The Circuit Detective    -  Various Electrical Tests

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Electrical Testing for Hot, Neutral, Ground, etc.


What Kind of Tester? page
Chart of Testers page
Is a Device/Fixture Good?
Is There Hotness at a Device, Fixture, Box, or Wire?
Is There Neutral or Ground at a Device, Fixture, Box, or Wire?
Testing For Shorts and Ground-Faults


Is a Device/Fixture Good?

Is a receptacle working? Best is to plug a good lamp or appliance in and see. A neon tester, receptacle tester, or volt tester may be handier, but they don't pull enough current to be sure the voltage is sustainable.
Is a light working? Screw in a bulb you know recently worked. A fluorescent fixture with more than one tube needs all brand-new tubes to test it reliably; also see this fluorescent troubleshooting site.
Is a switch working? If the switch is unable to turn on a good bulb, turn off the breaker, remove the wires from switch (and keep track of how they were connected), connect those wires to each other, and turn the breaker on. If the item now works, the switch (or another 3-way switch) or its connections to the wires was probably bad. Otherwise, assume the switch is OK.
Is a bulb good? Try it in a socket known to work. Otherwise test the removed incandescent bulb with ohmmeter: 4-200 ohms is a good bulb, but some good halogens indicate no continuity. Continuity testers vary and may not answer this question for bulbs of all wattages.
Is a fuse good? Best is to remove the fuse and test it with a continuity tester or ohmmeter; any substantial continuity means the fuse is good. If a round fuse is to be tested while in its fuseholder, slip one probe of a neon tester along side the fuse with the other to the palm of your hand. If it lights up, the fuse is good IF this is a 120-volt circuit AND IF this fuse is for the hot, not the neutral; neutrals in some old homes were fused. For a cartridge-shape fuse that is accessible while in place, touch the probes of a neon tester to the ends of the fuse; if no light shows, the fuse is good, otherwise not -- assuming at least one end of fuse IS hot -- so check that first.
Is a breaker good? If this question arises from losing power to the circuit, a short, overload, or open is more likely than a bad breaker. If more things in the house work with the breaker on than off, the breaker is fine; you have an open. Be sure the breaker isn't simply tripped. Force it firmly into a full Off position, then firmly On. You could repeat this with the wire removed from the breaker, especially if the breaker didn't stay on or you heard a hum or buzz when you put it on (if the breaker without the wire now stays on, and wouldn't before, it is fine and was responding to a short). If you have the wire removed, you may as well do this test also: if the turned-on breaker's screw reads hot for a neon or volt tester, it is probably good, especially if a light bulb or Wiggins tester responds when connected between that screw and a ground in the panel (otherwise the breaker is bad). But the best all-around test is to move the breaker's wire temporarily to a new or different breaker, turning both off while moving it. With that breaker on, if the problem has gone away, figure the old breaker was bad; otherwise it is good. One more test would be to turn the breaker off, remove it, reset it to On, and check with an accurate ohmmeter between its screw and its bus-clip. It is likely bad if it reads more than 5 ohms. Finally, one of the things that can make a breaker go bad is arcing from poor contact with the live busbar under it; in this case the new breaker should be installed in a different location in the panel.


Is There Hotness at a Device, Fixture, Box, or Wire?

[... Text from this point on may have disappeared (just as the whole website could some day), but you can download the whole website for offline use (with no disappearing text) for $10. Here's how. Here's why.]

Does hotness reach a certain receptacle or light? If your purpose in electrical testing is personal safety for working on your problem, a non-contact volt stick will alert you if there is some hotness present. (One exception is when you are checking an underground wire or cable you have uncovered.) When your purpose is to check the extent of a circuit or of an open hot, a neon tester will light up slightly for something hot. Have one of its probes in the palm of your hand. A non-contact voltage tester inserted in these same receptacle slots or light sockets will also indicate hotness. A voltmeter will indicate something's hotness, and even show how hot, but only if the other probe is touching something you independently know is grounded. None of these tests tells you whether the outlet or light has a good neutral or ground.
Does hotness reach a certain electrical box or terminal? Having removed the cover, you can touch an in-hand neon or non-contact tester to the side-screw terminals of any switches or receptacles, but to check deeper in a box, the non-contact tester will be the easiest, once you loosen any devices in the way. At a circuit breaker one neon probe to the breaker's screw and the other in your palm will light it up if hotness is there. Don't trust a non-contact volt tester to help at a breaker since many nearby things are also hot. These tests do not tell you whether good neutrals or grounds are present.
Is a certain hot or neutral able to carry a load? Occasionally testers will show good voltage between hot and neutral, whereas plugging in a lamp or attaching a light socket and bulb to hot and neutral will show you that either the hot or the neutral is inadequate to run real things. Which one is poor? Unless you distrust the groundedness of the ground wire, connect a load (at least a light bulb worth) between it and the hot; if that runs the bulb, the neutral is poor; otherwise the hot is.
Which wire is hot? A neon tester with one lead in the palm of your hand is best. If it lights a bit when you touch it to the metal of a wire, at least that wire is hot, whether is it meant to be or not. A non-contact tester isn't always able to be near one wire without also being near others. (Also it too often reads a wire as hot which is merely not grounded and has gathered some "phantom" voltage from a hot wire it runs through the house with; for example, the unhot traveler in a 3-way switch system.) When a neutral is open somewhere on a circuit, white wires in the non-working area of the circuit can often read as hot -- and are somewhat -- in addition to the true hot. And of course, switched wires are hot when switched on and not when switched off. The fact that a wire is not hot does not mean it is always that way, nor that it is a neutral. The fact that a wire is black does not mean it is meant to be always hot, and the fact that a wire is white does not mean it is not hot, even always.
Is the voltage from hot to neutral too high or low? A voltmeter would be touched between hot and neutral. 120 volts is the nominal normal voltage-to-neutral supplied by the power company. The actual measured voltage at your home will be a little different -- as much as about 5% higher or lower. More variation than this is abnormal. It could be something the power company should correct, or it might indicate a neutral connection problem in one of your circuits or in your main wires.
Is a low but non-zero voltage reading significant? When a zero or 120-volt reading is expected but something in between is indicated by a voltmeter (5-100 volts), this can mean a connection is poor somewhere. However, it could be due to phantom voltage; such voltage, when shorted to ground, gives no spark and should be ignored.


Is There Neutral or Ground at a Device, Fixture, Box, or Wire?

Does "neutralness" reach a certain receptacle or light? If you trust the hot there, plugging in a lamp or screwing in a good bulb shows whether the neutral is good. If the hot is questionable, bring a good hot via an extension cord to where you can attach both it and the neutral in question, to the wires of a socket to run a bulb. A less reliable indication that a neutral is healthy is if a continuity tester or ohmmeter shows strong continuity between it and the ground wire; this should be done with the breaker off.
Does neutral reach a certain electrical box? Approach this as stated for the previous question. However, the neutrals at switch boxes are often less accessible for contacting or attaching to. In such cases, wire connectors may have to be undone to test. The breaker of the circuit(s!) involved in the box should be off until everything is ready for the testing. If neutrals are separated in order to test, it is normal, if you think about it, for only one of these whites to then test out as the neutral.
Is a ground wire good? If a light socket and bulb attached from hot to neutral works and works attached from hot to ground, the ground is good; if it works from hot to neutral but not from hot to ground, the ground is bad. An outlet, neon, or volt tester may indicate some groundedness, but these do not tell you for sure that a ground is good. For what to do about a bad or missing ground, you might want to refer to my discussion of Home inspection findings.


Electrical Testing For Shorts And Ground-Faults:

Is a hot-to-neutral short present? The breaker itself -- tripping off -- is the best test of the shorting. Video. If the circuit uses a fuse, don't repeatedly replace it to test the short unless you use a main pull-out or disconnect ahead of it to recontact the short. The arcing of a short would do damage to a round fuse's holder. The matter of whether it is the neutral that the hot is shorting to can be determined by disconnecting that circuit's neutral from the panel's neutral bar, capping it, and seeing that the short is then gone. An ammeter clamped around the hot wire at the breaker or fuse can also confirm that it is tripping/blowing for high current. Clamped around the circuit's neutral, it would also show that the short is indeed running hot-to-neutral. An ohmmeter showing 0-5 ohms between suspected wires will tend to mean the short is still there, but since lightbulbs and motors out on the circuit could give such a low resistance reading, I recommend against giving such a test much weight. Give a continuity tester even less weight for the same reason. Where to apply this test in a circuit.
Is a hot-to-ground short present? If a breaker or fuse has been tripping/blowing, it will be the best indicator, and a procedure corresponding to that mentioned above regarding the hot-to-neutral short, will apply. However, rather than capping the ground when it is removed from the neutral bar, to keep you safe and to keep it from completing the short circuit, it must be kept from any contact with you or any sort of bare metal -- wires or otherwise. Consider too that some shorts to ground will not even use the circuit's ground wire, but send current to a pipe or the earth... If the short is only tripping a GFCI receptacle, that is the best indicator (where to apply this test in a circuit).
Is a neutral-to-ground fault present? No regular breaker will trip for this. A GFI receptacle or GFI breaker will, and is the best way to keep testing. If an ohmmeter showed up to 30,000 ohms from the ground to load white(s), (disconnected from the line white), this MIGHT mean the fault is still present. Where to apply this test in a circuit.
Is this receptacle dead from a tripped GFI receptacle somewhere? A GFI receptacle (but not a GFI breaker) disconnects both hots and neutrals when it trips. So an ohmmeter or continuity check between the neutral slot and the ground hole of a totally dead regular-looking receptacle will tell whether a tripped GFI outlet somewhere is likely the cause. Normally a dead receptacle, with no hotness showing, retains continuity between neutral and ground (assuming the ground is good), since both are connected to the neutral bar in the panel. But one that is downstream from a tripped GFI receptacle should show no continuity. See Is a GFI to blame?


© 2007-2013 Larry Dimock

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