Pros: Small, cheap, and handy to use.
Cons: Can be misleading, give false confidence if not properly understood. Only basic symptoms of circuits.
These testers are simple to use, and just as simple to read with their three lights. However, using them can be potentially unsafe and misleading if electrical circuits are not truly understood, or if other problems are lurking in the wiring.
After plugging in the device into an outlet, and then by looking at the lights, there are six possible conditions:
1. Only the middle orange light is lit - this is an "Open Ground" condition. An open ground means that there is no ground connection on the outlet (this would be the 3rd lower hole on the outlet). For 2 prong plugs, this is not a problem. But, if a using a device that requires 3 prongs, then the ground is needed to protect both the equipment and the user from a potential shock. Air conditioner units are very commonly a cause of shock to people in similar situations. If comfortable with looking and doing some minor work, make sure that the breaker is off [double check!], and then take the outlet out of the box, looking for any loose bare wires. Also check that all connections are tight, the wire nut connectors are also tight, and that all is secure. Check other outlets on the same circuit until all are observed.
2. Only the right orange light is lit - this is an "Open Neutral" condition, which means that there is juice coming from the hot lead, but its not completing a circuit... unless you touch the wires, which of course could result in shock and/or death.
3. No lights are lit - this is an "Open Hot" condition. This indicates that there is no power to the outlet, which could be caused by a blown fuse, a switch is turned off, or the breaker is flipped off. However, caution must be used, so do NOT necessarily trust that there really is no power, as this "open hot" could be just a temporary situation - such that if playing w/ the wires without the breaker turned off for sure [i.e.: just trusting this device], a shock or death could result.
4. The far left red and the left orange lights are lit - this is a condition where the "Hot and Ground Are Reversed". This can lead to a shock &/or death, and/or cause any equipment on the circuit to be damaged - since such indicates the potential that all three wires are wired wrong! Of all the ways to miswire an outlet, this could well be the most dangerous! Must use extreme caution, as even the screw on the cover of the outlet can be hot; i.e.: carrying an electrical current. Shut off the breaker immediately, and recheck all wiring to insure proper connections are made. If not sure, contact a qualified electrician for assistance.
5. The far left red and the middle orange lights are lit - this is a condition where the "Hot and Neutral Are Reversed". This can cause a shock, as the two wires [the lead and the neutral] are on the wrong side of the plug - they are reversed. For some appliances, this is not a big deal - however, for some this can definately be a source for some unpleasant situations. To fix this kind of problem, you must first shut off the circuit breaker, remove the cover and then the outlet from the box, and then switch the sides that the wires are attached to the outlet. For example, the black wire should be attached with the brass screw, whereas the white wire should be attached via the silver screw. This situation is also called 'reversed polarity'. Again, this is not necessarily a dangerous situation, especially when compared to the above miswiring fault. However, damage can occur with some electronics; even though many appliances will function perfectly well with reversed polarity, in any case, it's not a good thing.
6. The two orange [middle and right] lights are lit - this is normal, good condition. Congratulations! ;)
The seventh function is found by pressing the large red button that is ontop of the device. If the GFCI is working properly, then the circuit will be tripped, shutting off the main breaker. Simply reset the breaker, and the outlet is good until checked again [recommend that check these outlets once a month].
Now... if really interested in having a better tester, do NOT use this device, and instead buy a Ground Impedance Tester - and of course follow all instructions. Many electricians consider the 'three-light' testers to be nothing more than glorified night lights... ;) Therefore, they don't use these, as misleading results can be acquired, besides these simple devices do not tell the entire story as to what is going on in your electrical wiring. Recall that what you can discover is only as good as the tools you use.
A 'ground circuit impedance tester' gives you the ability to really troubleshoot the electrical quality problems on AC wiring and grounding systems. While the main function of this device is to detect and measure the impedance of the equipment grounding conductor or neutral from the point of the test back to the source neutral-ground bond, it can also test, verify, and detect:
a) wiring errors (reversed polarity, open equipment grounding conductor, and open neutral),
b) the measurement of voltage,
c) neutral-to-ground and isolated ground shorts, such as neutral impedance. A high impedance, found along the neutral conductor path, can cause two distinct problems:
* Electrical fires, due to overheated conductors and their connections; and
* Common-mode noise, due to neutral and ground wiring issues.
In any case, the three-light circuit testers are handy, cheap, and quick 'n dirty to use. However, realize that there are more potential gremlins hiding in your circuits than what this simple device can find... and while some of these gremlins are relatively benign, others can cause serious problems. Therefore, go ahead and use this, but, for real peace of mind [not to mention the safety & security of your family, house, & belongings] have the electrical wiring professionally checked out with a "ground circuit impedence/continuity tester".
Bottom line is this: must respect the power and danger of electricity - just because electricity is used all the time around us, since its such a major part of our everyday life, never let your guard down. If unsure, take the time and spend the Too many deaths and injuries, besides loss of belongings, are attributed to poor wiring and/or not being careful with electrical devices or repairs. One good site to recommend would be: http://www.osha.gov/SLTC/smallbusiness/sec14.html
PS: there is also a saying, "Black to brass, to save your tail" or some such... methinks that you can get it to be more poetic if so desired. ;) However, just because this is normally, and by code should be the case [black is live, white is neutral, bare is ground] does NOT mean that it was wired that way to start with by the original person that did the wiring. So, again, use caution.