Arc Flash Questions

[robh 12]

Hey all, I am going to be attending an arc flash training class in the middle of July put on by the engineering company that did the arc flash analysis on our facility. Just wondering if anybody had any unanswered questions I might be able to get answers for. I started to look through some of the previous posts here at MR. PLC but there was 2 pages of them related to arc flash. Not a lot of time to browse today, but I will when time permits. P.S. baseball fans---My Beavers won the CWS! Woo Hoo

[TimWilborne 108]

1. If a distribution system was being installed today, what would be the best way to lower the incident energy at the main transformer. Any specific fuses that are best in the primary and secondary? 2. Are there any problems or nuisance fuse blowing that you may run into by lowering the incident energy right on your main? Thanks TW

[geniusintraining 2]

Ya, when is NFPA 70e going to be mandated? by osha, I like the idea of safety but that one sucks....full leathers to check a control circuit? (if I have 480 with in x amount of feet) and I do, every thing I have is 480 or 24vdc

[TimWilborne 108]

It already is. It did not actually ever have to be in OSHA guidelines to be implemented but it is listed now. See THIS for explanation.

[geniusintraining 2]

Being "Law" and being "mandated" you have to agree they are not the same... This is a very good read, http://mikeholt.com/mojonewsarchive/NEC-HT...sh~20040512.php I don't want to dress up in a suit every time I go into a cabinet

[Ken Moore 23]

Great article. We are currently in the Arc Flash Study phase, and any info we can get is helpful.

[gravitar 3]

Yup, very interesting article. And I'm probably the only one that notices things like this, but did they really need to make separate PPE categories for WITH and WITHOUT underwear?! And for that matter, they only mention cotton underwear.. How about thongs or silk boxers? Wait, maybe I shouldn't go there :)

[robh 12]

This information was given to me by an engineer who works for the company that did our Arc flash analysis on our facility. Some stuff may not be the same outside of Oregon. I say this because the federal requirements for arc flash gloves to be tested is every 6 months. Here in Oregon it is every 3 months if the gloves are being used and if the gloves have not been used in that 3 months they are allowed to be used once before they have to be sent out and tested. I was able to get an answer to the first question but not the second.... 1. (Engineers answer) I think that the best way is to place a transformer differential current relay (87T) in the system with a zone extending to the load side of the main over current device on the secondary side. These relays are very fast operating (<0.01 seconds) for relatively small currents so they limit the duration that an arc will occur. In the applications that I have seen these used the Arc Flash hazard ratings were zero (0). I also asked the question about the Cat ratings on my Fluke meters and what they mean while we were looking at them in the class. Here is the answer that I got... ---In response to your question concerning the Cat. III rating on your multimeter. I asked around about the cat. III rating on your fluke. This rating is an over voltage rating that can be found in EIC 1010-1 and ANSI/ISA S82.01-1994. Basically put, the over voltage surges in a system vary with respect to where in the system you are. For example the over voltage surges that can be expected from a duplex outlet are much smaller than the over voltage surges that could occur at the main service entrance conductors due to the available fault currents, proximity to the utility line, and conductor sizing in the system. For this reason meters are rated according to the locations that they expect to be used. A cat. I rated meter is not rated to be utilized for voltage measurements at the service entrance conductors however, a cat. III is. If you took a facility and mapped it out on a piece of paper, there would be a region surrounding the main entrance conductors and main switchboard that was cat. III, another region encompassing the panel boards downstream (Cat. II) and a third region that would extend down to everything else (cat. I). Another thing that I learned is about the PPE that can be worn. Carhartt has a very good looking line of FR (Flame resistant) work clothes that I think will be better than the coveralls my company has purchased. It looks like alot of them are rated high enough to work on the Class 3 hazards we have. Another thing is that the PPE ratings are additive. So if you have a shirt that has a FR rating of 6 and a coat that has a rating of 12 then you are rated at 18. I will try to post more stuff that I learned as I go back through my notes. Hope this is helpful.

[TimWilborne 108]

Thanks for the info What exactly is the differential current relay?

[robh 12]

Sorry for the slow response. We had a major shut down 10 days, then I went fishing in Alaska for a week. So I am just getting back in to the swing of daily life! This is a long and complicated answer to this question . I am looking for a better one and waiting for the P.E.s email.http://gauss.ffii.org/PatentView/EP453196

[TimWilborne 108]

Yes, please share the P.E.s explaination, hopefully in a little more laymans terms Thanks TW

[robh 12]

I emailed my P.E. and got the out of office reply saying he is on vacation till the 28th of this month. So it might be awhile before I get a better answer.

[TimWilborne 108]

Must be nice

[robh 12]

Hey TW, I haven't abandoned this. Still haven't got any info from the P.E. I am going to email him again. From what I have been able to gather about this little device is that it senses an arc (I have no idea how or what it senses) and cuts power at the main OCD. Rob

[TimWilborne 108]

I've got a company doing some engineering of some new distribution. I will ask them about it too and report back if they can help Thanks TW

[robh 12]

TW, Arc flash guy finally replied back to me: A transformer differential relay compares the currents going into the transformer to the currents coming out of it. The "zone" of protection is based on the location of the current sensors. If there is a fault anywhere within the zone of protection, the relay will de-energize the transformer to protect it. The benefit of this protection is it's very fast, which is quite important when reducing the time that incident energy is released. The drawback is that an upstream protective device and sensors must be installed to de-energize the transformer (cost/utility owned equipment). Rob

[TimWilborne 108]

Thanks. I think the cost would be minimal in the long run compared to the cost of additional PPE and time

[BobLfoot 302]

Thanks Rob - We're about to enter the Arc Flash gambit as well. Your info and this thread will be helpful.

[Ken Moore 23]

I uploaded a power point show on electrical safety to the down load section: LINK There are a couple of slides, with a little Vendor advertising of safety equipment, but all in all, a pretty informative show. It does a decent job of explaining the hazards of Arc Flash, and gives a little info on arc flash studies. I plan on using it at a safety meeting. It's rather large, around 12.5 megs.

[paulengr 44]

Nuisance fuse blowing: No. When you fuse a motor, you use current limiting fuses. These have a current-let through curve that approximates the motor very well except at the very tail end, where a traditional overload device is more appropriate. These fuse types cover the "dead short" situations very well. Also, you need the overload block because when you lose a single phase leg, the overload block will usually react quicker than the fuses to disconnect the other 2 phases before the last couple fuses blow, but this is not always the case. The motor protection is about the best that money can buy if you set it up this way. Also, I prefer eutectic overloads to thermal-magnetic or solid state. Thermal-magnetic overloads are not as good at protecting the motor in the first place and need routine maintenance to adjust them. Electronic overloads are easily fooled by production folks. They can throw the main disconnect and restart an overheated motor immediately without adequate time for the coils to cool. Half a dozen cycles later and the motor is fried. You can't do that to a eutectic overload. On a transformer or other non-motor circuits, the appropriate device is a "fast blow" fuse. I've seen the fuse manufacturers offer countless variations on the theme, but it mostly appears to me that it's just a scheme to get away from "standard" fuse packages. Everybody sells an RK-5 fuse package for instance. But if you choose a real oddball, chances are you've locked yourself into a particular fuse manufacturer. Two things I have learned through experience. First off, don't bother with resettable fuses. That's just asking for trouble. Second, don't bother relying on any of the "hot spot" or nipples that stick out or other "fuse blown" indicators. Simply put, they don't work. I have seen countless examples of both false positive and false negative indications with these devices. Although this may be in the <10% case, it causes needless troubleshooting errors just because somebody didn't use a multimeter to verify. Problems with nuisance tripping: This is strictly a circuit breaker phenomena. A fuse blows because the energy exceeded it's rated capacity. The only exception is that once in a while I've seen some fuses installed in the 1970's that seemed to have tripped because they apprently exceeded their useful life. This may be a "useful lifespan" type situation. Circuit breakers cause a serious OSHA/ANSI violation. You are required to find the source of the problem before re-energizing the circuit. With a circuit breaker, it is just as likely that the device is the problem as there is a real issue in the circuit. Under the "find the source of the problem" rule, simply resetting the breaker is a no-no. With fuses however, there are no "false trips". If you get false trips after lowering your maximum incoming current, then you need to go back to the coordination study and figure out what your actual loads are.