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 Post subject: question - for which equipment is arc flash study required
PostPosted: Tue Aug 25, 2015 1:06 pm 
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Joined: Tue Aug 25, 2015 12:36 pm
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First of all, sorry - I am a safety guy and new this topic. I have read NFPA 70E requirements and have attended two-day seminar on the code. I am putting together some guidance for developing safety programs. When it comes to arc flash, I get stumped.
I am pretty sure you don't need to consider arc flash for simple light switch - and at the other end, you definitely will need to do that for main power distribution panel. There is a cut-off somewhere - where is it?
In one manufacturer's literature, I read arc flash study is needed for switchgears, panelboards, MMC, etc. (no mention of any other type of electrical equipment, such as transformers)
Can you help me understand when is arc flash study (or use of tables) needed? thank you - Niru


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 Post subject: Re: question - for which equipment is arc flash study requir
PostPosted: Wed Aug 26, 2015 8:08 pm 
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It's not quite as easy as it seems.

The short answer is that you have to do a study on "everything" over 50 V. Mind you, this may not be a very detailed study.

As an example, let's say the firm in question is an electrical service contractor. They work all the time on other people's equipment. It is flat out unrealistic for such a company to perform a detailed engineering study on everything they work on. They are at the mercy of the host employer for that. In this specific case there are a set of tables in 70E (and the NESC) that give some guidance for handling equipment where an engineering study has not been performed. In practice according to an analysis that was done on real arc flash incidents it appears that the table approach adequately protects workers about 50% of the time, which is significantly better than the 90% failure rate when nothing is done at all, and not nearly as good as the 100% success rate using IEEE 1584.

An engineering study does NOT mean that a detailed calculation must be done on any and all equipment. But the key areas of concern may surprise you:
1. In 2009 in Georgia OSHA documented two electricians who were disassembling a temporary construction panel BEFORE the utility came to shut the power off. One died and one was seriously injured. The panel voltage was unspecified but is most likely 240/120.
2. Up to 130 VDC, a study performed at Kinetrics lab commissioned by Duke Power showed that if the electrodes are very close together (1/2") but not too close, there is enough energy to just reach the 2nd degree burn threshold.
3. Several similar tests have been performed by private utilities and EPRI and the aggregate result is published in the NESC standard that 4 cal/cm^2 arc rated PPE is enough to survive any type of equipment under 250 V. This is actual testing. There is no model that accurately represents what happens under 250 V, and since utilities are now required by OSHA to mandate arc flash PPE for pretty much all work under all circumstances, there is no longer a need to look at a lower threshold in the utility business.
4. Currently IEEE Standard 1584 has a lower cutoff of "less than 240 V" (208 V) fed by a single transformer smaller than 125 kVA. Based on preliminary test work performed by a joint NFPA/IEEE study (data is not publicly available), this threshold is likely to be lowered but as of right now it's really the only consensus safety standard we have for a lower voltage/current cutoff. Thus for low voltages it is possible to do a quick survey and document whether the incident energy is "<1.2 cal/cm^2" or "<4 cal/cm^2" by combining these two results. Any study which reports results using IEEE 1584 below 250 V should be viewed with extreme suspicion because the study only used a single data point because none of the other lab tests that it is based on could sustain an arc for long enough to get a valid data point. Fortunately the IEEE cutoff captures the vast majority of lighting panel installations. This exception was removed from 70E a few years ago because it is method-specific but still lives on in IEEE 1584.
5. For the most part once a transformer exceeds 1500 kVA for 480 or 600 V unless the impedance is extremely high, it is not practical to use primary protection to protect against an arc flash on the secondary side. Thus the incident energy rating on the first breaker/disconnect is frequently high enough that PPE is simply not available. The most practical solution aside from smaller transformers right now is to use relaying on the secondary side to trip the primary side protection.
6. In the same conditions at 4160 V, the maximum transformer size increases up to around 20,000 kVA. Thus somewhat nonintuitively the most dangerous equipment tends to be 480 or 600 V equipment. As voltage increases, the incident energy tends to decrease on an equal power capacity (kVA/MVA) basis.
7. Another significant factor is how much the equipment design tends to "focus" the arc flash. The decrease in incident energy with distance is vastly less for switchgear compared to open air wiring.
8. The list that you alluded to in terms of equipment is from the National Electric Code. It gives a list of equipment which are examples only. Labels are required on anything that gets worked on while energized "frequently" (typically taken to mean 1 or more times per year). Labels are not required on other items but an arc flash study would still have to be done when the work occurs.
9. Not every device has to be looked at and a top-down approach tends to be used. When looking at arc flash from an overall point of view, it would seem that the farther one gets away from the source of energy, the lower the short circuit current and thus the lower the arc flash should be but this does not hold true for all cases. It is definitely the case once the incident energy is so low that the overcurrent protection devices (fuses or breakers) won't trip for at least 2 seconds. At that point, longer distances result in lower arc flash. However when the trip time is less than 2 seconds, longer distances tend to lower the short circuit current but since this also increases the trip time, paradoxically incident energy increases with distance. The second case to consider is that depending on the distances involved, the increase may be modest enough that the impact on arc flash is inconsequential and thus experienced engineers can frequently make a judgement call as to when to stop looking further. This works unless there is a transformer involved, which typically increases incident energy on the downstream side.
10. Not all equipment is worked on energized. For instance most plants do not work on motor terminations while energized. Energized testing is typically done at the starter where the test points are readily accessible, and lockout is performed at the starter as well. Thus the motor is almost never worked on live except possibly to test for absence of voltage, and the motor is downstream of a circuit breaker or fuses that implement short circuit (instantaneous) protection. Thus not so much in terms of arc flash but for very practical reasons motor terminations are not typically labelled.

Thus as I said...it is "everything above 50 V". In practice the vast majority of electrical equipment (the loads) can be eliminated because standard practice is not to work directly on much of that equipment live. Low voltage loads can sometimes be a concern but can be identified almost as a category because there are no valid detailed study methods available and scant testing results.. And finally a "top down" approach with the distribution system can quickly identify some areas that need not be analyzed as cutoffs are reached by the protective devices. This can still leave a large (and expensive) amount of work, but it cannot be avoided.

Finally we come back to the "tables vs. calculations" argument. The table approach would seem to be quick, cheap, and easy. But there are three problems with the approach:
1. The tables have maximum assumed cutoffs (trip times) that have to be checked. The labor necessary to do this is identical to the calculation approach so there is no actual cost savings if the table method is done right.
2. In practice the table approach correctly predicts the required PPE 50% of the time. The calculation approach correctly predicts required PPE 100% of the time. From a safety point of view, the choice is pretty clear.
3. Even overcoming these issues, the table approach is necessarily (but not entirely) "overly conservative". The PPE requirements specified in the calculation approach will almost always be much less than what the table approach requires. Thus there are clear operational and safety improvements by using the calculation approach.


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 Post subject: Re: question - for which equipment is arc flash study requir
PostPosted: Mon Aug 31, 2015 7:40 am 
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Wow, that is fantastic information - will be VERY useful. I certainly appreciate that very much. I will study your information carefuelly, and will repost if something is not clear. Once again, thank you very much and sorry for later response - I was away for a while. Niru


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