A clamp meter is a great tool to have when looking for a current value or amperage reading while working with electrical AC circuits. It's not magic, and learning to use it safely and effectively is relatively easy. Here's a brief explanation of what’s happening.
How a Clamp Meter Picks Up a Reading
These devices offer a quick way of reading the current going through an electrical conductor, from a few amps to thousands, without physically tapping into it. That is accomplished by the use of a ferrite core clamp at the top of the meter (Figure 1). The clamp is spring-loaded and opens by pressing a lever just below the tactile barrier while wrapping it around a current-carrying conductor (Figure 2).
The reading is picked up by induction, using the same basic principle applied in transformers, where an AC voltage is applied to a primary winding to create a magnetic force field around a ferrite core at the center of the transformer, which in turn is induced into the wrapped-around secondary winding.
The transformer built into the current meter is different from that of a voltage transformer in design with its primary winding consisting of only one or very few turns that can be of heavy gauge wire coil or just a simple flat turn looped around a ferrite core.
Current transformers can “step-down” the current level of the secondary from thousands of amperes down to standard output. The ratio of the loops between the primary and the secondary coils produces a secondary AC output inversely proportional to the current measured in the primary winding. The reduced resulting output makes it possible for the tester to monitor the current flow through the AC power line, by factoring the value for an accurate reading on the meter’s dial or digital readout.
Working With Live Wires - Safety Comes First!
Before using the meter, the exterior casing should be checked for damage or missing parts. It should not be used in those events or if the leads have damaged insulation or exposed metal.
Industrial grade safety gloves must be worn when using the clamp meter.
When using the clamp meter to measure with the jaws, both probes must be removed from the meter.
Hands and fingers must always be kept behind the clamp meter’s tactile barrier (Figure 3).
Special care must be taken when dealing with voltages exceeding 60 VDC or 30 volts AC RMS for increased risks of shock.
It is extremely important to remember that in order to take measurements with a clamp meter, the wires involved have to be live, so extra care must be taken to handle or move wires with insulated tools only if there isn’t room enough to easily wrap the jaw around it.
Clamp meters should always be used to take measurements from well-insulated conductors but never from bare live wires!
Circuits should always be disconnected from their source and high voltage capacitors discharged before testing resistance, continuity, or diodes.
The connection between the meter leads and the circuit being tested should be removed and the meter powered off before opening up the case.
Important Tips About the Clamp Meter
Here are a few details and particularities about the clamp meter, its care, and its handling as a quick reference:
The meter should never be used or stored in environments with excessive heat, humidity, or exposure to magnetic fields.
The meter’s batteries and battery indicator should be checked before taking it in the field. A weak battery could result in false readings.
When measuring current flow, always clamp around one conductor only. If both wires are clamped, each current flow going in the opposite direction will cancel the other one out.
If the reading seems incorrect, the selection of the function switch should be checked before anything else, and the AC must be a regular sine wave without spikes and not square waves.
Most models of clamp meters will indicate a low reading when it’s below 0.5 A. Refer to step #1.8.
On AC/DC meters, the “zero” function should be used to remove the DC offset from the measurement for more accuracy.
The meter should be turned off when not in use and its batteries removed when stored for an extended period.
Steps 1-10 on How to Measure AC Current Using the Clamps.
Since analog clamp meters are not much used any more, this procedure will describe the operation of the digital type clamp meter. It is also obvious that the biggest difference between different meter models is the jaws and the functions they will vary in effectiveness functionality.
1.1 - Remove the probes or leads from the meter.
1.2 - Switch the meter on by turning the dial (Figure 4).
1.3 - If the meter has DC function, turn the dial to either AC or DC amps.
1.4 - The DC setting should be set on the “zero” function while measuring DC current. This will remove DC offset from the measurements for more accurate results
1.5 - Set the measuring range to the highest setting on the scale.
1.6 - Open the jaws with the release lever and put it around one of the two conductors through which the current is to be measured.
1.7 - Release the lever to close up the jaws’ loop closing the two halves of the core together generating the magnetic field.
1.8 - The conductor must line up between the alignment marks on the jaws. If the clamp meter doesn’t have the alignment marks, The conductor should be centered inside the loop formed by the jaws.
1.9 - The displayed result is the AC current. Whenever readings seem inaccurate, however, check which range the meter is set to, before anything else.
1.10 - If the result is too small for proper display, a makeshift patch cable can be used to increase the turns of the primary winding. As adding an extra turn to the primary will double the output result, and adding two turns triples it (Figures 5,6, and 7), the resulting reading just needs to be divided by the number of added turns (the heater elements’ physical properties from a toaster in these examples cause slight variations of 0.8 amps).
Four Extra Steps when Using the Meter with the Test Leads.
The test leads are required to get readings on AC and DC voltages, to measure resistance, continuity, and test diodes, exactly the same way it's done with a regular multimeter.
2.1 - The black lead is connected to the COM jack and the red lead to the other (V, Diode, or Ω) jack (Figure 8).
2.2 - The function has to be set on the proper position and the range at the right level or even higher.
2.3 - If the range is set too high for an accurate reading, the meter should be removed first or switched off before rotating to the right setting on the dial.
2.4 - The meter can then be turned back on and replaced over the wire to get a better reading.
The proper use of a clamp meter is easy once you understood how it works, and it only gets easier the more you keep experimenting with it!