What is A Clamp Meter?
An electrical meter with integral AC current clamp is known as a clamp meter, clamp-on ammeter or tong tester.
In order to use a clamp meter, only one conductor is normally passed through the probe; if more than one conductor is passed through then the measurement would be the vector sum of the currents flowing in the conductors and would depend on the
phase relationship of the currents. In particular if the clamp is closed around a two-conductor cable carrying power to equipment the same current flows down one conductor and up the other, with a net current of zero. Clamp meters are often sold with a device that is plugged in between the power outlet and the device to be tested. The device is essentially a short extension cord with the two conductors separated, so that the clamp can be placed around only one conductor.
The reading produced by a conductor carrying a very low current can be increased by winding the conductor around the clamp several times; the meter reading divided by the number of turns is the current, with some loss of accuracy due to inductive effects.
Clamp meters are used by electricians, sometimes with the clamp incorporated into a general purpose
multimeter.
It is simple to measure very high currents (hundreds of amperes) with the appropriate current transformer. Accurate measurement of low currents (a few milliamperes) with a current transformer clamp is more difficult.
Less-expensive clamp meters use a rectifier circuit which actually reads mean current, but is calibrated to display the
RMS current corresponding to the measured mean, giving a correct RMS reading only if the current is a
sine wave. For other waveforms readings will be incorrect; when these simpler meters are used with non-sinusoidal loads such as the
ballasts used with
fluorescent lamps or
high-intensity discharge lamps or most modern
computer and
electronic equipment, readings can be quite inaccurate. Meters which respond to true RMS rather than mean current are described as “true RMS”.
Typical hand-held
Hall effect units can read currents as low as 200 mA, and units that can read down to 1 mA are available.
The Columbia tong test ammeter, manufactured by
Weschler Instruments, is an example of the iron vane type, used for measuring large AC currents up to 1000 amperes. The iron jaws of the meter direct the magnetic field surrounding the conductor to an iron vane that is attached to the needle of the meter. The iron vane moves in proportion to the strength to the magnetic field and thus produces a meter indication proportional to the current. This type of ammeter can measure both AC and DC currents and provides a true RMS current measurement of non-sinusoidal or distorted AC waveforms. Interchangeable meter movements can be installed in the clamping assembly to provide various full-scale current values up to 1000 amperes. The iron vane is in a small cylinder that is inserted in a space at the hinged end of the clamp-on jaws. Several jaw sizes are available for clamping around large conductors and bus bars up to 4
+1⁄2 inches (110 mm) wide
you can see parts of a clamp meter in picture below:
How does A clamp meter work?
An amp meter or ammeter is a device that measures current in a circuit in amperes which is a measurement of the movement of electrons over a point through time.
The first ammeters were galvanometers, which exploit the deflection of a needle by a current through a coil (magnetic field), accomplished via spring action; galvanometers can measure only direct current (DC). Moving iron ammeters can measure both DC and alternating current (AC) and replace the needle with a piece of iron which is acted on by deflection across the magnetic field. To measure larger currents, a shunt (which acts as a resistor) is added to the system; most of the current is redirected through the shunt and, because the resistance across the shunt is known, it remains possible to measure the current.
A clamp-on ammeter, also known as a current clamp or current probe, can be clamped around a conductor via its two jaws, allowing the monitor to get a reading of amperage. Generally speaking, clamp-on ammeters use their jaws to detect the conductor’s magnetic field, which acts on an iron vane, or a sensitive cylinder of iron, which provides a reading of current.
Non-clamp-on ammeters
The first ammeters were galvanometers, which exploit the deflection of a needle by a current through a coil (magnetic field), accomplished via spring action; galvanometers can measure only direct current (DC). Moving iron ammeters can measure both DC and alternating current (AC) and replace the needle with a piece of iron which is acted on by deflection across the magnetic field. To measure larger currents, a shunt (which acts as a resistor) is added to the system; most of the current is redirected through the shunt and, because the resistance across the shunt is known, it remains possible to measure the current.
Clamp-on ammeter
A clamp-on ammeter, also known as a current clamp or current probe, can be clamped around a conductor via its two jaws, allowing the monitor to get a reading of amperage. Generally speaking, clamp-on ammeters use their jaws to detect the conductor’s magnetic field, which acts on an iron vane, or a sensitive cylinder of iron, which provides a reading of current.
How To Use a Clamp meter?
Instructions
Measuring Current with a Digital Clamp Meter
1-Remove power from the circuit, if possible. Working on a hot circuit involves extra risk to the user and all equipment involved.
2-Isolate a wire carrying the current you wish to measure.
3-Open the clamp portion of the meter and then close it around the wire (and only that wire) you wish to measure current passing through.
4-Set the clamp meter to read the current type (AC or DC) and the expected range in milliamps or amps. If you don’t know the range, guess high, and dial down as needed.
5-Return power to the circuit and read the digital display, writing the value down to avoid repeating the process. If the reading fluctuates, give it a minute to settle down.
6-Remove power from the circuit. Then remove the clamp meter.
Using a digital clamp meter is an uncomplicated and safe way to measure current in a circuit. Unlike regular meters, the clamp type does not put the meter into the circuit, but measures the field induced by a current passing through the clamp. It is suitable for moderate (<480V) voltage and current applications.
Tips & Warning
This type of meter is not for measuring current on a circuit board or for measuring small (<100 milliamps) current. This type of meter is not for precision current measurement. If a precise current reading is required, use an in-circuit meter.
-When working with electricity, always use proper safety techniques.
Ordinary clampmeters used to measure AC currents work on the principle of electromagnetic induction caused by the alternating current flowing in the conductor which reverses direction causing a dynamically changing magnetic field. However, in DC conductors, the current flows in a fixedpolarity. Consequently, the magnetic field around the conductor is fixed and does not change. Hence, a conventional clamp meter will register no reading.
DC clampmeter works on the principle of the Hall Effect. The Hall Effect, named after Edwin Hall who discovered it 1879, states that when a conductor carrying current is placed in a magnetic field, a potential is induced across the conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current. It is caused as the charge 
carriers, electrons or holes, experiencea force known as the Lorentz force and are pushed to the sides of the conductor.
