The reading produced by a conductor carrying a very low current can be increased by winding the conductor around the clamp several times the meter reading divided by the number of turns is the current, with some loss of accuracy due to inductive effects.Ĭlamp meters are used by electricians, sometimes with the clamp incorporated into a general purpose multimeter. This could be clamped around standard two- or three-conductor single-phase cables to provide a readout of the current flowing through the load, with no need to separate the conductors. As electrical cables for equipment have both insulated conductors (and possibly an earth wire) bonded together, clamp meters are often used with what is essentially a short extension cord with the two conductors separated, so that the clamp can be placed around only one conductor of this extension.Ī relatively recent development is a multi-conductor clamp meter with several sensor coils around the jaws of the clamp.
In particular if the clamp is closed around a two-conductor cable carrying power to equipment, the same current flows down one conductor and up the other the meter correctly reads a net current of zero. Only one conductor is normally passed through the probe. Pushing the large button at the bottom opens the lower jaw of the clamp, allowing the clamp to be placed around a conductor.Īn electrical meter with integral AC current clamp is known as a clamp meter, clamp-on ammeter, tong tester, or colloquially as an amp clamp.Ī clamp meter measures the vector sum of the currents flowing in all the conductors passing through the probe, which depends on the phase relationship of the currents. The Rogowski coil gives a voltage proportional to the rate of change of current in the primary cable, so more signal processing is needed before the sensed values can be displayed.Ī multimeter with built-in clamp. It has the advantage of better linearity, having no core to saturate, it can be made flexible, and does not require any magnetic or electrical contact at the opening end. This coreless transformer is used in clamp meters and power monitoring loggers. Resembling a current clamp in appearance and function is the Rogowski coil current sensor. This type was often used with oscilloscopes, and with high-end computerized digital multimeters, however, they are becoming common place for more general use. The Hall effect type is more sensitive and is able to measure both DC and AC, in some examples up to the kilohertz (thousands of hertz) range. The calibration of the instrument is clearly non-linear. The vane is usually fixed directly to the display mechanism of an analogue (moving pointer) clamp meter. Due to its physical size it is generally limited to power transmission frequencies up to around 100 Hz. In the iron vane type, the magnetic flux in the core directly affects a moving iron vane, allowing both AC and DC to be measured, and gives a true root mean square (RMS) value for non-sinusoidal AC waveforms. In this mode, the coil forms the primary and the test conductor the secondary. Usually, the injection probe is specifically designed for this purpose. This type may also be used in reverse, to inject current into the conductor, for example in electromagnetic compatibility susceptibility testing to induce an interference current. When measuring current, the subject conductor forms the primary winding and the coil forms the secondary.
Like any transformer this type works only with AC or pulse waveforms, with some examples extending into the megahertz range. The conductor it is clamped around forms the other winding. A wire coil is wound round one or both halves, forming one winding of a current transformer. Types of current clamp Current transformer Ī common form of current clamp comprises a split ring made of ferrite or soft iron.
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