The Circuit Detective

Choosing an Electrical Tester

Whether and Why to Test

Things CALLED testers are hardly the only way to learn the status of things in your electrical system. My Testers chart lists some of the most common, available, and simple testers. But a tester is like a question. If you aren't asking an appropriate question, the answer may be puzzling or misleading. The brief comments in that chart are to guide you, generally, in picking the right tester for the right question. Sometimes more than one tester will need to be used, and sometimes only in a certain order. So my further comments here will elaborate what the chart says and alert you to issues you may not be aware of. I am writing in general about what sort of tests are needed for different situations. For specific suggestions on good ways to test for different things you are welcome to consult How to test for what. But I think it will help if you first understand the logic behind those suggestions. Video. When testers will be helpful, use them according to the manufacturer's instructions, common sense, and safety.

Anytime you are about to use a tester -- I could say "are TEMPTED to use a tester" -- ask yourself what your purpose is. Suppose that you answer that you want to know, "Is this thing hot?" Even within this question there are different possible purposes. You may just want to know if you are personally safe handling a wire, screw, etc. Or you might want to know as part of troubleshooting a problem, whether you are dealing with a healthy hot wire that is capable of doing its job in the circuit. Or instead, you might have a reason to be interested in how hot it is, interested in measuring its degree of hotness -- its voltage. But these different purposes call for different testers. There is no generic "hotness tester." This is because the person's concept of hotness is not clear until they clarify their intent.

This chart may help clarify your testing purpose:

So, for instance, a non-contact tester or a neon tester can tell you when it is probably safe and when it is probably not safe to touch a wire. Other testing can make these probabilities certain. But these two testers cannot determine for you whether a wire they say is hot can carry more than a tiny bit of current, because they don't use much (neon) or any (non-contact) current themselves. Sometimes a poor connection upstream from a wire that registers as hot this way, won't let enough current through to run anything important.

Other electrical testers that don't draw much current are the 3-prong receptacle tester and the voltmeter. So they can't tell us that a circuit is good in every respect, but they will tell us if a circuit is bad in some respects.

On the other hand, a solenoid-type tester or an ordinary turned-on lamp will tell whether the circuit, at that point, is capable of running a load. But when they don't light up or vibrate, this is not, by itself, any indication that everything there is dead and safe.

For outages, you might use the chart above generally as follows. After confirming with C that there is a problem, start with A and work your way from there to B to C again. If B indicates that something should now carry a load (work) but C shows that it will not, you can bring a known working non-GFCI hot and neutral via extension cord and perform the C test from it to the supposedly good neutral and hot (respectively), to see which is actually poor. Before doing so, be sure you will not be testing from hot to hot, since 240 volts might be involved and blow out your 120-volt test item (bulb).

Of the six testers mentioned so far, only the first two can indicate some hotness independent of the presence of a good ground or good neutral. The non-contact and neon testers can do this by using your body as a slight source of ground. (Lately, the simple neon testers that will light up for voltage when you are the "ground" have become less available; not all neon testers will do this now; see a simple one in this video.) When the other four testers fail to show voltage between wires, they do not automatically let you know which is bad, the hot or the neutral/ground. The 3-prong tester attempts to overcome this difficulty by simultaneously checking voltage from the hot to each of the other outlet holes -- neutral and ground. This does enlighten you if at least one or the other of those holes is indeed grounded. But if neither is grounded, the tester is suggesting that the hot hole isn't hot -- "open hot" -- which may not be true at all. For more, see 3-prong testers.

The last two electrical testers I want to comment on are the ohmmeter and the continuity tester. They both try to send battery current through a path by connecting to two points believed to be two ends of such a path. Unlike the testers above, then, these two are primarily power sources. They let you know if their power has succeeded in finding a complete path, by using that power to move a needle or light an indicator bulb. I do not find the continuity tester as helpful as an ohmmeter because the continuity tester will light up -- or buzz, if it is that type -- for paths that have a certain level of resistance but not for resistances that are a bit higher. Even the times I use an ohmmeter are usually limited to checking if an ordinary light bulb or fuse is bad.

If I am checking a switch, I usually have a 120-volt hot available and simply see if it gets through the switch when On and not when Off. If I want to see if a wire at one outlet goes to another outlet, I tend to use a 120-volt source. Continuity and ohm testers might be used for the same purposes with the circuit off, but I see them as notoriously confusing unless the person knows exactly what the results will mean in each situation. Performing tests just out of curiosity is generally distracting. To understand the readings you get might mean mapping out an entire circuit and understanding the resistances of many things on that circuit. I think you'd rather concentrate on solving your problem.

Finally, here are two important things to note when using testers:

1. One note is on testing for hotness, neutralness, or groundedness. Most often when testers indicate one of these conditions, this will also mean that the wire (or other metal) will be able to carry the current needed to run lights and appliances (or to carry a short). But occasionally the wire will not sustain the needed voltage; contact is not good enough somewhere. Even 120 volts measured by a voltmeter may shrink down instantly when a real-life load tries to run.
2. In addition, many testers may encounter a different condition, " phantom voltage." It will tend to register as 50 volts or less but might read almost as high as the usual voltage of the home (120 volts). It would not be able to shock you. It seems to be generated onto a wire that is neither hot nor neutral, by a hot wire that is bundled with it -- in the same cable, for instance. To be sure a reading is phantom, see that it will not run a light bulb, then that it does not spark when shorted to ground. Then ignore it.