GB2258095A

GB2258095A - Residual current device

Info

Publication number: GB2258095A
Application number: GB9116367A
Authority: GB
Inventors: Paul Victor Brennan
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-07-26
Filing date: 1991-07-26
Publication date: 1993-01-27
Anticipated expiration: 2011-07-26
Also published as: GB9116367D0; GB2258095B

Abstract

A residual current device incorporates a phase sensitive detector (5) the reference signal of which is derived by way of a phase shifter (3) from the supply phase-to-earth voltage so that the DC output voltage of the phase sensitive detector (5) is proportional only to the resistive component of any leakage current to earth and is insensitive to leakage current taking a capacitive path to earth. This has the important benefit that steady-state leakage current through innocuous capacitive leakage paths such as arising from distributed phase-to-earth cable capacitance and interference suppression capacitors does not contribute to the occurrence of nuisance tripping. A time delay circuit (8) following threshold comparator (7) renders the device immune to transients irrespective of amplitude. <IMAGE>

Description

RESIDUAL CURRENT DEVICE This invention relates to a type of residual current device.

Residual current devices (RCDs) are well known and are based on a current balance transformer on which the phase and neutral conductors of a mains supply are wound in opposition to each other so that the net flux developed in the transformer core is dependent on the imbalance in the currents carried by the phase and neutral conductors.

The current balance transformer is coupled either magnetically or electrically to a relay which opens if the magnetic flux exceeds a certain threshold. Thus any imbalance in phase and neutral currents due to a leakage path to earth gives rise to a flux in the transformer core which, if it exceeds a certain threshold, causes the opening of the relay contacts to interrupt the supply.

RCDs offer protection against electric shock taking a path through the body to earth and against any electrical fault condition to earth.

However, they are prone to being triggered by non-fault conditions such as mains transients or steady-state current leakage through a capacitive path to earth. The latter condition arises due to two principle causes: distributed capacitance between phase and earth conductors in cables and phase to earth capacitors used for interference suppression purposes in electrical appliances. To date, RCDs respond only to the magnitude of the earth leakage current and are therefore unable to distinguish between current taking a resistive or capacitive path to earth.

According to the present invention, a means is provided to detect only the component of leakage current in phase with the supply phase-toearth voltage so that the RCD is insensitive to leakage current taking a capacitive path to earth and is therefore far less likely to be falsely tripped by steady-state capacitive leakage currents.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying diagram, figure 1, which shows a block diagram of the modified RCD allowing immunity to capacitive leakage current.

Referring to figure 1, the phase (P) and neutral (N) conductors of the supply system are passed through the core of a current balance transformer (1), (or wound around the core with an equal number of turns but opposite winding polarities). The output of the sensing coil of the current balance transformer is applied to an amplifier (6) and the output of the amplifier is applied as an input to a phase sensitive detector (5). A fraction of the phase-to-earth (E) supply voltage is derived from a potential divider (2) and is applied via a phase shifter (3) to a comparator (4) the output of which is a logic level signal locked to the phase and frequency of the supply phase-to-earth voltage, which is applied to the second input of the phase sensitive detector (5).By proper adjustment of the phase shifter (3) the DC value of the phase detector output is proportional to the magnitude of the leakage current taking a resistive path to earth and, in principle, is completely insensitive to the component of leakage current representing a capacitive leakage path to earth. A particularly suitable choice of phase sensitive detector is a sample-and-hold detector which is capable of a rapid response with minimal output ripple. It should be noted that items 2, 3, 4 and 5 of figure 1 are not normally included in RCD designs.

The output signal of the phase sensitive detector (5) is applied to a threshold comparator (7) which responds when the magnitude of this signal exceeds a certain threshold, Vth. The comparator output is then passed via a time delay circuit (8) to suppress any transients to a latch (9) which, in the event of a fault, interrupts the supply to a relay (10) thus opening the relay contacts (11) and interrupting the mains supply. It is important that the time delay circuitry (8) is placed after the threshold comparator (7) so that the RCD is immune to transients exceeding a preset time duration irrespective of the amplitude of the transient.

The circuit may be expanded by duplicating the phase shifter (3), comparator (4) and phase detector (5) circuitry, where the second phase shifter is adjusted to produce a 900 phase-shifted reference voltage with respect to the original reference voltage, and an additional threshold comparator (7) is incorporated to allow separately adjustable sensitivity to both resistive and capacitive leakage currents. As a minor variant on the design, the potential divider (2) may be placed between the phase and neutral conductors with some small effect on circuit performance depending on the installation earthing arrangements. Also, the design could be triplicated to protect a three-phase supply.

Claims

1. A residual current device comprising a current balance transformer providing a voltage proportional to the imbalance between phase and neutral currents which is amplified and applied to the input of a phase sensitive detector, the other input of which is derived from a fraction of the supply phase-to-earth voltage via a phase shifter and comparator, the output of the phase sensitive detector being applied to a further comparator and, via a time delay, to a latch controlling the opening of a pair of relay contacts in order to interrupt the supply in the event of a fault condition.

2. A residual current device, as claimed in claim 1, where the correct setting of the phase shifter allows the device to be completely insensitive to steady-state capacitive leakage current to earth and only sensitive to the resistive component of such leakage current.

3. A residual current device, as claimed in claims 1 & 2, where the inclusion of time delay circuitry after threshold detection allows immunity to transients below a preset time duration irrespective of their amplitudes.

4. A residual current device, as claimed in claim 1, in which the duplication of the phase shifter, comparator, phase sensitive detector and threshold comparator allows separately adjustable sensitivities to both resistive and capacitive leakage currents.

5. A residual current device substantially as described herein with reference to figure 1 of the accompanying diagrams.