This invention concerns improvements in or relating to surge arresters (diverters) by which is meant a device, such as is commonly connected between power supply lines and earth, which under normal conditions represents an open circuit but which, in response to voltage overload conditions occasioned for example as the result of a lightning strike on a power line, rapidly switches to a conductive condition so as to divert the transient power surge to earth and then recovers to its former condition following cessation of the transient in question.

Conventional surge arresters commonly comp¬ rise, in accordance with the definition given in the International Electrotechnical Commission's recommend- ation document IEC 99-1 1970, a single or multiple gap connected in series with one or more non-linear resistors. Most surge arresters commonly comprise a series of gaps, associated with each of which may be provided means for expanding and thereby quenching an arc truck acros th ct v w

parallel grading circuit for grading the voltage dis¬ tribution across the series gaps in the event of an overvoltage occurring across the arrester; Figure 6 of British Patent Speci ication No. 1 232 005 for example shows one form of such a conventional surge arrester. The grading circuit has two prime functions, namely to ensure that the applied power frequency voltage is reasonably evenly distributed across all of the series- gaps and to maintian the required voltage distribution under impulse and switching surge con¬ ditions until gap sparkover occurs, and commonly comp¬ rises grading components such as linear resistors, non-linear resistors, and/or capacitors connected across the gaps, these grading components commonly being attached to the gaps as separate and non-integral components. Thus the series gap arrangement has comm¬ only had to be such as to permit the grading components to be suitably located with respect to, and electric¬ ally connected to, the series gaps. A more recent development in the field of surge arresters is the gapless or metal oxide type which can comprise (i) a number of non-linear metal oxide resistors in series, (ii) a single or multiple spark gap connected in series with one or more metal oxide resistors, or (iii) a number of metal oxide resistors in series with one or more additional such resistors electrically shunted by means of a spark gap or other by-pass component. The voltage grading in such surge arresters may be either inherent in the resistor arrangement as in the first and third poss¬ ibilities (i) and (iii) above∑πentioned, or as in the previously mentioned conventional arresters for the second possibility (ii) abovementioned. At normal service voltage the grading current in gapless arres- ters is predominantly capacitive and could be inadequate

to maintain reasonable voltage distribution without the addition of further grading elements. Under DC service voltage conditions, additional grading circuits would be required. The present invention resides in the concept of providing an electrically-resistive metal oxide glaze on the internal surface of the bore of the porcelain insulator housing normally employed for accommodating surge arrester components, and providing the surge arrester internal assembly with electrical connections between the gaps and/or grading components and the resistive coating at various locations throughout the length of the bore.

In accordance with the invention there ore, the resistive glaze in the internal bore of the por¬ celain housing is utilised, at least in part, as the grading circuit for the arrester series gaps. With otherwise conventional series gap/non-linear resistor type surge arresters, the conventional gap grading circuits will, in accordance with the invention, be replaced in whole or in part by the resistive glaze, and with metal oxide or gapless type arresters the resistive glaze or other layer can, in accordance with the invention, be used to reinforce the inherent gra- ding current by provision of electrical contact at regular intervals along the internal bore of the porcelain housing between the resistive glaze and the internal surge arrester components.

Proposals have been made in the prior art for the provision of an electrically-resistive coating on the bore of the porcelain housing of a surge arres¬ ter. In British Patent Speci ication 1 0 8 385 there is described a surge arrester comprising an assembly of series-connected non-linear resistors and spark gaps housed in the bore of a porcelain housing having a

resistive coating, comprised of a semi-conducting glaze or engobe, extending over the whole of the inner surface -of the housing. A similar arrangement is described in Swiss Patent Specification 319 390. In each of these prior art arrangements, the only connections between the electrically resistive layer provided on the internal surface of the housing and the internal components of the surge arrester, namely the assembly of series connected spark gaps and resistor blocks, are made at the top and bottom ends of the complete assembly. The resistive layer thus can have a voltage grading ef ect only when a plurality of such surge arresters are connected together in series, and has no grading effect in regard to the individual internal components of an individual surge arrester. Moreover, the prior art arrangements of British Patent Specification 1 098 385 and Swiss Patent Speci ication 319 390 aforementioned suffered from the disadvantage that since the only connections between the resistive layer and the arrester internal assembly were at the top and bottom ends of the assembly, differing voltages existed in service between inter¬ mediate positions on the internal components and the adjacent resistive layer leading to local electrical discharges within the arrester and partial destruction of the resistive layer. On account of these disadvant¬ ages, the proposals of British Patent Specification 1 098 385 and Swiss Patent Specification 319 390 were not practical and have not, to the Applicants knowledge, been commercially implemented..

The invention, together with features and advantages thereof, will best be appreciated from consideration of the following description of exemp¬ lary embodiments of the invention contrasted with prior art arrangements all of which are renresented

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schematically in the accompanying drawings wherein:-

Figure 1 schematically illustrates a prior art arrangement comprising linear-resistor grading circuits ; Figure 2 schematically illustrates a prior art arrangement comprising non-linear resistor grading circuits ;

Figure 3 schematically illustrates a prior art arrangement comprising non-linear resistor and capacitor grading circuits;

Figure 4 schematically illustrates a first embodiment of the invention; and

Figure 5 schematically illustrates a second embodiment of the invention. Referring first to the prior art arrangements of Figures 1, 2 and 3 these will be familiar to those • possessed of appropriate skills in the surge arrester art. The arrangements shown each comprise four series gaps GI to Q4 with grading circuits comprising respec- tively series connected linear resistors R_, to R. conn- ected to shunt the gaps (Fig. 1), series connected non-linear resistors NLR., to NLR. connected to shunt

1" 4 the gaps (Fig. 2), and combinations of non-linear resistors R. to LR and capacitors C„, C coupled to the gaps (Fig. 3). In each case, non-linear voltage- dependent resistors (not shown) may be series connected with the gaps as is well known. These prior art arrangements being well known, no further description relating thereto is considered necessary for the pur- poses hereof.

Figure 4 represents schematically a first emb¬ odiment of the invention comprising an otherwise con¬ ventional series gap, linear grading resistor type surge arrester as shown schematically in Figure 1, but wherein the resistors R etc. are comprised by sections

of a metal oxide resistive glaze such as of oxides of tin or iron or combinations thereof provided on the internal surface of the bore of the insulator housing. In Figure 4, the shedded porcelain insulator housing is designated 2, the electrically resistive glaze is designated 1, resilient contact fingers to the glaze 1 are designated 3 _. and the spark gaps are designated 4. As will be appreciated by those skilled in the surge arrester art, voltage-dependent non-linear resistors could be series-connected with the respective spark gaps intermediate adjacent gaps, the resistors constituted by the glaze shunting the spark gaps.

Figure 5 illustrates schematically a metal oxide or gapless arrester wherein the metal oxide resistive glaze on the internal surface of the porce¬ lain housing is used to reinforce the inherent grading current by virtue of' electrical contact being made at regular intervals along the internal bore of the housing. In Figure 5 _. reference numeral 1 designates the resistive glaze, 2 designates the housing, 3 des¬ ignates the contacts to the glaze 1, and 4 designates the non-linear resistors.

A comparison of the arrangements according to the invention as illustrated in Figures 4 and 5 with the surge arresters of prior art British Patent Spec¬ ification 1 098 385 and Swiss Patent Speci ication 319 390 will show that the invention differs from the prior art primarily in that electrical contact is established by means of resilient electrically-conductive compo- nents (contact fingers) between the components of the internal surge arrester assembly and the resistive or semi-conducting glaze provided on the internal surface of the porcelain housing, the contact fingers making firm and positive electrical connection to the glaze by virtue of their resilience.

The present invention is considered to afford the advantages listed hereunder:-

(i) the use of the metal oxide resistive glaze can eliminate at least a proportion of the expensive grading components conventionally provided; (ii) the internal assembly of the arrester is simplified; (iii) the use of the resistive glaze as a comp- onent of the grading circuits provides for a greater surface area for dissipating grading circuit watts loss thereby enabling greater grading currents to be utilised with resultant enhanced performance; (iv) the uniform resistive glaze will addition¬ ally sex-ve to screen internal components from external influence and this will reduce internal ionization levels; (v) the invention can readily be adapted to conventional- porcelain insulator housings without requiring dimensional changes ; (vi) the internal assembly of conventional surge arresters can be utilised with only the minimum of modification to provide for the electrical contacts to the resistive glaze;

(vii) the risk of individual grading component failure is reduced;, and (viii) the grading current is more practically monitored in service. The invention thus is considered to represent a considerable advance in the surge arrester art. As will be appreciated by those possessed of appropriate knowledge and skills pertinent to the surge arrester art, the foregoing description and discussion is exem- plary only of the possibilities inherent to the invention and various modi icationsand variations can be made with¬ out departure from the spirit and scope of the invent!