GB2110016A - Detector for electric earth- leakage currents, and method of detecting such currents - Google Patents

Abstract

Earth-leakage current in an electric installation or equipment is detected in a fail-safe manner using a core balance transformer CB with its detector coil tuned by a capacitor CI to form a resonant circuit. An amplifier ICIa provides positive feedback to generate resonant oscillations, and a window detector 4 is responsive to alternating current in the resonant circuit and provides an output signal only when the amplitude of such current is above a predetermined threshold level but below a predetermined trip level. The circuit is normally tuned to a frequency different from mains frequency (say 40 Hz) and the oscillation amplitude is small compared with the signal induced in the detector coil of the core balance transformer by a fault of the magnitude required to be detected. When protecting a polyphase line, lock-out facilities, to prevent re-energisation so long as the fault exists, may be provided by coupling an artificial neutral point to the input of detector 4. The impedance of the earth leakage path will then prevent the detector from proving an output. <IMAGE>

Description

SPECIFICATION Detector for electric earth-leakage currents, and method of detecting such currents This invention relates to a detector for earthleakage currents (also called ground fault currents) in electric equipment or installations, and to a method of detecting such currents. It is concerned primarily, but not exclusively, with a detector and a method for use in specially hazardous situations, such as in mines, petroleum installations, and industrial installations handling flammable materials. In such circumstances, the first requirement is fail safe characteristics (i.e. that any fault in the detector will result in a fault indication, whether or not there is any fault elsewhere in the installation) and the second is reliablility.

The normal conventional means of prime detection of earth-leakage currents is a corebalance transformer, comprising an annular magnetic core through which all the loadcarrying conductors supplying the installation (or both load-carrying conductors in a simple single-phase installation) pass and which is linked by a detector coil; under fault-free conditions, the vector sum of the currents in the load-carrying conductors is zero, and no nett voltage is induced in the detector coil; but if current is flowing in the protective earth conductor (which does not pass through the magnetic core) or otherwise flowing to earth an imbalance results and a voltage appears across the detector coil.

It will be evident that this prime detector lacks fail-safe characteristics, in that (for example) an open-circuit or short-circuit fault in the detector coil will suppress any output voltage, giving a fault-free indication when a fault may in fact be present. Various means have been used to give fail-safe characteristics to the detector as a whole, and the present invention provides a new technique for doing so that is particularly simple (and therefore reliable), stable and readily adjustable to provide a required sensitivity.

In accordance with one aspect of the invention, an earth-leakage detector comprises a core-balance transformer having a detector coil which is tuned by a capacitor to form a resonant circuit; an amplifier connected to the resonant circuit to amplify current flowing therein and having a positive feed-back connection to cause oscillation in the said resonant circuit; and means (herein called a "window detector") responsive to alternating current in the said resonant circuit and providing an output signal only when the amplitude of such current is above a predetermined threshold level but below a predetermined trip level.

The invention includes a method of detecting earth-leakage current in an electric installation or equipment comprising providing a core-balance transformer having a detecting coil that provides a signal responsive to such current; tuning the detector coil with a capacitor to form a resonant circuit; amplifying current flowing in the resonant circuit and providing positive feedback from the amplifier to cause oscillation in the resonant circuit; and generating an output signal only when the amplitude of current in the resonant circuit is above a predetermined threshold level but below a predetermined trip level.

The amplitude may be expressed in terms of current or voltage and may be observed at any appropriate point(s) in the circuit.

The detector should be so designed that the oscillation amplitude under fault-free conditions is small compared with the signal level induced in the circuit by an earth-leakage current of the smallest value required to be detected; the trip level of the window detector must correspond to that induced signal level, and the threshold level of the window detector should be just below the level of the normal oscillation. Preferably the tuned frequency of the resonant circuit differs from the mains frequency of the installaion (or the equipment) in which the detector is used, so that the circuit will be non-resonant under fault conditions.

Any suitable analog or digital window detector can be used, provided it is fail-safe, but a particularly preferred form of window detector comprises a flip-flop circuit that is bistable when the amplitude in the resonant circuit is between the threshold level and the trip level but becomes unistable if the amplitude goes outside that range.

The oscillation of such a circuit can be made conditional also on the earth fault impedance of the circuit to provide lock-out facilities to prevent re-energising while a fault is uncorrected.

The invention will be further described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a simplified (incomplete) circuit diagram illustrating the principles of operation of one form of detector in accordance with the invention; and Figure 2 shows a full practical circuit for one specific embodiment of this form.

As seen in Fig. 1, the phase conductors 1 of the installation to be protected pass from the contacts RL of a circuit-breaker relay R/LA through the core 2 of a core balance tranformer CB, provided in conventional manner with a test coil connected to any available AC supply of appropriate voltage through a test switch T.

The detector coil of the core balance transformer CB forms a resonant circuit with capacitor C1 and this is driven in oscillation by an amplifier CIa and feedback resistor R3, all forming part of an oscillator 3. Output is taken, say, from the output side of the amplifier and (so that no fault in the amplifier can lead to a steady d.c. output) is passed through a d.c. blocking filter B1 comprising a capacitor and two diodes, providing a pulsed d.c.

input to a window detector 4.

This consists of a suitable flip-flop circuit having one reversal of state conditional on the input being above its threshold value and the other reversal of state conditional on the input being below its trip value; hence oscillation in the flip-flop is only maintained when the input is within its required range. The output of the flip-flop is taken, via another d.c. blocking filter B2, to the operating coil relay of R/LA, and normally holds the relay contacts closed.

Referring now to the practical circuit diagram in Fig. 2 and 3, the detector coil of CB has an effective inductance of 3.4H and reso nates with Cl (4.7,up) at about 40 Hz. The amplifier ICIc is one quarter of standard inte grated circuit type 348. Resistors R1 (2.2 kQ), R2 (8.2 kS2) and R3 (100 kQ for 300 mA sensitivity, 220 kSk for 100 mA sensitivity) control the operation of the amplifier and its feedback to produce oscillation at resonant frequency with an amplitude of 0.5V across the capacitor.This is fed via resistor R4 (56 k52) to an inverting amplifier IClb (another quarter of the integrated circuit no. 348) whose gain is determined by the ratio of its feedback resistor R6 (240 kQ for 300 mA sensitivity at 50 Hz mains frequency, 560 k52 for 100 mA at 50 Hz, 820 k2 for 100 mA at 60 Hz) to R4.

The output of IClb, being the amplified 40 Hz oscillation in fault-free conditions and an amplified mains-frequency signal under earthleakage fault conditions, is fed through a d.c.

blocking filter comprising C3 (220 nF) and diodes D1 and D2 (each IN 4148) and a smoothing circuit comprising R7 (15 k#), C4 (220 nF) and R8 (680 k to pin 9 of lclc (another quarter of integrated circuit no. 348).

The other input pin (10) of lC1c is maintained at one of the two pre-set voltages by potential dividers comprising R10 (6.8 kE2) in one limb and R9/R19 (equivalent value about 10 kS2) in the second limb, with R12 (470 S) added in parallel with the second limb or not depending on the condition of IC2 (standard integrated circuit type 555) as described below. The output of lC1 c (at pin 8) is + 15V, and represents only the relative polarity of the two inputs.

This output charges C5 (1 nF) through resistors R (30 kS3) and Rl4 (56 kQ), and so tends to drive input pins 2 and 6 of IC2 towards + 15V as the case may be. The characteristics of IC2 are such that when the input on pins 2 and 6 is positive and over 5V the output on pin 3 is low and pins 7 and 1 are effectively switched together; when the input is negative or below 5V positive, the output is high and pin 7 is switched off (so as to float) C6 (100 nF) is added to guard against latch-up on switch-on.

Assuming a starting condition in which C4 and C5 are both discharged, the output of lCl c will be at + 15V and will charge C5 to this voltage; pin 7 remains switched to pin 1 and holds pin 10 of lClc at its lower pre-set voltage; the circuit remains unistable until the voltage on pin 9 of lClc rises above that on pin 10 when IC1c changes state and C5 discharges (and eventually changes to opposite polarity).On reaching 5V at its input, IC2 changes state and pin 7 floats, allowing pin 10 of lCl c to rise to its higher pre-set voltage; normally this will now be above the voltage on pin 9 and lClc reverses again, and when this results in charging C5 above + 5 volts IC2 also reverses switching pin 10 of lClc back to its lower preset voltage, whereupon the cycle repeats.

This oscillation continues so long as pin 9 of ICle remains at a voltage between the two pre-set voltages of pin 10. This produces an output at pin 3 of IC2 which is passed through a d.c. filter comprising C7 (47 ,zF), D3 and D4 (both IN 4001), smoothed by C8 (1 juF) and fed to the contactor relay and R/LA (12V, 285). Rl3 (100 kS2) provides some hysteresis in the output circuit and R20 (15S2) damps transients on lock-out switching (see below).

On occurrence of an earth fault above detection level, pin 9 of lClc rises above the upper pre-set voltage of pin 10, and switching of pin 10 to that voltage fails to reverse the state of IClc, so that oscillation ceases, releasing the relay; and on open-circuit of the detector coil or any other fault that suppresses oscillation of the resonant circuit, pin 9 falls below the lower pre-set voltage on pin 10, and oscillation ceases through failure of switching pin 10 to that lower voltage to reverse the state of lCl c.

Components R5 (100 kE2) and C2 (47 nF) ensure that the system fails safe in the event of short-circuit failure of an integrated-circuit voltage regulator in the power supply under which circumstances it couples ripple from the supply to the input of IC1 band so produces a trip signal.

Lock-out function is provided by connection to false neutral point by diodes D7, D8 (each 1N 4148), R21 and R22. DlO is a 15 volt Zener diode which prevents any outgoing voltage higher than + 15 volts peak, or any incoming voltage higher than + 15V (to provide outgoing intrinsic safety and prevent the circuit from possible incoming overvoltage).

R21, R22 and D10 are contained within a separate potted barrier unit. Cl 2 (1 1Lf) avoids false tripping when switching into high capacitance lines. Earth fault impedance is effectively in parallel with R14 and C5 and prevents the input to IC2 from reaching its switching voltage.

Conventional circuits are used to provide d.c. voltages of + 15V and - 15V for the integrated circuits and say 24 volts AC for the test coil (only shown in Fig. 1).

Claims (8)

1. An earth-leakage detector comprising a core balance tranformer having a detector coil which is tuned by a capacitor to form a resonant circuit; an amplifier connected to the resonant circuit to amplify current flowing therein. and having a positive feedback connection to cause oscillation in the said resonant circuit; and a window detector responsive to alternating current in the said resonant circuit providing an output signal only when the amplitude of such current is above a predetermined threshold level but below a predetermined trip level.

2. An earth leakage detector as claimed in Claim 1 in which the window detector comprises a flip-flop circuit that is bistable when the amplitude in the resonant circuit is between the threshold level and the trip level but becomes unistable if the amplitude goes outside that range.

3. An earth-leakage detector substantially as describd with reference to Fig. 1 of the drawings.

4. An earth-leakage detector substantially as described with reference to and as shown in Fig. 2 of the drawings.

5. A method of detecting earth-leakage current in an electric installation or equipment comprising providing a core balance transformer having a detector coil that provides a signal responsive to such current; tuning the detector coil with a capacitor to form a resonant circuit; amplifying current flowing in the resonant circuit and providing positive feedback from the amplifier to cause oscillation in the resonant circuit; and generating an output signal only when the amplitude of current in the resonant circuit is above a predetermined threshold level but below a predetermined trip level.

6. A method as claimed in Claim 5 in which the tuned frequency of the resonant circuit differs from the mains frequency of the installation or equipment.

7. A method as claimed in Claim 5 or Claim 6 comprising generating the output signal by means of a flip-flop circuit that is bistable when the amplitude in the resonant circuit is between the threshold level and the trip level but becomes unistable if the amplitude goes outside that range.

8. A method of detecting earth-leakage current in an electric installation or equipment substantially as described with reference to either figure of the drawings.