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Author Topic: GFCI Question  (Read 7417 times)

Guy Holt

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Re: GFCI Question
« Reply #10 on: May 08, 2015, 02:26:36 pm »

If you are at an event that falls under Art 525 "Carnivals, Circuses, Fairs and similar events" 525.23 requires that all non-locking receptacles used for assembly and re-assembly must be GFCI protected. 

Since the NEC expanded the scope of when GFCIs are required, we have seen electrical inspectors begin to require them where they never had before and you might too.  For instance, movie base camps that typically consist of numerous trailers (for hair, make-up, wardrobe, dressing rooms, and star accommodations, as well as catering, lighting, grip, and camera), never used to be required to be ground fault protected. But, because they are similar to a traveling carnival, they now require that GFCIs be used not only on 15 and 20 Amp circuits, but also 30 & 50A circuits supplying the trailers. For the larger 50A/240 circuits, the inspectors are looking for GFCIs on the generator panels. Access to a movie base camp is tightly controlled and not open to the general public, yet the inspectors want them to be GFCI protected.  This is a problem because the movie blimped generators are typically not equipped with 50A/240 circuits, nor are film style distro boxes. What we do instead is to use an “Interlocking Ground Fault System”, like that illustrated below, that consists of GFCIs of varying sizes and trip levels.


An interlocking system of GFCIs, like that illustrated above, can be very effective in providing the Class A personnel protection at the load required by code, while at the same time providing protection at the generator for the operator. Employing an Interlocking Zone Ground Fault Protection scheme is very much like employing a zone defense in basketball. The distribution system is broken up into zones wherever there is a change in wire size or branch circuit. A GFCI sized for the over current device used to protect the branch circuit is positioned downstream of the over current device. The result is a cascade of interlocking protective zones starting at the power source and ending at the load. As you can see in the table below, there exist a wide variety of GFCI devices for this purpose - ranging from Class A devices with fixed 6mA trip levels and devices with user adjustable trip settings from 5- to 50mA. Adjustable devices set for a trip level of 20mA meets the UL943 standard for Class C protection of equipment but is not suitable for personnel protection (Note: devices with adjustable trip levels are capable of providing personnel protection to UL943 Class A specifications when set for a trip level of less than 6mA and time delay of 100 milliseconds, but are not Class A devices because the UL Standard for Class A requires a fixed threshold of 6mA).


The forward zone 120V and 250V single phase GFCIs must employ Class A devices with trip settings of less than 6 mA to assure the safety of personnel. As long as that is the case, the rear zone 208/240V multi-phase GFCI can employ higher trip settings (typically 10 or 20mA.) The choice whether to use a trip setting of 6-, 10-, or 20mA depends on the circumstances and the level of protection desired - but some type of rear zone GFCI protection is a good idea.



Left: Bender 400A 3 Phase Adjustable GFCI. Middle: Shock Block 400A 3 Phase Adjustable GFCI. Right: 100A 3 Phase Adjustable GFCI.

The Bender Corporation offers a device that can be very useful in these situations. The trip level of their RCMA420 ground fault monitor can be adjusted steplessly from 10 mA to 500 mA. An adjustable time delay is also available, from 0 to 10 seconds. And a digital display shows the measured fault current in real-time.  These features enable the user to eliminate nuisance tripping from system noise by enabling them to first assess the level of system noise and then set a trip threshold above the noise floor that will trip in the event of a real fault event. Because the UL Standard for Class A requires a fixed threshold of 6mA, the RCMA420 ground fault monitor can not serve as the exclusive ground fault protection in a system, but if set to a tolerable shock level (less than 20mA with a very short time delay) it can offer a degree of protection against accumulated ground leakage and eliminate nuisance tripping,  while Class A devices at the individual loads offer the Class A personnel protection required by code. 

To satisfy inspectors, small distros  supplied by 50A/240V receptacles or Honda EU6500s  can be outfitted with a Class A single phase 100A Shock Block. This approach would likely eliminate the need to put GFCIs on your rack packs. For more detailed information on how to design such a system, see my IA Ground Fault Protection workshop available at http://www.screenlightandgrip.com/html/481_GFCI_Workshop.html.

Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com
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Adam Ellsworth

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Re: GFCI Question
« Reply #11 on: May 11, 2015, 04:11:43 pm »

This is a very helpful and timely thread. This is a relatively rural area and inspections here are usually infrequent. When they happen, it's a quick visual check of my gear for obvious problems before they move on to food vendors. But I just had my first outdoor event of the season this weekend in a city park (my 7th year there) and the electrical inspection was considerably more detailed.

He not only verified every circuit in the system had a GFCI, but also made me unplug every piece of equipment and demonstrate that the ground plug had not been cut off. He even hand-checked the tightness of my stage stringers (which are almost new.) Luckily this was just a community event and the PA was a flown self-powered VRX rig and K12 monitors - no amp racks - so everything was U-ground edison. It only took a few minutes and I passed inspection without issue.

But now I'm worried about larger events coming up. Like referenced earlier in this thread, I've provided GFCI for stage power for several years, but my amp racks and lighting power do not. Really, very little of our gear is even available outdoor-rated.

Should I be proactive and give the AHJ a call to talk it over? I hate to call attention to something they hadn't considered, but "hiding" is not a legitimate approach to safety either. I hadn't considered something like a 100-amp 3-phase GFCI breaker on the whole system until I saw this thread, but that might be the path of least resistance towards compliance. But the chance of a nuisance trip would make me nervous, or even a legitimate problem like a leaky guitar amp being able to shut down the whole show. Even a spilled beverage, with a master GFCI doing what it's supposed to do, wouldn't indicate which branch circuit was the problem. It could even be in the truss... you could spend a half-hour turning on breakers one by one until the GFCI tripped again. But the only other option would be replacing every breaker in the distro with a GFCI.

I'd love to hear more about what other people are doing.
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Jamin Lynch

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Re: GFCI Question
« Reply #12 on: May 12, 2015, 11:23:21 am »

Since the NEC expanded the scope of when GFCIs are required, we have seen electrical inspectors begin to require them where they never had before and you might too.  For instance, movie base camps that typically consist of numerous trailers (for hair, make-up, wardrobe, dressing rooms, and star accommodations, as well as catering, lighting, grip, and camera), never used to be required to be ground fault protected. But, because they are similar to a traveling carnival, they now require that GFCIs be used not only on 15 and 20 Amp circuits, but also 30 & 50A circuits supplying the trailers. For the larger 50A/240 circuits, the inspectors are looking for GFCIs on the generator panels. Access to a movie base camp is tightly controlled and not open to the general public, yet the inspectors want them to be GFCI protected.  This is a problem because the movie blimped generators are typically not equipped with 50A/240 circuits, nor are film style distro boxes. What we do instead is to use an “Interlocking Ground Fault System”, like that illustrated below, that consists of GFCIs of varying sizes and trip levels.


An interlocking system of GFCIs, like that illustrated above, can be very effective in providing the Class A personnel protection at the load required by code, while at the same time providing protection at the generator for the operator. Employing an Interlocking Zone Ground Fault Protection scheme is very much like employing a zone defense in basketball. The distribution system is broken up into zones wherever there is a change in wire size or branch circuit. A GFCI sized for the over current device used to protect the branch circuit is positioned downstream of the over current device. The result is a cascade of interlocking protective zones starting at the power source and ending at the load. As you can see in the table below, there exist a wide variety of GFCI devices for this purpose - ranging from Class A devices with fixed 6mA trip levels and devices with user adjustable trip settings from 5- to 50mA. Adjustable devices set for a trip level of 20mA meets the UL943 standard for Class C protection of equipment but is not suitable for personnel protection (Note: devices with adjustable trip levels are capable of providing personnel protection to UL943 Class A specifications when set for a trip level of less than 6mA and time delay of 100 milliseconds, but are not Class A devices because the UL Standard for Class A requires a fixed threshold of 6mA).


The forward zone 120V and 250V single phase GFCIs must employ Class A devices with trip settings of less than 6 mA to assure the safety of personnel. As long as that is the case, the rear zone 208/240V multi-phase GFCI can employ higher trip settings (typically 10 or 20mA.) The choice whether to use a trip setting of 6-, 10-, or 20mA depends on the circumstances and the level of protection desired - but some type of rear zone GFCI protection is a good idea.



Left: Bender 400A 3 Phase Adjustable GFCI. Middle: Shock Block 400A 3 Phase Adjustable GFCI. Right: 100A 3 Phase Adjustable GFCI.

The Bender Corporation offers a device that can be very useful in these situations. The trip level of their RCMA420 ground fault monitor can be adjusted steplessly from 10 mA to 500 mA. An adjustable time delay is also available, from 0 to 10 seconds. And a digital display shows the measured fault current in real-time.  These features enable the user to eliminate nuisance tripping from system noise by enabling them to first assess the level of system noise and then set a trip threshold above the noise floor that will trip in the event of a real fault event. Because the UL Standard for Class A requires a fixed threshold of 6mA, the RCMA420 ground fault monitor can not serve as the exclusive ground fault protection in a system, but if set to a tolerable shock level (less than 20mA with a very short time delay) it can offer a degree of protection against accumulated ground leakage and eliminate nuisance tripping,  while Class A devices at the individual loads offer the Class A personnel protection required by code. 

To satisfy inspectors, small distros  supplied by 50A/240V receptacles or Honda EU6500s  can be outfitted with a Class A single phase 100A Shock Block. This approach would likely eliminate the need to put GFCIs on your rack packs. For more detailed information on how to design such a system, see my IA Ground Fault Protection workshop available at http://www.screenlightandgrip.com/html/481_GFCI_Workshop.html.

Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com

So the whole trailer has to be on a GFCI? Don't all those travel trailers come with GFCI's built in already from the manufacture in the bathrooms and sink kitchen areas?
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Daniel Levi

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Re: GFCI Question
« Reply #13 on: May 12, 2015, 01:35:35 pm »

In reply to the above post, where I work (a holiday camp with 750+ static caravans and ~50 powered touring pitches) every power outlet (a mix of 16/32A CEE sockets) has both a MCB and a RCD (miniature circuit beaker and residual current device (GFCI)) even though there are proper fuse boxes in the vans with an RCD. Remember that the cable to the caravan/travel trailer needs protection as well.
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Jonathan Johnson

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Re: GFCI Question
« Reply #14 on: May 12, 2015, 04:23:10 pm »

So the whole trailer has to be on a GFCI? Don't all those travel trailers come with GFCI's built in already from the manufacture in the bathrooms and sink kitchen areas?

As I understand Guy's post, the general use branch circuits (where personnel connect tools, equipment, hair dryers, soldering irons, etc.) would be protected on each branch circuit by individual GFI/RCD factory set to a fixed 6mA trip current. These GFI/RCD devices would be built into the circuit breakers of the final distro.

The cabling to the distro itself would be protected by an upstream adjustable GFI/RCD set to a slightly higher trip current, perhaps 10mA. This would prevent a fault on an individual branch circuit causing a trip of the larger system.

If there are several distros connected to a single power source (such as a genset), each distro would be fed by a separate GFI/RCD (set to 10mA in this example). Then a master CGI/RCD at the genset, set to perhaps 20mA, provides protection for the distribution to the sub-distros.

Note that only the branch circuit GFI/RCDs provide personnel protection, due to the code requirement of 6mA. But that's OK, because personnel are not going to be connecting tools or equipment to the upstream disconnects once the distribution system is energized. The upstream GFI/RCD are there for equipment protection. By increasing the trip current as you go up the tree (toward the power source) you prevent "one little oops" killing the whole show. You also prevent cumulative leakage currents -- say, 3mA on circuit 14, 4 mA on circuit 19, 2 mA on circuit 21, for a total of 9 mA -- from tripping the upstream GFI/RCD.

This is the same concept as a typical tree of overload trip currents we're used to dealing with: 20A on a branch circuit, 100A at the distro, 400A at the service, and so on, except that in this case we are concerned with ground fault currents rather than overload currents.
« Last Edit: May 12, 2015, 04:25:39 pm by Jonathan Johnson »
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Guy Holt

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Re: GFCI Question
« Reply #15 on: May 12, 2015, 04:41:09 pm »

Remember that the cable to the caravan/travel trailer needs protection as well.

That is the reason why the inspector wants to see GFCIs on the generator power output panel as well as 15, 20, & 30A 120V receptacles.

... the chance of a nuisance trip would make me nervous, or even a legitimate problem like a leaky guitar amp being able to shut down the whole show. Even a spilled beverage, with a master GFCI doing what it's supposed to do, wouldn't indicate which branch circuit was the problem. It could even be in the truss... you could spend a half-hour turning on breakers one by one until the GFCI tripped again. But the only other option would be replacing every breaker in the distro with a GFCI.

Since a rear multi-phase GFCI, like the 100A 3-Phase model mentioned above, blankets the entire distribution system and not just an individual zone, it is subject to the cumulative current leaks of all the zones taken together, which for a large distribution system can approach 6mA even under the best conditions. For this reason, and to avoid the nightmare scenario described above, it is usually set at the higher Class C trip setting of 20mA, but with a quicker response time of 25 milliseconds. This combination of trip level and response time still offers protection for personnel, but will accommodate the cumulative current leaks of a typical distribution system.  Even with a Class C GFCI on the main feeder trunks, code still requires that you use forward placed Class A devices.


A major advantage to combining smaller Class A GFCIs on branch circuits  with  the blanket approach of a Class C device on the main feeder trunk in the tiered interlocking zone approach described above is that if a GFCI trips it doesn't take the entire rig down, throwing everyone in the dark. You also know immediately what zone the leak occured in and so are more able to isolate the source of the leak while the show goes on. For truss lights supplied via six-channel Soco cable, Bender offers Class A GFCI configured for Socapex Cable (pictured above.) So in the end, there are GFCIs available to tier just about any type of distro system, even splitter boxes supplied by a 50A/240V receptacles, so there is no good reason not to use them IMO.  For more detailed information on how to design such a system, see my IA Ground Fault Protection workshop available athttp://www.screenlightandgrip.com/html/481_GFCI_Workshop.html.

In the final analysis, for a Ground Fault protection system to work it has to be used. If the approach chosen only results in nuisance tripping because of too low trip thresholds or misplacement of the GFCIs than they will be pulled out of service and offer no protection at all. So, it is better to over engineer a Ground Fault protection system than try to make do with too little.

Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com
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Re: GFCI Question
« Reply #15 on: May 12, 2015, 04:41:09 pm »


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