GFCI protection requires extra attention in a construction environment.
The GFCI has no doubt saved many people from electrocution, so it would be easy to start thinking GFCIs will always protect you. By design, they should provide ground-fault protection. But they may not if you don't maintain them and don't test them before use. Consider the following example of poor construction site maintenance and the fatal consequences.
Joe Brown knew all about GFCIs. In fact, Joe always made sure he plugged his power tools into a GFCI-protected receptacle. While installing a robotics system, Joe needed to mount a switch on a control panel. He plugged his drill into a GFCI-protected receptacle, and 10 min later his foreman found him dead. What happened?
We'll look at what killed Joe in a moment, but let's first talk about GFCIs. Do you always test a GFCI receptacle before using it? Many people don't — and that's a potentially dangerous mistake. Except for recently introduced models that go dead when the GFCI section fails, these receptacles can lose their ability to provide ground-fault protection without indication, which means they can provide power without protection. For this reason, always test a GFCI receptacle before using it (see Testing a GFCI below).
Why use a GFCI?
OSHA requires GFCIs for all 120V, 15A, and 20A receptacles you use on a construction site, unless they're part of the permanent building structure. In 527.6(A) the NEC duplicates the OSHA requirement, but includes 30A receptacles also, with the exception of some small generator arrangements. Any outdoor receptacle must also be on a GFCI.
Since the receptacles on the ends of portable cords aren't part of the building structure, you must use a GFCI for any portable cord (Photo, right) on a construction site. The added protection of a GFCI isn't a substitute for the grounding requirements of OSHA 29 CFR 1926 or of the grounding requirements of Art. 250. In 527.6, you'll find detailed requirements for use of GFCIs on construction sites. Let's look at why those requirements exist and what a GFCI really does for you.
Metal enclosures and power tools have something in common: ground. If a hot wire shorts to the enclosure or to the power tool, you get a ground fault. That ground fault will trip the breaker or blow the fuse, opening the circuit. The following four consequences of relying on this action for your protection could prove lethal to you:
Too much time.
The time curve of an overcurrent protection device (OCPD) balances equipment uptime against equipment protection. The ride-through that this curve permits will almost always exceed what a human can endure.
Too much current.
The human body's tolerance for electrical current is just barely more than 10mA. That's not close to what it takes to trip a 400A breaker.
Possible ground break.
A break in the ground system — or in the ground wire of your portable cord — won't announce itself with trumpets and light. In fact, you'll never know about such a break without specifically testing for it, and how often are you likely to do that? When this break occurs, you now have a difference of potential — and a shock hazard.
Poor grounding practices.
Even if the grounding system has no breaks, it might be faulty in design or construction. Code violations like unbounded electrodes and load side neutral ground bonds are so common that some grounding experts consider them normal. A facility with proper grounding is the exception, not the rule.
Double-insulated tools provide an added layer of protection, but they can provide a false sense of security, too. The tool body isn't the only potential source of a shock. Heavy moisture conditions can create an ionized path around the insulation or between you and some other shock source.
Then there's the portable cord to consider. Seasoned electricians use contractor-grade portable cords and handle them carefully. You should inspect these cords before and after each use — you never know if someone has run over a cord or damaged it by some other means.
When you can't be sure the device you're working on is properly grounded — and a breaker won't protect you — you need to use a device that interrupts the circuit if a hazardous level of current leaks to ground. That device must measure that leakage even if there is no ground. Enter, the GFCI.
A GFCI trips in .025 sec on a current imbalance of 4mA to 6mA. It uses a small current transformer on the neutral and another on the hot to continually monitor the differential. When the difference between the incoming hot current and the return neutral current reaches a value between 4mA and 6mA, the GFCI trips. Because the device doesn't measure anything on the ground wire, you can use it on 2-wire circuits, but be sure to follow the labeling requirements of 406.3(D)(3).
We often think of GFCIs as receptacle devices, but they're also incorporated into breakers and portable equipment. Let's take a closer look at receptacles, though. On the terminals of every GFCI receptacle and in the instructions that come with it, you'll see a distinction between line and load. Remember, line comes from the breaker, not a downstream load. You can feed downstream loads from the load side of the GFCI. A GFCI protects all the receptacles downstream of it, but only if you correctly wire to line and load.
Keep GFCI protection in perspective. It won't protect you from line-to-line contact. For example, if you hold an energized hot in one hand and a neutral in the other, the results could be fatal. The same is true if you hold two hots. The GFCI provides protection against the ground fault, and a GFCI protection plan should be part of a larger strategy of electrical safety (see GFCI Safety Rules below).
What happened to Joe?
The forensic examination showed Joe used a high-quality portable cord. But he had loaned it to another contractor, who had run it unprotected across a roadway. Joe didn't know the cord had been crushed, but he might have discovered the damage if he had inspected the cord and noticed the phasing tape wrapped around it. Maybe Joe was careless in his inspection because he was counting on the GFCI, which he also failed to test. A storm the previous night had damaged the GFCI — had Joe tested it, he would have found it needed replacing. He could have replaced it with one of the spare GFCIs his company always kept on-site. Upon plugging in his damaged portable cord, Joe would have been unable to get the reset button on the front of the new GFCI to stay in. This would have led him to look at his cord and replace it with an undamaged one, thereby preventing his electrocution.
Portable cords are inherently safe, and GFCIs inherently protect you. But all of the built-in safety can't protect you if you don't follow GFCI usage rules. You can end up like Joe, or you can make safety your first priority and always be sure your equipment is operating properly. The decision is yours.
Sidebar: Testing a GFCI
Press and release the test button, then plug a lamp into the GFCI.
Push and release the reset button — the lamp should light.
Press the test button. The reset button should pop out, and the lamp should extinguish. If the GFCI tripped but the light stayed lit, the line and load connections are probably reversed.
Correct the wiring and test again.
Don't use any GFCI that doesn't pass this test.
Sidebar: GFCI Safety Rules
Place GFCIs upstream of any same-circuit receptacles to be protected on all 110V and 125V circuits available to your crew. Note that pigtailing the supply wires allows for a path around the GFCI. Install and wire per the instructions that come with the unit.
Test GFCIs before first use, when there is evidence of damage, and after any repairs. In the absence of the preceding conditions, conduct tests at least once every 3 mo.
Use construction-grade portable cords and power tools, and inspect them before use. Store and route portable cords in ways that minimize damage to them.
Store power tools in protective cases — don't just dump them in a gangbox.
Never paint a GFCI. This voids most warranties and may defeat the protective features of the device.
For more rules, see NEC 527.6.