GB2159354A

GB2159354A - Electrical protective devices

Info

Publication number: GB2159354A
Application number: GB08408586A
Authority: GB
Inventors: John Whitehurst
Original assignee: BIRMID QUALCAST
Current assignee: BIRMID QUALCAST
Priority date: 1984-04-03
Filing date: 1984-04-03
Publication date: 1985-11-27
Also published as: GB2159354B; GB8408586D0

Abstract

A residual current protective device for fitting in a plug of an electrical appliance includes a core balance transformer (C1) for sensing an imbalance between currents flowing in the live and neutral conductors connected to the load, and an electronic switch (TR1) responsive to an output of the sensing means for short circuiting the power supply to blow a fuse (F2) normally protecting the appliance. <IMAGE>

Description

SPECIFICATION Electrical protective devices Earth leakage circuit breakers (ELCB's) are already available in two types, the first to protect an entire supply such as that in a domestic dwelling, and the second as an individual unit that can be plugged into a socket thereby protecting the supply of that socket. More recently these devices have become known as residual current devices (RCS's).

The devices operate by measuring any imbalance of current flowing in the live and neutral conductors. If the imbalance exceeds a predetermined level indicating a leakage of current to earth, a relay is operated to disconnect the power. These relays usually incorporate a double pole switch to disconnect both the live and neutral conductors as a precaution in case the socket is incorrectly wired.

The maximum permissible current and tripping times are specified by British Standards in order to provide maximum protection against electric shock while at the same time preventing nuisance tripping that could be caused by spurious signals on the mains supply.

Unfortunately the majority of domestic dwellings do not at present have a protective supply, and the cost of individual ELCB's or RCD's is high, particularly when compared to the cost of small portable electrical appliances where this type of protection would be particularly useful.

The present invention relates to a protective device which provide the same level of protection as an ELCB or RCD and which can be incorporated within the plug of an electrical appliance to provide a new type of electrical safety plug.

In accordance with the invention, an imbalance in the current between the live and neutral conductors is detected as in an ELCB or RCD, but instead of tripping a relay to open a switch in the power circuit, the output from the current sensing means actuates an electronic switch to provide a short circuit across the power supply to blow the fuse formally protecting the appliance.

The short circuit current preferably does not flow through the actuating circuit so that these components are not damaged when the fuse blows. The protective circuit may, however, include a back-up fuse having a slightly higher rating than the fuse normally protecting the appliance. Accordingly, if say a 1 3 amp fuse were fitted to the appliance inadvertently instead of the recommended 3 amp fuse, the user would still be protected.

A device embodying the invention would be less expensive than conventional ELCB's or RCD's and would provide the same level of protection since the fuse would blow much more rapidly than is normally the case if the load protected by the fuse begins to draw too large a current.

Because it would be virtually impossible to blow two fuses of the same rating simultaneously, the protective device would normally be a single pole device. However, the electronic circuit within the device may include means for detecting whether a socket is correctly wired and means responsive to an error in the wiring for producing a visual warning, for example in the form of a red light, or alternatively rendering the appliance inoperative.

One example of the invention is illustrated in the accompanying drawing in which the sole figure is a circuit diagram of a residual current protective device for fitting in the plug of an electrical appliance, such as a lawnmower.

A sensing coil C1 wound on a toroidal core is biased at half rail by an inverter 11. The resulting output is dependent on the difference between the currents flowing in the neutral and live conductors. This output is amplified and fed to a comparator logic circuit which acts on the half cycles of the alternating current to produce high or low logic signals which are fed to a unijunction transistor Ol and a pulse amplifier Q2, Q3 to turn on the triac TR 1 whenever the imbalance between the currents exceeds a predetermined level.

The resulting short-circuit will blow the replaceable 3A fuse F2. If the 3A fuse has been inadvertently replaced by a 1 3A fuse, the short-circuit will blow the 5A fuse.

1. A residual current protective device for fitting in the power lead of an electrical appliance operated from a power supply, the appliance being normally protected by a fuse and the device comprising means for sensing an imbalance in the current flowing in the live and neutral conductors of the power supply, and means responsive to an output from the sensing means for short-circuiting the power supply to blow the said fuse.

2. A device according to Claim 1 in which the short-circuiting means comprises an electronic switch.

3. A device according to Claim 1 or Claim 2 further comprising a back-up fuse also protecting the appliance but having a slightly higher rating than the said fuse.

4. A device according to Claim 2 in which the sensing means includes a sensing coil wound on a toroidal core, the live and neutral conductors of the supply passing through the core.

5. A device according to Claim 4 in which the output of the sensing coil is amplified and fed to a comparator logic circuit which provides logic signals to a unijunction transistor and pulse amplifier to turn on the electronic switch whenever the imbalance between the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Electrical protective devices Earth leakage circuit breakers (ELCB's) are already available in two types, the first to protect an entire supply such as that in a domestic dwelling, and the second as an individual unit that can be plugged into a socket thereby protecting the supply of that socket. More recently these devices have become known as residual current devices (RCS's). The devices operate by measuring any imbalance of current flowing in the live and neutral conductors. If the imbalance exceeds a predetermined level indicating a leakage of current to earth, a relay is operated to disconnect the power. These relays usually incorporate a double pole switch to disconnect both the live and neutral conductors as a precaution in case the socket is incorrectly wired. The maximum permissible current and tripping times are specified by British Standards in order to provide maximum protection against electric shock while at the same time preventing nuisance tripping that could be caused by spurious signals on the mains supply. Unfortunately the majority of domestic dwellings do not at present have a protective supply, and the cost of individual ELCB's or RCD's is high, particularly when compared to the cost of small portable electrical appliances where this type of protection would be particularly useful. The present invention relates to a protective device which provide the same level of protection as an ELCB or RCD and which can be incorporated within the plug of an electrical appliance to provide a new type of electrical safety plug. In accordance with the invention, an imbalance in the current between the live and neutral conductors is detected as in an ELCB or RCD, but instead of tripping a relay to open a switch in the power circuit, the output from the current sensing means actuates an electronic switch to provide a short circuit across the power supply to blow the fuse formally protecting the appliance. The short circuit current preferably does not flow through the actuating circuit so that these components are not damaged when the fuse blows. The protective circuit may, however, include a back-up fuse having a slightly higher rating than the fuse normally protecting the appliance. Accordingly, if say a 1 3 amp fuse were fitted to the appliance inadvertently instead of the recommended 3 amp fuse, the user would still be protected. A device embodying the invention would be less expensive than conventional ELCB's or RCD's and would provide the same level of protection since the fuse would blow much more rapidly than is normally the case if the load protected by the fuse begins to draw too large a current. Because it would be virtually impossible to blow two fuses of the same rating simultaneously, the protective device would normally be a single pole device. However, the electronic circuit within the device may include means for detecting whether a socket is correctly wired and means responsive to an error in the wiring for producing a visual warning, for example in the form of a red light, or alternatively rendering the appliance inoperative. One example of the invention is illustrated in the accompanying drawing in which the sole figure is a circuit diagram of a residual current protective device for fitting in the plug of an electrical appliance, such as a lawnmower. A sensing coil C1 wound on a toroidal core is biased at half rail by an inverter 11. The resulting output is dependent on the difference between the currents flowing in the neutral and live conductors. This output is amplified and fed to a comparator logic circuit which acts on the half cycles of the alternating current to produce high or low logic signals which are fed to a unijunction transistor Ol and a pulse amplifier Q2, Q3 to turn on the triac TR 1 whenever the imbalance between the currents exceeds a predetermined level. The resulting short-circuit will blow the replaceable 3A fuse F2. If the 3A fuse has been inadvertently replaced by a 1 3A fuse, the short-circuit will blow the 5A fuse. CLAIMS

5. A device according to Claim 4 in which the output of the sensing coil is amplified and fed to a comparator logic circuit which provides logic signals to a unijunction transistor and pulse amplifier to turn on the electronic switch whenever the imbalance between the currents exceeds a predetermined level.

6. A device according to Claim 2 in which the electronic switch comprises a triac.

7. A residual current protective device substantially as herein described with reference to the accompanying drawings.