GB2227098A - Current probes - Google Patents

Abstract

A conductor (16) is connected to a generator of calibrating and degaussing signals (not illustrated) in order to apply calibrating and degaussing signals to a current probe (21) designed to sense current in a further conductor (also not illustrated). The conductor (16) is preferably carried in an insulating tube arranged in a loop (12-17), the loop including spur portions (14) by which the tube can be supported. The current probe (21) may have a magnetic circuit (27) with hinged interdigitated jaws and Hall-effect devices may be provided on opposite sides of the magnetic circuit connected differentially. <IMAGE>

Description

ELECTRICAL TESTING APPARATUS Most electrically powered or electrically controlled devices have a complicated wiring system and it is often desired to test the system without breaking through the insulating material covering the cables or disconnecting parts of the system. Current probes have been developed for this task, usually comprising a ring which is connected around a cable of the system, the ring feeding signals into the system and taking measurements out of the system inductively.

Current probes are subject to distortion due to accumulated magnetic fields and also require calibrating. Usually degaussing and calibrating windings are contained in the probe itself, but this makes the construction bulky. It is inconvenient if the probe has to be taken to a central calibrating and degaussing station.

The present invention provides a means for overcoming these disadvantages by providing the combination of a current probe and means for generating calibrating and degaussing signals; the probe however is not connected directly to the generating means, but instead the invention provides a conductor separate from the probe and connected to the generating means for carrying the calibrating and degaussing signals, the probe being applicable to the conductor to receive the signals in the same way as it receives currents to be measured.

Calibrating signals are passed through the conductor and with the probe applied to the conductor, the probe readings are checked against the calibrating signals. Similarly for degaussing purposes, alternating degaussing signals which initially saturate the probe and then decrease in amplitude are passed through the conductor and by induction cause the probe to be cleared of stored magnetic flux. With this arrangement the generator and the conductor connected to it can be mounted close to the conductor whose currents are to be measured by the probe, so that the probe can easily be degaussed and calibrated in conjunction with normal measurements.

The conductor may be in the form of a loop for example Ushaped and may be mounted in a hollow tube to support and insulate the conductor. The tube is preferably provided with means for affixing the conductor on other structures, such as those in which the ordinary cable forum on which the probe is to be used is situated, to assist the mounting of the conductor as mentioned in the preceding paragraph.

Another problem encountered with current probes is that of magnetic reluctance and fringing. These reduce the effective gain of the magnetic circuit and increase the (undesired) response of the probe to external fields; they occur because there must be a gap in the probe to allow it to be passed right around the cable to be tested (since in practice it is impossible to pass the loop of the probe along the cable from one end without obstruction).

The current probe therefore may have a ring-shaped head, the head comprising two parts hinged together, both the end of the parts remote from the hinge and the hinge itself being interdigitised. In this way the closed ring does not have a plane interface between the two parts, but an interface which is zig-zag in shape. The reluctance and fringing is thereby reduced due to the improved alignment and self-shielding effect at their joint.

A further problem with sensing heads such as current probes is that Hall Effect sensors carried on the head are subject to error from temperature drift; they are also subject to error from flux leakage in the magnetic circuit causing external magnetic fields to alter the flux around the core.

The probe head may therefore have two Hall Effect sensors, connected in opposition, arranged on opposite sides thereof.

External fields and temperature effects tend to affect the two sensors equally and so do not contribute to the differential measurment from the head. On the other hand the local field tends to have different effects on the sensors on opposite sides of the head, and so the differential reading will give a good response to the local field.

An example of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of a loop structure with a current probe in place on the structure, Figure 2 is a diagrammatic representation of the cores of the head of Figure 1, and Figure 3 illustrates the head in relation to various fields.

In Figure 1, a hollow loop structure 11 is formed from a series of tubes in a rectangular configuration. The top tube 12 extends beyond the side tubes 13 to provide lugs 14 by which the structure can be affixed to a support. The support might be a part of the test equipment. The structure at the top of the side tubes has openings 15 through which a conductor 16 can enter and leave the structure, the conductor passing through the side tubes 13 and lower tube 17, forming a U-shape. The conductor 16 is connected to a generator of calibrating and degaussing signals which is conventional and is not shown.

The current probe 21 encircles the conductor carrying the current to be measured. It is seen in Figure 1 encircling the conductor in the lower tube 17 of the structure. The probe 21 comprises an annular head 22 and an output device 23. The head 22 is in two semi-circular hinged halves 24, the interfaces of the two halves being interdigitised - ie they have interleaving fingers 25 spaced along the axial direction, both at the hinged end and at the other end. The hinge pin 26 passes through the fingers 25 at the hinged end. Each half of the head has a core 27 of magnetic material with a shield 28, and the fingers 25 in the ends of the core 27 provide a low reluctance join between the two halves.

Each half of the head contains a Hall Effect device 31, the two devices being connected in opposition. Figure 3 illustrates the local measured field EM due to current passing through a conductor and an external stray field EF. The devices are connected differentially to a amplifier 32. As can be seen the external field EF affects the Hall Effect devices 31 equally so that the output of the amplifier 32 is unaffected by the stray field EF. The devices 31, being mounted on opposite sides of the head 22 receive equal and opposite signals from the local field generated by the conductor 17 encircled by the head 22, and so the differential output of the amplifier 32 is the sum of the magnitudes of the signals picked up by the devices 31.

Claims (9)

1. A current probe assembly comprising a generator of degaussing and calibrating signals, a conductor connected to the generator for carrying the signals and a current probe separate from the conductor which probe can be applied to said conductor to sense the signal carried thereby.

2. An assembly as claimed in claim 1 wherein the conductor is mounted within an insulating tube.

3. An assembly as claimed in claim 2 wherein the tube and the conductor within it follow a U-shaped path.

4. An assembly as claimed in claim 2 or claim 3 wherein said tube is provided with means for affixing the conductor on other structures.

5. An assembly as claimed in claimn 3 and claim 4 wherein said fixing means comprises spur portions extending from said U-shaped path.

6. An assembly as claimed in claim 3 or any claim dependent thereon comprising two Hall effect devices, one on either side of the path, connected differentially.

7. An assembly as claimed in any one of the preceding claims wherein the probe comprisies a pair of jaws for-encircling the conductor.

8. An assembly as claimed in claim 7 wherein said jaws are hinged, said jaws each having a hinged end and a free end, at least one of said ends of the two jaws being interdigitated.

9. A current probe assembly substantially as herein described with reference to the accompanying drawings.