... it would be best to use GFCIs on branch circuits rather than feeders. A 50 amp 4 wire receptacle could be used either way.
While true in this particular case, it is not necessarily true in larger distribution systems. In motion picture distribution we typically use GFCIs on both the main feeder trucks as well as branch circuits in what is called an an “Interlocking Ground Fault System.”
We use an interlocking system consisting of GFCIs on branch circuits as well as the feeder trunks because a large distribution system is likely to have numerous ground leaks, none of which will trip a Class A device, but combined together can deliver a deadly shock. Capacitive reactance in the generator, a poorly insulated or nicked feeder cable, a defective distro box, bad plug, or deteriorated insulation in a lamp housing will all leak a little current (to name just a few causes.) While these individual faults, commonly referred to as “system noise,” may not be enough to trip a branch circuit GFCI, over a sizable distribution system they can all run together (as illustrated above), like many ground springs running together to form a stream, as they all return to the same source - the generator. Under such circumstances, someone touching the generator can receive a severe shock while someone else touching a lamp with a minor fault up stream will just experience a little tingle.
An “Interlocking Ground Fault System” like that illustrated above, consisting of GFCIs of varying trip levels, 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. 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 for the reason cited above.
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.
For more detailed information on how to design an interlocking ground fault system for temporary installations, 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
