GB1559498A

GB1559498A - Gas-blast circuit breakers

Info

Publication number: GB1559498A
Application number: GB18161/76A
Authority: GB
Original assignee: Associated Electrical Industries Ltd
Current assignee: Associated Electrical Industries Ltd
Priority date: 1976-05-04
Filing date: 1976-05-04
Publication date: 1980-01-23
Also published as: ZA772344B; DE2719135A1

Description

(54) IMPROVEMENTS IN OR RELATING TO GAS-BLAST CIRCUIT BREAKERS (71) We, ASSOCIATED ELECTRICAL INDUSTRIES LIMITE, of 1 Stanhope Gate, London W1A IEH., a British Company, 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 gas-blast circuit breakers and to contact arrangements therefor.

According to the invention a gas blast circuit breaker incorporates a contact arrangement having a stationary contact of tubular form, and a movable contact structure comprising a tubular contact holder having an internal diameter greater than the external diameter of the stationary contact, and being supported coaxially therewith, the contact holder supporting within it a movable contact and being movable axially between an open position in which it is spaced from the stationary contact, and, a closed position in which it surrounds the adjacent end of the stationary contact and the latter is engaged by the movable contact, at least the outer surface of the end of the contact holder nearest the stationary contact being of insulating material at least over a region which extends beyond the adjacent end of the movable contact.

When the contacts of a circuit breaker are closed on to a high voltage circuit, a flash-over tends to occur, causing current to flow, before the contacts actually make metallic contact. The precise position of the movable contact structure at which this flash-over, or pre-arcing as it is generally known, takes place depends, for any particular circuit breaker, on the applied voltage, the higher the voltage the further the moving contact is from the stationary contact when pre-arcing commences.

By forming the outer surface of the contact holder of insulating material over at least a region which extends beyond the adjacent end of the movable contact prearcing between the stationary contact and the exterior of the holder is effectively prevented, so that the arc is constrained to strike on to the inner surface of the holder and is confine within the normal arcing region. This is of particular advantage at high voltages and currents as we have found that with a metallic contact holder the longer arcs have a tendency to strike on to its exterior surface, arcs expanding outwards as contact closure continues, which can, in some cases, cause considerable damage to surrounding parts of the circuit breaker.

The contact holder can, for example, be formed of fibre reinforced synthetic resin, although any other suitable insulating material may be used. Whilst it may be convenient for the whole of the contact holder to be formed of insulating material, only said region at the forward end of the holder, that is to say the end directed towards the stationary contact, need be so formed.

However the contact holder can be formed of conducting material having an insulating coating for example of synthetic resin formed on its outer surface at least over said end region.

The moveable contact is conveniently formed as a plurality of inwardly biassed contact fingers disposed around the interior of the contact holder which fingers permanently engage the exterior of a stationary tube forming part of the moveable contact structure and, in the closed position of the contact arrangement, also engage the stationary contact thus forming a conducting bridge between the stationary contact and said stationary tube.

A tubular structure of magnetic material for example steel may be disposed within and be spaced from the internal surface of each of the stationary contact and the stationary tube forming part of the movable contact structure but being conconnected to the inwardly directed end of the contact or tube, as described and claimed in the co-pending patent application No. 6456. 176 (Serial No. 1 511 791).

Preferably the contact holder is surrounded by a stationary annular piston and the whole of the contact arrangement is surrounded by a co-operating blast cylinder supported from, so as to move with, the movable contact structure, so that as the contacts are opened gas within the cylinder is forced inwards into the space between the contacts and through one and/or both contacts, thus lengthening any arcs that are formed, leading to a more rapid extinction.

Conveniently, the circuit breaker may be housed within an enclosure, which may possibly be weather-proof to permit outdoor installation, and/or the enclosure may be gas-tight, to allow the use of arcextinguishing gases other than air.

The invention will be further explained by way of example only and with reference to the accompanying drawings, of which :- Figure I represents schematically an axial section through the circuit breaker in the fully closed position, Figure 2 shows part of the section of Figure 1, with the circuit breaker in an intermediate position, and Figure 3 is a section similar to that of Figure 2, with the circuit breaker in the fully open position.

Referring now to Figures 1 to 3, a circuit breaker comprises a tubular insulating housing 1 in which is accommodated a stationary contact assembly 2 and a cooperating contact assembly 3 projecting coaxially towards each other from opposite ends of the housing. The stationary contact assembly 2 comprises a tubular member 4 of copper to the inner end of which is secured a nozzle 5 through which gas is exhausted when the contacts are opened as will subsequently be described.

The outer end of the tubular member 4 terminates in a flange 6 by which the contact assembly 2 is supported within the housing 1.

The other contact assembly 3 also comprises a tubular member 10 of copper, to the inner end of which is secured a nozzle 11 through which gas is exhaused when the contacts are opened, the opposite end of the member 10 terrninating in a flange 12 by which the assembly 3 is supported within the housing, in a similar manner to the assembly 2. The second contact assembly 3 also includes a tubular contact holder 13 which surrounds the member 10 and carries within it a contact arrangement consisting of a plurality of contact fingers 14 each biassed inwards by a spring 15 at each end. The contact fingers 14 engage the tubular member 10, and the holder can be moved axially into a closed position as shown in Figure 1 in which the contact fingers also make contact with the tubular member 4, thus bridging and establishing an electrical connection between the members 10 and 4.

The contact holder 13 is mounted on a carriage 24 slidably supported on the tubular member 10, and the carriage is connected to a rod 17 extending outwards from the housing and coupled to an operating mechanism (not shown) for moving the carriage so as to effect the opening and closure of the circuit breaker contacts.

A blast cylinder 18 is also attached to the carriage 24, and surrounds an annular piston 19 fixed to the flange 12 by rods 16. The free end 20 of the cylinder extends inwards towards the outer surface of the tubular member 4 as shown in Figure I.

The free end of the contact holder 13 also carries an insulating annulus 21, for example of polytetrafluroethylene (PTFE,) which contacts the outer surface of the tubular member 4 when the contacts are closed, so that on the initial opening of the contacts the gas within the cylinder is compressed until the anulus 21 leaves the nozzle, 5 (Figure 2), the gas then being forced into the arc chamber between the nozzles 5 and 11, and then through the nozzles and tubes 4 and 10, so that it rapidly extinguishes any arcs that are formed.

It has been found that when the contacts of a device as above described are closed on to a high voltage circuit flashover occurs and current starts to flow before the contacts actually make metallic contact. However, by forming the contact holder 13 of insulating material such as polyethyleneteryphthalate reinforced synthetic resin. in accordance with the invention, this pre-arcing is restricted to between the nozzle 5 and the moving contacts within the contact holder 13, and is prevented from being struck between the stationary contact nozzle 5 and the external surface of the contact holder 13, as could occur if the latter were formed of metal. This is a particular advantage in circuit breakers designed to close on to very high short circuit currents, since arcs struck on to the external surface of a metallic contact holder tend to expand outwards and could damage the inside of the blast cylinder 18.

The contact holder may alternatively be formed by any other suitable insulating material. lt will, however, be appreciated that instead of the whole of the contact holder being formed of insulating material only its forward end, that is to say the end region directed towards the stationary contact need be so formed, the length of said region being adequate to ensure that pre-arcing on to the outer surface of the remainder of the holder is effectively prevented.

In a modification the holder is formed of metal, but has an insulating coating formed on its outer surface, at least over its forward end region.

Conveniently a tubular structure of magnetic material, such as steel, as shown at 22 is disposed within and is spaced from the internal surface of each of the two tubular members 4 and 10, but is connected to the inner end of the respective member in accordance with the invention forming the subject of co-pending Patent Application No. 6 456/76 (Serial No. 1511791).

When the contacts are opened the magnetic field which is formed within the tubular structures 22 serves to drive the roots of the arcs that are formed between the contacts along the interior of the structures, this lengthening the arcs, which are thereby more readily extinguished.

Claims

WHAT WE CLAIM IS :- 1. A gas blast circuit breaker incorporating a contact arrangement having a stationary contact of tubular form, and a movable contact structure comprising a tubular contact holder having an internal diameter greater than the external diameter of the stationary contact, and being supported coaxially therewith, the contact holder supporting within it a movable contact and being movable axially between an open position in which it is spaced from the stationary contact, and a closed position in which it surrounds the adjacent end of the stationary contact and the latter is engaged by the movable contact, at least the outer surface of the end of the contact holder nearest the stationary contact being of insulating material at least over a region which extends beyond the adjacent end of the movable contact.
2. A circuit breaker according to Claim 1, in which the contact holder is made substantially wholly of insulating material.
3. A circuit breaker according to Claim 1, in wihch said region of the contact holder at its end nearest the fixed contact is made substantially wholly of insulating material.
4. A circuit breaker according to Claim I in which the contact holder is formed from conducting material and has an insulating coating formed on the end of its outer surface which is nearest the fixed contact at least over said region.
5. A circuit breaker according to Claim 4, in which substantially the whole of the outer surface of the contact holder is coated with said insulating material.
6. A circuit breaker according to Claim 2 or Claim 3, in which the insulating material is formed from fibre reinforced synthetic resin.
7. A circuit breaker according to Claim 4 or 5 in which the insulating material is a synthetic resin.
8. A circuit breaker according to any one of the preceding claims, in which the movable contacts are formed as a plurality of inwardly biased contact fingers disposed around the interior of the contact holder, the fingers permanently engaging the exterior of a stationary tube forming part of the movable contact structure and disposed coaxially with but spaced from said stationary contact.
9. A circuit breaker according to Claim 8 in which, in the closed position of the contact arrangement, the contact fingers engage the stationary contact, thus forming a conducting bridge between the sta tionary contact and the said stationary tube.
10. A circuit breaker as claimed in any preceding claim in which the contact holder is surrounded by an annular piston and the whole of the contact arrangement is surrounded by a blast cylinder cooperating with the annular piston, to produce a blast of gas between the contacts as they are opened.
11. A circuit breaker according to Claim 10 in which the annular piston is stationary, and the blast cylinder is supported from, so as to move with, the movable contact structure, so that as the contacts are opened gas within the cylinder is forced inwards into the space between the contacts and through one and/ or both contacts, so that in use any arc that is formed is lengthened thereby thus leading to its more rapid extinction.
12. A circuit breaker according to Claim 8, in which a tubular structure of magnetic material is disposed within and is spaced from the internal surface of each of the stationary contact and the stationary tube forming part of the movable contact structure, but is connected to the inwardly directed end of the contact or tube as the case may be.
13. A circuit breaker as claimed in any preceding claim, in which the contact arrangement is mounted within an enclosure.
14. A circuit breaker according to Claim 13, in which the enclosure is a gastight enclosure, and contains an insulating gas.
15. A circuit breaker substantially as hereinbefore described with reference to the accompanying drawings.