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Electrical safety: Is a three-lamp circuit tester a valid tester or night-light?

By Ken Michaels, F.D. Hayes Electric Co.

Popular with electricians and electrical inspectors, this device is inexpensive. You can easily use it to test a receptacle for correct polarity. But is it giving you a false sense of security?

Some three-lamp circuit testers prove our industry's old adage: The bitter taste of poor quality remains long after you've forgotten the sweet smell of low price.

Sure, we'll spend thousands of dollars on harmonic analyzers, power line monitors, oscilloscopes, and spectrum analyzers. But this equipment finds only a small percentage of power quality problems, primarily those related to voltage quality events.

On the other hand, investing a few dollars in a three-lamp circuit tester to research wiring and grounding problems can cause you more problems. These represent the vast majority of power quality failures.

Yes, a three-lamp circuit tester is inexpensive. But the truth is: These devices can be very inaccurate and give you a "CORRECT" indication when, in fact, the outlet has one or more problems. Following the indications given by the circuit tester can lead to additional problems later.

Even IEEE 1100-1992 (IEEE Recommended Practice for Powering and Grounding Sensitive Electronic Equipment) provides a warning in its definition of a receptacle circuit tester:

"Receptacle circuit tester. A device that, by a pattern of lights, is intended to indicate wiring errors in receptacles. Receptacle circuit testers have some limitations. They may indicate incorrect wiring, but cannot be relied upon to indicate correct wiring." (Italics added for emphasis.) Some manufacturers even admit the inaccuracy of the product, printing a disclaimer on the package to caution you of its limitations.

So why are some three-lamp testers ineffective?

If you took one of these devices apart, you'd find nothing more than three resistors connected to the three blades of the device. These are ballast resistors, and they prevent each light from burning out when connected across 120VAC.

Two factors contribute to the tester's inaccuracy: circuit wiring capacitance and leakage current from the equipment plugged into the circuit under test. This means the three-lamp circuit testers can provide erroneous indications of the wiring conditions on practically every circuit! To show you the deficiencies of the devices, let's consider these two factors.

Circuit wiring capacitance

Assume you install two conductors for a 120V circuit (a hot and a neutral) and use the metallic conduit as an equipment-ground conductor. The circuit conductor has a distributed capacitance between it and the metallic conduit. If the equipment ground conductor pigtail from the metallic box to the receptacle becomes open, a capacitive voltage exists between the hot conductor and equipment ground terminal at the receptacle. This capacitive voltage causes the tester's lamp (which is connected across these conductors) to light, thus giving an erroneous CORRECT WIRING indication. This open ground condition represents a serious safety hazard during faults, while being detrimental to the operation of electronic equipment. Yet, the three-lamp circuit tester indicates this circuit is good!

Leakage current from equipment

Let's take this a step further. Suppose you connect equipment to the circuit (with the open ground condition). Now, there's no path for leakage current to flow. But there will be one: When you plug the outlet tester into the receptacle. The leakage current will now flow through the lamp connected between the neutral and ground, giving you an incorrect indication of REVERSE POLARITY.

So you follow the tester's indication, and reverse the hot and neutral. Now, you've created an actual reversed polarity. But when you test the receptacle again, the tester indication is CORRECT WIRING.

The original problem was an open ground, and it still exists. Only now, a reverse polarity condition compounds this original problem.

Use a ground circuit impedance tester instead

Your ability to troubleshoot power quality problems on AC wiring and grounding systems is only as good as the tools you use. Instead of choosing the three-lamp circuit tester, use a ground impedance tester. Yes, this more sophisticated device's primary function is to measure the impedance of the equipment grounding conductor or neutral (grounded conductor) from the point of the test back to the source neutral-ground bond. But, it can do additional testing as well:

-Detection of wiring errors (reversed polarity, open equipment grounding conductor, and open neutral),

-Measurement of voltage

-Verification of neutral-to-ground and isolated ground shorts

With this diagnostic tool, you'll get accurate information on which you can base your power quality solutions.

Don't be misled. When checking building wiring, use a ground impedance tester to verify correct wiring and low equipment ground impedances. Use the three-lamp circuit tester as a night-light!

Acknowledgement: Thanks to Ed Cantwell of PowerTek Services, Wheeling, Ill., for his contribution to this article.

This article originally appeared in Penton’s EC&M magazine.