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Analyzing Failures In AC Outlet Testing & What It Means For Pro Audio

It might come as a shock to your system, but all testers are not created equal, and the damage can be expensive.

Some “DIYers” and even electricians take shortcuts when replacing receptacles in older residences and stages by installing a grounded NEMA 5-15 outlet and strapping the ground screw to the neutral screw on back of the outlet, creating a classic bootleg ground situation.

Although this practice is a Code violation, it occurs more often than you think. And because many electrical inspectors rely on a simple 3-light “cube” tester to verify correct wiring connections at outlets, any RPBG outlets can go undetected.

Testing With A 3-Light Tester

Figure 2: A 3-light “cube” tester is unable to detect a reverse polarity bootleg ground outlet.

An RPBG outlet will test as correctly wired using any 3-light tester (Figure 2), because there’s no reference to actual earth potential.

What makes an RPBG outlet so dangerous is that any appliance (refrigerator), sound gear (guitar amplifier), or even an RV or boat plugged into an RPBG appears to operate normally.

However, the chassis of the “grounded” appliance is now directly connected to the “hot” wire in the outlet with a low-impedance current path, and anyone touching the body of the appliance will be electrically biased to a full 120 volts.

That situation itself is not dangerous due to the “pigeon on the power line effect.”

Figure 3: Even a voltmeter will fail to identify a reverse polarity bootleg ground outlet if we simply measure from hot-to-ground, ground-to-neutral, and hot-to-neutral.

But if a person touches the door handle on the refrigerator, strings of the guitar, or metal door of the RV plugged into an RPBG outlet while also touching anything that’s correctly earth grounded, they will receive a potentially deadly shock (around 100 mA of current at 120 volts).

Just 10mA of current through your body will result in a painful shock, and 100 mA of current for a few seconds is probably lethal if not immediately treated by a defibrillator.

These potential shock currents flow through the ground contact of the outlet, and avoid the neutral and hot contact current paths. So any GFCI outlet wired on a branch circuit extension downstream of an RPBG outlet will probably not sense unbalanced H-N currents and thus won’t trip as designed.

In fact, because the GFCI can’t disconnect its own ground contact from the now electrified ground wire, even if it trips, there will still be the full branch current available for the ground fault path up to the circuit breaker trip current (typically more than 20 amps).

Testing With A Voltmeter

Figure 4: A ground impedance tester can identify a receptacle with a bootleg ground, but it can’t tell if the outlet has correct or reverse polarity.

Don’t think a voltmeter is any “smarter” than a 3-light cube tester. If you simply measure from H-G, G-N, and H-N using any analog or digital voltmeter, you’ll find an RPBG outlet will appear normal (Figure 3).

The only hint there’s a bootleg ground is that the G-N reading will be very close to 0 volts, while in a loaded branch circuit with correctly isolated G-N outlets, we’ll likely see at least a half (1/2) to a few volts difference between ground and neutral due to voltage drops in the neutral bus.

Testing With An Impedance Tester

Now that we know a 3-light tester and a voltmeter can’t identify RPBG outlets, what other device can we try? A ground loop impedance tester, perhaps?

Although it can properly identify a bootleg ground due to the G-N impedance being too low, it cannot identify an RPBG either.

In Figure 4, the ground impedance tester is indicating a (F)alse bootleg ground, but the device can’t tell if the outlet has correct polarity or reverse polarity. It only indicates the false ground with a flashing (F) because the ground-to-neutral impedance is too low.

But if an RPBG outlet is feeding an extension outlet on that same branch circuit, the extra resistance of the added wire length would fool the ground impedance tester, indicating 100 percent Code compliance. Even a visual inspection of a “properly wired” extension outlet fed from an RPGB outlet probably wouldn’t turn up any obvious problems. Note that its neutral and ground connections are both at 120 volts with respect to earth potential.

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