Test lightPosted on: May 27, 2018, by : xnlamp
Unliketong ammeterswhich 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.
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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.
Recall of one model of voltage detector pen
Light-addressable potentiometric sensor
Rotary variable differential transformer
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.
,(Pacific Media Publishing 2012),ISSN1204-8011Vol. 18 No. 2 March/April 2012 page 89
This page was last edited on 14 April 2018, at 14:26.
A hand-held test lamp necessarily puts the user in proximity to live circuits. Accidental contact with live wiring can result in ashort circuitorelectric shock. Inexpensive or home-made test lamps may not include sufficient protection against high-energy faults. It is customary to connect a test lamp to a known live circuit both before and after testing an unknown circuit, to check for failure of the test lamp itself.
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 acontinuity testerlamp, or the lamp will be destroyed. Sometimes aflashlight(torch) is field-modified or factory-manufactured with test leads, to allow the flashlight to be used as a continuity tester.
In theUK, guidelines established by theHealth and Safety Executive(HSE) provide recommendations for the construction and use of test lamps.Probes must be well-insulated, with minimal exposure of live terminals, with finger guards to prevent accidental contact, and must not expose live wires if the test lamp glass bulb is broken. To limit the energy delivered in case of a short-circuit, test lights must have a current-limitingfu搜索引擎优化r current-limiting resistor and fuse. The HSE guidelines also recommend procedures to validate operation of the test light. When a known live circuit is not available, a separate proving unit that provides a known test voltage and sufficient power to illuminate the lamp is used to confirm operation of the lamp before and after testing a circuit.
The test light is an electric lamp connected with one or twoleads.Often, it takes the form of ascrewdriverwith the lamp connected between the tip of the screwdriver and a single lead that projects out the back of the screwdriver. By connecting the flying lead to an earth (ground) reference and touching the screwdriver tip to various points in the circuit, the presence or absence of voltage at each point can be determined, allowing simple faults to be detected and traced to their root cause. For higher voltages, astatiscopeconsisting of a neonglow tubemounted on a long insulating handle can be used to detect AC voltages of 2000 volts or more.
Neon-lamp type tester, which has no amplifier; this type requires a direct metallic contact to the circuit to be tested.
is a piece ofelectronic test equipmentused 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 amultimeter, 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.
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Areceptacle 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, missingelectrical groundor neutral, and others. However, leakage current through surge protectivemetal oxide varistorsconnected between neutral and ground of apower stripcan give a false indication that a ground connection exists.
Amplified electronic testers (informally calledelectrical tester pens,test pens, orvoltage 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 agroundreference (throughstray capacitanceto ground), at which point the indicatorLEDwill light up or a speaker will buzz, if the conductor being tested is live. Additional energy to light the lamp and power the amplifier is supplied by a small internal battery, and does not flow through the users body.
These types of testers can be used onseries-connectedstrings of miniChristmas lightsto 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 bypassshuntwhich completes the circuit.) Flipping the sets plug over and reinserting it in the outlet will cause the opposite end of the set or circuit to register instead.
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.
Standard Check for Ungrounded Outlets Using Neon Tester Can Yield False Results
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 andcapacitively coupledto 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.
American Electricians Handbook, Eleventh Edition
Linear variable differential transformer
A non-contact tester which senses electric fields cannot detect voltage inside shielded or armored cables (a fundamental limitation due to theFaraday cageeffect). Another limitation is that DC voltage cannot be detected by this method, since DC current does not pass through capacitors (in thesteady state), so the tester is not activated.
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 orHVACcontrol.
For low voltage work (for example, inautomobiles), the lamp used is usually a small, low-voltageincandescent light bulb. These lamps usually are designed to operate on approximately 12 V; application of an automotive test lamp on mains voltage will destroy the lamp and may cause a short-circuit fault in the tester.
A low-cost type of test lamp only contacts one side of the circuit under test, and relies on stray capacitance and current passing through the users 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 anelectric socket). Aneon lamptakes very little current to light, and thus can use the usersbody capacitanceto earth ground to complete the circuit.
Forline voltage(mains) work, the lamp is usually a smallneon lampconnected in series with an appropriateballastresistor. These lamps often can operate across a wide range of voltages from 90V up to several hundred volts. In some cases, several separate lamps are used with resistivevoltage dividersarranged to allow additional lamps to strike as the applied voltage rises higher. The lamps are mounted in order from lowest voltage to highest, this minimalbar graphproviding a crude indication of voltage.
, McGraw Hill, 1987ISBN0-07-013932-6pages 1-56 through 1-57
When the device is placed near a live conductor, acapacitive voltage divideris established, comprising the parasitic capacitance between the conductor and the sensor, and between the sensor to ground (through the users body).When the tester detects current flowing through this divider, it indicates the presence of voltage.
What Do You Know About Capacitive Voltage Sensors?, Fluke Corp, Retrieved 6 October 2015
Neon screwdriver test light in use. Current flows through a high ohm resistor and the lamp and the distributed capacitance and resistance of the users body.