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Author Topic: GFCI class questions  (Read 8087 times)

Don Gspann

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GFCI class questions
« on: July 04, 2017, 01:04:16 PM »

Has any research been posted regarding using anything other than Class A GFCI's in audio?
Search results showed minimal information. Is Class D or E suitable for outdoor PA applications, or is Class A per power drop still best?
What about if the outdoor outlet is already GFCI, and your extension cables also have their own?

Are there any GFCI's that are between the 6ma and 20 ma options?
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Stephen Swaffer

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Re: GFCI class questions
« Reply #1 on: July 05, 2017, 12:56:55 PM »

Class A  GFCi's are designed for personnel protection-all others are varying degree's of equipment protection.  Since 6 mA is a physiological threshold, if the purpose ( as is the case in outdoor audio) is personnel protection, then class A is the only adequate protection.

As has been discussed on here before, since equipment leakage currents add, the most trouble free would be to have a dedicated GFCI for each 120 VAC circuit.  You could even go to each device, but keep in mind that the GFCI only protects cables and equipment after it, so an extension cord plugged into a non-GFCI receptacle with a GFCI adapter at the end would be a hazard.
« Last Edit: July 05, 2017, 11:25:31 PM by Stephen Swaffer »
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Steve Swaffer

Guy Holt

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Re: GFCI class questions
« Reply #2 on: July 05, 2017, 09:04:55 PM »

...  keep in mind that the GFCI only protects cables and equipment after it, so an extension cord plugged into a non-GFCI receptacle with a GFCI adapter at the end would be a hazard.

For this reason, a better approach is a tiered ground-fault protection strategy with a more lenient ground fault device placed upstream to protect the entire distribution system and Class A GFCIs  at the loads.

Has any research been posted regarding using anything other than Class A GFCI's in audio?... Are there any GFCI's that are between the 6ma and 20 ma options?

There is an informational note in ANSI E1.19 – 2015, Recommended Practice for the Use of Class A Ground-Fault Circuit Interrupters (GFCIs) intended for personnel protection in the entertainment industry, that reads: “RCD and ELCBs can be used to mitigate ground-fault risks on circuits where GFCIs are not required.” Since NEC 445.20: “Ground-Fault Circuit-Interrupter Protection for Receptacles on 15-kW or Smaller Portable Generators” only requires the use of the GFCIs on 15 A and 20 A, 125 V receptacles, a technician is free to use a European style RCD on the 240V twist-lock output of small portable generators as a means of providing blanket ground fault protection. Even though these devices are rated for higher voltages and have higher trip thresholds (usually 10 or 30 mA), when used properly, they can provide valuable additional protection against the risk of electrocution when using small portable generators.

The major difference between RCDs and GFCIs is that the trip threshold in RCDs is user-adjustable while, according to the UL 943 standard, GFCIs must follow a fixed current-time relationship. In place of the fixed curve of UL 943, the IEC 60947-1 standard stipulates a maximum breaking time depending on the set trip threshold. But, since the trip times of RCDs are very short in comparison to the UL 943 curves they are no less safe.  For instance an RCD set for a trip threshold of 10 mA must trip within 300 ms, compared to approximately four seconds required by the UL 943 curve. Even though the curves of the IEC standards for RCDs are different than those stipulated by UL 943, they fall within the UL 943 curve and therefore are no less safe.

The break time stipulated in standard IEC 60947-1 for a trip threshold of 30 mA is rapid enough to avoid permanent organ damage and ventricular fibrillation. For this reason, a trip threshold of 30 mA has become the internationally accepted norm for RCDs intended to provide personnel protection against the risk of electrocution. For more details on the use of RCDs in a tiered ground fault protection strategy see my Production Power on a Budget series on ground fault protection strategies for Honda generators for Protocol which is available at http://www.screenlightandgrip.com/html/hd_plug-n-play_pkg.html.

Guy Holt, Gaffer
ScreenLight & Grip
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Lyle Williams

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Re: GFCI class questions
« Reply #3 on: July 12, 2017, 04:28:31 AM »

30mA is the threshold used in some other parts of the world.

6mA isn't a magic number, just the number the US chose.
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John Roberts {JR}

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Re: GFCI class questions
« Reply #4 on: July 12, 2017, 08:46:33 AM »

30mA is the threshold used in some other parts of the world.

6mA isn't a magic number, just the number the US chose.
There can be some variability between how individual humans are affected by shock events.

6mA is a relatively conservative safety threshold (should be low enough to avoid muscle contraction that can aggravate exposure and injury), but hopefully enough current to reliably indicate only real faults.

Not as reliable as some would like, but false trips are far less dangerous than ignoring real faults.  :o

JR
 
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Mike Sokol

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Re: GFCI class questions
« Reply #5 on: July 12, 2017, 12:02:20 PM »

30mA is the threshold used in some other parts of the world.

6mA isn't a magic number, just the number the US chose.

30mA at 50/60 Hz just about guarantees ventricular fibrillation after a few seconds of exposure
20mA is the level where all your muscles contract and you can't let go of what's shocking you
10mA is a very powerful shock that can be dangerous to small children and elders with heart issues
5mA is a pretty good jolt and will certainly get your attention, but probably not electrocute you
1mA is listed as the lower limit of actually feeling a shock

Since a wet human body has somewhere around 1,500 ohms impedance hand-to-hand or hand-to-foot, you can easily do the math (Volts / Resistance = Current) and see that 120 volts can supply an 80mA fault current through you, which is certainly lethal if you can't let go of the current source.  And if someone is shocked and unconscious CPR should always be started right after you call 911 and request an ambulance. Don't wait around for something to happen. Get it done...
« Last Edit: July 12, 2017, 12:17:43 PM by Mike Sokol »
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John Roberts {JR}

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Re: GFCI class questions
« Reply #6 on: July 12, 2017, 12:57:49 PM »

30mA at 50/60 Hz just about guarantees ventricular fibrillation after a few seconds of exposure
20mA is the level where all your muscles contract and you can't let go of what's shocking you
10mA is a very powerful shock that can be dangerous to small children and elders with heart issues
5mA is a pretty good jolt and will certainly get your attention, but probably not electrocute you
1mA is listed as the lower limit of actually feeling a shock

Since a wet human body has somewhere around 1,500 ohms impedance hand-to-hand or hand-to-foot, you can easily do the math (Volts / Resistance = Current) and see that 120 volts can supply an 80mA fault current through you, which is certainly lethal if you can't let go of the current source.  And if someone is shocked and unconscious CPR should always be started right after you call 911 and request an ambulance. Don't wait around for something to happen. Get it done...
I have personally felt hundreds of microamps so fraction of 1 mA is possible.

Not to quibble but my understanding is that the health of the individual's heart can alter sensitivity to shock injury.

It may also be worth noting that most (serious) mains hazards are relatively low impedance voltage sources, not pure currents (some have significant series impedance and look more like currents). How much current the meat puppet is exposed to depends on multiple factors (like skin moisture, and how tightly you grip or press into the energized conductor). This is why the grizzly old electricians with dry callused hands can lightly touch energized wires without dropping dead.

Energized water hazards (like a bathtub) my involve voltage gradients due to conductivity of water. The purity of that bathwater can also have an impact.

Good luck Mike getting to the bottom of this accident, the news reportage is rarely accurate or well informed. People who do know may be apprehensive about talking if liability is involved.

JR
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Mike Sokol

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Re: GFCI class questions
« Reply #7 on: July 12, 2017, 02:26:47 PM »

I have personally felt hundreds of microamps so fraction of 1 mA is possible.
Certainly true. And just like a 1 dB change is audio is what the average person is supposed to be able to hear, I do fractional dB changes all the time and can certainly hear them in a relatively good (but not perfect) listening environment. You can train yourself for this, as well as how to notice low current shocks below 1mA. But that 1 mA number is what the books say.
 
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Not to quibble but my understanding is that the health of the individual's heart can alter sensitivity to shock injury. And someone with a weak heart is certainly more at risk.
Yes, and as I noted, children and the elderly are more susceptible to heart fibrillation from relatively low currents below 10 mA, and perhaps as low as 5 mA.

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It may also be worth noting that most (serious) mains hazards are relatively low impedance voltage sources, not pure currents (some have significant series impedance and look more like currents). How much current the meat puppet is exposed to depends on multiple factors (like skin moisture, and how tightly you grip or press into the energized conductor). This is why the grizzly old electricians with dry callused hands can lightly touch energized wires without dropping dead.
Yeah, I've met a few of the old guys who seemed to never get shocked. However, my sweaty hands almost guarantee me a big jolt, so I learned my lesson early.

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Energized water hazards (like a bathtub) my involve voltage gradients due to conductivity of water. The purity of that bathwater can also have an impact.
Yes, but even pure water is contaminated once we put our body salts into it. And there are accidents every year from sump-pumps that create voltage gradients in the basement with a few recent deaths.

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Good luck Mike getting to the bottom of this accident, the news reportage is rarely accurate or well informed. People who do know may be apprehensive about talking if liability is involved.
Thanks... Sometimes the reporters call me out of the blue to learn about what might have happened, but most of the time everyone just circles the wagons. I've sent an inquiry to the news room, so maybe they'll respond.
« Last Edit: July 12, 2017, 06:42:20 PM by Mike Sokol »
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Lyle Williams

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Re: GFCI class questions
« Reply #8 on: July 13, 2017, 05:59:06 AM »

We are all obliged to comply with the rules in the places we live.  I am not suggesting that anyone not comply with their own local rules.

I don't think that the life-saving performance of 6 vs 30mA devices is very different.  30mA seems common in the rest of the world, and they seem to work just fine.

Nuisance trips may encourage people to not use GFCI/RCD.   The end result is that they are waiting for the breaker to trip.  Effectively a 150000mA trip current.
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John Roberts {JR}

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Re: GFCI class questions
« Reply #9 on: July 13, 2017, 10:19:37 AM »

We are all obliged to comply with the rules in the places we live.  I am not suggesting that anyone not comply with their own local rules.
I think this is one area where we should do more than required, if we can.
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I don't think that the life-saving performance of 6 vs 30mA devices is very different.  30mA seems common in the rest of the world, and they seem to work just fine.
AFAIK RCD protection is generally applied on branches not outlets, and reportedly suffer from nuisance trips too (from residential moisture and the like), and 240V mains would generate 2x the current from the same impedance leak.   

I wouldn't mind less than 6mA trip current, but suspect there would be too many nuisance trips... I'm get unexplained trips around the house on two of my three GFCI outlets (so far).
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Nuisance trips may encourage people to not use GFCI/RCD.   The end result is that they are waiting for the breaker to trip.  Effectively a 150000mA trip current.
150A?

Ideally per outlet GFCI and perhaps carry spare drops with GFCI built in to work around faulty GFCI outlets. (The more common these become, the more common problems will be encountered.) Thousands of false trips are worth it, to save one life (even the talent).

JR
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Re: GFCI class questions
« Reply #9 on: July 13, 2017, 10:19:37 AM »


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