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 Post subject: Labeling requirement for 50V and above.
PostPosted: Thu Sep 06, 2018 3:51 pm 
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Joined: Sat Apr 04, 2009 9:50 pm
Posts: 104
Location: San Antonio, TX
The NFPA 70E-2018 establishes when electrical equipment requires an arc flash label:

Three requirements that need to be satisfied simultaneously:

1. A list of type of equipment SUCH AS switchboards, MCCs, panel-boards, etc.
2. Equipment likely to be exposed (i.e. opened due to inspection, maintenance, service, etc.)
3. Equipment energized.

It does not establishes the voltage of the equipment.

But, in the definition of electrical hazard, it establishes that equipment less than 50 V is not considered hazardous. Also, the electrical safe working condition is only applied for equipment 50 V and over.

Should we conclude then that arc flash labels should be placed only on equipment that satisfy the following 4 requirements?

1. A list of type of equipment SUCH AS switchboards, MCCs, panel-boards, etc.
2. Equipment likely to be exposed (i.e. opened due to inspection, maintenance, service, etc.)
3. Equipment energized.
4. 50 V and over


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 Post subject: Re: Labeling requirement for 50V and above.
PostPosted: Sat Sep 08, 2018 3:28 pm 
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You've caught a couple strange things in the somewhat contorted wording. Yes it doesn't given a lower cutoff but then later in the standard it is clear that there is a 50 VAC (100 VDC) cutoff for shock. None for arc flash but the Committee is still hung up on proving a negative and won't just accept that even below 130 VDC there's not enough of an arc flash to exceed the limits (1.2 cal/cm2) established in the rest of 70E. You get similar language in terms of the energized work permit requirement followed almost immediately by an exception for troubleshooting and similar work that is done energized. So unfortunately this is a place where you need to read from back to front instead of front to back.

ALSO the list of equipment that must be labelled comes from NEC (NFPA 70), not 70E. NEC also specifies that it is equipment that requires frequent maintenance so no reason to label a junction box that only gets opened once a decade, nor to label equipment that does not need an arc flash study. Of course you get into the catch-22 with unlabelled equipment, which is when someone does need to access it, what is the rating? So it's not as cut and dry as the standard makes it out to be.


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 Post subject: Re: Labeling requirement for 50V and above.
PostPosted: Mon Sep 10, 2018 8:01 am 
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Joined: Tue Jan 19, 2010 2:35 pm
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For me, if you do a study, it needs to include just about everything. But you need to use common sense as well.

During training classes, I'll ask the students "why don't we label light switches and receptacles? After all, there is a shock and arc flash hazard associated with these devices." Of course, the question is designed to get them to think about when and where electrical hazards exist. Further, if we were to label everything, the labels lose their intended purpose and become just another warning to be ignored. Look at the tags on extension cords or Christmas lights. Who actually reads these? However, as far as arc flash labeling goes, my view is if there's a shock hazard, it should be labeled. If the arc flash hazard is below 1.2 calories AND the device has a voltage warning label that's in good condition, I don't see that labeling is necessary. Otherwise, label. Finally, if there is an accident, you'll likely be asked why didn't you label this device? I'd want solid justification for not putting a shock and arc flash warning label on the device.


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 Post subject: Re: Labeling requirement for 50V and above.
PostPosted: Wed Sep 12, 2018 10:42 am 
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To finish this up the other example I use is why don't we erect meteor shields on Earth? After all we all know that the hazard from being struck by a meteor, even a very small one, is 100% fatal every time. The reason is that the likelihood is very small.

Arc flash is like that, too. With shock all that is required is to get close enough to something exposed and shock is all but guaranteed. But with an arc flash the key word here is arc. Something has to happen to cause an arc. There are three reasons for this to happen. The first is that someone is doing some kind of activity that has a really high propensity to form an arc. An example is installing, removing, or digging through wires while they are energized. We can determine which activities these are before they are actually done so this list of activities can be determined in the office.

The second reason is that the equipment fails on it's own without any direct involvement of personnel and spontaneously arcs. In this scenario, merely being in the wrong place at the wrong time is sufficient to cause a major injury. From an absolute point of view, these failures have to exist. After all I'm sure we have all gone out to troubleshoot equipment and found that an arcing fault that happened while nobody was around caused it to fail. But at the same time I have been unable to document any cases in the OSHA databases of someone just walking by. The 70E Committee has also stated that the likelihood of this condition is so unbelievably low that they don't even consider it a risk. Furthermore if this was a serious problem, the various consumer safety groups would declare electrical equipment to be a serious public health hazard and ban it's use. What's worse if you accept the premise that electrical equipment is inherently dangerous then there is simply no way to support using it and it needs to be completely banned, right down to ripping out light switches and sitting in the dark. I think the jury might still be out on flash lights when taking this approach.

The third reason is that something has changed in the condition of the equipment so that it has already failed but that the failure is hidden. The failure is not revealed until an otherwise normal operation causes the arcing fault to happen. To take an example from OSHA in a parking deck the operators used switching duty breakers to turn the lights off at the end of the shift. At an earlier time an electrician worked on the electrical panel. To block off an empty hole in the breaker panel, the electrician closed a piece of sheet metal in the door instead of securing it with fasteners. When an operator turned off the breakers at the end of the shift, the sheet metal strip fell down into the bus bars of the lighting panel and triggered an arc flash, severely injuring the parking garage operator. Many other causes such as fork truck damage, water/dust/oil infiltration, all can set up a condition where otherwise normal operation such as closing in a disconnect or breaker sets off an arc flash. In the vast majority of these cases, simple visual inspection looking for obvious signs of failure can very easily detect the vast majority of potential failures of this kind such as looking for leaking fluids or black char marks around doors and openings, and doing recommended routine preventative maintenance. Granted there can be completely hidden failures particlarly when it comes to poor maintenance practices but simple visual inspections eliminate the vast majority of these.


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 Post subject: Re: Labeling requirement for 50V and above.
PostPosted: Fri Sep 14, 2018 3:39 am 

Joined: Fri Sep 07, 2018 1:59 am
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Hidden failures are so annoying to pinpoint. Are preventative maintenances pretty much the best defense against those?


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 Post subject: Re: Labeling requirement for 50V and above.
PostPosted: Sat Sep 15, 2018 8:56 am 
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SylvesterM wrote:
Hidden failures are so annoying to pinpoint. Are preventative maintenances pretty much the best defense against those?


The best tools by far are predictive maintenance tools. For example taking oil samples once a year in an oil filled transformer can reliably detect failures as they are occurring inside a transformer well before it needs to be taken out of service. These exist for a lot of equipment such as vibration analysis for rotating machinery as well as PdMA for motors. The other big advantage of nearly all of these approaches is that they work while the equipment is in service so no or very little impact to normal operation.

But with a lot of electrical equipment such as a circuit breaker there is no such tool, and that's where we get into hidden failures. In fact that's the case with a lot of equipment. In this case we have to do something to it, usually offline, to test operation of the equipment. The vast majority of electrical equipment (over 80% according to large scale maintenance studies done) fails essentially purely randomly with a fairly consistent rate. So knowing the desired risk rate (maximum percentage chance of failure on demand) and if we have an estimate of the failure rate of the equipment, we can easily determine an inspection/testing frequency where we periodically go in and inspect to find failures before they become a problem while in service.

There are of course exceptions. Some sensors have diagnostic data. For instance it is usually a lot easier to figure out when a continuous level sensor stops working compared to a float switch because we can monitor the level and notice if either it isn't changing, or it is pegged at an extreme low or high value. And the level sensor itself may also have diagnostics that detect problems with operation. All of these contribute to a lower risk of hidden failures compared to the simple float switch.

Another exception is that when all of the equipment is the same brand and model, sometimes manufacturer design errors cause multiple failures simultaneously. A customer of mine is having this problem with several GE Powerbreak II insulated case breakers where the trip units are all failing at about the same time. Yet another exception is when the testing procedure is only able to detect a percentage of the defects. In general it is usually not possible to detect every failure with most equipment, no matter how rigorous the testing is. Accepting this and building in some redundancy or tolerance for failures makes a far more robust system and far easier testing regimen.


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