Clamp Meters

Clamp meters are electrical testers featuring wide jaws that are able to clamp around an electrical conductor. Originally designed as a single-purpose tool for measuring AC current, clamp meters now include inputs for accepting test leads and other probes that support a wide range of electrical measurements. Indispensable as a test tool, the jaws of a clamp meter facilitate work in tight spaces and permits current measurements on live conductors without circuit interruption.

Though clamp meters are closely related to multimeters, they are not simply multimeters with inductive jaws replacing test leads. Generally, multimeters can be seen as voltage-measuring instruments with some current-measuring ability while clamp meters are current-measuring instruments with some voltage-measuring ability.

Clamp Meter Technology

Clamp meters rely on the principle of magnetic induction to make non-contact AC current measurements. Electric current flowing through a wire produces a magnetic field. Since alternating current frequently reverses polarity, it causes dynamic fluctuations in the magnetic field which are proportional to the current flow. A current transformer inside the clamp meter senses the magnetic fluctuations and converts the value to an AC current reading. This type of measurement is convenient for measuring very high AC currents.

Direct current, however, flows through conductors in a fixed polarity. Consequently, the magnetic field around the conductor does not change and conventional clamp meters will register no reading. DC clamp meters work on the principle of the Hall Effect. Hall effect sensors sense the magnetic field caused by current flow which causes a small voltage across the Hall effect sensor. That voltage, which is proportional to current is then amplified and measured.

Clamp meters often include other sensors such as voltmeters, ohmmeters, etc. which increase the versatility of the instrument. These other sensors use test leads which plug into the clamp meter. As only current measurements can be made with the clamp, other measurements do not benefit from the non-contact nature of the clamp.

Clamp Meters Features

Clamp meters are often available with a number of features that make it easier to take accurate readings and handle the resulting data. Of course, higher end clamp meters are more likely to include these advanced functions.

True RMS: Since alternating current reverses directions several times per second, it is represented as a sine wave. Because the amplitude of the sine wave changes continuously over the wave period, current measurements may vary quite a bit at different points in time. True-RMS (root mean square) converts AC signals into DC signals of equivalent value for more stable and accurate AC readings.

IP rating: Ingress Protection Rating classifies and rates the degree of protection of enclosures against the intrusion of moisture and foreign bodies. Adequately protected instruments are suitable for use in a wider range of environments. NEMA ratings are another enclosure rating system that is commonly used.

PC interface: Increasingly, clamp meters are including serial ports (PC interfaces) as a means to easily transfer data from the meter to a computer where further analysis or report generating can occur. Common interfaces include Ethernet, USB, FireWire, or RS-232. Often, software is also available to help organize data once it has been transferred to a computer.

Data logger: Internal memory capable of holding a number of measured values for later recall.

Inrush: The Inrush function provides users an accurate measurement of high current surges that flow into motors during startup. This measurement can be critical when troubleshooting problems such as nuisance trips of over-current protection devices.

Autoranging display: Autoranging units automatically sets the correct measurement range saving users from having to adjust switch positions while trying to position the clamp and take a measurement.

Clamp Meter Measurements

Though the clamp is designed solely for current measurement, most clamp meters accept input from test leads or other probes which greatly increases the types of measurements of which they are capable and making the clamp meter a much more versatile instrument.

• AC/DC current: Current, measured in amperes, is the flow of an electric charge and is the most basic measurement of a clamp meter. Current can be used to generate heat as well as magnetic fields, which are widely used for motors, inductors, and generators. Most modern clamp meters can measure both AC and DC current.
• AC/DC voltage: Voltage, measured in volts, is the difference in electrical potential of a unit charge transported between two points, or more simply, how much electrical energy is delivered if a certain number of electrons are transmitted through a circuit. Voltage can represent either a source of energy or energy used, stored, or lost.
• Resistance: Resistance, measured in ohms, is the opposition to passage of an electrical current through a conductor. Resistance is determined by the material and shape of the conductor.
• Continuity: Continuity is a quick "pass/fail" resistance test that distinguishes between an open or closed circuit. Generally, a continuity test emits a beep when a closed circuit is detected eliminating the need to look at the meter as the test is performed.
• Capacitance: Capacitance, measured in farads, is the ability of an object to store an electrical charge. Any object that can be electrically charged exhibits capacitance.
• Frequency: Frequency, measured in hertz, refers to the rate at which oscillations of alternating current occur in an electric power grid. Generally, power systems in North America use a 60 Hz frequency.
• Power Factor: Power factor is an advanced measurement defined as the ratio of real power flowing to the load, to apparent power in the circuit. In an electrical system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred.
• Temperature: Many clamp meters accept input from temperature probes or thermocouples for contact temperature measurements.

Things to Consider When Selecting a Clamp Meter

• What measurements and ranges are needed?
• Are jaw sizes important for your application?
• Does the meter need protection from dirt or moisture (IP or NEMA ratings)?
• What resolution and accuracy are required?
• What accessories (test leads, probes, etc.) are needed?
• Does the clamp meter have the correct safety rating for the work you are doing?

If you have any questions regarding clamp meters please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.