A digital clamp meter is a device that measures current in amperes (A) by magnetic induction. This type of measurement is non-intrusive and very convenient. The digital clamp meter is squeezed open, and the cable whose current needs measurement is enclosed in the clamp meter loop. The clamp meter is removed after the current reading is made, and all this can be done without having to interrupt the continuity of the cable under test. In old methods an ammeter has to be inserted, which meant disrupting the current to do the measurement.
The basic principle behind the digital clamp meter is magnetic induction. A piece of wire with electrical current flowing through it will also have a magnetic field with intensity in proportion to the current flow. This magnetic field can induce current flow in a second wire. If the second wire is properly engineered, then a second current in proportion to the current on the first cable can be produced. The clamp couples the magnetic field from the first wire to the second wire.
Magnetic core-grade metal is used to make the digital clamp meter and provides a good and linear reading on the second cable, which may be visualized as the “pickup” coil. The clamp makes up a closed magnetic circuit, where the intensity of the magnetic flux in the core is in proportion to the sampled current. A clamp meter can be thought of as a transformer where the primary winding is the cable under test and the secondary winding is the measurement or sampling winding.
Alternating current is directly measurable using the induction-type digital clamp meter. With proper analog-to-digital conversion and good design, a clamp meter can become a digital clamp meter. The analog-to-digital converter circuitry, together with the numeric display, makes up the key digital portion of the meter.
When direct current (DC) has to be measured, the induction-type clamp meter may not work. Induction-type clamp meters rely on the alternating characteristic of the current to induce current from the measured to the sampling winding. For direct current, devices like the Hall effect transducers are used to measure static, or non-changing, magnetic fields.
The digital clamp meter is very useful for rapidly checking load currents and transient load currents via a maximum hold feature. For instance, in maintaining high-power motors, the start-up currents can surge to a certain average level, such as 35 A. If a suspected motor surges to 70 A, corrective maintenance may be needed.