GB2123567A

GB2123567A - A method and apparatus for checking the integrity of a wire

Info

Publication number: GB2123567A
Application number: GB08218805A
Authority: GB
Inventors: Gerald Anderson
Original assignee: Redland Automation Ltd
Current assignee: Redland Automation Ltd
Priority date: 1982-06-29
Filing date: 1982-06-29
Publication date: 1984-02-01

Abstract

An inductor (101, 102) is positioned adjacent or about wire (106) so that its inductance is influenced by a change in the resistance of the wire (106) or the resistance of a connection (108, 109) at an end of the wire (106), or by a deterioration in the isolation of the wire (106), the inductor (101, 102) being connected to form part of an oscillation circuit; and detector means (113) connected to the oscillation circuit is arranged to produce a predetermined signal when a predetermined shift in the frequency or phase of an oscillation in the oscillation circuit, due to a change in the said resistance or the said isolation, is detected. <IMAGE>

Description

SPECIFICATION A method and apparatus for checking the integrity of a wire The present invention relates to a method and to apparatus for checking the integrity of a wire.

Current practice in the inspection of wires, particularly earth wires on intrinisically safe systems, requires periodic checks of the integrity of the wire. This practice is inconvenient and, unless greater care is taken, or checks are made at frequent intervals, it can be unreliable and may be insensitive to gradual deteriorations in the wire or connections at its ends.

According to a first aspect of the present invention there is provided a method of checking the integrity of a wire in which: an inductor is positioned adjacent or about the wire so that its inductance is influenced by a change in the resistance of the wire or the resistance of a connection at an end of the wire or by a deterioration in the isolation of the wire, the inductor being connected to form part of an oscillation circuit; and detector means connected to the oscillation circuit is arranged to produce a predetermined signal when a predetermined shift in the frequency or phase of an oscillation in the oscillation circuit, due to a change in the said resistance or the said isolation, is detected.

Preferably, the inductor comprises a coil wound on a toroid made from ferromagnetic material, e.g. ferrite.

Preferably, the toroid is positioned so that the wire being checked passes therethough.

A check wire may be connected between the ends of the wire being checked, the check wire not passing through the toroid.

The oscillation of the oscillation circuit may be compared with that of a reference oscillation circuit, the said predetermined signal being produced when a predetermined shift between the frequencies or phases of the oscillations in the two circuits is detected.

In a preferred method, an auxiliary coil is wound around the toroid; the ends of the auxiliary coil are intermittently electrically connected, and the oscillation of the oscillation circuit when the auxiliary coil is on open circuit is compared with that of the oscillation circuit when the ends of the auxiliary coil are electrically connected, the said predetermined signal being produced when a predetermined shift between the frequencies or phases of the oscillations at these two times is detected.

The wire being checked may be an earth wire.

The said predetermined shift may occur when the said resistance increases.

The said predetermined shift may occur when the isolation of the wire deteriorates.

A further ferromagnetic toroid having a further coil wound thereon may be positioned so that both the wire being checked and the check wire pass therethrough, the further coil being connected to form part of a further oscillation circuit; and the or further detector means may be arranged to produce a further predetermined signal when a predetermined shift in the frequency or phase of the oscillation in the further oscillation circuit, due to a deterioration in the isolation of the wire being checked, is detected.

According to a second aspect of the present invention there is provided wire integrity checking apparatus comprising: a ferromagnetic toroid for encircling a wire to be checked; a coil wound on the toroid and connected to form part of an oscillation circuit; and detector means for producing a predetermined signal when a predetermined shift in the frequency or phase of an oscillation in the oscillation circuit, due to a change in the resistance of the wire or in the resistance of a connection at an end of the wire or deterioration in the isolation of the wire is detected.

The detection means may comprise a reference oscillation circuit and comparator means for comparing the oscillations in the two said circuits, the said predetermined signal being produced when a predetermined shift between the frequencies or phases of the oscillations in the two said circuits is detected.

In a preferred embodiment, an auxiliary coil is wound on the toroid; means are provided for intermittently electrically connecting the ends of the auxiliary coil; and comparator means are provided for comparing the oscillation of the oscillation circuit when the auxiliary coil is on open circuit with that of the oscillation circuit when the ends of the auxiliary coil are electrically connected, the said predetermined signal being produced when a predetermined shift between the frequencies or phases of the oscillation at these two times is detected.

The detection means may be arranged to produce the said predetermined signal when the said resistance increases.

The detection means may be arranged to produce the said predetermined signal when the isolation of the wire being checked deteriorates.

The apparatus may further comprise: a further ferromagnetic toroid for encircling both a wire to be checked and a check wire connected between the ends of this wire; a further coil wound on the further toroid and connected to form part of a further oscillation circuit; and the or further detector means for producing a further predetermined signal when a predetermined shift in the frequency or phase of an oscillation in the further oscillation circuit, due to a deterioration of the isolation of the wire being checked, is detected.

The present invention thus provides a method and apparatus for an on-line inspection of the integrity of a wire. As will be described below, preferred embodiments of the invention provide a method and apparatus for continuously testing whether the wire being checked is on open circuit or whether the resistance of connections at the ends of the wire is increasing. Preferred embodiments also provide a method and apparatus for continuously checking whether the isolation of wire has deteriorated.

The present invention will now be described, merely by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of part of wire integrity checking apparatus according to the present invention, Figures 2, 3 and 4 show examples of situations in which the apparatus shown in Figure 1 may be used, Figure 5 shows a modified form of the apparatus shown in Figure 1 being used in a situation similar to that shown in Figure 2, Figures 6 and 7 are block diagrams of two kinds of detection circuitry which may be used in modified apparatus such as that shown in Figure 5, and Figure 8 shows a circuit diagram of a circuit such as that shown in Figure 7.

Figure 1 shows a ferrite toroid 1, such as a Mullard FX3313, around which is wound a coil 2 of eleven turns, or so. The coil 2 may be formed from 0.3 mm diameter enamel insulated wire and arranged so as to have an inductance in the range 70-1000 ,uH. Two semi-conductor diodes 3 and 4 are connected in parallel across the coil 2 to limit the energy in the coil 2 to a safe level. The coil 2 is connected by a cable 5 to detection circuitry (not shown in Figure 1).

The coil 2 is connected to a timing capacitor in, the detection circuitry so as to form part of a tuned oscillation circuit which is continually oscillated by the detection circuitry. If a wire 6 is passed through the toroid 1, the detection circuitry will "see" a different inductance in dependence upon whether the ends of the wire 6 are an open circuit, are short circuited, or connected together through a resistance. Thus, the apparatus can be used to detect changes in the inductance of the coil 2 due to a change in the resistance of the wire 6. This may be done by comparing the oscillations of the said oscillation circuit with those of a tuned reference oscillation circuit and detecting any shift between the phases or frequencies of the oscillations of the two circuits.Such detection circuitry is wellknown, for instance in respect of detection apparatus, and will not therefore be described in any detail. Descriptions of examples of suitable circuitry will be found in British Patent Specifications Nos. 1,338,062, 1,561,641, 1,588,531, 2,063,538, 2,065,946, and 80/36701 (Publication No.

It will be appreciated that such detection circuitry can be arranged to detect an increase or a decrease in the inductance of the coil 2 and can be arranged to detect rapid or slow changes in the inductance of the coil. A decrease in the inductance of the coil 2 would, for instance, indicate that there was an accidental short-circuit on the wire, whereas an increase in the inductance of the coil 2 would indicate an open circuit or an increase in the resistance of the wire.

The detection circuitry may also be arranged to neglect relatively slow changes in inductance, for instance those due to ambient changes such as temperature variation. The detection circuitry used may be of the digital or micro processor type, these being particularly suitable when changes over a long period of time are to be detected. The manner in which the apparatus shown in Figure 1 and the detection circuitry described above can be used to check the integrity of a wire will be further described with reference to the examples shown in Figures 2, 3 and 4. Similar parts in these figures are given similar reference numerals but are increased by 100 for each figure.

Figure 2 shows a ferrite toroid 101 such as that shown in Figure 1 being used to check the integrity of an earth wire 106 which connects a drum 107 to earth when the drum 107 is being filled to prevent a charge building up on the drum.

The earth wire 106 is connected to the drum 107 by a bonding clip 108, passes through the toroid 101, and is connected to a stake 109 driven into an earth mat 110. An earth integrity check wire 111 is also connected to the clip 108 and is connected to an auxiliary stake 112 driven into the earth mat 110. The earth integrity check wire 111 does not, however, pass through the ferrite toroid 101. The earth wire 106 and the check wire 111 thus form a single turn winding through the toroid 101.A coil 102 on the toroid 101 is connected to detection circuitry 113 and this is arranged to detect an increase in the inductance of the coil 102 caused, for instance, by breakage of the earth wire 106 or a deterioration in the connection between the earth wire 106 and the clip 108 or between the earth wire 106 and earth.

The detection circuitry 11 3 is connected to an alarm (not shown) and/or an automatic shut down device (not shown) which is actuated when the detection circuitry produces a predetermined signal.

The arrangement described above only checks the integrity of the earth wire 106 and not that of the connection between the clip 108 and the drum 107. However, if the check wire 111 is connected directly to the drum 107, for instance via its own bonding clip (not shown), or a spring contact (not shown) underneath the drum 107, the connections to the drum 107 form part of the single turn through the toroid 101 and would also, therefore, be checked by the apparatus.

A similar application to that described above is shown in Figure 3 in which the integrity of.an earth wire connected to a tanker 214 being loaded or unloaded is checked. The arrangement is very similar to that shown in Figure 2 except that instead of providing an additional check wire a connection through earth is used to complete a single turn through a toroid 201, the tanker 214 being connected to earth both by an earth wire 206 and a static discharge chain 215. For this system to be feasible the distributed resistance RD of the connection through earth should not be so high as to make the increase in resistance of the single turn due to a breakage of the earth wire 206 so small that it is difficult to detect.

A third application of the present invention is illustrated in Figure 4. This Figure shows an intrinsically safe system in which one or several pieces of equipment are earthed via a barrier busbar 316, the busbar 31 6 being connected to earth via an earth wire 306 and a stake 309 in an earth mat 310. The busbar 316 provides protective earthing for intrinsically safe field equipment which is separated from safe area equipment by one or more Zener barriers 317.

Relatively high voltages (within prescribed limits) may occur in the safe area equipment so this equipment is housed in a protected area in which no explosive mixture which could be ignited by such voltages should exist. Explosive atmospheres may, however, be present in the field equipment and for this reason this equipment is designed to operate at low power and to be unable to store sufficient energy to produce a spark which could ignite an explosive atmosphere. The Zener barriers 31 7 limit the available energy to the intrinsically safe circuits in the field equipment and the earth connection to the busbar 31 6 prevents the circuits from becoming charged to high voltages which could create high energy sparks if an accidental earth occurred.The present invention may be used to check the integrity of the earth connection to the busbar 31 6 and to check for an earth "invasion" in the system or other deterioration in the isolation of the system.

An earth integrity check wire 311 is connected to the busbar 31 6 and is connected to earth by an auxiliary stake 312 in the earth mat 310. A first toroid 301A encircles the earth wire 306 and detects any change in resistance thereof in a manner similar to that described in relation to Figures 2 and 3. A second toroid 301 B encircles both the earth wire 306 and the check wire 311.

As both of these wires pass through the toroid 301 B a change in their resistance will have no effect upon the inductance of a coil 302B wound on the toroid 301 B, but if there is an earth "invasion" in the field or safe area equipment the connection through earth between the equipment and the earth mat 310 thus formed will complete a conductive loop effectively wound on the toroid 301 B and this will affect the inductance of the coil 302B. Thus, the first toroid 301A checks the integrity of the earth wire 306 and the second toroid 301 B checks for an earth "invasion" in the field or safe area equipment. Such an earth "invasion" may comprise an illegal earth connection 31 8 such as those shown in dashed lines in Figure 4, or an equipment fault which reduces the isolation of the system.An earth "invasion" is a potential hazard as it provides a conductive loop into which inductive energy can be coupled, and this may cause a high current to flow. Alternatively, earth fault currents from elsewhere may enter the intrinsically safe system causing hazardous voltages. In the system shown in Figure 4, it is preferable to connect the coils 302A and 302B to separate detectors 31 3A and 31 3B and to arrange for the oscillations produced in the two coils 302A and 302B to be of different frequency so as to avoid any interaction between the two coils.

Figure 5 shows a modified form of the apparatus shown in Figure 2. An auxiliary coil 419 is wound on a toroid 401 and connected via a resistance RTEST to two contacts 420 which are arranged to be connected together when a relay 421 is actuated. An FET or other electronic switch may be used instead of a relay. An earth wire 406 and check wire 411 form a low resistance single turn winding on the toroid 401 so when the relay 421 is actuated and the two contacts 420 are connected together, the detection circuitry 412 "sees" little or no increase in the inductance of a coil 402.However, if the resistance of the loop formed by the earth wire 406 and the check wire 411 goes high, for instance because of corrosion at the connection between the earth wire 406 and a stake 409, the detection circuitry 413 will "see" an increase of inductance when the contacts 420 are disconnected as the effect of the auxiliary coil 419 on the inductance of the coil 402 will no longer be small compared to that of the turn formed by the earth and check wires 406 and 411.

The value of RTEST can be adjusted to alter the sensitivity or threshold of the apparatus, the effect of the auxiliary coil 419 on the inductance of the coil 402 being similar to that of the earth loop when the resistance of the earth loop equals R,,,dn2, where n is the number of turns of the auxiliary coil 419 around the toroid 401. The resistance of any electronic switch used in place of the relay 421 will, of course form part of REST.

The relay 421 is controlled by a timer 422 and may be actuated periodically, for instance once every ten seconds. This modified apparatus can thus provide a periodic quantitative evaluation of the resistance of the turn formed by the earth and check wires 406 and 411.

Two alternative forms of detection circuitry which may be used in a system having an auxiliary coil as described above will be described with reference to Figures 6 and 7. These circuits compare the oscillations of the oscillation circuit when the auxiliary coil is short-circuited with those of the oscillation circuit when the auxiliary coil is an open circuit and do not, therefore, require a reference oscillation circuit as do the embodiments described above.

Figure 6 shows a block diagram of a circuit for detecting a shift in the frequency of oscillation of an oscillation circuit comprising a coil 502 wound on a toroid 501 and two capacitors C1 and C2 connected in parallel with the coil 502. An amplifier 523 with automatic gain control is connected in parallel with the capacitor C1. The amplifier 523 is thus connected in a feed-back arrangement with the oscillation circuit to maintain oscillations in the circuit. The connection between the amplifier 523 and the coil 502 is connected to a first switch S1 which alternately connects the oscillation circuit with a first binary counter 524 and a second binary counter 525.

The binary counters 524 and 525 are connected to control circuitry 526 and controlled by a time base signal from a first oscillator 527 which is preferably a crystal oscillator. An auxiliary coil 519 wound on the toroid 501 is connected in series with a resistance RTEST and the ends of the coil can be connected together through the resistance RTEST by a second switch S2. The two switches S1 and S2 are controlled together, for instance by a relay or electronic switch (not shown), the operation of the relay or electronic switch being controlled by clock signals fed to the control circuitry 526 from a second oscillator 528, these clock signals preferably having a frequency in the range 0.1 to 20 Hz.The switches 51, and S2 are arranged so that switch S1 connects the oscillation circuit to the first binary counter 524 when the switch S2 is open, and connects the oscillation circuit to the second binary counter 525 when the switch S2 is closed.

The binary counters 524 and 525 are connected to a digital comparator 529 which compares the counts in the two counters and produces a predetermined signal of the appropriate sign if the counts differ, this signal being used to actuate an alarm relay 530.

Thus, in use, when switch S2 is open the first binary counter 524 is connected to the oscillation circuit, and a count is established therein which represents the frequency of the oscillations in the oscillation circuit when the auxiliary coil 519 is on open circuit. When switch S2 closes, the second binary counter 525 is connected to the oscillation circuit and a count is established therein which represents the frequency of the oscillations in the oscillation circuit when the ends of the auxiliary coil 519 are connected together through the resistance RTEsT. If the closure of the switch S2 affects the oscillations in the oscillation circuit because of an increase in resistance of a wire 506 being checked, then the counts in the two counters will differ as the count in counter 525 will be greater than that in counter 524 and the alarm relay 530 will be actuated.

Alternatively, if RTEST is high and n is small, e.g.

RTEsT=l 0OS2 and n=1, the apparatus can be used to check for a deterioration in the isolation of the wire being checked as this will cause the count in counter 524 to be greater than that in counter 525.

Figure 7 shows a block diagram of a circuit for detecting a shift in the phases of oscillations of an oscillation circuit comprising a coil 602 wound on a toroid 601 and a capacitor C3 connected in parallel with the coil 602. One end of the coil 602 is'connected to earth and the other end to an amplifier 623 with automatic gain control. The output of the amplifier 623 is connected to a buffer circuit 631 which is also connected to an oscillator 632 producing clock pulses having a frequency in the order of 1 50 KHz. The oscillator 632 and the buffer circuit 631 are connected to a phase comparator 633. The output of the phase comparator 633 is connected to one side of a comparator 634 and, when a switch S101 is closed, to a hold capacitor C4 which is also connected to the other input of the comparator 634.The other side of the capacitor C4 is connected to earth. An auxiliary coil 619 wound on the toroid 601 is connected in series with a resistance RTEST and the ends of the coil can be connected together through the resistance RTEST by a switch S102. The two switches S101 and S102 are controlled together for instance by a relay or electronic switch (not shown), the operation of the relay or electronic switch being controlled by clock signals from an oscillator 628.

These clock signals have a frequency in the range 0.1 to 20 Hz and are also used as a strobe input to control the operation of the comparator 634.

The switches S101 and S102 are arranged so that when switch S102 is open the phase comparator 633 is connected only to the comparator 634, and when switch S102 is closed the phase comparator 633 is also connected to the hold capacitor 634. The comparator 634 is arranged to produce a predetermined signal when the inputs it receives from the hold capacitor C4 and the phase comparator 633 differ, and the predetermined signal is arranged to actuate an alarm relay 630.

Thus, in use, when switch S102 is closed, the hold capacitor C4 is connected to receive and store a signal representative of the phase difference between the oscillation of the oscillator 632 and that of the oscillation circuit when the auxiliary coil 619 is short-circuited through the resistance RTEsT. When switch S102 is open, a signal representative of the phase difference between the oscillation of the oscillator 632 and that of the oscillation circuit when the auxiliary coil 61 9 is on open circuit is fed to one input of comparator 634 and this is compared with the signal stored in hold capacitor C4. If the opening of the switch S102 affects the oscillation of the oscillation circuit because of an increase in resistance of a wire 606 being checked, then the comparator 634 will produce the said predetermined signal to actuate the alarm relay 630.

The circuit shown in Figure 7 can be made sensitive to an increase or to a decrease in the inductance of the oscillation circuit by adjusting the oscillation of oscillator 632 with respect to that of the oscillation circuit. If the frequency of oscillation of oscillator 632 is greater than that of the oscillation circuit, then a decrease in inductance of the oscillation coil will reduce the phase difference between the oscillations.

Alternatively, if the frequency of oscillation of oscillator 632 is less than that of the oscillator circuit, an increase in inductance of the oscillation circuit will reduce the phase difference between the oscillations.

Figure 8 is an auto-tuning version of a circuit such as that shown in Figure 7. The circuit diagram shown in Figure 8 gives details of the construction of this circuit, but a brief description of some of the more important parts thereof will be given. A phase lock loop integrated circuit IC1 corresponds to the oscillator 632 of Figure 7. The frequency of this oscillator is governed by resistors R6 and R7, capacitor C4 and the DC voltage on Pin 9. An output frequency signal is fed from Pin 4 to a phase comparator input Pin 14 (which corresponds to comparator 633) and to an automatic gain control transistor Q3 (which corresponds to amplifier 632) via a resistor R8.

The collector current of resistor Q3 passes through a resistor R10 and a winding 702 on a toroid 701. This winding 702 is tuned by a capacitor C5 (corresponding to capacitor C3 of Figure 7). The voltage developed across resistor R10 and the tuned oscillation circuit is amplified by an amplifier LA3 and the voltage at the output of this is rectified by a diode D3 which develops a voltage across a capacitor C6. The greater the amplitude the higher the voltage. A resistor R10 is provided to develop a minimum voltage when the circuit is completely off tune. This voltage is applied through a resistor R9 to the.base of transistor Q3 to provide automatic gain control action.The voltage at the collector of transistor Q3 is fed to a phase shifting network with approximately 450 lag or lead as shown by network 1 or network 2. These networks are designed to provide approximately 450 shift over the possible working range of the oscillator integrated circuit IC1. The output from the network is fed to a comparator LA2 which drives the other phase comparator input 3 of integrated circuit IC1.The phase comparator output from Pin 2 is fed through a transistor Q2 (which corresponds to switch S101) and loop filter components resistor R2, capacitor C2 and resistor R3, to the oscillator control Pin 9. When transistor 02 is switched ON the loop oscillator will lock so that it is on one side or other or the resonance of the winding 702 and capacitor C5.The side will be determined by which network is used. When transistor Q2 is OFF any change in the inductance of the winding 702 will produce a phase change at Pin 2 which is filtered by resistor R1 and capacitor C1 and applied to an amplifier LA1 (which corresponds to amplifier 634). The output of amplifier LA1 will go HIGH provided the change of phase is in the right direction.

An oscillator (which corresponds to oscillator 628) consisting of a comparator LA4, a resistor R13, a resistor R14 and a capacitor C7 generates a signal having a frequency of about 1 Hz. This switches transistors Q2 and Q4 (which correspond to switch S102) ON and OFF together. When transistor Q2 is ON, the loop locks and transistor Q4 in conjunction with a test resistance RTEST and an auxiliary coil 719 sets up the reference condition. When transistors Q2 and Q4 are both OFF there may be a change in inductance depending on the loop resistance of the wire 701 being tested. If this change is in the correct sense the output of amplifier LA1 will go HIGH and this will be clocked into a flip flop integrated circuit IC2 as the output of comparator LA4 rises to switch Q2 and Q4 ON again. This substitutes the strobe input in Figure 7.

The iatched high output of integrated circuit IC2 turns transistor Q1 ON through resistor R4 and operates a relay 730 to give an alarm. If the winding 702 should go open circuit then current flows through transistor Q3, resistor R5, and diode D1 and turns transistor Q1 ON to operate the relay 730 to give an alarm. This is a fail safe feature.

The part numbers for the integrated circuits shown in Figure 8 are proprietory numbers used by RCA and Motorola. Further description of the function of these integrated circuits is not included as they are employed in a standard manner.

As will be apparent to a man skilled in the art, the present invention can provide a method and apparatus for checking the integrity of earth wires and other connections, or for checking for isolation faults in many kinds of systems.

The present invention is particularly suitable for checking the integrity of an earth wire as the amplitude of signals induced in the wire being checked by the coil wound on the toroid can be kept very low, for instance less than 30 mV peak to peak. Signals of such magnitude are, of course, compatible with the requirements of intrinsically safe systems. As described above, with the present invention it is also possible to use more than one set of apparatus on a single conductor without interference therebetween by simply using different oscillation frequencies in the two sets.

Claims

1. A method of checking the integrity of a wire in which: an inductor is positioned adjacent or about the wire so that its inductance is influenced by a change in the resistance of the wire of the resistance of a connection at an end of the wire, or by a deterioration in the isolation of the wire, the inductor being connected to form part of an oscillation circuit; and detector means connected to the oscillation circuit is arranged to produce a predetermined signal when a predetermined shift in the frequency or phase of an oscillation in the oscillation circuit, due to a change in the said resistance or the said isolation, is detected.

2. A method as claimed in claim 1 in which the inductor comprises a coil wound on a toroid made from ferromagnetic material, e.g. ferrite.

3. A method as claimed in claim 2 in which the toroid is positioned so that the wire being checked passes therethrough.

4. A method as claimed in claim 3 in which a check wire is connected between the ends of the wire being checked, the check wire not passing through the toroid.

5. A method as claimed in any preceding claim in which the oscillation of the oscillation circuit is compared with that of a reference oscillation circuit, the said predetermined signal being produced when a predetermined shift between the frequencies or phases of the oscillations in the two circuits is detected.

6. A method as claimed in any of claims 2 to 4 in which: an auxiliary coil is wound around the toroid; the ends of the auxiliary coil are intermittently electrically connected; and the oscillation of the oscillation circuit when the auxiliary coil is on open circuit is compared with that of the oscillation circuit when the ends of the auxiliary coil are electrically connected, the said predetermined signal being produced when a predetermined shift between the frequencies or phases of the oscillations at these two times is detected.

7. A method as claimed in any preceding claim in which the wire being checked is an earth wire.

8. A method as claimed in any preceding claim in which the said predetermined shift occurs when the said resistance increases.

9. A method as claimed in any of claims 1 to 7 in which the said predetermined shift occurs when the isolation of the wire deteriorates.

10. A method as claimed in claim 4 or any of claims 5 to 9 when dependent on claim 4 in which: a further ferromagnetic toroid having a further coil wound thereon is positioned so that both the wire being checked and the check wire pass therethrough, the further coil being connected to form part of a further oscillation circuit; and the or further detector means are arranged to produce a further predetermined signal when a predetermined shift in the frequency or phase of the oscillation in the further oscillation circuit, due to a deterioration in the isolation of the wire being checked, is detected.

11. A method of checking the integrity of a wire substantially as hereinbefore described with reference to the accompanying drawings.

1 2. Wire integrity checking apparatus comprising: a ferromagnetic toroid for encircling a wire to be checked; a coil wound on the toroid and connected to form part of an oscillation circuit; and detector means for producing a predetermined signal when a predetermined shift in the frequency or phase of an oscillation in the oscillation circuit, due to a change in the resistance of the wire or in the resistance of a connection at an end of the wire or a deterioration in the isolation of the wire is detected.

13. Apparatus as claimed in claim 12 in which the detection means comprises a reference oscillation circuit and comparator means for comparing the oscillations in the two said circuits, the said predetermined signal being produced when a predetermined shift between the frequencies or phases of the oscillations in the two said circuits is detected.

14. Apparatus as claimed in claim 12 in which: an auxiliary coil is wound on the toroid; means are provided for intermittently electrically connecting the ends of the auxiliary coil; and comparator means are provided for comparing oscillations of the oscillation circuit when the auxiliary coil is on open circuit with those of the oscillation circuit when the ends of the auxiliary coil are electrically connected, the said predetermined signal being produced when a predetermined shift between the frequencies or phases of oscillations at these two times is detected.

1 7. Apparatus as claimed in any of claims 12 to 1 6 comprising: a further ferromagnetic toroid for encircling both a wire to be checked and a check wire connected between the ends of this wire; a further coil wound on the further toroid and connected to form part of a further oscillation circuit; and the or further detector means for producing oscillations in the further oscillation circuit and a further predetermined signal when a predetermined shift in the frequency or phase of oscillations in the further oscillation circuit, due to a deterioration of the isolation of the wire being checked, is detected.

14. Apparatus as claimed in claim 12 in which: an auxiliary coil is wound on the toroid; means are provided for intermittently electrically connecting the ends of the auxiliary coil; and comparator means are provided for comparing the oscillation of the oscillation circuit when the auxiliary coil is on open circuit with that of the oscillation circuit when the ends of the auxiliary coil are electrically connected, the said predetermined signal being produced when a predetermined shift between the frequencies or phases of the oscillation at these two times is detected.

1 5. Apparatus as claimed in any of claims 12 to 14 in which the detector means is arranged to produce the said predetermined signal when the said resistance increases.

16. Apparatus as claimed in any of claims 12 to 14 in which the detector means is arranged to produce the said predetermined signal when the isolation of the wire being checked deteriorates.

17. Apparatus as claimed in any of claims 12 to 1 6 comprising: a further ferromagnetic toroid for encircling both a wire to be checked and a check wire connected between the ends of this wire; a further coil wound on the further toroid and connected to form part of a further oscillation circuit; and the or further detector means for producing a further predetermined signal when a predetermined shift in the frequency or phase of an oscillation in the further oscillation circuit, due to a deterioration of the isolation of the wire being checked, is detected.

1 8. Wire integrity checking apparatus substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

New claims or amendments to claims filed on 15 Feb 1983.

Superseded claims 1,5, 6, 10, 12, 13, 14 and 17.

New or amended claims:

1. A method of checking the integrity of a wire in which: an inductor is positioned adjacent or about the wire so that its inductance is influenced by a change in the resistance of the wire or the resistance of a connection at an end of the wire, or by a deterioration in the isolation of the wire, the inductor being connected to form part of an oscillation circuit; and detector means is connected to produce oscillations in the oscillation circuit and to produce a predetermined signal when a predetermined shift in the frequency or phase oscillations in the oscillation circuit, due to a change in the said resistance or the said isolation, is detected.

5. A method as claimed in any preceding claim in which oscillations of the oscillation circuit are compared with those of a reference oscillation circuit, the said predetermined signal being produced when a predetermined shift between the frequencies or phases of the oscillations in the two circuits is detected.

6. A method as claimed in any of claims 2 to 4 in which: an auxiliary coil is wound around the toroid; the ends of the auxiliary coil are intermittently electrically connected; and oscillations of the oscillation circuit when the auxiliary coil is on open circuit are compared with those of the oscillation circuit when the ends of the auxiliary coil are electrically connected, the said predetermined signal being produced when a predetermined shift between the frequencies or phases of the oscillations at these two times is detected.

10. A method as claimed in claim 4 or any of claims 5 to 9 when dependent on claim 4 in which: a further ferromagnetic toroid having a further coil wound thereon is positioned so that both the wire being checked and the check wire pass therethrough, the further coil being connected to form part of a further oscillation circuit; and the or further detector means are arranged to produce a further predetermined signal when a predetermined shift in the frequency or phase of oscillations in the further oscillation circuit, due to a deterioration in the isolation of the wire being checked, is detected.

12. Wire integrity checking apparatus comprising: a ferromagnetic toroid for encircling a wire to be checked; a coil wound on the toroid and connected to form part of an oscillation circuit; and detector means for producing oscillations in the oscillation circuit and a predetermined signal when a predetermined shift in the frequency or phase of oscillations in the oscillation circuit, due to a change in the resistance of the wire or in the resistance of a connection at an end of the wire or a deterioration in the isolation of the wire, is detected.

13. Apparatus as claimed in claim 12 in which the detection means comprises a reference oscillation circuit and comparator means for comparing the oscillation in the two said circuits, the said predetermined signal being produced when a predetermined shift between the frequencies or phases of the oscillations in the two said circuits is detected.