Clamp meter

How it works

Current in a conductor produces a magnetic field around it. The jaws of a clamp meter close into a ring that concentrates this field. In a current-transformer clamp the jaws are wound with a secondary coil and work like a transformer with one primary turn, which is the conductor being measured. Fluke gives the example of a 1000-turn secondary, in which 1 ampere in the conductor produces 1 milliampere at the meter’s input.

A Hall-effect clamp has a small air gap in the jaw tips with a semiconductor sensor in it that responds to the magnetic flux directly; Fluke says these clamps measure both ac and dc current up to about 1000 hertz and must be zeroed before a reading to remove offsets. A flexible clamp uses a Rogowski coil, a wound air-cored coil with no iron, whose output is proportional to the rate of change of current and is integrated by the meter; it works only with ac current.

Anatomy

Jaws: Hinged ferrite-iron jaws, concealed under the plastic molding, that surround the conductor and concentrate its magnetic field.
Jaw trigger or release lever: Opens the jaws to admit the conductor.
Tactile barrier: A ridge that keeps the fingers behind it when the jaw is used on live conductors.
Rotary dial: Selects current, voltage, resistance and other functions.
Display and function buttons: Hold, Min Max, Zero, Inrush and similar functions on some models, as listed in Fluke’s guide to the symbols.
Test lead jacks: For voltage and resistance measurements with probes.

Variants

Current-transformer (ac) clamp: Rigid jaws wound with a secondary coil; ac only.
Hall-effect (ac/dc) clamp: A sensor sits in a gap between the jaw tips and the clamp measures dc as well as ac.
Flexible clamp (Rogowski coil): A thin flexible coil that wraps around cables or busbars that a rigid jaw cannot fit; ac only. Fluke cites a model reading up to 2500 A ac.
Milliamp process clamp: Reads low-level control-loop current such as 4 to 20 mA without breaking the loop; Fluke lists models for HVAC controls.
Hot-stick current transformer: An insulated-handle device for measuring current in high-voltage conductors, described in Schweitzer’s patents of 1945 and 1954.

Specifications

Secondary turns ratio, example · turns: 1000 : 1Fluke’s worked example for a current-transformer clamp; actual ratios vary by model.
Hall-effect frequency range · hertz: dc to about 1000As stated by Fluke.
Inrush measurement · samples per interval: about 400 samples over 100 msFluke’s description of the inrush function.
Jaw opening · mm: Varies by model and has not been sourced here.

Made to

IEC 61010-1
IEC 61010-2-032
IEC 61010-2-033

Safety and care

Safety

Keep the fingers behind the tactile barrier when measuring with the jaw, avoid explosive gas and vapor and damp or wet places, and examine the probes for damaged insulation and exposed metal before use.
When connecting test probes, connect the common probe before the live probe and disconnect the live probe first.
Do not use a flexible current sensor if the inner contrasting insulation color is showing, and do not fit or remove the flexible probe on hazardous live conductors.
Schweitzer’s 1945 patent records that opening a hinged split-ring current transformer around an energized conductor was hazardous to the operator, and that voltages as low as 110, 220 and 440 volts had caused severe and sometimes fatal injuries to operators of such devices.

Care

Use only the specified battery type, properly installed, and replace the batteries as soon as the low-battery indicator appears to avoid false readings.
Replace damaged test probes before using the meter, and have the meter serviced only by qualified personnel with the specified replacement parts.

Tips

Clamp one conductor, not the whole cable
Fluke notes that current flowing in opposite directions cancels. The jaws read the net current, so clamping a cable that carries both the supply and return conductors reads near zero, and one conductor must be isolated inside the jaw.1
Zero a dc clamp with the jaws closed and empty
On models with a Zero button, wait for the display to stabilize with the dial at dc current, with the jaws closed and no conductor inside, then press Zero. Fluke explains that Hall-effect clamps pick up the earth’s field and other nearby fields, so the reading is zeroed to remove the offset before measuring.1 2
Close the jaws completely and square
The Chauvin Arnoux article says a gap or misalignment of the jaws of an iron-core clamp can produce significant reading errors, one reason it gives for choosing flexible sensors in cramped spaces.1
Read the arrow as the direction of the current
Fluke says an arrow is placed near the jaw opening on current clamps. On a dc clamp the arrow pointing toward the load gives a positive sign and pointing toward the source a negative sign, and on power and power-quality measurements the orientation matters.1
Arm the inrush function before switching the load on
Fluke says only meters with an inrush button can measure it: the meter is armed, triggered by the starting current, and takes about 400 samples over 100 milliseconds to calculate the starting current. The value is recorded for later comparison, and Fluke says it helps locate a startup problem in the motor or the starting circuit.1 2
Center the conductor in a flexible probe and keep other cables away
Fluke’s instructions for flexible current probes are to center the conductor in the probe and, if possible, to avoid measuring close to other current-carrying conductors. Chauvin Arnoux adds that a flexible coil can be looped around a small cable several times to multiply the reading and improve low-end measurement.1 2
Cycle through maximum, minimum and average with Min Max
Fluke says the first push of Min Max shows the maximum input, and later pushes show the minimum and the average, in current, voltage and frequency modes.1 2

History

Earliest documented

c. 1930

not stated (object held by the Science Museum Group, London)

The earliest dated object located is a split-core transformer used with a hot-wire ammeter in high-voltage circuits, cataloged by the Science Museum Group as made in 1930. Chauvin Arnoux claims a hinged-jaw transformer clamp in 1937, and General Electric announced a portable hook-on volt-ammeter in October 1940.1 2 3

The clamp meter answers a limit of the ammeter. A conventional ammeter sits in series with the circuit, so the circuit has to be opened. Fluke describes the clamp as an integration of a hinged jaw into a meter, so that current can be measured without disconnecting or deenergizing the circuit. The principle is the current transformer, in which, as Fluke explains, the conductor is the single primary turn and the coils on the jaws are the secondary. A hinged or split core allowed the transformer to be fitted around a conductor already in place, and the Science Museum Group holds a split-core transformer of about 1930 used with a hot-wire ammeter on high-voltage circuits.

Who first made the hinged clamp is a matter of company claim and sparse records. Chauvin Arnoux says it invented the split-core hinged-jaw transformer clamp in 1937, and the surviving evidence of the period is a General Electric announcement in 1940 of a hook-on volt-ammeter with a selector switch and a rectifier instrument in a molded case. Schweitzer’s patent filed in 1942 refers to split-ring transformers already on the market and regards them as hazardous, since the core had to be opened by hand near live conductors. None of these sources gives an inventor for the first instrument.

Safety shaped the early forms. Schweitzer’s 1945 patent records injuries at ordinary voltages when operators opened hinged transformers on energized conductors, and sets as its aim a device that keeps the operator at a distance. His 1954 patent describes a fixed-gap core on an insulating handle for conductors at 2300 volts and above. General Electric’s molded handle was described as providing insulation and a maximum of safety for the operator.

The medium of measurement then changed. An iron-core current transformer does not read direct current. Fluke’s Hall-effect clamps put a semiconductor sensor in an air gap in the jaw and measure ac and dc, and Blain’s 1985 patent addresses the residual magnetization of the core in a clamp-on instrument for dc current, adapted for ac. The Rogowski coil, first used in 1912 according to Fluke, supplied a flexible alternative with no iron core, no saturation and a wide frequency range, but it reads only ac.

The clamp meter has also absorbed the multimeter. Chauvin Arnoux says clamp meters have evolved to include digital displays, Hall-effect sensors for dc, voltage and resistance functions and, on recent models, power, power factor and harmonics; Fluke defines a clamp meter as a basic digital multimeter integrated with a current sensor. The standard that now governs the class is IEC 61010-2-032. O*NET’s Tools Used table lists clamp-on ammeters for 2 of the 6 occupations mapped to this tool in the archive’s crosswalk.

Timeline

1912
Fluke dates the first use of the Rogowski coil, the air-cored coil later used in flexible clamps, to 1912; Chauvin Arnoux says it was invented that year by the German physicist Walter Rogowski.1 2
c. 1930
A split-core transformer for use with a hot-wire ammeter in high-voltage circuits is made; it is cataloged by the Science Museum Group.1
1937
Chauvin Arnoux says it invents the transformer clamp, the first current clamp to use a split-core hinged-jaw system; the clamp was soon combined with an ammeter into a stand-alone “tong tester”, according to the company.1
1940
General Electric places on the market the Type AK-1 portable hook-on volt-ammeter for alternating current and voltage, for conductors up to 2 inches in diameter, accurate within 3 percent and operated with one hand.1
1942
Edmund O. Schweitzer, Jr. files the application for a current transformer that can be placed on a conductor at a distance, because split-ring transformers then on the market had to be opened by hand; the patent issues in 1945.1
1949
Schweitzer files for improvements to a core structure with a fixed air gap carried at the end of an insulating handle, for measuring current in conductors at 2300 volts and above; it issues as US 2,686,898 in 1954.1
1985
Aurele Blain’s US Patent 4,513,246 issues for a clamp-on ammeter that restores the core to a predetermined magnetic state for each measurement, so that it can measure dc current, with additional circuitry adapting it for ac.1
2012
IEC publishes IEC 61010-2-032:2012, the safety standard for hand-held and hand-manipulated current sensors, which measure current without opening the circuit.1

Where the sources disagree

Who made the first hinged split-core current clamp, and when?

Chauvin Arnoux invented the transformer clamp in 1937, the first to use a split-core hinged-jaw system.1
Chauvin Arnoux UK, in an article it authored
A split-core transformer was made about 1930 for use with a hot-wire ammeter in high-voltage circuits; the record does not name a maker.1
Science Museum Group
Hinged split-ring current transformers were already on the market by 1942; no inventor or date is given.1
E. O. Schweitzer, Jr., US Patent 2,375,591

Gaps in the record (5)

No patent or catalog confirms the 1937 Chauvin Arnoux transformer clamp; the date rests on the company’s own article.
The first clamp-on ammeter by any maker, with an inventor and date, has not been identified.
The Science Museum Group record for the c. 1930 split-core transformer does not name the maker or say whether it was hand-held.
When digital clamp meters and Hall-effect clamps first appeared has not been sourced.
Jaw opening and current range specifications have not been sourced for typical meters.