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 Post subject: Finger-Safe Guards
PostPosted: Thu Sep 18, 2008 12:39 pm 

Joined: Thu Sep 18, 2008 12:35 pm
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Does adding Finger-Safe guards to a panel reduce the Arc Flash hazard?
ie. Putting plexiglass ove all exposed 480 vac connections.


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PostPosted: Thu Nov 13, 2008 10:15 am 
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Location: Michigan
We have been recently debating that too; unfortunately I don't have a solid answer and can only offer the conclusion that most of us where I work seemed to have arrived at.

I believe finger-safe guards designed to prevent electric shock due to direct contact ultimately help prevent arc flash events from occuring (ie. dropped tool ect.).
I don't believe the guard could contain the blast or reduce the incident energy level however it may prevent the incident from happening in the first place. In the event that an arc flash actually does occur, that finger-safe guard probably does not have an ATPV rating and therefore is just one more material present in the arc blast.

So the question remains, to guard or not to guard? I would use the covers but not consider them a safety factor when determining the arc flash hazard unless they are listed for that purpose by the manufacturer.


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PostPosted: Mon Nov 17, 2008 9:35 am 
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jonp20001 wrote:
Does adding Finger-Safe guards to a panel reduce the Arc Flash hazard?
ie. Putting plexiglass over all exposed 480 vac connections.


Assuming the plexiglass is voltage rated, that would be one way to reduce the approach distances (not arc flash exposure) cited in NFPA 70E. This is a legitimate concern (and benefit) because control system work often encroaches on the these approach boundaries for 480V. Barriers might allow a person to work on low voltage control wiring in a cell without needing the precautions of Restricted or Prohibited 480V approach boundary work.

The prescriptive Arc Flash PPE table in NFPA70E recognizes similar reduction in probable contact, with lower prescribed PPE not based on calculated exposure. That is testamony to safety improvements such barriers would achieve though there is no method given for applying this to the calculated exposure.


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 Post subject: Re: Finger-Safe Guards
PostPosted: Mon Nov 26, 2018 5:25 am 

Joined: Mon Nov 26, 2018 5:17 am
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Plexiglas guards can also make it harder to trouble shoot the equipment, you can not do thermography through Plexiglas.


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 Post subject: Re: Finger-Safe Guards
PostPosted: Tue Nov 27, 2018 8:11 am 

Joined: Fri Sep 28, 2018 7:32 am
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Location: Evansville, IN
My belief is that plexiglass "finger-safe" guards are a good means of protecting against electric shock due to accidental contact with energized parts. My issue is that these "shields" can give personnel a false sense of safety. Temperatures of an arc event are around 35,000 degrees while the melting point of plexiglass is 320 degrees. In the event personnel are accessing a panel while energized they should not become complacent due to the "guarding" as it will in no way contain the blast from an arc event and in reality will only add to the shrapnel.


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 Post subject: Re: Finger-Safe Guards
PostPosted: Fri Nov 30, 2018 2:37 pm 
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My belief is that plexiglass "finger-safe" guards are a good means of protecting against electric shock due to accidental contact with energized parts. My issue is that these "shields" can give personnel a false sense of safety. Temperatures of an arc event are around 35,000 degrees while the melting point of plexiglass is 320 degrees. In the event personnel are accessing a panel while energized they should not become complacent due to the "guarding" as it will in no way contain the blast from an arc event and in reality will only add to the shrapnel.


Old thread here but...

Fundamentally an arc flash is an arcing fault. An arc flash risk (called a "hazard" in 70E) is when there is more than a very remote likelihood of an arcing fault. One obvious condition is if there are exposed conductors and energized work is being conducted. There are other conditions but this is the most obvious one. If there is no more than a very remote chance of causing an arcing fault then there is no arc flash risk (hazard). Again, this is the way 70E defines it and makes it clear that this is not an absolute consideration. Examples that 70E gives where an arcing fault would not occur are just walking by and normal operation of equipment. An extreme example of non-energized work which could cause an arc flash would be for instance fishing cables into an energized cable space. Finger guards are good examples of how to convert exposed conductors into guarded ones which removes the exposure. This allows for instance the ability to safely take voltage measurements using probes that can poke around the corners of the guard while not leaving anything exposed and thus no shock or arc flash hazard (with an appropriate Category meter and the probe tips in place).

Once you get an arcing fault, to put it simply, it makes a mess. It can eat through conductors, all plastics, metals, etc. The pressure developed is essentially linear and bounded only by the pressure limits of the enclosure which in practice gets to a maximum of about 2-10 PSI before the doors are gone according to just about every peer reviewed study on the subject I can find. After that the pressure drops down to a very minimal value.

However, the 35,000 degree claim is utterly bogus. Tammy Gammon needs to go back and reread the scientific papers that she is quoting from when she throws this one around because it's totally wrong. First you mixed your units a little. 35,000 degrees is C not F. It's 20,000 C. It comes from a series of scientific papers on arcs that show that the arc core is a plasma. Scientifically plasmas are sort of characterized by temperature so the properties of the plasma require this value. The paper also shows that the arc core is fully contained within a magnetic field that restricts the diameter of the arc core to a couple millimeters in diameter MAXIMUM. But the same papers also state very clearly that nearly all of the heat from the arc core is absorbed by the surrounding air and says absolutely nothing about the surrounding conditions. The heat radiated out away from the arc core though in no way creates a 35,000 F temperature at a distance of 12 or more inches away. That's utter nonsense. If that were the case then basically ALL solid materials would not only be melted but the electrons would be blown off and they'd be literally vaporized leaving nothing left.


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 Post subject: Re: Finger-Safe Guards
PostPosted: Wed Dec 05, 2018 9:06 pm 
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A couple of thoughts:

1. Covered does not not mean insulated, but, it sure helps.

2. Finger safe is IP20, this means that a 12 mm ball cannot touch live parts when the conductor is inserted.


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 Post subject: Re: Finger-Safe Guards
PostPosted: Mon Dec 10, 2018 6:40 pm 
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Flash wrote:
A couple of thoughts:

1. Covered does not not mean insulated, but, it sure helps.

2. Finger safe is IP20, this means that a 12 mm ball cannot touch live parts when the conductor is inserted.


Finger safe is a little more complicated than just the sphere test. There is also the "mechanical finger" test which allows bigger holes but then you can stick your finger in and not go any further.

Also missing from IEC is say an "arm safe" test...can you fit your arm through an opening and not be able to touch the conductors, or similar to the sphere test is the opening too small to fit more than a finger into it? This would allow for a much greater range of conditions such as use of chain link fence or various perforated, chicken wire, or square hole covers that allow plenty of ventilation, are inexpensive, with much larger holes than 10 mm, yet still prevent access to energized conductors. The finger safe test is applicable for small control power terminals but falls apart when addressing larger equipment configurations.


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 Post subject: Re: Finger-Safe Guards
PostPosted: Mon Jan 07, 2019 4:30 pm 
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PaulEngr wrote:
Flash wrote:
A couple of thoughts:

1. Covered does not not mean insulated, but, it sure helps.

2. Finger safe is IP20, this means that a 12 mm ball cannot touch live parts when the conductor is inserted.


Finger safe is a little more complicated than just the sphere test. There is also the "mechanical finger" test which allows bigger holes but then you can stick your finger in and not go any further.

Also missing from IEC is say an "arm safe" test...can you fit your arm through an opening and not be able to touch the conductors, or similar to the sphere test is the opening too small to fit more than a finger into it? This would allow for a much greater range of conditions such as use of chain link fence or various perforated, chicken wire, or square hole covers that allow plenty of ventilation, are inexpensive, with much larger holes than 10 mm, yet still prevent access to energized conductors. The finger safe test is applicable for small control power terminals but falls apart when addressing larger equipment configurations.


If you can post that data, it would be interesting


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 Post subject: Re: Finger-Safe Guards
PostPosted: Tue Jan 08, 2019 10:41 pm 
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Flash wrote:
PaulEngr wrote:
Flash wrote:
A couple of thoughts:

1. Covered does not not mean insulated, but, it sure helps.

2. Finger safe is IP20, this means that a 12 mm ball cannot touch live parts when the conductor is inserted.


Finger safe is a little more complicated than just the sphere test. There is also the "mechanical finger" test which allows bigger holes but then you can stick your finger in and not go any further.

Also missing from IEC is say an "arm safe" test...can you fit your arm through an opening and not be able to touch the conductors, or similar to the sphere test is the opening too small to fit more than a finger into it? This would allow for a much greater range of conditions such as use of chain link fence or various perforated, chicken wire, or square hole covers that allow plenty of ventilation, are inexpensive, with much larger holes than 10 mm, yet still prevent access to energized conductors. The finger safe test is applicable for small control power terminals but falls apart when addressing larger equipment configurations.


If you can post that data, it would be interesting


http://www.ee.co.za/article/standard-te ... ester.html

Test "fingers" are pretty readily available. You'd have to get a copy of the IEC standard to see specifically what the test finger data is.


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