Current clamp

A current probe, also clamp meter, clamp or colloquially called clamp-on ammeter is a measuring instrument for indirect measurement of electric current. Whereas in the direct measurement of the circuit must be separated in order to switch the current measuring device in the electric line, this is not required for the indirect measurement of the current probe as it is registered, the magnetic effect of the phase current. Thanks to a pincer-like separable iron core can comprise conductor or conductor rail, without separating the circuit. Therefore be also measured with systems which can not be disabled for purposes of measurement. The measurement is isolated from ground. A current probe measures the amperage of a conductor when it is accessible individually. Enclose them a lead cable with rear ladder, they observed only leakage currents.

• 2.1 Built-in Display
• 2.2 Display on accessories

Operation

AC measurement

When AC current clamp diameters the transformer principle is applied. The clamp acts here as a magnetic transducer / converter. Thereby, the fixed and the movable arm of the clamp is in the closed state of the transformer core, the conductor to be measured and the primary winding of the coil in the measuring device, the secondary winding. The current in the conductor magnetizes the core, inducing a current in the secondary winding which is proportional to the conductor current. The output of the secondary coil is so large that it can operate directly ( scaled appropriately ) instrument (eg with moving iron ). The energy to drive the measuring element is taken from the current to be measured circle.

Allstrommessung

Ready DC current probe can not be built according to the above principle because of the lack of alternating fields. Here Hall sensors or magnetic field-dependent ( magnetoresistive ) resistors, which may also detect static magnetic fields, arranged in an air gap of the core. The generated weak signals must be amplified electronically. Therefore these measuring instruments are powered via batteries or power supplies with energy. These meters are suitable for alternating currents.

Furthermore, a measurement principle may be used with the compensation method. In this case, a current flowing through compensating winding current is controlled so that the magnetic flux in the core will ideally be zero. The compensation current is reported to the police; the current flowing (as described above in the transformer principle ) underpinned the turns ratio accordingly. The advantage here is that any non-linear properties such as the core or the Hall sensor hardly have influence on the measurement. Also in this case necessary for the control of the compensation current and a power supply.

There is also a historical, rarely used form of current clamp meter, which basically is a suitable for both AC and DC current moving iron, the magnetic circuit is formed by the forceps. In this respect it resembles the outside of today's design, but not a Hall or magnetoresistive sensor is used to determine the field strength used, but brought the force of the magnetic field mechanically for display. For DC or AC often different scales are mounted on the measuring elements. These devices are only suitable for relatively large currents and have high measuring accuracy. For measuring range change separate moving iron works are pluggable placed in the magnetic circuit.

Display of the measured value

Built-in Display

The measured values ​​of current clamps in accordance with the principle of a transformer can be visualized directly on moving iron works. It can also be used Drehspulmesswerke, but where a rectifier must be installed.

Newer versions often have digital displays for which the measured value must be converted by means of electronic circuits only. Such devices therefore require (in addition to the measured variable ) an additional power supply.

As noted above, the DC current clamps suitable in principle need amplifier for weak signals of the magnetic sensors. So anyway, since electronic modules and an additional power supply are already available, they are usually equipped because of the small additional expense with digital displays.

Indication of additional devices

There are also current probes without built-in display ( current clamp ). Apart from its structure is basically the same as in the other embodiments. Instead of a meter here sockets or cables are available, over which the measured value is transmitted to oscilloscopes, chart recorder or meter, for example. Each device has a fixed transmission ratio, about the true magnitude of the current must be calculated. For example, is a clamp having a gear ratio of 100:1 at a current of 20 A of output current from 200 mA. There are also versions with built-in current - voltage converter ( in the simplest case as a passive load (resistance) ), the right to a voltage input (usual input of an oscilloscope ) can be connected.

Execution of the core

The split core of the current probe consists of laminated electrical steel sheets ( at higher frequencies, ferrites) and is usually isolated in addition, there is indeed often measured with such devices on non-insulated busbars. The contact surfaces of the two core parts are polished and fitted with teeth together to keep magnetic losses as low as possible. The movable part of the core is connected with a hinge to the fixed part and can be opened by means of a lever. During the measurement, a closing spring presses the two core parts together.

Measuring range

The measuring range of current clamp diameters is bounded below, as at smaller currents the magnetic effect is so weak that the iron core is not magnetized enough. Will you still measure small currents with them, so you can make several turns of the conductive line by the tongs. Is inserted, for example, to be measured in line 10 turns around the core, the result is the ten-fold indication. Back to top of the possible measuring range is limited only by the size of the forceps opening, as currents in the kA range extremely large conductor cross sections or busbars and thus correspondingly large openings require pliers.

Special design for multi-core cables

Current probe designed for the measurement of a single conductor. Apart from leakage currents, the magnetic fields of the outgoing and return conductors pick up. Therefore the measured wire must be accessible separately for multi-core cables, which in some cases requires stripping the cable sheath.

But in the vicinity of the magnetic fields are different, depending on whether the sensor is located closer to one or the other line. Multi-sensor current probe offset the signals differently arranged coils. They are more expensive and less accurate, but can measure normal power lines with return cable.