A clamp meter is a portable device used for making electrical measurements. In its most basic form, the device measures current and voltage. Most modern clamp meters are digital rather than analog, and the basic functions are supplemented by other electrical measurement functions common to digital multimeter (DMM) devices. Some electricians prefer a clamp meter to a digital multimeter because many digital multimeters do not have a built-in current transformer. Clamp meters use a clamp-type meter for measuring current. The jaws of the clamp are coupled around a conductor carrying the current to be measured.
AC Current Measurement
AC current measurement is one of the primary functions of a clamp meter. AC stands for alternating current, which is the type produced by most generators. Current is measured by placing the conductor through the meter’s clamp. This allows the clamp meter to measure AC current without having to disconnect the conductor. Some clamp meters can also use a detachable flexible current probe that allows AC current to be read from conductors that are too large to fit in the clamp. AC current is measured through a basic transformer action.
AC/DC Motor Testing
Clamp meters can be used in testing both AC and DC motors. The testing may be comprised of current testing, AC/DC voltage testing and capacitance measurement. Capacitance measurement tests the controllers and power supplies of motors to measure the electricity used when the motor is starting.
The ammeter is the part of a clamp meter that measures the current through a conductor. Analog ammeters use the current to move a needle on a gauge that is set to display the amperage of the current. Digital ammeters convert the analog amperage into digital signals that activate an LED display.
A continuity check is one of the basic tests that can be performed with a clamp meter. Continuity determines whether an electrical circuit is open or closed. A closed circuit is one in which an electrical signal may be transmitted, whereas an open circuit is one that is broken or blocked, impeding the signal. A circuit that does not check out as having continuity may often be fixed by replacing a broken or disconnected wire.
Current/Voltage Measurement Resolution
The resolution of a clamp meter is the minimum amount of change in current or voltage that can be read by the clamp meter. A resolution of 0.1 A measures changes in hundredths of amps, while a resolution of 1 mA measures changes in thousandths of amps. Resolution is often confused with accuracy, but resolution is, instead, a measurement of precision. However, more precise clamp meters are usually more accurate.
DC Current Measurement
DC stands for direct current, the type of current usually produced by batteries. DC current is measured by Hall effect sensors, which are different sensors than those used to measure AC current. DC current can only be measured by the clamp because the Hall effect chip is set in the gap between the jaws of the clamp.
Diodes are connected to an electrical circuit to ensure that electricity flows in only one direction through the circuit. A diode check can reveal if a diode is malfunctioning by conducting electricity in the opposite direction. It can also discover the voltage loss experienced by electricity as it passes through the diode.
The display count of a clamp meter refers to how large or how accurate a measurement can be taken and shown on the LED display. Display count is stated in terms of x00. The x is a whole number that represents the maximum top digit before a unit of precision is lost. For instance a 2000 display count has four total digits that will measure voltage up to 19.99. At 20 volts, the precision drops, so the display would read 020.0.
Display digits are the number of digits the display of a clamp meter will show. Half digits mean only a 1 or 0 will be displayed as the final digit.
The hold function stops the clamp meter from making a new reading while freezing the currently displayed value.
Clamp meters made for measuring HVAC equipment usually include the following measurement applications: volts (AC/DC), AC current, continuity, resistance and temperature. In addition, a diode check is often available on HVAC clamp meters.
The jaw opening specification of a clamp meter is the maximum width of a conductor that can fit between the jaws of the clamp when fully open.
The measurement rate, or measuring rate, of a clamp meter is the number of times readings are made each second. The measurement rate of most clamp meters is between 2 measurements per second and 4 measurements per second. The measurement rate may be variable on clamp meters with minimum frequency settings.
A milliamp meter is a type of clamp meter that can read very small currents measured in milliamps. One milliamp is equivalent to 1/1000 of an amp.
The min/max function of a clamp meter records the lowest and highest readings taken since the function was started.
Measurements made by clamp meters are often referred to as non-contact measurements because it is not necessary for the device to physically touch the conductive line or break the circuit.
A non-linear load is an electric current that changes in amperage without a proportional change in voltage. Non-linear loads draw current in pulses instead of smooth waveforms. Clamp meters that are specified as RMS, or root mean square, read non-linear loads more accurately than clamp meters that do not use RMS.
Peak function is a clamp meter feature that measures both the maximum and minimum current or voltage at the time the feature is activated.
Shock and Vibration
Some clamp meters have a feature that allows them to measure the acceleration and velocity of vibrations in electronic equipment.