgear. We use remote racking so operators are outside of the flash hazard boundary during racking. The breaker control switches on the front of the cubicles only work when the breaker is in the test position, not racked in. All normal switching is done from a control panel located outside of the flash hazard boundary.


I agree with zog, but I agree with ron even more:

If you compare the added costs of arc resistant switchgear to the costs of one arc flash accident the solution becomes clear. IMO arc resistant gear will not even be an option in the future, it will be standard. There was a time you could get the seat belt option on a car.

Installations need to have good headroom for the blast to go upward. Some allow you to duct the blast out, but you need to be near an exterior wall and have heavy duct.
They only work when the doors are closed. Although it happens the other way too, most accidents that I see have a door open with someone working on it......

It's fairly easy to accomodate the plenum and ducting. The major issue I have with the usefulness of arc res gear for personnel protection is that it only works with the doors closed. We use remote racking and remote switching, so what purpose does arc resistance really serve? The only case where it helps is if a breaker trips while an operator happens to be inside the flash hazard boundary. Considering substations are locked and unmanned an operator is only in front of a given breaker about 5 minutes per month, so this is a very remote possibility. The other issue is this is still rackable switchgear and racking mechanisms jamb and require personnel to open the doors and fix the problem. This means that well-designed relaying schemes are required to protect personnel when they are fixing problems in the gear and have the doors open.

The other side of the coin is that the threshold for incident energy that causes 2nd degree burns is 1.2 cal/cm, and I haven't yet seen a relaying scheme* that can reduce incident energy this low in any non-trivial industrial installation. The ultimate goal is to reduce incident energies to below 1.2 cal/cm, and until we've reached that point we should apply any available technologies that have a chance of protecting personnel, even if they only protect personnel when the doors are closed.

*There is something called the Arc Vault that can lower incident energy below 1.2 cal/cm, but it isn't strictly a relaying device and I'm not sure that it is available for purchase yet.

My opinion is that if you have a proper relaying scheme with transformer differential, bus differential, and keyed instantaneous maintenance switches on feeders then the choice of whether to buy metal clad or arc resistant is dependent on management's philosophy on safety. There are many companies where safety programs exist for regulatory compliance only, but there are a growing number of facilities that are seriously and actively concerned with worker safety. If you are at one of these facilities then buy arc res gear.

 

[BJ Conner]

Have you worked with it, i.e spec'd or installed?

It is typically built more ruggedly incurring additional costs --- do you think it is worth it?

What's the typical price difference?

What is the largest size CPT you would feel comfortable with spec'ing inside a vertical section?

It seems to have some limitations v. non-Arc-Resistant which may be a great check on management from cramming so much in such a limited space putting the operator more at risk.

What are your general thoughts on it?

IT was developed in Europe. From what I've read it's manditory there ( when they say europe I assumed it was the CE but it may not be every country).
My bet is that it will be here as well. How long it will take is the question.

 

[weressl]

Have you worked with it, i.e spec'd or installed?

It is typically built more ruggedly incurring additional costs --- do you think it is worth it?

What's the typical price difference?

What is the largest size CPT you would feel comfortable with spec'ing inside a vertical section?

It seems to have some limitations v. non-Arc-Resistant which may be a great check on management from cramming so much in such a limited space putting the operator more at risk.

What are your general thoughts on it?

Just think about what extra will it actually buy you in addition to what a remote operator already provides?!

Cost? They say 10-20%, but you have to increase your building height and provide a ventillation duct to exhaust the exploding gasses.

In 20 years I really would like to see the cummulative economic data on what extra did we spent; how many occasion did this actually 'operated'; how many times it actually 'saved' life; what extra cost of maintenance did it requires; added cost to operate, etc. Reflect and statistically equalize that data with the same for existing non-arc resistant gear.

 

[TxEngr]

I've had experience with purchasing and installing 15 KV Arc Resistant Swithgear. I've also had two experiences with faults in AR Gear. The equipment was about a 10% adder but I didn't even let the purchasing people see the difference - spec the arc resistant gear at the beginning. Like the others have said - what is your employees safety worth! In both incidents of faulted gear, water in some form got into the gear and caused the fault internally. In both cases, the relaying cleared the fault quickly with little damage to the gear. However in both cases, the arc flash energy was vented out the top of the gear which I suspect was a great aid in limiting the damage to the gear. I would definitely buy it again. In fact, any future medium voltage starters I buy will be specified as arc resistant now that that equipment is available.

 

[dbuckley]

IT was developed in Europe. From what I've read it's manditory there ( when they say europe I assumed it was the CE but it may not be every country).
My bet is that it will be here as well. How long it will take is the question.

This sounds similar to what a UK electrician would call "form 4" construction, whereby each breaker is in its own "box", the busbars are in their own box, and terminations are separate from the breakers and from other terminations.

Thus you can terminate a circuit with the breaker off in safety, as no other (live) terminals are exposed.

I've not seen it at MV (not an area I'm familiar with) but it's almost ubiquitous at LV.

There's a picture somewhere I still haven't found of a Siemens 6300A form 4 switchboard overloaded to its AIC rating, you can see the busbars glowing through the grill at the top, and the smoke rising, its a great picture...

 

[jdsmith]

I've had experience with purchasing and installing 15 KV Arc Resistant Swithgear. I've also had two experiences with faults in AR Gear. The equipment was about a 10% adder but I didn't even let the purchasing people see the difference - spec the arc resistant gear at the beginning. Like the others have said - what is your employees safety worth! In both incidents of faulted gear, water in some form got into the gear and caused the fault internally. In both cases, the relaying cleared the fault quickly with little damage to the gear. However in both cases, the arc flash energy was vented out the top of the gear which I suspect was a great aid in limiting the damage to the gear. I would definitely buy it again. In fact, any future medium voltage starters I buy will be specified as arc resistant now that that equipment is available.

Was the plenum vented through performations in the top of the plenum or was the plenum ducted outside? On our 38 kV arc res gear we vented the top of the plenum and we had to have 48" of empty vertical space above the plenum vents. This basically requires the bulding be an extra 4 ft taller, which adds a little bit of cost. We looked at ducting the plenum outside instead, but then there are issues with water entering the equipment. The best way to duct the plenum outside is to slope it down from the switchgear to the wall penetration, but there's still a chance that moisture can be driven into the switchgear due to differential pressures developing from wind and HVAC systems.

 

[SG-1]

This sounds similar to what a UK electrician would call "form 4" construction, whereby each breaker is in its own "box", the busbars are in their own box, and terminations are separate from the breakers and from other terminations.

Thus you can terminate a circuit with the breaker off in safety, as no other (live) terminals are exposed.

I've not seen it at MV (not an area I'm familiar with) but it's almost ubiquitous at LV.

There's a picture somewhere I still haven't found of a Siemens 6300A form 4 switchboard overloaded to its AIC rating, you can see the busbars glowing through the grill at the top, and the smoke rising, its a great picture...

Here in the United States we call "form 4" Metal Clad. Opposed to Metal Enclosed. All Arc Gear that I know of is Metal Clad. It is my understanding that the steel used in standard European switchgear is much thinner than our ANSI rated gear. This was a major factor causing Europe to develop the arc gear first.

 

[weressl]

Here in the United States we call "form 4" Metal Clad. Opposed to Metal Enclosed. All Arc Gear that I know of is Metal Clad. It is my understanding that the steel used in standard European switchgear is much thinner than our ANSI rated gear. This was a major factor causing Europe to develop the arc gear first.

In fact the ANSI arc resistant gear - specification only exists for MV equipment - the construction modification is minimal when compared to the Metal Clad gear. All the basic elements were already there for most manufacturers, eg. the segregation of the individual functional compartments. Two issues were needed to be addressed:

1. Securing confinement of the arc related pressure, and
2. Directing 1. into a defined area.

 

[SG-1]

In fact the ANSI arc resistant gear - specification only exists for MV equipment - the construction modification is minimal when compared to the Metal Clad gear. All the basic elements were already there for most manufacturers, eg. the segregation of the individual functional compartments. Two issues were needed to be addressed:

1. Securing confinement of the arc related pressure, and
2. Directing 1. into a defined area.

Some of the early test units that were "blown up" looked like King Kong was inside and want out really bad.