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 Post subject: 120V 20A circuit / 50 Volts or greater -when is NO PPE reqd
PostPosted: Fri Apr 24, 2015 3:37 pm 
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If Category 1 PPE is the lowest under NFPA 70E - 2015, per 130.7.C (16) and per the same Table no.

and 130.5 requires "if a hazard exists, that an assessment be done" and the NFPA 70E-2105 handbook commentary after 130.5 indicates that "a potential arc flash hazard is presumed to exist" when there are exposed circuit parts operating at 50V or higher.

and there is no methodology in Annex D for single phase 120V...

Does that mean that PPE 1 is required when dealing with a 120V / 20A circuit feeding a power supply in a control panel?

Similarly wouldn't PPE 1 be required in a receptacle j-box...how about simply extracting a plug from a receptacle since we are exposed to exposed energized parts 50V or higher.

Where is the "get out of PPE" card for 120V - 20A circuits or isn't there one?

HRC 0 used to cover this but I understand a reason for the removal of Category 0 was the hazard of non arc rated clothing igniting due to sparks/arcing event even at low calories.... with natural fiber clothing igniting at 700 ish degrees and immediately burning at over 1400 degrees - burning natural fiber clothes were a temperature multiplier.

So at 50 Volts or greater - when is no PPE required?

Can anyone shed some light on this?

Thanks all!


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 Post subject: Re: 120V 20A circuit / 50 Volts or greater -when is NO PPE r
PostPosted: Fri Apr 24, 2015 3:54 pm 
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70E takes the stance that it is a SAFETY standard, and that essentially someone else develops and propagates standards for incident energy analysis. There are basically 3 methods for quantifying the incident energy hazard, though the risk still has to be handled differently. They are:
1. Tabular methods. 70E provides tables, and so does NESC. The NESC tables are very applicable for <250 VAC but in general limit it to 4 cal/cm^2 for everything, and so does 70E. IEEE 1584 suggests that it is less than 1.2 cal/cm^2 for circuits under 240 VAC fed by a single transformer that is under 125 kVA. This will be revised at some point in the future but it is still up in the air.
2. Calculations. This doesn't work. Low voltage arcs tend to be unstable and/or self-extinguishing. IEEE 1584's raw data includes only a single value at 208 V because that's the only one where a stable arc was formed. So we can stop right there.
3. Actual test data. This is also the underpinning of the NESC tables.

The way I handled it is this:
1. Test data on DC systems shows <=1.2 cal/cm^2 for 125 VDC or smaller systems with a 20 kA current rating, plenty bi cover typical 60 cell substaiton batteries. Use Ammerman or Neal/Doan calculation above this range, which is very conservative (off by 200% or more compared to actual test data).
2. Under 50 VAC, don't bother.
4. 50-208 VAC and fed by one transformer <125 kVA or smaller or equivalent in short circuit current, <1.2 cal/cm^2, as per IEEE 1584.
5. For anything else up to 250 VAC, <4 cal/cm^2 as per NESC and 70E tables.

It's not the same as an actual "calculation" but since there is no calculation method right now, this is the best I can do.


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 Post subject: Re: 120V 20A circuit / 50 Volts or greater -when is NO PPE r
PostPosted: Sat Apr 25, 2015 8:49 am 
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BGnan wrote:
and 130.5 requires "if a hazard exists, that an assessment be done"


The requirement in 130.5 is to perform a Risk Assessment to determine if a hazard actually exists.

Per the Article 100 definition, a Risk Assessment:
Quote:
...estimates the potential severity of injury or damage to health, estimates the likelihood of occurrence of injury or damage to health, and determines if protective measures are required.


Effectively this is why there is now a Table 130.7(C)(15)(A)(a), where NFPA70E pretty much acknowledges that not all 'interactions' with energized equipment results in an arc flash event requiring PPE.


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 Post subject: Re: 120V 20A circuit / 50 Volts or greater -when is NO PPE r
PostPosted: Sat Apr 25, 2015 1:16 pm 
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Thank you - I see where in that table - seven tasks up from the bottom on page 147 of the handbook - it specifically indicates that PPE is NOT required for work on control circuits with exposed energized conductors 120V or below - where there is no other exposed equipment over 120V - thanks again!


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 Post subject: Re: 120V 20A circuit / 50 Volts or greater -when is NO PPE r
PostPosted: Mon Apr 27, 2015 9:53 am 
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Very interesting discussion. Everyone is talking about arc flash PPE. Let's not forget that shock PPE is required above 50 volts and 70E requires both risk assessments (arc flash and shock).


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 Post subject: Re: 120V 20A circuit / 50 Volts or greater -when is NO PPE r
PostPosted: Tue Apr 28, 2015 5:40 am 
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rdj wrote:
Very interesting discussion. Everyone is talking about arc flash PPE. Let's not forget that shock PPE is required above 50 volts and 70E requires both risk assessments (arc flash and shock).


Yes, it does. However at low voltages again things are a bit ugly. Up to 300 V, the restricted approach boundary is "avoid contact". Thus shock protection would be required if intentional or unintentional contact would occur. 130.7(C)(7)(a) is clear that this means voltage rated rubber gloves when contact can/may occur. So following the ANSI Z10 hierarchy of controls, this is the least desirable method of protection (PPE) and other methods are recommended whenever/wherever possible. The means to do so would be either to isolate or insulate the circuit parts from the person. Examples would include using insulated probes on a multimeter or insulated tools. 70E is pretty weak in going into details on this because there are about a dozen different work methods. Similar to OSHA 1910.269 and Subchapter S, the distances come from IEEE Standard 516, "Guide for Maintenance Methods on Energized Power Lines", and that standard has extensive discussion about work methods and explanations of for instance primary vs. secondary insulation.

Why mention this? Because a casual reader would come to the conclusion that arc flash PPE is the one and only way to provide protection from arc flash when in fact the risk assessment process as well as ANSI Z10 points out that PPE should only be used when all other options are exhausted. Similarly it would appear from casual reading of 70E that the one and only method for shock protection is via rubber gloves when in fact just the opposite is required.


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 Post subject: Re: 120V 20A circuit / 50 Volts or greater -when is NO PPE r
PostPosted: Wed Apr 20, 2016 8:51 am 
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I'm not an electrician or an electrical engineer. Is there a amperage below which there is no shock hazard for under 50V?


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 Post subject: Re: 120V 20A circuit / 50 Volts or greater -when is NO PPE r
PostPosted: Wed Apr 20, 2016 9:26 am 
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BarryB wrote:
I'm not an electrician or an electrical engineer. Is there a amperage below which there is no shock hazard for under 50V?


NFPA 70E does not require shock protection below 50V. Current doesn't come into the equation for shock hazard, just voltage.


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 Post subject: Re: 120V 20A circuit / 50 Volts or greater -when is NO PPE r
PostPosted: Wed Apr 20, 2016 1:21 pm 
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BarryB wrote:
I'm not an electrician or an electrical engineer. Is there a amperage below which there is no shock hazard for under 50V?


70E uses voltage as does the standards that it comes from. However if you really must know...this all comes from the work of Charles Dalzeil. He was the first one to study the effects of electricity on organisms. His work proved two things. First that electrical shock effects are proportional to body weight and second he was able to determine the thresholds for various effects such as fibrillation based on shocking animals (thus the need for the correlation...not enough available human subjects for some of the more severe testing!)

There are more details out there if you google his name but essentially mammals are highly susceptible to shocks at around 50-60 Hz (below or above this point is less of a hazard). If we are not considering tissue damage then below 0.08 s, there is little danger because the pulse doesn't last long enough to interfere with normal heart rhythm and above 5 seconds if heart fibrillation does not occur, it won't occur.. For human size subjects at around 100 mA or less, fibrillation will not occur after 5 seconds.

Now we need to get from here to a voltage. IEEE standard 80 specifies this as 1,000 ohms while the corresponding IEC standard uses a variable with a minimum of 650 ohms. Taking 100 mA as fatal and using Ohm's law (V=IR), we arrive at a fatal threshold of 65-100 VAC. In practice there are no known fatalities due to electrical shock below 50 V except for some very questionable data from a couple cases in China which were not even reported as specific values but a range of values.

50 V is pretty much the standard everywhere and it is extremely pervasive. Telephone and data systems use 48 V including Power-over-Ethernet (PoE) which uses 48 V as the standard voltage. Most DC batteries for vehicles are 6, 12, 24, or 48 V.

The only place I know of where 50 V is not the accepted standard is New Brunswick province in Canada which uses a 28 V standard (albeit with a huge number of implementation problems) although the rest of the country is 50 V.

There have been some potential medical problems reported by welders with open circuit voltages in the range of 50-150 V that purport to cause all kinds of strange neurological disorders but I haven't seen anything conclusive from any of it.

Also a word about DC is in order. With DC there is not a fibrillation issue and roughly 1 A is necessary to cause tissue destruction. Instead of a fibrillation hazard DC standards are usually 100 VDC. The concern in this case is that there is a severe amount of pain caused when de-energization occurs and the pain threshold varies from one individual to another but has been somewhat arbitrarily set at 100 VDC.


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