GB2186738A - Fail safe surge arrester - Google Patents

Abstract

A surge arrester for protecting equipment has a pair of line electrodes (2) at the ends of a conventional gas- filled enclosure (4). The end-cap (3) of each electrode (2) is attached to a washer (5) which provides an air gap, and each washer (5) is mounted between its associated end cap (3) and a secondary end connection (6) in the form of a cap filled with soil solder (7). On sustained overheating the solder (7) melts and fills the void in the associated mica washer (5) thus shorting the associated line electrode to earth via a wire (8). <IMAGE>

Description

SPECIFICATION Fail safe surge arrester The present invention concerns gas-filled surge arresters.

Gas-filled surge arresters are two or three electrode devices which normally have electrode spacings of about 1 mm and are filled with a gas at below atmospheric pressure. They break down when a voltage in the region of 250 volts is applied and the resulting arc voltage across the arrester is about 20 volts. As a result, they can be used to protect sensitive electronic equipment against overvoltage arising from lightning and AC power crosses from high voltage mains.

Arresters of this kind have two potential failure mode problems. Firstly, if an AC power cross is sustained, giving for example a continuous current through the arrester of say 10 amps, the resulting 200 watts dissipation in the arrester will cause it to overheat and possibly set fire to its surroundings.

Secondly, if the arrester develops a leak, its gas pressure rises to atmospheric and the resulting breakdown voltage may be as high as several Kilovolts, so that the arrester does not provide adequate protection.

It is known to compensate for these problems by making two additions to the arrester or its mounting.

The heat generated by the arrester during a power cross can be used to melt a solder slug and lead to the shorting of the electrodes, as is described in, for example, U.K. Patent Specification No. 1,410,836 Alternatively, the heat may soften a plastic covering of a sprung wire to enable it to short the electrodes, as is described in U.K. Patent Specification No.

2,078,025B.

In either case the shorting of the electrodes leads to the elimination of the heat generated by the arrester and the fire hazard is reduced.

The other problem of loss of vacuum can be minimised by having a secondary external atmospheric air gap connected between the electrodes. In order to achieve an acceptable breakdown voltage, the gap must be of the order of 0.1 mm, and the resulting breakdown voltage of about 1000 volts provides some degree of protection if the arrester 'vents' (i.e. leaks). The small spacing can be achieved by inserting thin pieces of plastic or mica between the secondary electrodes, the insulating material having a hole in the middle through wich the breakdown can occur.

The present invention has for an object to use both of these basic techniques in a simple and inexpensive manner.

Accordingly the present invention consists in a surge arrester for protecting electronic equipment against overvoltages and comprising two line electrodes each connected to an end cap, and mounted in a gas-filled enclosure to provide breakdown points to earth on the occurrence of an overvoltage, the arrester further including a secondary air gap between each line electrode and the means for connecting it to the line to be protected, and solder located adjacent each secondary air gap, the arrangement being such that the secondary air gap affords a degree of protection if the arrester fails through leakage, and if the arrester is subjected to a continuous current which causes overheating, the solder melts to fill the secondary air gap and shorts the electrode to earth.

In accordance with a feature of the invention each secondary air gap is provided by a washer mounted on the end cap connected to its associated washer, and the solder is held in a metal cup secured to the washer and provided a secondary end connection.

In order that the present invention may be more readily understood, and embodiment thereof will now be described by way of example and with reference to the single figure of the accompanying drawing, which is a section through a surge arrester constructed in accordance with the invention.

Referring now to the drawing this shows a surge arrester comprising a tubular centre body 1 and line electrodes 2 joined to end caps 3. The device is sealed by brazing the electrodes to tubular ceramic members 4. The minimum spacing d between the electrodes is about 1 mm and the whole is filled with argon at about 0.1 atmosphere. The end caps or electrodes 3 are connected to the lines L1 and L2 of the protected equipment and the centre body 1 is connected to earth. Breakdown in the event of an overload can occur either between L or L2 to earth, or L1 to L2.

Attached to the end caps 3 are mica washers 5 approximately 0.1 mmthickwith a 1 mm hole in the middle. The secondary end connections 6 comprise metal cups filed with soft solder 7 which butt onto the mica washer. The wire 8 provides a path from the cups 6 to earth. The cups 6 are mechanically fixed to the washers 5 by, for example, a glue or epoxy resin.

In the event of a power cross, one or both of the line electrodes 2 will overhead and this heat will be transferred via the end caps 3 to the solder 7. The solder will melt and fill the void in the mica washer, thus shorting the electrode 3 to earth via the wire 8.

The dissipation in the arrester will thus cease and prevent the fire hazard.

In the event of a leak occurring in the arrester, the secondary air gap 3 to 6, via the mica washer 5 and solder 7, will come into play and will ensure that the breakdown voltage between line and earth does not rise above 1 KV.

It will be appreciated that the arrester just described inciudes both of the known safety features already described in an exceptionally simple and compact manner. Thus the hole in each mica washer 5 functions in a dual manner, providing both a secondary, external air gap and affording a route for excess current to be connected to earth on overheating of the arrester.

The described arrangement also has the advantage that it solves a problem inherent in solder/short systems which is the need to ensure that the electrodes will short whatever the orientation of the arrester with respect to gravity. In the present embodiment the volume which has to be filled by the molten solder is very small as it is determined by the thickness of the mica washers 5.

Thus the expansion of the solder on heating and melting will ensure that it will move into the available space even if the arrester is in a vertical position with the overheated end down.

It will be appreciated that whilst mica is the preferred material for the washers 5 other suitable materials may be used. Furthermore, there may be variations in the washer dimensions depending on the size of the air gap required.

Claims (7)

1. A surge arrester for protecting electronic equipment against overvoltages, the arrester comprising first and second line electrodes each connected to an end cap, means for connecting said line electrodes to a line to be protected, a tubular centre body at the ends of which said line electrodes and end caps are mounted and defining a gas-filled enclosure to provide breakdown points to earth in the event of an overvoltage, a secondary air gap defined between each said line electrode and the means for connecting it to the line to be protected, and a fusible element associated with each said secondary air gap, the arrangement being such that each secondary air gap provides a degree of protection if the arrester fails through leakage, and if the arrester is subjected to a continuous current which causes overheating, one or both of the fusible elements melts to fill the secondary air gap associated therein to short the respective line electrode to earth.

2. A surge arrester as claimed in Claim 1, wherein each secondary air gap is defined by a washer.

3. A surge arrester as claimed in Claim 2, wherein each fusible element is mounted in a secondary end connection, each washer being mounted between its associated end cap and its associated secondary connection.

4. A surge arrester as claimed in Claim 3, wherein each said secondary end connection comprises a metal cap housing the fusible element.

5. A surge arrester as claimed in Claim 4, wherein each said washer is made of mica.

6. A surge arrester as claimed in Claim 5, wherein each fusible element is solder.

7. A surge arrester substantially as hereinbefore described with reference to the accompanying drawings.