GB2345187A - Metal oxide varistors - Google Patents

Abstract

A metal oxide varistor 10 is constrained against the expulsion of material therefrom upon explosion by being at least partially shielded by absorbing material 12 , for example silver sand. Preferably, the varistor 10 , an associated fuse 20 and the absorbing material 12 are contained in a housing 18 to form a replaceable module. An additional protection network or networks of varistors and fuses can be connected across the fuse 20 in order not to leave the electrical supply unprotected if the varistor 10 fails.

Description

METAL OXIDE VARISTORS This invention relates generally to metal oxide varistors, and is particularly concerned with protection measures, both mechanical and electrical, which can be used in the event of failure of the varistors.

Metal oxide varistors (hereinafter referred to just as "varistors"for the sake of simplicity) are used extensively to absorb transient over-voltages, particularly in ac power systems. The varistors function by becoming increasingly conductive as the applied voltage across them is raised. They are capable of conducting very large peak currents, often of the order of thousands of amperes, for pulse durations of the order of tens to hundreds of microseconds.

In the conducting state, the voltage across a varistor is appreciable, amounting to several hundred volts in the case of devices used for mains supply services. Consequently, over-voltages result in power being dissipated within the varistor. Over-voltages which persist much longer than the short duration transients mentioned above result in serious over-dissipation of heat in the varistor, leading to failure.

This failure is quite often catastrophic: the device coating or package bursts and varistor material is expelled, contaminating anything in the vicinity. This contamination can produce further damage. The expelled varistor material is deposited on any surface in its path, and is electrically conductive. If the surface is for example a printed circuit board with conductive copper tracks carrying mains supply voltages, then the conductive material which is deposited can bridge the mains supply, heat up rapidly and then burst into flames. The printed circuit board coatings and other components provide further fuel.

Although an enclosure can be used to contain the damage and prevent this from becoming a fire hazard, it is desirable to avoid varistor material causing contamination.

It is therefore one object of the present invention to provide means whereby contamination following varistor failure is eliminated or at least substantially reduced.

Various measures are currently used to try to reduce the risk of such contamination: 1) The most commonly used measure is to provide over current protection for the varistor by the provision of a fuse or other device which performs a similar function.

The fuse rating must be such that it will withstand transient currents, allowing the varistor to perform its normal protection function, while breaking the flow of current during a prolonged over-voltage event before the varistor fails explosively. However, in practice, bursting of the varistor is not always prevented by this means.

2) Some varistors are completely encapsulated in a rigid material. By enclosing the varistor in a rigid encapsulation, such as an epoxy resin, the varistor can in principle be constrained so that bursting is delayed long enough for the over-current protection device to operate. However, this method carries the risk that if it is unsuccessful, then the resulting explosion can be of considerably greater violence.

3) A conformal coating can be applied to the printed circuit board. This method accepts that varistor material can be deposited on the printed circuit board, but seeks to prevent subsequent ignition by covering all conductive parts with a non-conductive coating. The weakness of this method, as with the method last mentioned above, is that severe contamination can result.

More seriously, it is difficult to guarantee the integrity of the conformal coating over a period of time.

If both the coating and the varistor should fail, then the conformal coating will increase the fire hazard.

In accordance with one aspect of the invention there is provided a metal oxide varistor constrained against the expulsion of material therefrom by being at least partially shielded by an electrically con-conductive, non-flammable absorbing material.

Preferably, the varistor is enclosed with the absorbing material in a housing to form a replaceable module.

A second problem which arises as a result of varistor failure is the loss or reduction of protection for the electrical supply. Varistor failure results in the device tending towards a short circuit, and therefore able to sustain only a small limiting voltage, due to the destruction of the many semiconductor junctions which are present at the varistor material grain boundaries. Most commonly, protection against failure is conventionally provided by a fuse or other overcurrent protection device. However, once the fuse or other device has blown, the supply is no longer protected.

It is therefore an object of the present invention to provide measures whereby the supply is not left unprotected in the event of failure of a varistor and of the over-current protection device.

In accordance with another aspect of the invention there is provided a protection circuit for an electrical supply, comprising a first varistor and a first fuse or other overcurrent protection device connected thereto, and an additional protection network comprising a second varistor and a second fuse or other over-current protection device, the additional network being so connected to the first varistor that the additional protection network exerts a protective function if the first varistor fails.

Preferably, the first varistor is connected in series with the first fuse or other over-current protection device, and the additional protection network is connected in parallel with said first fuse or other over-current protection device.

In order that the invention may be more fully understood, a number of embodiments in accordance with the invention will now be described by way of example and with reference to the accompanying drawings.

In the drawings: Fig. 1 is a schematic illustration of a first embodiment of protective device in accordance with the invention; Fig. 2 is a schematic diagram of a second embodiment of protective device in accordance with the invention; Fig. 3 is a schematic illustration of a third embodiment of protective device in accordance with the invention; Fig. 4 is a schematic illustration of a fourth embodiment of protective device in accordance with the invention; Fig. 5 is a schematic illustration of a protective device in accordance with the invention in the form of a replaceable module ; Fig. 6 shows a conventional varistor circuit for use with a protected supply; Fig. 7 shows a conventional form of circuit protection using a parallel combination of fuses and varistors; Fig. 8 shows a conventional circuit arrangement using a neon indicator; Fig. 9 shows an alternative known use of a neon indicator; Fig. 10 shows a conventional method of combining two such indicators; and, Fig. 11 shows an electrical circuit arrangement in accordance with the invention which provides protection for the electrical supply.

Referring first to Fig. 1, this shows a varistor, indicated generally at 10, which may be either a standard disc with a thin coating, or an unencapsulated disc. Around the varistor 10 is provided absorbing material 12. In Fig. 1 the absorbing material totally surrounds the varistor. The absorbing material is electrically non-conductive, is nonflammable and is mechanically flexible in order to absorb the shock of an explosion of the varistor. The absorbing material 12 must also have sufficient volume, coupled with a large surface area, in order to absorb the varistor material following an explosion without allowing a conductive deposit to form on the printed circuit board 14 on which the device is mounted. One suitable absorbing material 12 is sand, and especially silver sand.

If the varistor is a standard disc with a thin coating, or an unencapsulated disc, this minimises the material which can be ejected into the absorbing material 12 in the event of an explosion. The advantages of the use of an absorbing material around the varistor is that any explosion is contained, and contamination of the equipment enclosure is avoided. Also, the absorbing material, by virtue of its nature, prevents further ignition. Desirably, the varistor 10 and absorbing material 12 are manufactured as a replaceable module. This then means that economic repair of the protected equipment is possible, just by replacing the module.

Fig. 2 shows a modifie arrangement in which the varistor 10 is only partially surrounded by absorbing material 12, i. e. on the top and around one or more sides. Here, the varistor 10 is positioned at the edge of the printed circuit board 14.

By partially constraining the varistor 10, the expulsion of material in the event of an explosion is directed away from the conductive parts of the printed circuit board, in that it follows the path of least resistance in the direction away from the conductive components, i. e. towards the left in Fig.

2. In this way, the material of the varistor which is expelled can be directed into an area which is designed to absorb the contamination. This structure uses less absorbing material 12 than the embodiment in Fig. 1, because the absorbing material now no longer needs to surround the whole of the varistor. The degree to which the varistor is surrounded by absorbing material is a matter of design choice, depending upon the location of the varistor on the printed circuit board and other structural considerations.

Fig. 3 shows a further embodiment in which the varistor 10 is encapsulated within a rigid material 16, such as an epoxy resin, in order to delay bursting of the varistor and increase the likelihood of operation of an associated fuse or other over-current device. The encapsulating jacket 16 is then surrounded by absorbing material 12 as in Fig. 1.

Fig. 4 shows a further embodiment, similar to that of Fig. 2, but with the varistor 10 again encapsulated in an epoxy resin jacket 16. In Fig. 4, as in Fig. 2, by providing the absorbing material 12 around only part of the encapsulation, material from any explosion is directed along a path away from the conductive parts of the printed circuit board 14.

Fig. 5 shows a further embodiment in accordance with the invention. Here, the varistor disc is indicated again at 10 and the absorbing material 12 is provided within an enclosure 18. This provides a convenient replaceable module.

Additionally, a fuse 20 or other over-current protection device is housed within the enclosure 18. The fuse 20 can be a commercially produced cartridge fuse, a fuse wire, or a thermal disconnection such as a spring-loaded solder joint which fails at the melting point of the solder. By providing the absorbing material 12, such as sand, within the enclosure 18 one absorbs any stress, flammable conductive material and any sparking.

The enclosure 18 is provided with three connector pins 22, appropriately connected to the varistor 10 and to the fuse 20.

Although not shown in the drawing, one or more indicating devices could also be included within the enclosure 18.

Further reference to such indicating devices will be made hereinafter.

As mentioned above, it is a further object of the present invention to overcome the loss of protection or reduction of protection which occurs as a result of failure of a varistor.

This is with a view to protecting the electrical supply.

Varistor failure results in the device tending towards a short circuit, and thereby only being able to sustain a small limiting voltage, due to the destruction of the many semiconductor junctions which are present at the varistor material grain boundaries. Protection against this condition is conventionally provided by a fuse or other over-current protection device, such as shown at 20 in Fig. 5. The term "fuse"is used herein to mean"fuse or any other over-current protection device".

Fig. 6 is a diagrammatic representation of a conventional arrangement in which a varistor 10 is connected in series with a fuse 20 across a protected supply between terminals A and B. Once the fuse 20 has blown, then the supply is no longer protected.

In some cases it is known to provide additional protection by a parallel combination of fuses and varistors, as shown for example in Fig. 7. Here, two varistors 10 and 10'are respectively connected in series with fuses 20 and 20', in parallel across the supply at terminals A and B.

However, in the event of a prolonged or continuous overvoltage, all the varistors 10,10'are likely to fail shortcircuit and the fuses 20,20'blow open-circuit, again leaving the supply unprotected.

This condition can be indicated by means of an indicator 24 as shown in Fig. 8. Typically, the indicator 24 can be a neon lamp as shown, or a light emitting diode (LED), sometimes associated with electronic circuitry, but here shown in its simplest form in series with a resistor 26 to limit the current. The electronic circuitry (not shown) may typically be employed to operate a relay, to give what is known as a "volt-free contact"for the purpose of providing remote indication of failure. With the circuit as shown in Fig. 8, the neon indicator 24 is normally lit and is extinguished when the fuse 20 has blown.

Another known arrangement is shown in Fig. 9 where the neon indicator 24 and resistor 26 are here connected across the fuse 20. With this circuit arrangement, the indicator 24 is normally"off"and only lights when the fuse 20 has blown and the varistor 10 is still present with a low limiting voltage.

Fig. 10 shows another known circuit arrangement, combining the circuits of Figs. 8 and 9, with neon indicators 24 and 24'and series-connected resistors 26 and 26' respectively connected across the fuse 20 and varistor 10.

With this circuit arrangement, the neon indicator 24'is normally lit. Neon indicator 24 lights under the condition just described above. If both indicators 24 and 24'are unlit, this means that the fuse 20 has blown and that connection to the varistor 10 has been lost, or alternatively that the power source is"off".

In accordance with the present invention electrical means are provided to overcome these problems and ensure that the supply is not left unprotected when the varistor fails. This involves providing an additional varistor protection network, so connected that it is brought into action when the first varistor fails. This is effective provided that the failed resistor is still connected in its near short-circuit condition. Fig. 11 illustrates how this can be achieved.

Here again, a varistor 10 and fuse 20 are connected across a supply provided at terminals A and B. A second varistor 30 is here connected in series with a second fuse 32 across the first fuse 20. The terminal of the varistor 30 which is remote from the second fuse 32 is connected to a point intermediate the first varistor 10 and first fuse 20. This differs from the circuit arrangement shown in Fig. 7 where one just has a parallel combination of fuses and varistors.

The circuit shown in Fig. 11 has a number of benefits.

The supply at terminals A and B is not left unprotected if the varistor 10 fails. Also, if the second varistor 30 is provided with a higher limiting voltage than that of the first varistor 10 (prior to failure of the latter), it is less likely to be damaged by a prolonged or continuous overvoltage.

This principle can be extended, by connecting a varistor and fuse network across each successive fuse 20,32,... If progressively higher limiting voltage varistors are used, then damage becomes progressively less likely. Indicators, such as referred to earlier, can be applied to each network in the manner described above. In conjunction with the use of higher limiting voltage varistors, the status of the indictors can be used to gauge the magnitude of the over-voltage which caused the failure.

In principle, the network described above can be placed in parallel with similar networks to increase the peak surge current which the protection device can handle.

It is also to be noted that the three connection arrangement indicated by the connection pins 22 in Fig. 5 lends itself to the addition of further varistors plus fuses, in the manner just described. Also, the indicator or indicators referred to above can be incorporated within the enclosure 18 (Fig. 5), visible from outside the enclosure.

Claims (20)

CLAIMS:

1. A metal oxide varistor constrained against the expulsion of material therefrom by being at least partially shielded by an electrically non-conductive, non-flammable absorbing material.

2. A metal oxide varistor as claimed in claim 1, in which the absorbing material is sand.

3. A metal oxide varistor as claimed in claim 2, in which the absorbing material is silver sand.

4. A metal oxide varistor as claimed in any preceding claim, in which the varistor is at least partially encapsulated, with the encapsulation being at least partially enclosed within the absorbing material.

5. A metal oxide varistor as claimed in any preceding claim, in which the encapsulation is of a rigid material.

6. A metal oxide varistor as claimed in claim 5, in which the encapsulating material is an epoxy resin.

7. A metal oxide varistor as claimed in any preceding claim, enclosed with the absorbing material in a housing to form a replaceable module.

8. A metal oxide varistor as claimed in claim 7, which has a fuse or other over-current protection device connected thereto and also enclosed by the absorbing material within the housing.

9. A metal oxide varistor as claimed in claim 7 or 8, which has an illuminable indicator device connected thereto.

10. A metal oxide varistor as claimed in claim 9, in which the indicator device is located within the housing.

11. A metal oxide varistor as claimed in any of claims 7 to 10, having three connector pins for connection to an additional protection network comprising at least one varistor and fuse.

12. A protection circuit for an electrical supply, comprising a first varistor and a first fuse or other overcurrent protection device connected thereto, and an additional protection network comprising a second varistor and a second fuse or other over-current protection device, the additional network being so connected to the first varistor that the additional protection network exerts a protective function if the first varistor fails.

13. A protective circuit as claimed in claim 12, in which the first varistor is connected in series with the first fuse or other over-current protection device, and the additional protection network is connected in parallel with said first fuse or other over-current protection device.

14. A protection circuit as claimed in claim 12 or 13, in which the second varistor has a higher limiting voltage than the first varistor.

15. A protection circuit as claimed in claim 12,13 or 14, in which a sequence of additional protection networks are connected in turn across the fuse or other over-current protection device of a succession of such additional protection networks.

16. A protection circuit as claimed in claim 15, in which progressively higher limiting voltage varistors are provided in the sequence of additional protection networks.

17. A protection circuit as claimed in any of claims 12 to 16, which includes indicator means to indicate failure of a varistor.

18. A protection circuit as claimed in claim 17 when dependent on claim 16, in which the status of the indicator means is arranged to gauge the magnitude of the over-voltage which caused failure of the varistor.

19. A metal oxide varistor substantially as hereinbefore described with reference to the accompanying drawings.

20. A protection circuit for an electrical supply, substantially as hereinbefore described with reference to the accompanying drawings.