BY ROY KAMPMEYER
The ground fault circuit interrupter (GFCI) is used at fire incidents, construction sites, dive stations, marinas, and anywhere else where there is a need to protect a power tool or light user. Whenever an electrical leakage path occurs from the AC (alternating current) power source internally from the tool or light through the tool’s metal housing to the hand of the worker and thence to ground (or a water environment), the circuit is complete. How severely this shock affects the user varies. All GFCIs must be able to shut down the AC source voltage supply to the user’s tool on detecting a current leakage.
 Photo courtesy of Power Electronic Systems, Inc. (PES) |
The GFCI discussion below is limited to firefighters who work in the worst possible circumstance-a water environment with a good leakage path to ground.
To ensure safety, the Occupational Safety and Health Administration (OSHA) requires using portable GFCIs on construction sites. These units are typically provided with a single output receptacle and a cable to attach to the AC source (120 volts). The worker plugs an electrical tool into the receptacle and proceeds with his activities. In a similar fashion, a firefighter plugs his tool into the AC outlet on the fire apparatus. If the apparatus has no reliable GFCI protection, the user should consider a portable GFCI between the tool and apparatus outlet.
Throughout this discussion, it is assumed that the firefighter is providing a path for passing electrical currents from the tool through his body back to the wet surface. If this path is incomplete, then no accident could occur. Leakage currents can get through small holes in the insulation (rubber) in the suit and gloves.
In light of the preceding, erring on the side of life safety is the correct route to follow, even if your equipment is all new and should be trouble-free.
In homes and shops, GFCI receptacles are mounted in baths, kitchens, basements, near pools, and in outdoor locations. The built-in GFCIs internal circuitry is identical to that in the same manufacturer’s portable GFCI. In some newer locations, a GFCI breaker is mounted in the distribution box, and a separate wire runs to a number of receptacles, thus providing a similar mode of downstream protection.
The discussion below is based on commercially available portable GFCIs. Such units can be placed at a convenient site on or near the apparatus.
HOW MUCH IS TOO MUCH?
Regarding electrical shock, the overall effect of current leakage depends on the amount of fault current and the length of time it is applied to a body. These effects have been studied through experimentation using various animals but not extensively on humans.
The International Electrotechnical Commission (IEC) has performed pioneering studies in this field and assumes that experimental data from studies of dogs and electrical shock provide a margin of safety when applied to humans. The present Underwriters Laboratories Inc. (UL) specifications for GFCIs are based on extrapolations and distillations of such data derived by researchers in the field. The body resistance values assumed are not inclusive for values to the heart, nor are any data offered for a subject in water. Also, minimal but permanent effects on humans resulting from electrical currents passing through the body have not been researched, except in reference to electrotherapy.
Little data are available from actual deaths by accidental electrocution in regard to the amount of fault current and for how long it was applied to the victim’s body. Nonlethal shocks are presumed to be of no consequence, except that the subject should “be more careful next time.”
UL’s spec values for the calibration of GFCIs are thus based on animal studies. The figures range from an exposure of 6 milliamps (mA) of fault current for 5.6 seconds to a maximum exposure of 264 mA for 0.025 seconds. These figures mark the end limits of a curve of allowable fault currents, at which point the GFCI should immediately shut down.1
The effects of current passing through the human body, again derived from tests on animals and their reactions, have been placed in three categories:2
- “Involuntary startle reaction” may result from a 0.5 mA fault current. In this case, the firefighter might drop a tool or let go of an object supporting him.
- “Muscle tetanization” or the inability to let go may result from a fault current of 5 mA lasting “more than a few seconds.”
- “Ventricular fibrillation” (arrhythmia) may result from a fault current of 20 mA lasting for “more than a few seconds.”
What occurs beyond these time limits is open to interpretation. Arrhythmia (i.e., irregular heartbeat) causes the heart to jump around, eventually causing total heart stoppage and death. It is unlikely that another firefighter will be immediately available to help; and even if he were, he would risk exposing himself to an electrocution hazard.
The confusing statistics, conditions, and results of animal electrocution studies are in no way related to humans and how the safety people are trying to provide proper specifications to avoid death by electrocution. It is not entirely their fault; they are not medical experts.
The American Medical Association (AMA) has not studied human electrocution because insufficient data are available. Often, in a death resulting from electrocution, the cause of death may be recorded as heart failure or some other cause unrelated to the actual accident. Additionally, usually no one was there to measure the amount of current leakage and the time the victim was exposed to the fault current.
Until more information becomes available, the average firefighter must rely on the control characteristics of the GFCI for protecting his life. It is essential that the firefighter use a GFCI in addition to keeping his tools and other accessories in good working order.
Some GFCI designers feel that the UL specs are not stringent enough and that the shutdown time should be less than stated for the 6 mA threshold point. Lower-cost units may shut down in 0.025 second, whereas the all-electronic types may do so in less than 0.005 second.
Another factor to add to the confusion is that not all people have the same sensitivity to fault currents and may react differently to the same current level. Age, weight, and medical condition are also factors.
USING A GFCI
On fire apparatus, a generator supplies the AC power. The question that always arises is the relationship of the common or neutral of the single-phase AC source to chassis and then to some path to ground. The neutral wire from the AC source should not be grounded to the generator frame. If the neutral is isolated from everything, then there is no way a fault current can get to ground from the hot lead, through a tool, through a user’s body to ground and back to the hot lead-a so-called current loop. The firefighter is thus protected in this special situation. The only current loop is the actual load current of the tool through its power source.
If the neutral wire is grounded on purpose, then there is a possible fault current loop through the firefighter’s body and back through the generator’s low-output resistance to the hot lead. This is the one that could kill you.
So how is the neutral wire actually connected? The operating personnel may not know. Some have offered a solution to this: use an isolation transformer between the AC source and the user. Neither of the transformer’s secondary leads are grounded, so this isolates the selected neutral lead. Or, a GFCI can be inserted in series with the AC power lead to the power tool. If a fault current is detected, it shuts down the power to the tool and the firefighter is safe.
Which to do? The usual AC power supplied would be 120 V AC at maximum of 15 amps or 1,800 watts. It could be 20 amps (2,400 watts). Using an isolation transformer to supply this much power would make it large and heavy, but it would negate the need for a GFCI. However, now another element is introduced: the possibility of corrosive activity or aging of the transformer’s internal insulation. Any insulation defects would show up as a current path to the neutral lead and violate the desired isolation effect.
This leaves us with the only other solution. Use a GFCI alone. The GFCI introduces no alternate path to the neutral AC power lead. If the AC power neutral is ungrounded, it offers nothing, except that if the neutral eventually contacts ground, then the GFCI protects the user.
On the fire apparatus, the portable GFCI can be plugged into the AC power source receptacle. The AC cable of the firefighter’s tool is plugged into the GFCI, which may have multiple receptacles for a drill, a light, and additional tools.
Each time you use a GFCI, test it to ensure proper operation. To do so, push the TEST button to activate the GFCI. The test button applies a known resistance and, therefore, a fault current and trips the GFCI to demonstrate it is working properly. If the GFCI does not activate (i.e., shut down), you should not use it and should obtain another unit.
Each time the GFCI is activated, either initially turned on or tested, it has to be reset manually to reinitialize it and get it ready to detect a possible ground fault.
A power cable length running from the GFCI receptacle to the user’s equipment may exhibit a leak that could set off the GFCI, which would interpret the leakage as a ground fault. The GFCI does not determine the origin of the leakage, since it analyzes only the power cables. It detects any path from the hot wire to a ground (water). Hence, it is imperative that the power cables be of such proven quality that they cannot be a source of a ground fault. The entire cable must be tested in the water and out of it to eliminate it as a source of a ground fault before replacing it. Also, the cable’s length could be the culprit. The small leakage to a ground from a 10-foot cable would not be detected, but the cumulative effect of a current leakage in a 500-foot length could produce enough leakage to set off the GFCI. Often, the cable’s capacitance causes the leakage. This is between the hot lead and ground again. At 60 hertz, its reactance could be sufficient to conduct leakage current, just like a ground fault.
The other place to monitor is the connection between the firefighter’s equipment and the power cable. Tools should always be inspected to ensure they have a good seal against water encroachment, leaving only internal leakage (because of aging) as the culprit.
CHOOSING A GFCI
Except for cost, selecting a GFCI is probably the largest stumbling block for most safety officers. How do you determine which is the best product to protect a firefighter’s life? Operational reliability is one consideration. Will the unit still perform properly after several years of use? Does the manufacturer indicate the type of internal circuitry used and its reliability? Are you confident that what you purchase has the highest reliability?
At a minimum, a GFCI unit should be UL approved and certified and bear the seal of UL or another certified national testing lab that tests to UL standards.
However, UL approval of a GFCI does not necessarily indicate its reliability over the long term. The device may be tested to many extremes of current and voltage to prove that it can take it. Tests also will attempt to “burn it up” by electrically overloading it to the point at which it will ignite. No tests are performed to predict the product’s future reliability; this is up to the manufacturer.
Beware when reading the GFCI’s label or instructions. Many manufacturers of commercial GFCIs state that the devices won’t reset if they fail; they will shut down permanently. If the unit uses relays for switching to shut down the GFCI, the contacts may weld together permanently, leaving the circuit closed and allowing current to flow, thus violating this premise and overriding the circuit interrupt mechanism.
On the other hand, electronic switching offers a significant improvement over relays, which have a failure rate 20 times greater than that of GFCIs with electronic switching. Since the semiconductors used in electronic switching have no moving elements and no attendant metal migration on contacts as the relays do, they offer more reliability.
Also, reliability is further enhanced if the electronic switching design incorporates dual channels of detection and switching mechanization.
Many users say that if their GFCI fails when tested, they just replace it with a new one. Although it makes sense to test the unit periodically to confirm it operates correctly, what about the time between these tests? If the unit malfunctions when it’s actually in use, the result could be deadly!
Like many electrical products, the higher the device’s quality (e.g., involving redundant circuitry and more durable construction), the higher the cost. American manufacturers often go offshore to reduce the manufacturing cost and thus the product’s price. So where the GFCI is manufactured will have much bearing on the price.
Before purchasing a GFCI, ask the seller appropriate questions regarding its construction, design, and reliability; contact the manufacturer if necessary to resolve your questions. Valuable information concerning GFCIs is also available on the Internet. Don’t just walk into an electrical store and grab a GFCI off the shelf.
Other considerations in selecting a GFCI include the number of output receptacles, the current and voltage ratings, internal overcurrent shutdown, portability and mechanical characteristics, ease of use, a bright color for easy identification, whether it can be used for outdoor usage, and if is protected against weather and temperature extremes.
Endnotes
1. UL 943, Standard for Ground-Fault Circuit-Interrupters, paragraph 24.1. Underwriters Laboratories Inc., Standard for Safety, Sept. 2000, continuously revised.
2. Reilly, J. Patrick. Electrical Stimulation and Electropathology. (Cambridge University Press, 1992). 434.
ROY KAMPMEYER is president of Power Electronic Systems, Inc. (PES) of Lansdale, Pennsylvania, which specializes in power-related design and product development.
