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:
nollij said:
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:
zog said:
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.
ron said:
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.