Product type
A clamp meter reads the current in a conductor by enclosing it in a magnetic circuit rather than by inserting anything into the line. The conductor stays where it is, the supply stays on, and the reading appears on the display, which is the whole reason the type exists: on a feeder, a distribution board or a machine that cannot be shut down, there is no opportunity to break the circuit and put an instrument in series with it.
A clamp meter reads the current in a conductor by enclosing it in a magnetic circuit rather than by inserting anything into the line. The conductor stays where it is, the supply stays on, and the reading appears on the display, which is the whole reason the type exists: on a feeder, a distribution board or a machine that cannot be shut down, there is no opportunity to break the circuit and put an instrument in series with it.
Measuring from outside the conductor brings its own set of limits. The jaw has to close around the conductor, so the conductor’s diameter is a specification; the reading comes from the field the current produces, so a small current is a small field and the bottom of the range is where the instrument struggles; and two conductors carrying the same current in opposite directions cancel, which is why a single conductor has to be enclosed rather than a cable carrying both.
Within the type the decision that matters most is whether the jaw measures direct current as well as alternating. It is not a question of resolution or of price alone. The two are built on different sensors and behave differently, so an instrument bought for a mains feeder will not substitute for one needed on a battery bank or a drive output, and the difference cannot be recovered after the fact.
An alternating-current jaw is a current transformer. The magnetic circuit of the jaws couples the field of the conductor into a secondary winding inside the instrument, and the current induced there is proportional to the current in the conductor. The arrangement is passive and draws nothing from the circuit, but it responds only to a changing field, which is why a transformer jaw reads nothing on a steady direct current.
A jaw that also measures direct current uses a Hall-effect element in a gapped magnetic circuit, or a fluxgate arrangement, to detect the static field the current produces. The gap and the element make the jaws more sensitive to position: where the conductor sits between them, and whether they are fully closed, both move the reading in a way a transformer jaw is largely immune to. That is why the direct-current figure is normally specified with a lower accuracy than the alternating one on the same instrument, and why the zero has to be set before a small direct-current reading is trusted.
The signal then passes through the same conversion decision as any other alternating measurement. An instrument that averages the rectified waveform reads correctly on a sine wave and low on anything else, while a true-RMS instrument computes the heating equivalent and agrees with the other instruments on the same installation. Variable-speed drives, switching supplies and dimmers put that choice on the specification rather than in the manual.
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
|---|---|---|
| Sensor type | A current-transformer jaw for alternating current, or a Hall-effect jaw for alternating and direct current | Whether a DC feeder, a battery bank or a drive output can be measured at all. It is decided by the sensor and cannot be added later. |
| Jaw opening | From about twenty-five millimetres upwards | The largest conductor or bar that fits between the jaws. A conductor that does not fit cannot be measured with the instrument, whatever its range. |
| Count and resolution | Around two thousand counts, four thousand, or six thousand | The smallest change visible on a given range. A higher count helps on the top range rather than at the bottom of the current span. |
| Alternating response | Averaging, or true root mean square | Whether readings taken on drives, switching supplies and other non-sinusoidal loads are comparable with readings taken on clean mains. |
| Additional functions | Resistance, continuity, capacitance, frequency, temperature and a non-contact voltage detector | How many instruments the job has to carry. On most service work these replace a second meter rather than extend the first. |
| Safety category and voltage class | Category ratings stepping up towards the supply entry point | What the instrument is rated to survive if a transient arrives while the jaws are around a live conductor. This is a safety limit and not an accuracy figure. |
| Display form | A numeric display, or a display with a bar graph or a starting-current capture | Whether a fluctuating or starting current can be read. A motor start is over before a numeric display settles. |
The sensor type and the jaw opening are decided first, because between them they set the list of jobs the instrument can be taken to.
| Decision | What settles it | What to ask |
|---|---|---|
| Whether the circuit can be interrupted | The consequence of switching the supply off | Where it cannot be, clamping is the only method available and a series meter is out of the question. |
| Whether direct current is involved | The nature of the supply or the load | A battery bank, a photovoltaic string, a drive output or a DC supply needs a Hall-effect jaw; a mains feeder does not. |
| The conductor to be enclosed | The diameter of the cable, or the width of the bar | The jaw opening has to exceed the conductor with the jaws fully closed, since a partially closed jaw reads low. |
| The smallest current of interest | Whether a leakage or a standby figure is wanted | A small direct-current reading needs the instrument zeroed first, and small alternating currents sit near the bottom of the jaw’s useful range. |
| The waveform at the load | Whether the load is resistive or electronic | A drive or a switching supply requires a true-RMS instrument for the reading to be comparable with others taken on the site. |
| The point of measurement in the installation | The transient level at that point | A distribution board and a low-voltage electronic circuit do not carry the same transient energy, and the rating has to match the point of use. |
| Whether a starting current has to be captured | The presence of motors | A capture or peak-hold function is specified with the instrument, since a numeric display alone cannot report a start. |
If any of these are still open, send what is known. A quotation can be issued against a duty description and refined once the gaps are filled.
These are the types an enquiry for a clamp meter usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a clamp meter |
|---|---|
| Digital multimeters | Measure current by inserting a shunt into the circuit, so the load has to be interrupted and the whole current passes through the instrument. They are more accurate on a low current and stop at a few amperes; a clamp meter trades some accuracy for the ability to measure without breaking the line. |
| Power meters | Read voltage and current together and compute power, power factor and energy. A clamp meter answers what the current is, not what the load is doing with it. |
| Current meters | Carry one current value to a panel or a control room continuously, usually as a loop-powered indicator. A clamp meter is a portable instrument taken to the conductor. |
| Electrical safety testers | Apply a test voltage and measure the leakage that results, which proves insulation rather than measuring the current in service. The two are used at different stages and neither replaces the other. |
| Voltage testers | Establish whether a conductor is live rather than how much current it carries. Deciding that a circuit is dead is a voltage question and cannot be answered by clamping it. |
The boundary worth settling first is whether the conductor can be interrupted. Where it can, a series connection is the more accurate method; where it cannot, the clamp meter is not a compromise but the only instrument that will do.
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
Available products
3266TA (3 models) · Clamp Meters
Clamp meter range reading current through a 25 mm jaw without breaking the circuit, from an AC-only model up to an AC/DC instrument with true-RMS conversion, 6000 counts and a 100 mF capacitance range.
Also in Electrical Test & Measurement
Every type we quote for under Electrical Test & Measurement. A type with no link yet is still quoted for - it is described on the Electrical Test & Measurement overview, and gets a page of its own as soon as a model is published against it.
Frequently asked
Because the two are built on different sensors. A transformer jaw responds only to a changing field, so it reads nothing on a steady direct current; a Hall-effect element detects the static field and can therefore measure direct current as well. The difference is in the instrument rather than in the setting, so the requirement has to be settled before the model is chosen.
Less accurate, and unavoidably so. The clamp method has to work with the field the conductor produces, so the position of the conductor between the jaws, the completeness of the closure and any nearby field all affect the reading, and a direct-current jaw adds the element’s own offset. The method exists because the circuit cannot be interrupted, not because it measures better.
Power, power factor and energy, unless the instrument is specifically built to read voltage at the same time. A current figure on its own does not say how much work the load is doing, and a load at a low power factor can carry a large current for very little power. Where consumption is the question, a power meter answers it and a clamp meter does not.
The largest conductor to be clamped; whether direct current is involved; the currents of interest at both ends of the range; whether the load waveform is sinusoidal; the point of measurement and the safety category that goes with it; whether a starting current has to be captured; and which additional functions, if any, are wanted on the same instrument.
Send the specification and we will confirm the matching model, availability and lead time. Minimum order quantity is 1 pc.
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