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 Post subject: Medium Voltage and the Table
PostPosted: Wed Feb 13, 2008 3:10 pm 
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For the tasks that move into the Medium Voltage category, such as insertion of a CB into Switchgear 1KV and above - there are no 'NOTES' to define the limitation of fault current and clearing time.

How do I know when I can and can't do these tasks? Is it just a matter of seeing if the Incident Energy is greater than 40 cal. But then I would have to do an Engineering analysis, and the whole point of the Table was to avoide that. Comments.

How would you treat racking in a Main Switchgear breaker which is fed directly from the Utility, say at 13.8KV. Fault current analysis typically does not give you clearing times for utility gear. What if they are using a recloser, or have an automatic throwover scheme. The utility is not going to de-energize the street power because you want to rack out your MCB for maintenance. Does this mean utilities will now install air breaks on all incomings?

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PostPosted: Wed Feb 13, 2008 4:58 pm 
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Utility Disconnects

Haze10,

Having been a utility engineer for 28+ years and would say that I don't know of cases in which you could not schedule an outage with the utility to get your breaker out and then back in. There are always protection and disconnect devices at the utility supply that could be opened manually once you unload your gear. If it is a safety issue and you can give proper notice, I don't know of anyone who would not be glad to cooperate. Yes it would require an outage on the gear, but so does racking out the main. And you are correct in your concern since the utility protection is only there to protect the transformer from severe and continous overload, and in many cases is sized to allow for 150 to 200 percent of transformer rating......hence big arc on the secondary to operate! They do not want your power to be interrupted unless it has to be.

In MOST cases utility practice does not include the provision for reclosing on single or even multiple transformer banks at a single customer site. The primary protection is generally a fuse. Throwover equipment is expensive and is used only in specialized situations. You would probably know if this capability exists on your service and if not could ask your utility provider for more info.

There always exists the possibility that two or more customers are served from the same transformer installation, but with almost all services to a larger customer this will not be the case.

For smaller customers, acr flash is probably not a major issue anyway.

The utilities are facing much of the same safety issues as you are and should be understanding. At some point each entity must work with the other to make all of this stuff safer! If you do not have a relationship established with your electric service provider's rep, I think it is time to get to know him/or her.

Hope that helps.
Alan


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PostPosted: Wed Feb 13, 2008 5:02 pm 
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Location: Charlotte, NC
High Side Utility Protection

BTW....We get requests frequently for our high side devices and sizes from folks trying to do exactly what you are attempting with your system modelling. Your utility provider should be glad to get them for you and discuss any special operations that might be associated with your service.
Alan


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PostPosted: Wed Feb 13, 2008 8:42 pm 
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Alan,
thanks for the response. In this case, the utility has 13.8KV in the street as the community distribution service, and I have a downriser on my meterpole tapped directly to the 13.8KV. My 13.8KV Switchgear lineup had its MCB directly connected to the 13.8KV. There is an airbreak several poles down, and the utility could use that to drop me out, but would also loose the pole top transformer to two houses. Had I been there when this was installed, I would have insisted on having my own air break on the meter pole, or have set an air break pole upstream of the meter pole.

The 13.8KV is direct to the town's substation, about 1.5 miles away. There are two 23KV intrastate feeders that feed the substation. I know they have an automatic throw over scheme, because when we lose power, it always come back in 5 seconds. Plus the utility confirmed it is an auto throw over. I have to check if there is an auto reclose setting on the 13.8KV. I actually don't think we have ever lost that one. The 23KV come out of a 169KV sub about 10 miles away, and that's typically where all the troubles have been historically.

So in this case my MCB would not be going through a transformer, other than the 23KV to 13.8KV utility trainsformer at the switchyard 1.5 miles away, and I am sure they would have some type of over current on the 13.8KV, althought it is probably set high.

So I guess I could ask the utility for the settings to see if it would clear with less than 40cal.

Am I right about the table then, since no notes, the only rule to apply is the 40 cal rule.


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PostPosted: Thu Feb 14, 2008 6:03 am 
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Utility disconnects

Haze10,

Are you saying that the utility tapped your 13.8 kV dip directly to the overhead line with no fuse for overcurrent protection? If this is the case, a fault on your equipment would lockout the entire 13.8 kV circuit. WOW, extremely unusual if that's the case!

Please take a look at the pole and see if there are 3 open type fuse cutouts where they make the connection to your dip.

Also, standard practice would be to reclose the overhead circuit 2 or 3 times on 5 to 30 second intervals before locking out their equipment, in hopes of allowing the fault to clear on its own, or blowing the fuse if the fault is downstream from one.

Interesting,
Alan


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PostPosted: Thu Feb 14, 2008 11:55 am 
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Location: St. Louis, MO
haze10 wrote:
For the tasks that move into the Medium Voltage category, such as insertion of a CB into Switchgear 1KV and above - there are no 'NOTES' to define the limitation of fault current and clearing time.

How do I know when I can and can't do these tasks? Is it just a matter of seeing if the Incident Energy is greater than 40 cal. But then I would have to do an Engineering analysis, and the whole point of the Table was to avoide that. Comments.


Haze, I'm not sure if I understand the first part of your question. If you are using the tables, and there is no note, then you follow the table. It is written without qualification.

Having said that, I don't like the tables, and don't trust the tables.


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PostPosted: Thu Feb 14, 2008 12:13 pm 
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haze10 wrote:
In this case, the utility has 13.8KV in the street as the community distribution service, and I have a downriser on my meterpole tapped directly to the 13.8KV. My 13.8KV Switchgear lineup had its MCB directly connected to the 13.8KV.

The 13.8KV is direct to the town's substation, about 1.5 miles away. There are two 23KV intrastate feeders that feed the substation. I know they have an automatic throw over scheme, because when we lose power, it always come back in 5 seconds. Plus the utility confirmed it is an auto throw over. I have to check if there is an auto reclose setting on the 13.8KV.


Is it the 13.8kV breaker you are concerned with? If that is the case, it should be fairly easy to get available fault current up to that point or to your drop from the utility. As acobb said, most utilities are familiar with the arc flash concerns and information requirements for thier customers by now. They should also be able to give you fuse settings (if their are any). From there, an energy calculation is fairly straightforward. The utility may even be able to tell you.

If your concern is below the 13.8kV, at the main on the LV Switchgear after your transformer, chances are the calorie rating is pretty high. I would not trust the tables for this situation! (But at least you are covered, if you do decide to use them.) Try an online calculator if you have clearing times for the MV breaker.

As for the recloser, from an arc flash standpoint, I consider it a non-issue.It is also unlikely that the arc will re-initiate on the recloser operation. Of course, if the energy is high enough to trip the utility, then by the time the recloser operates, chances are you are sitting against the far wall! :eek:

haze10 wrote:
Am I right about the table then, since no notes, the only rule to apply is the 40 cal rule.


What 40 cal rule??!!


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PostPosted: Sun Feb 17, 2008 8:25 pm 
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I checked the line and the utility does have cutouts (non fused) and what looks to be an electronic trip unit. Its a large metal can with all three phases going in and coming out, with some kind of electronic box below on conduit. Definately not capacitors, so it is either an electric actuated air break or an overload.


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PostPosted: Sun Feb 17, 2008 8:27 pm 
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The 40 cal rule is in the guidebook which basically says no live work can be performed above this value because the blast pressure is too great.


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PostPosted: Mon Feb 18, 2008 5:47 am 
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The Utility Protection

haze10 wrote:
I checked the line and the utility does have cutouts (non fused) and what looks to be an electronic trip unit. Its a large metal can with all three phases going in and coming out, with some kind of electronic box below on conduit. Definately not capacitors, so it is either an electric actuated air break or an overload.


Haze10,
The box is most likely a recloser. It is probably set to lockout on the first trip though and not reclose. TCC curves are readily available for these if you can get the settings from the utility, ie. type, curve, pickup, time delay, and inst. (if there is one). I would expect that it is set to operate on 1 time delay shot. Using the tables for the one calc would not be difficult if this is the only one.

For a 13.8 system, unless it is an extremely "stiff" system, the fault duties are usually in the 5 to 7 kA range at the station and can be as low as 2 to 3 kA when you get to 3 or 4 miles from the substation. 10 kA+ on the bus is possible but not likely. Energy might not be as high as you expect. I would be surprised if you calculated an actual level of 40+. You would probably be better to do the calcs at 13.8 kV. My experience has been that the tables are extremely conservative, as one would expect. In most cases a 4 cal level in the tables (NESC) results in less than 1 or 2 cal when actuals are calculated....sometimes as low as .5!

This is not for a 3 phase "arc in a box" and for sure yours will be higher. I would still consider doing the calcs for your system instead of using the tables, especially if you do the one and see that the levels are much lower than expected. Check it out and let us know.

Hope this is useful,
Alan


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PostPosted: Mon Feb 18, 2008 7:22 am 
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haze10 wrote:
The 40 cal rule is in the guidebook which basically says no live work can be performed above this value because the blast pressure is too great.


I don't want to hijack the thread, but this is a pet peeve of mine. You can respond at http://www.arcflashforum.com/showthread.php?t=14

The 40 cal/cm2 limit is quoted all over the place, with no reference. There is no such restriction, that I can find, anywhere! It's not in the NESC, not in the NEC, not in the NFPA 70E, not in 1584! I haven't found it within OSHA documents, either. There is a mention in the NFPA 70E Handbook commentary, which is not enforceable. Yes, it's a good idea, but it just is not an required rule. If you have a reference, code section, please let me know.


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PostPosted: Tue Feb 19, 2008 4:29 pm 
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Couple of additional comments then:

In the MV Switchgear scenerio above, lets say that I accept the fact that I can not rack out or rack in the breaker without the utility opening the circuit.

Does that also mean that I cannot 'OPERATE" the breaker open and closed behind a closed metal enclosure even if the Incident Energy came out to be higher than 40cal/cm2.


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PostPosted: Tue Feb 19, 2008 6:16 pm 
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CB Operation

haze10 wrote:
Couple of additional comments then:

In the MV Switchgear scenerio above, lets say that I accept the fact that I can not rack out or rack in the breaker without the utility opening the circuit.

Does that also mean that I cannot 'OPERATE" the breaker open and closed behind a closed metal enclosure even if the Incident Energy came out to be higher than 40cal/cm2.


Haze10,

Table 130.7 lists the operation of circuit breakers in gear 1kV and above as a Cat. 2 with the doors closed.....but I expect you already know that. If you do the calcs, doesn't that assume arc flash/arc blast exposure without a metal barrier? Has any lab done tests/calcs for a scenario with the doors closed? It doesn't seem reasonable to attempt to extrapolate risk from an exposed arc to an arc behind closed doors.

I would believe that you either rely on the tables, in this case a Cat 2, or do the calcs and live with them.....having already said that I do not believe the calcs are designed for this task. They are designed to illustrate the heat energy directly on the worker, not through a metal door. Unless the blast "blows your doors off" I would expect that your Fruit of the Looms might just end up needing a good cleaning!

Have you done the calcs and found 40+ or are you just raising a good question? The reason that I ask is from my previous post. Unless you would rather not know, I really think that the calcs would give you a much lower level than the tables at 13.8 kV.

Anybody else?
Alan


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PostPosted: Thu Feb 21, 2008 6:32 pm 
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Thanks Alan, you are probably right. I'll contact the utility, but I will probably have a wait to get the info. I'll let you know what I find out.


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