A test light, test lamp, voltage tester, or mains tester is a very simple piece of electronic test equipment

Electronic test equipment

Electronic test equipment is used to create signals and capture responses from electronic Devices Under Test . In this way, the proper operation of the DUT can be proven or faults in the device can be traced and repaired...

 used to determine the presence or absence of an electric

Electricity

Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 voltage

Voltage

Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

 in a piece of equipment under test.

Two-contact test lights

The test light is simply an electric lamp connected with one or two insulated

Electrical insulation

thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...

 wire

Wire

A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...

 leads. Often, it takes the form of a screwdriver

Screwdriver

A screwdriver is a tool for driving screws and often rotating other machine elements with the mating drive system. The screwdriver is made up of a head or tip, which engages with a screw, a mechanism to apply torque by rotating the tip, and some way to position and support the screwdriver...

 with 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, a statiscope consisting of a neon glow tube

Gas-filled tube

A gas-filled tube, also known as a discharge tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Although the envelope is typically glass, power tubes often use ceramics, and military tubes often use glass-lined metal...

 mounted on a long insulating handle can be used to detect AC voltages of 2000 volts or more.

For low voltage work (for example, in automobile

Automobile

An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

s), the lamp used is usually a small, low-voltage incandescent light bulb

Incandescent light bulb

The incandescent light bulb, incandescent lamp or incandescent light globe makes light by heating a metal filament wire to a high temperature until it glows. The hot filament is protected from air by a glass bulb that is filled with inert gas or evacuated. In a halogen lamp, a chemical process...

. 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.

For line voltage (mains) work, the lamp is usually a small neon lamp

Neon lamp

A neon lamp is a miniature gas discharge lamp that typically contains neon gas at a low pressure in a glass capsule. Only a thin region adjacent to the electrodes glows in these lamps, which distinguishes them from the much longer and brighter neon tubes used for signage...

 connected in series with an appropriate ballast resistor. 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 resistive voltage dividers arranged to allow additional lamps to strike as the applied voltage rises higher. The lamps are mounted in order from lowest voltage to highest, this minimal bar graph providing a crude indication of voltage.

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 short circuit

Short circuit

A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....

 or electric shock

Electric shock

Electric Shock of a body with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to denote an unwanted exposure to electricity, hence the effects are considered undesirable....

. 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.

In the UK, guidelines established by the Health and Safety Executive

Health and Safety Executive

The Health and Safety Executive is a non-departmental public body in the United Kingdom. It is the body responsible for the encouragement, regulation and enforcement of workplace health, safety and welfare, and for research into occupational risks in England and Wales and Scotland...

 (HSE) provide recommendations for the construction and use of test lamps. Probes must be well-insulated, with minimal exposure of live terminals and 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-limiting fuse

Fuse (electrical)

In electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit...

 or 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.

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 only contacts 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 electric socket). A neon lamp

Neon lamp

A neon lamp is a miniature gas discharge lamp that typically contains neon gas at a low pressure in a glass capsule. Only a thin region adjacent to the electrodes glows in these lamps, which distinguishes them from the much longer and brighter neon tubes used for signage...

 takes very little current to light, and thus can use the user's body capacitance to earth ground to complete the circuit.

Screwdriver-type test lamps are very inexpensive, but cannot meet the construction requirements of UK GS 38. The long exposed shaft provides a shock hazard to the user, 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. 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.

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 ground

Ground (electricity)

In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....

 reference (through stray capacitance to ground), at which point the indicator LED will 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 user's body.

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 electrical 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 HVAC

HVAC

HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...

 control. AC detectors can be used even if no current is flowing through the wire in question.

A non-contact tester which senses electric fields cannot detect voltage inside shielded or armored cables (a fundamental limitation due to the Faraday Cage

Faraday cage

A Faraday cage or Faraday shield is an enclosure formed by conducting material or by a mesh of such material. Such an enclosure blocks out external static and non-static electric fields...

 effect). Another limitation is that DC voltage cannot be detected by this method, since in this case the stray capacitance of a user's body does not pass sufficient current to activate the detector circuit.
These can be used on strings of mini Christmas lights to detect which bulb has failed and broken the circuit, causing the set (or a section of it) to not 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 show as good, as long as the shunt

Shunt (electrical)

In electronics, a shunt is a device which allows electric current to pass around another point in the circuit. The term is also widely used in photovoltaics to describe an unwanted short circuit between the front and back surface contacts of a solar cell, usually caused by wafer damage.-Defective...

 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.

Outlet tester

An outlet tester plugs into an outlet, and can detect some types of wiring errors. The particular error in wiring is shown by a combination of three lights. Errors include reversed hot/neutral, missing electrical ground or neutral, and others.

Continuity tester lights

A lamp and battery can be used to test for contact closure or wire continuity. Care must be taken to insure that all circuits are completely de-energized before use of a continuity tester

Continuity tester

A continuity tester is an item of electrical test equipment used to determine if an electrical path can be established between two points; that is if an electrical circuit can be made...

 lamp, or the lamp will be destroyed. Sometimes a flashlight

Flashlight

A flashlight is a hand-held electric-powered light source. Usually the light source is a small incandescent lightbulb or light-emitting diode...

(torch) is field-modified or factory-manufactured with test leads, to allow the flashlight to be used as a continuity tester.

External links

• Recall of one model of voltage detector pen