There are several cases when current measurement is necessary. In many of these cases it is practically impossible to break an electrical circuit in order to connect a digital multimeter. Another limitation of using a DMM is that common multimeters have a restricted current range (most of them can measure up to 20 Amperes).
In these cases where current measurements on a live conductor without circuit interruption must be performed, or a high current must be measured, the instruments used are called Clamp Meters.
What is a Clamp Meter?
Clamp Meters are electrical instruments which have two jaws which open and can be clamped around an electrical conductor. In this way it is easy to perform live current measurements without having to shut down an electrical installation.
There are clamp meters which can only measure AC current and others which can also measure DC current. This is because the principal of current measurement is different in AC and DC mode. AC current measurement is based on the transformer principal and the measurement is obtained by the magnetic flow generated by the AC current flowing through the conductor. On the other hand, Hall elements are used for DC current measurements.
True RMS Clamp Meters
Nowadays there are many types of clamp meters. There are True RMS clamps which are perhaps more expensive but they can measure AC current more accurately. There are clamps with flexible jaws which are very useful in cases where there is not enough space for a traditional clamp meter. And, of course, modern clamp meters are not solely able to measure current.
Depending on the model they can also measure AC and DC voltage, Resistance, Continuity, Capacitance, Frequency, etc. A clamp meter is a very useful instrument in many cases. Almost every electrician carries one!
Clamp Meter Safety
In order to perform accurate and safe measurements, the user should always consult the clamp meter's manufacturer's instructions. It is very important to know how to use the clamp meter correctly. Depending on the make and model, some clamp meters may have special functions or handling; below we can mention some general rules regarding the usage of a clamp meter.
General Clamp Meter Usage Rules;
Always consult the clamp meter's manual in order to determine the maximum conductor size which the meter is capable of measuring. Also, the manual shall mention if the clamp meter is suitable for measuring insulated or bare wires.
- Make sure that the jaws of the instrument are completely closed, firmly touching one the other. If there is an open space between the jaws, the measurement might not be correct.
- When measuring DC current, the Clamp meter usually has an offset at zero current. This offset must be zeroed (DC current clamps have a zero button) before beginning the measurement. Since DC current clamps are sensitive of the earth's magnetic field, it is important not to change the clamps orientation after zeroing.
- It is important to keep the jaws clean and in good condition.
- As with any other measuring instrument, it is important to have the clamp meter calibrated according to manufacturer's specifications, in order to ensure that it operates within its limits.
Clamp Meter Applications
Clamp meters are used in many applications. In residential appliances, it is often necessary to measure the loads of individual branch circuits or to ensure that a circuit breaker operates correctly. In commercial and industrial environment, the three phase circuits should be checked. Each phase should have more or less the same current. In all appliances it is critical to measure leakage currents.
In industry, it is important to measure inrush currents of motors, or to know the peak load currents. All these measurements are easily performed with a clamp meter. Of course, safety precautions mentioned on the user manual shall be always taken into account since many clamp meters can measure up to several thousands of Amperes!
