The recommended instrument when measuring a wire without cutting it first is called a clamp meter.
A clamp meter is shaped like a clothespin and can clamp around live wires to measure the electrical current. The standard multimeter needs to be inserted in series with the circuit that is being measured, which means it first has to be cut.
However clamp meters can measure the current that is flowing in the wire they are clamped around as they can detect the magnetic field that the current is generating.
Clamp meters are capable of measuring the current flowing to an electronic device or machine still in operation. They come in many variants but are able to be classified by what they measure:
- AC current measurement
- DC current measurement
- Starting current measurement
- Leaking current measurement
- Continuity checks, diode measurement and resistance measurement
- Capacitance measurement and temperature measurement
These parameters cannot all be measured by the same clamp meter, so it is important to choose the clamp meter that is best suited to your particular application.
The basics of clamp meter measurement
It is easy to make use of a clamp meter, which just has to have the jaw sensor of the instrument clamped around a wire in order to measure the flowing current. The instrument should only be clamped around one single wire when measuring current as it is not possible to accurately measure current in a single phase circuit if it is clamped around two wires as the two wires carry current in opposite directions and thus generate counteracting fields.
However the instrument can sometimes be clamped around two wires when attempting to measure leakage, except with a ground wire. Leakage current flows through the insulation resistance of a load, usually with several microamperes or even less under normal conditions.
How to choose a clamp meter
As there are a number of variants of clamp meters, it is important to use a model that is suited to your specific purpose.
The type of current intended to be measured will determine the clamp meter type used. There are a number of clamp meters intended to measure load current, while others are designed for measuring leakage current. For ordinary current measurement, the former is used, while the latter are used during the likes of short circuits.
Differences in rectification method need to be observed when making use of a load current clamp meter. True RMS rectification and mean value rectification are the two usable methods of this, but care needs to be exercised as the latter cannot give an accurate measurement of the RMS values with distorted current waveforms.
AC or DC current being measured also influences the clamp meter type being used. The former can involve varying frequencies, so check the frequency band of the clamp meter.
The thickness of the cable that the clamp meter clamps to can also vary so the thickness needs to be ascertained beforehand.
The maximum current value that is associated with the load on the observed wire should fall within the current range of the clamp meter.
A range of other functions are also offered by clamp meters, making it essential for the instrument you choose to have all the required capabilities.