Clamp meter
A clamp meter (clamp-on ammeter) is a type of ammeter that measures electrical current without the need to disconnect the wiring through which the current is flowing.
Clamp meters are also known as tong testers or Amprobes (after one of the first vendors of such devices).
The most common forms of clamp meter are:
- A probe for use with a multimeter.
- A self-contained unit.
- A built-in part of a specialized multimeter used by electricians.
Pushing the large button at the bottom opens the lower jaw of the clamp, allowing the clamp to be placed around a conductor (wire).
In order to use a clamp meter, the probe or clamp is opened to allow insertion of the wiring, and then closed to allow the measurement. Only one conductor is normally passed through the probe; if more than one conductor were to be passed through then the measurement would be a vector sum of the currents flowing in the conductors and could be very misleading depending on the phase relationship of the currents. In particular, if the clamp were to be closed around a mains extension or similar cord, no current will be measured at all as the current flowing in one direction will cancel that flowing in the other direction.
In practice, nearly all clamp meters are used by electricians and the meters often include additional circuitry to allow the reading of voltage and, sometimes, resistance. The meters also often contain a mechanical pointer-locking device so that a reading can be taken in locations where the meter pointer can't be seen, the pointer then locked, and the meter brought out to a more-convenient place for reading. For the meter shown in the picture, the white push-button marked "lock" provides this function.
AC ammeters
Older clamp meters are AC devices only. The probe consists of a core of ferromagnetic material, which when closed forms the core of a transformer of which the wiring passing through the clamp is the primary winding. The instrument or probe contains a secondary winding of many turns. By measuring the current induced in this secondary winding, and taking account of the characteristics of the transformer and the frequency and waveform of the current being measured, the size of the current flowing through the conductor can be determined.
Less-expensive clamp meters use an average-detecting rectifier circuit that is then calibrated to read in RMS units; it is assumed in their design that the current is a sine wave of the local mains frequency, that is, either 50 or 60 Hz. If either of these assumptions are violated, incorrect readings may be obtained, so when such meters are used with non-sinusoidal loads such as the ballasts used with fluorescent lamps or high-intensity discharge lamps or most modern computer and electronic equipment, the readings produced by such meters can be quite inaccurate.
At the other extreme, probes sold for use with some computerized digital multimeters use true-RMS converters that will give accurate readings in almost any situation. These high-end meters may also replace the transformer secondary winding with a Hall effect sensor that allows accurate readings over a much wider frequency range, often extending from DC to thousands of hertz and occasionally, for small clamp meters with ferrite cores, extending into the megahertz range.
DC ammeters
Newer units often read both AC and DC current. A Hall effect device is used to detect the DC magnetic field induced into the clamp. The signal from the Hall effect device is amplified and displayed using a built-in digital voltmeter. Typical handheld units can read currents as low as 200mA, and units that can read down to 1mA are available.
Related devices
Clamp probes are also sold for use with oscilloscopes. In this case, there is no rectifier (converter) circuit and the current waveform is presented directly to the scope's input. Such probes are available in both Hall effect and inductive forms, for DC or AC applications.
The Columbia tong test ammeter, manufactured by Weschler Instruments, is a clamp-on ammeter with an iron vane movement used for measuring large AC currents up to 1000 amps. The iron jaws of the meter direct the magnetic field surrounding the conductor to an iron vane that is attached to the needle of the meter. The iron vane moves in proportion to the strength to the magnetic field and thus produces a meter indication proportional to the current. This type of ammeter can measure both AC and DC currents and provides a true RMS current measurement of non-sinusoidal or distorted AC waveforms. Interchangeable meter movements can be installed in the clamping assembly to provide various full-scale current values up to 1000 amperes. The iron vane is in a small cylinder that is inserted in a space at the hinged end of the clamp-on jaws. Several jaw sizes are available for clamping around large conductors and bus bars up to 4-1/2" (114mm) wide.
Scientific Basis of Clamp-on Ammeter
The Clamp-on Ammeter is possible because the magnetic field produced by a wire is proportional to the electric current passing through it. A more accurate statement is contained in Ampere's Law, which states that the circulation of the magnetic field around a closed loop is proportional to the electric current passing through that loop. By use of a soft ferromagnetic material within the clamp-on ammeter the magnetic field of the wire is enhanced. By use of Faraday's Law of electromagnetic induction applied to an ac current and ac magnetic field it is possible to measure the circulation through many parts of the loop defined by the clamp-on ammeter. Because of Ampere's Law, if the loop defined by the clamp-on ammeter is much bigger than the current-carrying wire enclosed by the ammeter, no matter where within the ammeter the wire passes, the reading will be the same. An ammeter with radius of one mile would give the same one amp reading if the wire were near one edge or at the very center. The clamp-on ammeter was conceived around 1912 by Walter Rogowski, and is also known as a Rogowski coil and a clip-on ammeter. Even earlier, in 1887 Chaddock designed a similar device but used it to measure magnetic fields rather than currents.