GB1582881A

GB1582881A - Electrical surge protection in telephone exchanges

Info

Publication number: GB1582881A
Application number: GB1940777A
Authority: GB
Original assignee: Post Office
Current assignee: Post Office
Priority date: 1978-05-09
Filing date: 1978-05-09
Publication date: 1981-01-14

Description

(54) ELECTRICAL SURGE PROTECTION IN TELEPHONE EXCHANGES (71) We, the POST OFFICE, a British corporation established by Statute, of 23 Holland Street, London W1P 6HQ, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed; to be particularly described in and- by the following statement: - This invention relates to apparatus for providing electrical surge- protection in a telephorie exchange main distribution frame.

In the development of modern telephone exchanges the reliability and rapidity of operation of, for example, reed relay and electronic switching equipment has been obtained.atthe expense of some robustness compared with the older Stronger switching equipment.

Thus, the heavy-duty relays utilised in Strowger equipment did not require lightning protection - except in those regions where lightning is particularly common such as Cornwall in the United Kingdom.

However, the ' changeover from older to newer, more delicate, switching equipment did not result in a corresponding e-xtendsion of lightning protection to those regions where lightning is less prevalent. The result of this was that switching equipment associ ated with an electrical surge path through a telephone exchange could bye rendered useless for a long period of time by a single lightning strike.

It is intended that apparatus according to the invention should provide lightning protection for currently installed switching equipment although it could conceivably be used in association with new installations.

The intended location of such lightning protection apparatus -is at the exchange main distribution frame, at the 'line' side of which, subscribers' lines are connected via a 'line' side distribution block and jumper wires to an 'exchange' side distribution block and hence to routing apparatus within a telephone exchange.

According to one aspect of the invention there is provided apparatus for providing electrical surge protection at a distribution block of a telephone exchange main distribution frame, said block having a plurality of first tags connected to a plurality of second tags through respective break jacks, said apparatus comprising a pair of end plates adapted to be secured to opposite end faces of said block so that each plate has a region extending beyond the boundary of the said end face, a conducting bar adapted to seat between extending regions of said plates in electrically condutive relationship therewith, and one or more plug means incorporating a voltage sensitive breakdown device, the or each plug means having first and second pins arranged and constructed for insertion into adjacent break jacks and ciip means arranged to engage said bar, said breakdown device when in a breakdown condition providing an electrically conductive path between said pins and said clip.

The plug means may have a flat moulded plastics body, said pins and clip means projecting from one edge face thereof. The pins can be formed of metallic leaf material bonded on to projecting plastics formers integral with the body. The clip means may be a generally U-shaped spring clip having its inside contour shaped to tightly engage the conducting bar. A part of said clip means within said body can be waisted so as to permit the clip to flex on being urged against the conducting bar. Said conducting bar may be of circular section. The end plates can be so shaped that, when fixed against their associated end faces, identification marks on the end faces are exposed.

The voltage sensitive breakdown device may be one of a gas discharge tube, an air-gap device or an energy absorbing Zener diode.

Accoding to another aspect of the invention' there is provided a distribution block of a telephone exchange main distribution frame incorporating electrical surge protection, said block comprising an insulating body having a plurality of first electrical tags connected to respective ones of a plurality of second electrical tags through respective break jacks, a pair of electrically conductive plates connected to opposite end faces of the block so that each plate has a region projecting beyond the boundary of the end face, an electrically conductive bar extending betwen and electrically connecting said projecting regions, and one or more plug means incorporating a voltage sensitive breakdown device, the or each plug means having first and second pins arraned and constructed for insertion into a pair of adjacent break jacks and clip means arranged to engage said bar, said breakdown device when in a breakdown condition providing an electrically conductive path between said pins and said clip.

The invention will be described now by way of example with reference to the accompanying drawings wherein Figure 1 is a perspective view of apparatus according to the invention and a distribution block of a telephone exchange main distribution frame before assembly of the apparatus relative to the block, and Figure 2 is a schematic plan view of the layout of elements in a plug means forming part of the apparatus.

Referring to Figure 1 a distribution block 10 of a telephone exchange main distribution frame comprises a plurality of planar elements 11 which are stacked one upon another and held in place by bolts 12. Each element of the block has tags 14 projecting from a face 15 of the block. Similar tags (not shown) project from an adjacent face 16 of the block. The tags 14 are connected to the tags on the face 16 by break jacks so that subscribers' lines connected to the tags 14 can be connected to exchange routing equipment connected to the other tags.

Extending into the block 10 from another face 18 is a matrix of break jack sockets 19.

Each of the sockets is formed where ends of conducting strips remote from tags 14 and the tags on the face 16 come into contact.

Lateral pairs of break jacks, for example, C and D, are associated with a single subscriber and routing path. The ends of the conducting strips form the break jacks 19 and can be forced apart by the insertion of flat pins. The usual function of the break jacks is as a test facility since a test circuit can be readily inserted without interrupting path continuity.

The present arrangement proposes using these break jack facilities to provide protection against electrical overvoltage condi tions.

To this effect contoured plates 2Q which are made of electrically conductive material are bolted to respective end faces 21 of the block 10 using the nut and bolt combing' tions 12 holding the individual elements 11 of the block in place. The plates 20 are formed with recesses 22 in order that the block 10 does not have to be completely dismantled to fit the plates. The plates are also cut away as at 24 in order that marks such as A and B printed on the end faces 21 of the block 10 and identifying the tags 14 can be readily seen. When fixed to the end faces 21 a front edge of the plates 20 extends both forwardly and laterally beyond the confines of the end face, thus overhanging the block. Holes are formed in the extreme ends of the overhanging edge in order to receive conducting, circular-sectioned bars 25. The bars can be secured to the plates by means of a screw or other fixture ensuring electrical contact between the bars 25 and the plates 20. The plates 20 are connected to an exchange earth busbar (not shown).

Plugs providing overvoltage protection are associated with each element 11. One such plug is shown schematically in Figure 2 and comprises a flat moulded plastics body 30 from which project two flat pins 31 for insertion into an adjacent pair of break jacks 19, and a clip 32 to grip the bar 25 when the plug is inserted into the break jacks 19. A gas discharge tube 35 (shown in schematic form in Figure 2) is wholly en closed within the plug body 30. The gas discharge tube has terminals which are electrically connected to the remote ends of the pins 31 and the clip 32. The clip 32 is formed of spring material and its projecting part is generally U-shaped and has an internal contour corresponding to the shape of the bar 25 to ensure a good and immedi ate contact when the plug is inserted into a pair of break jacks 19. A rear part of the clip is waisted as shown at 36 in order to allow the clip to move relative to the pins 31 in order to cater for tolerances permitted in locating the bars 25. The pins 31 are formed from metallic leaf material and bonded to projecting plastics formers integral with the body 30. Both the pins and the clip have their ends spot welded to the terminals of the gas discharge tube. The pins 31 are securely embedded within the housing 30 whereas the clip 32 is received in a housing permitting limited lateral movement within limits set by upstanding studs 38.

In use, with the plates 20 secured to the block 10 and one or more plugs 30 inserted into the break jacks 19, if an electrical surge occurs, for example, as a result of a light ning strike, one of the end terminals in the gas discharge tube becomes highly charged causing ionisation of the gas in the gas discharge tube thereby allowing passage of current from that end terminal through the clip 32, bar 25 and plate 20 to the exchange earth busbar.

As can be seen from the drawings, the example of block 10 shown has two pairs of break jacks in each element 11. A symmetrical arrangement has been utilised in which plugs extending down the lefthand side of the unit have the orientation shown while plugs inserted into break jacks on the righthand side (not, shown) will have a reversed orientation.

As described the gas discharge tube is wholly enclosed within the' plug body 30. As an alternative arrangement the tube can be located in an aperture in the plug body thereby allowing air to circulate around the tube for cooling purposes.

As shown schematically in Figure 2 the pins 31 and clip 32 are connected to the body of the gas discharge tube. It is envisaged that the gas discharge tube may have terminal pins projecting from it to which the pins 31 and clip 32 can be welded. Alternatively, instead of connection by welding, the gas discharge tube may be arranged so that it can be clipped into resilient contacts in a manner similar to a fuse in a domestic electrical- plug. The tube may also be soldered or crimped in position.

WHAT WE CLAIM IS: 1. Apparatus for providing electrical surge protection at a distribution block of a telephone exchange main distribution frame, said block having a plurality of first tags connected to a plurality of second tags through respective break jacks, said apparatus comprising a pair of end plates adapted to be secured to opposite end faces of said block so that each plate has a region extending beyond the boundary of the said end face, a conducting bar adapted to seat between extending regions of said plates in electrically conductive relationship therewith, and one or more plug means incorporating a voltage sensitive breakdown device, the or each plug means having first and second pins arranged and constructed for insertion into adjacent break jacks and clip means arranged to engage said bar, said breakdown device when in a breakdown condition providing an electrically conductive path between said pins and said clip.

2. Apparatus as claimed in claim 1 wherein said plug means has a flat moulded plastics body, said pins and clip projecting from one edge face of the body.

3. Apparatus as claimed in claim 2 wherein said pins are formed from metallic leaf material bonded on to projecting plastics formers integral with said body.

4. Apparatus as claimed in claim 2 or claim 3 wherein said clip means is a generally U-shaped spring clip having its inside contour dimensioned to tightly engage said conducting bar.

5. Apparatus as claimed in claim 4 wherein a part of said clip means within said body is waisted to permit the clip to flex on being urged against the conducting bar.

6. Apparatus as claimed in any preceding claim wherein said conducting bar is circular in section.

7. Apparatus as claimed in any preceding claim wherein the end plates are so shaped that they can be secured to the block by loosening bolts on the block and when secured against the end faces identification marks on the end faces are exposed.

8. Apparatus as claimed in any preced 'ing claim wherein the voltage sensitive breakdown device is one of a gas discharge tube, an air-gap device or an energy absorbing Zener diode.

9. Apparatus as claimed in any one of claims 2 to 8 wherein the breakdown device is embedded in the body.

10. Apparatus as claimed in any one of claims 2 to 8 wherein the breakdown device is located in an aperture in the body.

11. A distribution block of a telephone exchange main distribution frame incorporating electrical surge protection, said block comprising an insulating body having a plurality of first electrical tags connected to respective ones of a plurality of second electrical tags through respective break jacks, a pair of electrically conductive plates connected to opposite end faces of the block so that each plate has a region projecting beyond the boundary of the end face, an electrically conductive bar extending between and electrically connecting said projecting regions, and one or more plug means incorporating a voltage sensitive breakdown device, the or each plug means having first and second pins arranged and constructed for insertion into-a pair of adjacent break jacks and clip means arranged to engage said bar, said breakdown device when in a breakdown condition providing an electrically conductive path between said pins and said clip.

12. Electrical surge protection apparatus substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. discharge tube thereby allowing passage of current from that end terminal through the clip 32, bar 25 and plate 20 to the exchange earth busbar. As can be seen from the drawings, the example of block 10 shown has two pairs of break jacks in each element 11. A symmetrical arrangement has been utilised in which plugs extending down the lefthand side of the unit have the orientation shown while plugs inserted into break jacks on the righthand side (not, shown) will have a reversed orientation. As described the gas discharge tube is wholly enclosed within the' plug body 30. As an alternative arrangement the tube can be located in an aperture in the plug body thereby allowing air to circulate around the tube for cooling purposes. As shown schematically in Figure 2 the pins 31 and clip 32 are connected to the body of the gas discharge tube. It is envisaged that the gas discharge tube may have terminal pins projecting from it to which the pins 31 and clip 32 can be welded. Alternatively, instead of connection by welding, the gas discharge tube may be arranged so that it can be clipped into resilient contacts in a manner similar to a fuse in a domestic electrical- plug. The tube may also be soldered or crimped in position. WHAT WE CLAIM IS:

1. Apparatus for providing electrical surge protection at a distribution block of a telephone exchange main distribution frame, said block having a plurality of first tags connected to a plurality of second tags through respective break jacks, said apparatus comprising a pair of end plates adapted to be secured to opposite end faces of said block so that each plate has a region extending beyond the boundary of the said end face, a conducting bar adapted to seat between extending regions of said plates in electrically conductive relationship therewith, and one or more plug means incorporating a voltage sensitive breakdown device, the or each plug means having first and second pins arranged and constructed for insertion into adjacent break jacks and clip means arranged to engage said bar, said breakdown device when in a breakdown condition providing an electrically conductive path between said pins and said clip.