GB2144272A - Contact unit for electrical apparatus - Google Patents

Abstract

An electrical contact unit comprises a spring contact blade 13 for bridging a fixed contact 11 and a slidable contact 12. In order to ensure constant contact loads without extremely close machining tolerances being necessary, the contact blade 13 has a central longitudinal rib 14 which is inserted in a seat 19 in the fixed contact 11, and a plurality of pairs of arms extending transversely from the rib 14, each arm comprising an inclined portion 16 connected by an arc-shaped portion 17 to a second inclined portion 18 of opposite direction. The arc-shaped connection portion 17 presses elastically on the slidable contact 12 and the end 25 of the second inclined portion 18 presses elastically against the fixed contact 11. <IMAGE>

Description

SPECIFICATION Contact unit for electrical apparatus This invention relates to a contact unit for electrical apparatus, such as circuit breakers, isolators etc.

In particular, the present invention relates to those contact units for electrical apparatus which are required to withstand the passage of high-intensity currents, and comprise a fixed contact wall on which a mobile contact wall is slideable byway of one or more contact blades normally fixed to the fixed wall. The two contactwalls and the blades can be, for example, of flat or cylindrical shape. The relative movement between the two contact walls determines the opening or closure ofthe contact unit in known manner.

Known contact blades are in the form of a metal strip comprising a series of contact lugs formed by making transverse parallel cuts and bending torsionally with respectto the plane ofthe strip. The lugs adhere elastically against the fixed and mobile contact walls, to thus create a secure contact for the passage of current. However, contact units using such a blade are known to require high-precision machining both of the fixed contactwall and ofthe mobile contact wall in orderto keep within close tolerance limits the interspace between them which is to contain the metal blade. This latter must also be constructed with close machining tolerances.

In this respect, variations in the distance between the two contactwalls and in the extent bywhich the lugs ofthe contact blade are bent lead to corresponding variations both in the internal resistance and in the sliding load ofthe contact unit.

If machining is not of maximum precision, a series of contact units produced with the same structure could have technical characteristics which differ one from the other.

In addition to the problem of precision machining, there is the fact that any inaccuracy in the parallelism ofthe relative movement between the two contact walls could in time cause permament deformation of the highly stressed lugs, so asto result in an irreversible reduction in the points of contact, with obvious consequences. The object of the present invention is to obviate the aforesaid drawbacks of known contact units.

This object is attained according to the invention by a contact unit for electrical apparatus comprising a fixed contactwall on which a mobile contact wall is slidable byway of at least one elastic contact blade interposed between two respective parallel contact surfaces of said two contact walls, characterised in that said contact blade comprises a central longitudinal rib from which there extend appendices in the form of lugs, each constituted in succession by a first and a second portion joined together byan arc-shaped portion, the arc-shaped connection portion pressing elastically against one of said two contact surfaces, the terminal partofsaid second portion pressing elastically againstthe other of said two contact surfaces, said central rib being housed in a secure manner in a corresponding seat in one of said two contactwalls.

The characteristics of the present invention and its advantages overthe known art will be more apparent from the description of one embodiment given hereinafter by way of non-limiting example with reference to the accompanying drawing in which: Figure lisa cross-section on the line I-I of Figure 2 through a contact unit according to the invention; Figure 2 is a partial view of the contact blade of the contact unit of Figure 1 in the direction ofthe arrow F.

The contact unit of Figure 1, indicated overall by 10, comprises a fixed contact wall 11 and a mobile contact wall 12 between which an elastic contact blade 13 is interposed. The walls 11 and 12 pertain to two respectively fixed and mobile contact elements of an electrical apparatus, for example a circuit breaker.

The blade 13 comprises a central longitudinal rib 14 from which there transversely extends a series of separated pairs of projecting divergent appendices 15 in the form of lugs.

Each appendix 15 comprises a first rectilinear inclined portion 16which branches from the rib 14 and extends by way of an arc-shaped connection portion 17 to a second rectilinear portion 18 inclined in the opposite direction to the preceding.

The rib 14 is of U shape and is housed in a corresponding seat 19 in the fixed contact wall 11 by way of an insulating support 20, in such a mannerthat the blade 13 does not separate from the wall 11 during the relative contact opening and closure movement of the two walls 11 and 12.

The seat 19 is connected to an actual contact surface 22 ofthefixed wall 11 byway oftwo inclined U-shaped faces 21 which followthe corresponding profile of the pairs of appendices 15.

Over its entire area the mobile wall 12 comprises only one contact surface, this being indicated by 23 and parallel tothe surface 22.

As can be seen from Figure 1,foreach appendix 15 the arc-shaped connection portion 17 presses elastically againstthe surface 23, and the terminal stem 25 ofthe portion 18 presses elastically against the surface 22.

The portions 16 and 18 ofthe blade 13 have a specific purpose. The portion 18 transmits current between the contact surfaces 22 and 23, whereas the portion 16 proportions the load on each of the points of contact.

It has been verified experimentally that the shape of the appendices 15, in the light ofthe particular described purpose of its component parts, ensures a constant predetermined load on the points of contact even if the distance between the two contact surfaces varies, and prevents permanent deformation ofthe blade, by virtue ofthe particular elastic action and reaction ofthetwo portions 16 and 18.

The drawbacks described in the introduction are thus overcome. In orderto limit excessive expansion ofthe appendices 15, the wall 11 comprises two steps 24which laterally define the space occupied by the blade 13, and against which the stems 25 are halted should the blade 13 undergo large deformation.

The best operating compromise is obtained when: a) the angle a between the portion 16 and surface 23 is between 35 and 45".

b) the angle between the portion 18 and surface 23 is between 60 and 70".

c) the ratio ofthe elastic lever arm a ofthe portion 1 6to the elastic lever arm b of the portion 18 is between 2 and 3.

It is apparentthat modifications and/or additions can be made to the present embodiment, which is described by way of non-limiting example.

The shape ofthe projecting appendices of the blade can be modified provided that the particularcollaboration betweenthetwo appendix portions occurs.

The shape ofthe longitudinal rib and corresponding seat in the fixed contact wall can also be modified, provided that secure fixing ofthe blade to this latter is obtained.

In the illustrated example, the rib 14with its seat 19 is suitable for contact unit elements of cylindrical shape and for a sliding movement ofthe wall 12 on the wall 11 1 in 11 inthedirectionofthedoublearrowG.

The insulating support 20 is not always necessary.

In the case of direct contact between the seat 19 and rib 14 secondary current paths are created in addition to the main current paths along the portions 18.

Claims (7)

1. A contact unit for electrical apparatus, comprising a fixed contact wall on which a mobile contactwall is slidable byway of at least one elastic contact blade interposed between two respective parallel contact surfaces of said two contact walls, characterised in that said blade comprises a central longitudinal rib from which there extend appendices in the form of lugs, each constituted in succession by a first and second portion joined together byway of an arcshaped portion,thearc-shaped portion pressing elastically against one of said two contact surfaces, the terminal part of said second portion pressing elastically againstthe other of said two contact surfaces, said central rib being housed in a secure manner in a corresponding seat in one of said two contactwalls.

2. A contact unit as claimed in claim 1, characterised in that said appendices are disposed in longitudi nally successive pairs separate from each other, each of said pairs being constituted bytwo appendices diverging from said logitudinal rib.

3. A contact unit as claimed in claim 1, characterised in that said first and second connected portions are of rectilinear profile inclined in opposite directions to said contact surfaces.

4. A contact unit as claimed in calim 1, characterised in that said central rib and said corresponding seat have a U profile.

5. A contact unit as claimed in claim 1, characterised in that an insulating support is disposed between said seat and said central rib.

6. A contact unit as claimed in claim 3, characterised in thatthe angle between said first rectilinear portion and said parallel contact surfaces is between 35' and 45',the angle between said second rectilinear portion and said parallel contact surfaces is between 60" and 70 , and the ratio ofthe elastic lever arm of said first portion with respect two said central rib to the elastic lever arm of said second portion with respect two the arc-shaped connection portion is between 2 and 3.

7. A contact unit for electrical apparatus substantially as herein described with reference to the accompanying drawings.