As an Amazon Associate, Akermin earns from qualifying purchases. Product links on this page are affiliate links.
A clamp meter works by surrounding a conductor with a magnetic circuit and measuring the field that the current produces. In the common AC clamp, the jaws form a split iron core, and a secondary winding inside the meter produces a current proportional to the primary current, which is what the display reads. For DC, the core alone cannot work, because a steady current produces a steady field that does not induce a voltage, so DC-capable meters add a Hall effect sensor in the core gap. That is why a meter like the UNI-T UT219PV can read both AC and DC current while a simple AC clamp cannot. The method is non-contact and safe, but it measures the net field through the jaw, so adjacent conductors, jaw alignment and low currents all affect the result. A clamp meter is the right instrument for load current, leakage and motor current; it is not the right instrument for milliohm resistance, for which a four-wire milliohm meter is required.
The jaw is a transformer primary, and you are the secondary
Every clamp meter begins with a magnetic circuit. The jaws are not just a convenient shape; they are a split core, usually laminated iron or a ferrite, that closes around the conductor. When alternating current flows in that conductor, it produces an alternating magnetic field in the core. A secondary winding wound inside the meter sees that changing field and produces a small current proportional to the primary current. The meter measures that secondary current and scales it to the display.
This is a current transformer, and it explains several things at once. First, it only works for alternating current, because a transformer needs a changing field. Second, the ratio is fixed by the number of secondary turns, so accuracy depends on the core and the winding, not on the display electronics alone. Third, the jaws must close cleanly. A speck of dirt or a slightly sprung jaw introduces an air gap, and an air gap in a transformer core changes the coupling. The meter will still read, but not necessarily the same reading you would get with clean, closed jaws.
The transformer principle also explains why a clamp meter can be safe on live conductors. The conductor passes through the core without being interrupted, so there is no need to break the circuit, no need to touch a live terminal, and no need to insert a shunt. The insulation of the conductor and the insulation of the jaws do the separation. That is the whole safety argument, and it is why the jaw is rated for a category and voltage, not just the display.
DC current cannot use the transformer alone, which is why Hall sensors exist
Direct current produces a steady magnetic field. A steady field does not induce a voltage in a secondary winding, so a pure transformer clamp reads zero on DC no matter how much current is flowing. To measure DC, the meter needs a sensor that responds to the field itself rather than to its rate of change. The common solution is a Hall effect sensor placed in a gap in the core. The core concentrates the field, the Hall element produces a small voltage proportional to the field, and the meter amplifies and scales that voltage.
This is the difference between an AC clamp and a true AC/DC clamp. A meter that claims DC current without a Hall sensor or a similar field sensor is either not measuring DC or is measuring something else. The listing for the UNI-T UT219PV Solar Pro states AC/DC True RMS and non-contact current measurement, which is consistent with a Hall-based DC path. The listing for the Megger DCM1500S states 1500 A AC/DC and 2000 V DC, which is a solar-oriented specification: high DC voltage and high DC current, the two things a string or battery circuit presents.
Hall sensors bring their own problems. They drift with temperature, they have offset, and they are less sensitive than a transformer at very low currents. That is why DC clamps often have a zero or relative button, and why the lowest accurate range on a DC clamp is usually higher than on an AC clamp. The instrument is not faulty; the physics of the sensor sets the floor.
What the clamp actually measures: net field, jaw position and the low-current floor
A clamp meter does not measure the current in one conductor. It measures the net magnetic field passing through the jaw. If two conductors carrying current in opposite directions are inside the jaw, their fields partly cancel and the meter reads the difference. That is useful for leakage current, where the difference between live and neutral is the leakage, and it is a trap for anyone who clamps a whole cable and expects the sum of the conductors. The reading is the algebraic sum of the currents enclosed, not the current in any one of them.
Jaw position matters too. The core is designed so that a conductor near the centre gives the most accurate reading, and a conductor pressed against one side can read slightly differently. The effect is small on a large current and significant on a small one. For low currents, the recommended practice is to centre the conductor, close the jaws fully, and if the meter has a relative or zero function, use it with the jaws closed and no conductor inside before clamping.
The low-current floor is the specification buyers most often ignore. A clamp that reads 600 A full scale may have a resolution of 0.1 A and an accuracy of a few percent of reading plus a few counts. At 0.5 A, the counts dominate and the percentage error is large. If the job is checking a 4 to 20 mA signal or a small leakage current, a clamp meter is usually the wrong instrument; a multimeter in series, or a dedicated leakage clamp with a much lower range, is the right one. The listing for the EX655 states a 600 A AC/DC range and True RMS, but the listing does not publish a resolution or an accuracy figure, so the low-current behaviour cannot be judged from the bullets alone.
True RMS, crest factor and the waveforms that make a clamp lie
A clamp meter’s AC path is usually calibrated for a sine wave. If the current is a distorted sine, a phase-controlled load, a switched-mode supply or a variable frequency drive, the average of the rectified signal is not the same as the heating equivalent. A True RMS meter computes the heating equivalent, which is what matters for conductor sizing and for the thermal load on a component. A non-True RMS meter will read low on a distorted waveform, sometimes by a large margin.
True RMS is not a complete answer either. The crest factor is the ratio of peak to RMS, and a meter has a maximum crest factor it can handle before the reading becomes unreliable. A waveform with a high crest factor, such as a pulsed current, can exceed that limit even on a True RMS meter. The listing for the FLIR CM276 states a VFD mode that uses a low-pass filter to give accurate voltage, current and frequency readings where switching electronics affect them. That is the practical answer for drives: filter out the high-frequency switching component and read the fundamental, rather than trying to true-RMS the whole spectrum.
The lesson is that the waveform is part of the measurement. A clamp meter reading 10 A on a clean resistive load and 10 A on a drive are not measuring the same thing unless the meter is configured for the waveform it is seeing. The instrument’s bandwidth, its filtering and its RMS method all decide whether the number means what the reader thinks it means.
The clamp is one function; the meter around it decides the rest
Most clamp meters are also multimeters, and the quality of the voltage, resistance and continuity functions is separate from the quality of the clamp. The clamp is a current sensor; the test leads are a voltage divider and a current source. They share a display and a case, not a measurement principle. That is why a meter can be excellent on current and mediocre on resistance, or the reverse.
The category rating is the safety specification that matters most. A CAT III or CAT IV rating describes the transient environment the meter can survive, not the voltage it can read. A meter rated CAT IV 600 V, such as the UT219PV per its listing, is intended for commercial and industrial distribution, where transients are larger than at a socket outlet. A meter with a lower category rating may read the same voltage on a quiet day and fail dangerously on a bad one. The listing for the Megger DCM1500S states 2000 V DC and 1500 V AC, which is a solar string and battery specification; the category rating is Not stated in the listing bullets, so the buyer should not assume it from the voltage alone.
For resistance and continuity, the clamp is irrelevant. The test leads and the meter’s resistance range do the work, and the clamp jaws should be open and empty. If the question is whether a connection is good, the right tool may be a milliohm meter rather than a clamp, because a two-wire resistance measurement includes the lead resistance and the contact resistance of the probes. The article on milliohm meters for low resistance testing explains why the four-wire method exists and when it changes the answer.
Where the clamp meter stops being the right instrument
A clamp meter is a current instrument. It is at its best on load current, motor current, leakage current and battery or string current, where the conductor is accessible and the current is large enough to be above the low-current floor. It is at its worst on small currents, on DC below the Hall sensor’s offset, and on any measurement that requires breaking the circuit or measuring a voltage drop across a known resistance.
It is also not an alarm. A clamp meter displays a number; it does not warn, trip or log unless the listing says it does. The FLIR CM276 listing states infrared guided measurement with a 160 by 120 resolution and built-in LED worklights, which is a diagnostic aid for locating hot spots before working live, not a substitute for a thermal imager or for lockout. The listing does not publish a temperature accuracy, so the infrared function should be treated as a guide, not a calibrated measurement.
For voltage, resistance, capacitance, frequency and diode test, a clamp meter with a full DMM function, such as the UT219PV per its listing, can replace a multimeter on many jobs. But the clamp itself adds nothing to those measurements, and a dedicated multimeter is often more accurate and easier to hold. The article on multimeter versus clamp meter sets out the division of labour; the short version is that the clamp is for current without breaking the circuit, and the multimeter is for everything else. If the question is how a digital multimeter works, the explanation of the digital multimeter’s measurement chain covers the voltage divider and the analogue-to-digital converter that the clamp meter shares.
The one sentence to remember
A clamp meter measures the magnetic field a current produces, so it is accurate when the field is large, clean and centred, and it is unreliable when the current is small, the waveform is distorted or the jaw is not closed; use it for load current and use a multimeter or a milliohm meter for the measurements it cannot make.
Frequently Asked Questions
Can a clamp meter measure DC current?
Only if it has a DC-capable sensor, usually a Hall effect element in the core gap. A transformer clamp cannot measure steady DC because a steady field does not induce a voltage in the secondary winding. The listing for the UNI-T UT219PV states AC/DC, and the listing for the Megger DCM1500S states 1500 A AC/DC, so both are DC-capable by their published specification. A meter that lists only AC current is not.
Why does my clamp meter read zero on a live conductor?
The most common reasons are that the meter is set to DC and the current is AC, or the reverse; that the jaws are not fully closed; that the conductor is outside the jaw; or that the current is below the meter's low-current floor. A clamp meter measures the net field through the jaw, so two conductors carrying opposite currents can cancel. Check the function, the jaw closure and the conductor position before suspecting the instrument.
Is a clamp meter accurate enough for a 4 to 20 mA signal?
Usually not. A 600 A clamp has a resolution and an accuracy that are specified at a much higher current, and at 20 mA the counts and the offset dominate. The listing for the Generic EX655 does not publish a resolution or accuracy figure, so the low-current behaviour cannot be judged from the bullets. A multimeter in series, or a dedicated low-range current clamp, is the correct instrument for milliamp signals.
What does True RMS mean on a clamp meter?
True RMS means the meter computes the heating equivalent of the waveform, not the average of the rectified signal. On a distorted waveform, such as a phase-controlled load or a switched-mode supply, a non-True RMS meter reads low. True RMS is necessary but not sufficient: the crest factor and the meter's bandwidth also limit what it can measure, which is why drives often need a low-pass filter mode such as the VFD mode stated for the FLIR CM276.
Can I use a clamp meter to find a leakage current?
Yes, if the meter has a low enough range and you clamp all the conductors of the circuit together. The meter then reads the net current, which is the leakage. A standard 600 A clamp is usually too coarse for small leakage; a dedicated leakage clamp with a millamp range is the right tool. The principle is the same: the jaw measures the algebraic sum of the enclosed currents.
Does the jaw size affect the reading?
It affects what you can clamp and how well the conductor sits in the core. A larger jaw accepts thicker conductors and busbars, but the conductor should still be centred for the best accuracy. A conductor pressed against the jaw can read slightly differently, especially at low currents. The listing for the UNI-T UT219PV states a large jaw for industrial conductors; the listing does not publish the jaw dimensions, so the buyer should check the physical opening against the conductor size.