inside shielded or armored cables (a fundamental limitation due to the Faraday cage effect). Another limitation is that DC voltage cannot be detected by this method, since DC current does not pass through capacitors (in the steady state), so the tester is not activated. These types of testers can be used on series-connected strings of mini Christmas lights to detect which bulb has failed and broken the circuit, causing the set (or a section of it) not to light. By pointing the end of the detector at the tip of each bulb, it can be determined whether it is still connected at least on one side. The first bulb which does not register is likely the one just past the problem bulb. (Burnt-out bulbs will still show as good, if there is a bypass shunt which completes the circuit.) Flipping the set's plug over and reinserting it in the outlet will cause the opposite end of the set or circuit to register instead. Receptacle tester A receptacle tester (outlet tester or socket tester) plugs into an outlet, and can detect some types of wiring errors. The particular error in wiring is shown by various combinations of three lights. Detectable errors include reversed hot/neutral, missing electrical ground or neutral, and others. However, leakage current through surge protective metal-oxide varistors connected between neutral and ground of a power strip can give a false indication that a ground connection exists. Continuity tester lights A lamp and battery can be used to test for contact closure or wire continuity. Care must be taken to ensure that all circuits are completely de-energized before use of a continuity tester lamp, or the lamp will be destroyed. Sometimes a flashlight (torch) is field-modified or factory-manufactured with test leads, to allow the flashlight to be used as a continuity tester. See also Solenoid voltmeter References External links Recall of one model of voltage detector pen Electrical test equipment Electronic test equipment