GB2223368A

GB2223368A - Electrical safety apparatus

Info

Publication number: GB2223368A
Application number: GB8814428A
Authority: GB
Inventors: Barry Stanley Ford
Original assignee: EMLUX Ltd
Current assignee: EMLUX Ltd
Priority date: 1988-06-17
Filing date: 1988-06-17
Publication date: 1990-04-04
Also published as: GB8814428D0

Abstract

Safety apparatus, for protecting users of electricity against exposure to electrical shock, comprises an electrical cutout C responsive to severing of a cable F. A high frequency signal from a transmitter D normally reaches a receiver B which maintains cutout C closed. If the signal is not produced or not received, the receiver B changes state and cutout C opens. A filter A prevents the HF signal from reaching the supply E. The system is suitable for 2-core or 3-core cable. <IMAGE>

Description

Electrical Safety Apparatus This invention relates to safety apparatus for protecting users of electricity against exposure to electrical shock.

Various apparatus exist to protect users of electrical equipment. These apparatus disconnect the electrical supply if the current has been exceeded or if there is an imbalance in the current flowing between live and neutral for example. Whilst these methods protect the user there are times when the electrical supply can be cut and exposed and still be electrically alive. This could be lethal in a damp situation especially out of doors. A cutting blade on a lawn mower or hedge trimmer for example can on one pass sever one wire in a cable and in the next pass sever another wire. Because the blade has not been in contact with both wires at the same time and normally the blade is not earthed no existing protection device has operated. This gives rise to the situation of electrically live cables left exposed on the ground.If the operator should touch the exposed cable to make a repair their resistance would be measured by the existing protection apparatus as an electrical load and receive a lethal electric shock.

The present invention includes all novel disclosures present in this specification.

According to a first aspect the safety apparatus comprises an electrical cutout and receiver at the electrical supply end of an electical cable and a transmitter at the appliance end of an electrical cable.

According to a second aspect the safety apparatus comprises a transmitter, a receiver, an electrical cutout and a filter to remove an electrical supply if any cable is severed.

According to a third aspect the safety apparatus comprises a transmitter, a receiver, an electrical cutout and a filter to remove an electrical supply if any combination of cables should be shorted together.

According to a fourth aspect the safety apparatus comprises a transmitter, a receiver, an electrical cutout and a filter to remove an electrical supply if any cable should become intermittent in the supply of electricity.

The safety apparatus may be used in conjunction with existing protection devices, such as residual - current circuit breakers (RCCB) for example, but always with a fuse.

One example of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 shows one example of the safety apparatus.

Fig. 2 shows one example of a filter, cutout and receiver.

Fig. 3 shows one example of a transmitter.

Fig. 4 shows one example of a filter, cutout and receiver for two and three core cable operation.

In fig. 1, the safety apparatus comprises a filter A, a receiver B, a cutout C and a transmitter D.

The complete receiver section comprising A,B & C connects to an AC or DC electrical supply E. The transmitter D connects at the appliance end of the supply cable F. The cable F is a standard extention lead or permanently wired cable. The transmitter can be a plug and socket assembly (as shown) to connect to the appliance N.

In fig. 2, the complete receiver section A,B & C is shown with a load fuse G and receiver fuse H. The filter A is a 'T' bar filter tuned to remove a frequency of 110KHz (Band B, range 100KHz - 125KHz). The receiver B is a tuned circuit (e.g. a Signetics NE567 integrated circuit) set to a frequency of 110KHz coupled to the neutral and earth cables via a coupling device M (e.g.

capacitive and/or inductive coupling). Cutout C is a relay or contactor. For starting and reset a momentary switch I is included.

In fig. 3, the transmitter apparatus comprises straight through connections. A supply for an oscillator J is tapped from live and neutral via a direct current (DC) power supply K. A sine wave output from the oscillator J (e.g. a National Semiconductor LM1893 integrated circuit) is coupled to the neutral and earth via a coupling device L (e.g. capacitive and/or inductive coupling).

In fig. 4 a changeover switch 0 is included which allows a complete receiving section A,B & C to be able to operate with two or three core cable installations.

Operation of Electrical Safety Apparatus for Three Core Cable Systems In operation the contacts Y & Z are closed by energising the relay C coil via the momentary switch I.

This enables the electrical supply E to be fed to the appliance N. Once the transmitter D is powered up via its D.C. supply K the oscillator S injects a 110KHz sine wave into the neutral and earth cables via the coupling device L. The 110KHz sine wave is received at B which being tuned to the same frequency changes the state of its output and puts a holding voltage on relay coil C. The start switch I is then released and the appliance N can be used in its normal way. If the supply cable F neutral or live wire is cut between the transmitter D and receiver B the supply to the oscillator-J is lost and the 110KHz sine wave ceases. Directly the receiver B looses the 110KHz sine wave it changes state removing the voltage from relay C coil. This in turn opens contacts Y & Z de-energizing cable F making the system safe.If the system is in its normal operational state and the earth wire is cut the receiver B stops receiving the signal from the transmitter D. The receiver B in losing the signal changes its output state removing the voltage from the relay coil C.

The contacts Y & Z are opened by the relay C and isolating the cable F making the system safe.

The filter A is included to retain the 110KHz sine wave within the cable F and not allowing it to permiate into the low voltage supply cables E. This allows a number of units to operate in the same vicinity without interfering with each other.

Operation of Electrical Safety Apparatus for Two Core Cable Systems The operation of a safety system for two core cables is obtained in a similar way as for a three core cable system. The transmitter D output is this time coupled to the neutral and live wires and the receiver H receives the signals from the neutral and live wires. Once the neutral or live wire is cut the supply to the oscillator J is lost and the 110KHz sine wave is lost. The receiver in losing the signal changes state removing the supply voltage from relay R coil. This causes the contacts Y & Z to open removing the electrical supply from cable F making the system safe.

Claims

CLAItlS

1, Safety apparatus for protecting users of electricity against exposure to electrical shock, comprising: at least one electrical cable, said cable having an electrical supply end and an appliance end; and an electrical cutout and receiver at said electrical supply end , and a transmitter at said appliance end,

2. Safety apparatus for protecting users of electricity against exposure to electrical shock, comprising: a transmitter; a receiver; at least one electrical cable; and an electrical cutout and a filter to remove an electrical supply if any said cable is severed,

3. Safety apparatus for protecting users of electricity against exposure to electrical shock, comprising: a transmitter; a receiver; at least one electrical cable; and an electrical cutout and a filter to remove an electrical supply if any said cable should become intermittent in the supply of electricity.

4. Safety apparatus as claimed in any one of claims 1 to 3, in conjunction with at least one residual-current circuit breaker; and at least one fuse.

5. Safety apparatus, substantially as hereinbefore described with reference to and as shown in Fig. 1 of the accompanying drawings.

6. Safety apparatus, substantially as hereinbefore described with reference to and as shown in Fig. 2 of the accompanying drawings.

7. Safety apparatus, substantially as hereinbefore described with reference to and as shown in Fig, 3 of the accompanying drawings.

8. Safety apparatus, substantially as hereinbefore described wit reference to and as shown in Fig, 4 of the accompanying drawings,

9. A method of protecting users of electricity against exposure to electrical shock, comprising utilising safety apparatus as claimed in any one of claims 1 to 8.