Electrical safety barriers

Abstract: An electrical safety barrier which is adapted to pass a current returning from a hazardous area to a safe area but which will transmit negligible energy from the safe area to the hazardous area under fault conditions has in its return channel a fuse (F2), shunt diodes (22) and additional diodes (D3). These additional diodes (D3) are either conventional rectifier diodes positioned at the safe-area end of the return channel (Fig. 1) or are Schottky diodes positioned at the hazardous-area end of the return channel.

Description: This invention relates to electrical safety barriers and particularly to shunt-diode safety barriers as used for example for protecting electrical circuits and apparatus located in hazardous areas. The present invention is particularly concerned with the diode return type safety barrier. This may be used, in conjunction with another safety barrier or power supply, where a switch in a hazardous area is required to control the operation of a load, such as a relay or other device, in a safe area. It is an object of the present invention to provide an improved safety barrier which will pass a current returning from a hazardous area but which will transmit negligible energy in the other direction under fault conditions. This can be achieved by incorporating diode means within the return barrier. The return barrier can be used readily in combination with other barriers, and in practice often forms part of a 2-channel unit. Prior art diode-return safety barriers are known in which series diodes are provided at the hazardous-area end of the safety barrier. However, the disadvantage of having the diodes at the hazardous-area end of the safety barrier is that high power Zener diodes are required. It is an object of the invention to overcome this disadvantage. It is known from US-A-3813578 to provide a single-channel electrical safety barrier for providing safe interconnection between a safe area and a hazardous area. The circuitry includes Zener diodes to provide voltage limitation. DE-A-1816553 describes a two-channel safety barrier in which each channel has like elements. US-A-4099216 describes a safety barrier which includes blocking diodes, but at the hazardous end of one channel. In accordance with the present invention there is provided an electrical safety barrier adapted to pass a current returning from a hazardous area to a safe area but which will transmit negligible energy from the safe area to the hazardous area under fault conditions, the barrier comprising a first channel from the safe area to the hazardous area including first shunt-diode means, characterised by a return channel from the hazardous area including second shunt-diode means, fuse means and three series-connected diodes which have a high reverse voltage rating positioned on the safe area side of all said second shunt-diode means and in series with the fuse means, said three series-connected diodes both preventing current being fed into the hazardous area under fault conditions while permitting current flow in the other direction and also protecting the barrier against a reverse polarity input. The advantage thereby achieved is that by moving the diodes to the safe-area end of the barrier one can then use low power Zener diodes. In order better to illustrate the present invention reference is now made to the accompanying drawing which is a schematic circuit diagram of a shunt-diode safety barrier incorporating a diode-return channel in accordance with the present invention. In the embodiment shown in the drawing a switch 10 is located within a hazardous area and controls a load 12, such as a relay, located in a safe area, via a safety barrier which is indicated generally at 14. The safety barrier comprises a first channel having a fuse F1, a chain or chains 20 of Zener or other diodes connected in shunt, and a terminating resistance R1. The return channel from the hazardous area comprises a resistor R2, two or three low-power Zener diodes 22 connected in shunt, and a fuse F2. Incorporated within the return channel are three conventional rectifier diodes D3 connected in series and having a relatively high reverse voltage rating. These diodes D3 are located at the safe-area end of the safety barrier between the shunt diodes 22 and the fuse F2. This arrangement is appropriate in applications where the voltage drop is not critical. With the diodes D3 located at the safe-area end of the barrier, under fault conditions, the Zener diodes 22 cannot conduct in the reverse direction and the power dissipated in them is greatly reduced, thus allowing the use of smaller and therefore lower-cost Zener diodes. The present invention is concerned with the provision of diodes in a diode-return barrier, which may be used with conventional safety barriers, safety barriers which incorporate current-limiting circuits, and intrinsically safe power supplies.

Claims: An electrical safety barrier adapted to pass a current returning from a hazardous area to a safe area but which will transmit negligible energy from the safe area to the hazardous area under fault conditions, the barrier comprising a first channel from the safe area to the hazardous area including first shunt-diode means (20), characterised by a return channel from the hazardous area including second shuntdiode means (22), fuse means (F2) and three series-connected diodes (D3) which have a high reverse voltage rating positioned on the safe-area side of all said second shunt-diode means (22) and in series with the fuse means (F2), said three series-connected diodes (D3) both preventing current being fed into the hazardous area under fault conditions while permitting current flow in the other direction and also protecting the barrier against a reverse polarity input. A safety barrier according to claim 1, characterised in that said series-connected diodes (D3) comprise a plurality of rectifier diodes. A safety barrier according to claim 1 or 2, characterised in that said series-connected diodes (D3) are connected between the second shunt-diode means (22) and the fuse means (F2), with the fuse means being more remote from the hazardous area than said series-connected diodes. A safety barrier according to any of claims 1 to 3, characterised in that the first and second shunt-diode means (20,22) comprise Zener diodes.

Inventors: EPTON DAVID JOHN, HUTCHEON IAN CARRODUS, EPTON, DAVID JOHN, HUTCHEON, IAN CARRODUS
Applicants: MEASUREMENT TECH LTD, MEASUREMENT TECHNOLOGY LIMITED
Application year: 1988
Main classification: H02H9
IPC codes: H02H9