# PPE and VFD's



## paulengr (Oct 8, 2017)

JRockey said:


> First let me preface this post by saying I am by no means an expert on PPE/Arc Flash etc, etc. I am simply trying to learn more to protect my employees and still be able to offer a cost effective product for my customers.
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> ...




First let’s start at the top. OSHA requires the employer to protect employees against RECOGNIZED hazards. This includes the idea that there is ample research and engineering to back it up. If there is not a regulation, a Code aka a consensus safety standard, etc., then even if a hazard is known and exists, lacking direction, OSHA can’t enforce. That’s the case with arc blast and exploding pieces from electrical equipment. Electrolytic caps look very impressive when they explode but I haven’t seen much in terms of momentum behind the thin aluminum foil and paper inside. Second if you add a box in terms of arc blast you are virtually guaranteeing you will launch the door so you’re going from a potential hazard to a definite one. The enclosure pressurizes up to around 4-10 PSI before the hinges and fasteners instantly fail and the door flies off within 10-30 milliseconds, way too fast to get out of the way.

Second when it comes to shock hazards overall DC shock concern is the pain threshold when it lets go which is usually around 100 V or so, or burns (Joule heating) at 1-10
A or more. The bus caps at around 100 HP or so definitely can kill. There is a discharge resistor and most VFD manufacturers say wait at least 15 minutes and don’t trust the light. I agree...put your meter on the DC bus and test (test before touch as per OSHA) or on big ones you may want to setup probes with a discharge resistor for say 1-10 A.

Finally with VFDs there are three things to look at in terms of arc flash. First is the amount of energy stored in the bus caps which is in Joules so you can just convert to calories and then estimate the area of a sphere at the working distance and divide to get cal/cm2. That’s how the national research labs do it. Second is if you have a bypass. If the bypass fails then effectively the VFD is ignored so it is treated as if you just have a starter. Third is look at the maximum (overload) limit of the drive, usually about 150-300% of rated current. This is your maximum fault current draw for modeling purposes. If the current is higher, the transistors just become fuses. Don’t forget to include the semiconductor fuses. This means the Allen Bradley medium voltage drives that have an ancient design with inductors instead of bus caps are the most dangerous (inductors don’t dump the stored energy quickly) and fusing is critical. In practice in terms of arc flash the hazard is much less than an equivalent horsepower starter, particularly because the VFD electrically isolates both sides.

On a 12 MW common DC bus system running 690 VAC the arc flash was 1.0 cal/cm2 in the worst area. There is no standard for DC arc flash calculations and little testing either other than some tests that showed that the DC calculations are off by a huge margin (grossly overestimating the hazard), even Ammermans version.

PM if you need more.


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## Wiresmith (Feb 9, 2013)

paulengr said:


> First let’s start at the top. OSHA requires the employer to protect employees against RECOGNIZED hazards. This includes the idea that there is ample research and engineering to back it up. If there is not a regulation, a Code aka a consensus safety standard, etc., then even if a hazard is known and exists, lacking direction, OSHA can’t enforce.


i disagree, known hazard is known hazard

osha is not the safety expert in your field, you are

in a specialized field like the OP is describing he is the specialist and there are not standards for everything nor can there reasonably be


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## Wiresmith (Feb 9, 2013)

paulengr said:


> First let’s start at the top. OSHA requires the employer to protect employees against RECOGNIZED hazards. This includes the idea that there is ample research and engineering to back it up. If there is not a regulation, a Code aka a consensus safety standard, etc., then even if a hazard is known and exists, lacking direction, OSHA can’t enforce.


i disagree, known hazard is known hazard

osha is not the safety expert in your field, you are

in a specialized field like the OP is describing he is the specialist and there are not written standards for everything nor can there reasonably be, you have to use reasonable judgement

would a reasonable person with requisite knowledge in the safety of the field believe there is a reasonable hazard? if it has happened before in your shop and looked like it could hurt someone, then you should take some kind of step for sure


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## Wiresmith (Feb 9, 2013)

if the osha specialist says clearly your fine without out it, then that's good reasoning for not having the cabinet. but then again since your asking the question in the first place, if someone winds up getting hurt from it, then that is good reasoning for you should've had the cabinet.

i recommend you error on the side of caution in this area, your the specialist. if your not sure error on the side of caution, if your confident it's fine, it probably is.

but error-ing on the side of caution is not the real-world industry standard if thats what your asking for


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## paulengr (Oct 8, 2017)

Wiresmith said:


> if the osha specialist says clearly your fine without out it, then that's good reasoning for not having the cabinet. but then again since your asking the question if someone winds up getting hurt from it, then that is good reasoning for you should've had the cabinet.
> 
> i recommend you error on the side of caution in this area, your the specialist. if your not sure error on the side of caution, if your confident it's fine, it probably is.
> 
> but error-ing on the side of caution is not the real-world industry standard if thats what your asking for



Not true.

I’ll give you an example: arc flash on transmission lines. Does it exist? Obviously. But what is acceptable for protection? How far away or what PPE? The highest voltages with any test data are around 15-20 kV. At those voltages arcs are short so we can treat it somewhat as a point source. But we have no way to handle a six foot long arc. Geometry becomes a big consideration. Transmission line arc flash is in the same place low voltage equipment was in the 1980s. We know it exists but we have no way to quantify or address it. There are no standards, studies, or anything else. You can’t enforce if there is no known way to address it and that’s what I started with. Sure we have NESC but that’s pure conjecture and when you drill down to the footnotes based on impossibly short arcs for transmission voltages.

So whatever your solution is has to be accepted absent any other competing standard. OSHA can enforce arc flash to a point. It is now known as in there is a consensus safety standard for low voltage. In utilization (not .269) it is just the general duty clause. The employer is free to use any standard even one they make up as long as there is a standard for AC. It is a weak position if it’s not based on some kind of research or consensus standard. It is a much stronger position though to use a consensus standard and not just make something up where both the technical and implementation must be defended as opposed to the implementation of a standard only.

There are theoretical papers on DC arc flash. There is a smattering if test results that show the papers are totally wrong. We’re in the same place in DC as we were on AC in the 1990s. If a DC case went to court OSHA would lose because the science is far from well settled. That’s why OSHA started enforcement actions in 2006 when AC arc flash was pretty well settled for 480-600 V.

Vehicle collision avoidance exists today too on a lab basis but it cannot be mandated on truck fleets because a reliable system is far from existing despite what all the self driving car nuts claim. Again it’s not enforceable at this time despite the fact that the hazard is pretty well known.


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## Wiresmith (Feb 9, 2013)

paulengr said:


> We know it exists but we have no way to quantify or address it. \


the original poster, knows how to address his problem. 


obviously if an asteroid hits the guys building osha ins't going to site him for not wearing hardhats.


you don't need a study, no matter what any law says, you still have to go by reasonable judgement (what a reasonable person with requisite knowledge in the field would consider reasonable) 

your always encouraged to use common sense whether there is or not a standard on a subject


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## paulengr (Oct 8, 2017)

Here’s the problem with fir instance the DC formula in the 70E annex D.8. In testing it is at least 2-4 times higher than reality at 260 VDC. The equation is Volts x Amps x seconds times some other stuff. Arc voltage is almost fixed though. So as voltage increases the equation is increasingly wrong, and for anything other than a little single phase microdrive it will be completely insanely over protective, causing increased hazards from loss of dexterity, visibility, and heat exhaustion from a bull hockey equation some engineer pulled out of thin air that clearly does not represent anything in the real world.


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## gpop (May 14, 2018)

On new installs we use small fuses to bring the drive online. Once the drive is up it will do a self check. As for programming most drives can be programmed using some type of remote programming tool. 

Once the drive is up then the fuses can be set to the correct size and programmed then the drive can be started using the control wiring (plc). 

Ive seen drives blow and its impressive but in every case the arc force was vented backwards towards the mounting surface. The worst case is when the drive is mounted in a cabinet which redirects the force forwards (door open).


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