Three-Lamp Circuit Tester–Useful Tool or ‘Night-light’?

3 lamp

A friend of mine was fond of saying ‘the bitter taste of poor quality lingers long after the sweet aroma of low price is forgotten.’ I think a strong case could be made for that saying to apply to the widespread use of three-lamp circuit testers that are commercially available at any hardware store. But after 30 years of being in the business of Power Quality troubleshooting, I find it perplexing that the widely-known problems with these testers is still a well-kept secret when it comes to electricians, engineers, electrical inspectors, home inspectors, and just about anyone else who troubleshoots electrical systems.

Sure, we’ll spend thousands of dollars on high-end troubleshooting devices such as harmonic analyzers, power line monitors, oscilloscopes, spectrum analyzers, and many more in an effort to locate electrical problems. Armed with the knowledge that these only find a small percentage of power quality-related problems (less than 10%), it begs the question as to what is being used to test the quality of the electrical wiring. Most students in our classes tell us that they use the old standby…the three-lamp circuit tester.

‘Why?’, we ask. The response is almost universal…’they’re cheap’.

Yes, the three-lamp circuit tester is inexpensive. And, so I’m on the record here, I don’t want to misrepresent the validity of their product listing for safe use. They are indeed compliant with regulations on manufactured test equipment. But a product listing does not guarantee its indicated results.

And the simple truth is: these devices can be very inaccurate and give a CORRECT indication when, in fact the outlet has one or more problems…or give INCORRECT wiring indication when everything’s fine. On more than one occasion, some of those we speak to who have attempted to correct the wiring errors indicated by the circuit tester have admit to spending money on ‘problems’ that weren’t there or not correcting the problems at all.

When one considers that the vast majority of problems are caused, or worsened, by the polarity AND integrity of the wiring and grounding connections, it’s not a stretch to think of the money it can cost you in the long run, not to mention the underlying threat that shock or fire hazards go unnoticed!

So, why are these three-lamp circuit testers ineffective?

 Let’s start with the basic construction of them. If you take one apart, you’ll find that they are nothing more than three lights connected to the three blades of the device via resistors. These are commonly called ballast resistors, and they prevent each light from burning out when connected across 120VAC. That’s basically it! This is why it’s inexpensive…very few parts. But the simplicity is its downfall.

Two factors contribute to the tester’s inaccuracy:

  • Circuit capacitance.
  • Leakage current from loads on the circuit to be tested.

Circuit Capacitance

A capacitor is, by definition, two conductors separated by a dielectric (or insulator). We use them for filtering wanted/unwanted frequencies as well as for storing charges (or a voltage) between two points in an electrical circuit.

In electrical wiring, we can take this a step further and presume a capacitor can exist where two wires are separated by the insulation around one or more wires (or even the air!). This can be observed in the figure below.


3 Lamp


We have two circuit conductors for a 120VAC circuit – a hot and a neutral – and are using an equipment grounding conductor. As the length of the 120VAC circuit increases, the circuit conductor (hot) has distributed capacitance between it and the metallic conduit. The effects of the capacitance are diminished if all conductors are properly terminated and tightened. But, let’s assume the equipment grounding conductor pigtail from the metallic box to the receptacle becomes ‘open’. We can assume a capacitive voltage will now exist between either the hot or neutral conductors (or both) and equipment ground terminal at the receptacle.

This capacitive voltage causes the tester’s lamp (which is connected across these conductors) to light when they shouldn’t, thus giving an erroneous CORRECT WIRING indication. This open ground condition represents a serious safety hazard during fault conditions, while also being detrimental to the operation of electronic equipment. Yet, the three-lamp circuit tester indicates this circuit is GOOD.

The bad news doesn’t stop there…

Leakage Current

This sounds like a bad thing (and it can be) but it’s just another way of saying ‘capacitively-coupled current’. I tell people to imagine their equipment frame as a conductor, their internal electrical components as conductors (which they are), and the air between them as an insulator. If, over time, the barrier of the air ‘breaks down’ due to the build-up of dust, dirt, moisture, or insulation breakdown on wires between the frame and components, then some current will ‘leak’ on over to the equipment frame. At low levels, this current is harmless…but it’s there. Leakage current flows through our bodies all the time on some devices and we hardly feel it. Above certain levels, it can be harmful (that’s why we have Ground Fault Circuit Interrupters (GFCI’s)).  But we won’t address that now…

Suppose you connect a device that causes leakage current to an outlet (or a set of outlets) that are affected by a loose or open ground condition. You might think that there’s no path for leakage current to flow. But, once the three-lamp tester is plugged in at any outlet on the same string of outlets, guess what? The resistor between the ground blade and the light will provide a load to the circuit and falsely indicate a GOOD condition. Or, worse yet, it may cause one of the other lights to darken or illuminate when they shouldn’t, thus indicating a DIFFERENT condition.

This could lead you to ‘correcting’ a wiring condition that doesn’t exist or give you a false sense of security. In either case, it sows a seed of uncertainty that we don’t need in complex electrical systems, especially if we’re trying to troubleshoot wiring and grounding problems.

What to Do?

Your ability to successfully troubleshoot ac wiring and grounding problems (or even power quality problems) will hinge on two things: your methods (a later blog) and your tools. One of the most important tools in the bag will be a wiring polarity/impedance tester that can successfully indicate the wiring conditions at each outlet in a working electrical environment. This includes polarity AND integrity of the wiring.

I know what you’re thinking…‘iGround makes an impedance tester…it makes sense to bash the competition.’ Well, iGround isn’t the only company to make an impedance tester. Furthermore, NO tester is ‘perfect’ and finding some wiring conditions in an energized electrical environment requires a certain level of troubleshooting skills not taught these days, regardless of your tools. But, when the Institute of Electrical and Electronics Engineers (IEEE) Standard 1100 (Emerald Book) singles the three-lamp testers out specifically as what NOT to use for troubleshooting purposes, you know there’s something to it.

So, if you have a three-lamp circuit tester, don’t throw it out…keep it as a night-light…otherwise, you’ll be stumbling around ‘in the dark’, in more ways than one.