Test light
A test light, test lamp, voltage tester, or mains tester is a piece of electronic test equipment used to determine the presence of electricity in a piece of equipment under test. A test light is simpler and less costly than a measuring instrument such as a multimeter, and often suffices for checking for the presence of voltage on a conductor. Properly designed test lights include features to protect the user from accidental electric shock. Non-contact test lights can detect voltage on insulated conductors.
Two-contact test lights
The test light is an electric lamp connected with one or two
For low voltage work (for example, in
For
Incandescent bulbs may also be used in some electronic equipment repair, and a trained technician can usually tell the approximate voltage by using the brightness as a crude indicator.
Safety
A hand-held test lamp necessarily puts the user in proximity to live circuits. Accidental contact with live wiring can result in a
In the
Since energy to operate the test lamp is drawn from the circuit under test, some high-impedance leakage voltages may not be detectable using this type of non-amplified test equipment.
One-contact neon test lights
A low-cost type of test lamp contacts only one side of the circuit under test, and relies on stray capacitance and current passing through the user's body to complete the circuit. The device may have the form of a screwdriver. The tip of the tester is touched to the conductor being tested (for instance, it can be used on a wire in a switch, or inserted into a hole of an
Screwdriver-type test lamps are very inexpensive, but cannot meet the construction requirements of UK GS 38. If the shaft is exposed, a shock hazard to the user exists, and the internal construction of the tester provides no protection against short-circuit faults. Failure of the resistor and lamp series network can put the user in direct metallic contact with the circuit under test. For example, water trapped inside the screwdriver may allow enough leakage current to shock the user. Even if an internal short circuit does not electrocute the user, the resulting electric shock may result in a fall or other injury. The lamp provides no indication below the strike voltage of the neon lamp, and so cannot detect certain hazardous leakage conditions. Since it relies on capacitance to complete the circuit, direct-current potential cannot be reliably indicated. If the user of the screwdriver is isolated from ground and capacitively coupled to other nearby live wires, a false negative may occur when testing a live circuit, and a false positive when testing a dead circuit. False negatives may also occur in brightly lit areas which make the neon glow hard to see.
Non-contact voltage detectors
Amplified electronic testers (informally called electrical tester pens, test pens, or voltage detectors) rely on capacitive current only, and essentially detect the changing electric field around AC energized objects. This means that no direct metallic contact with the circuit is required. The user must touch the top of the handle to provide a
When the device is placed near a live conductor, a capacitive voltage divider is established, comprising the parasitic capacitance between the conductor and the sensor, and between the sensor to ground (through the user's body).[3] When the tester detects current flowing through this divider, it indicates the presence of voltage.
Some amplified testers will give a stronger indication (brighter light or louder buzz) to gauge relative strength of the detected field, thus giving some clues about the location of an energized object. Other testers give only a simple on/off indication of a detected electric field. Professional-grade testers will also have a feature to reassure the user that the battery and lamp are working.
Voltage detector pens are made for either line-voltage or lower-voltage (around 50 volt) ranges. A tester intended for mains-voltage detection may not provide any indication on lower-voltage control circuits such as those used for doorbells or
Unlike tong ammeters which sense changing magnetic fields, these detectors can be used even if no current is flowing through the wire in question, because they sense the alternating electric field radiating from the AC voltage on the conductor.
A non-contact tester which senses electric fields cannot detect voltage 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
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
References
- ISBN 0-07-013932-6pages 1-56 through 1-57
- ^ "Guide GS 38 - PDF edition" (PDF). HSE.
- ^ "What Do You Know About Capacitive Voltage Sensors?", Fluke Corp, Retrieved 6 October 2015
- ISSN 1204-8011Vol. 18 No. 2 March/April 2012 page 89