US2831913A

US2831913A - Transformer case grounding pad

Info

Publication number: US2831913A
Application number: US223762A
Authority: US
Inventors: Wilfred A Okenfuss
Original assignee: Wagner Electric Corp
Current assignee: Wagner Electric Corp
Priority date: 1951-04-30
Filing date: 1951-04-30
Publication date: 1958-04-22
Anticipated expiration: 1975-04-22

Description

A ril 22, 1958 w. A. OKENFUSS 2,331,913

TRANSFORMER CASE GROUNDING PAD Filed April 30, 1951 United States Pa ent 2,831,913 7 TRANSFORMER CASE GROUNDING PAD Wilfred A. Okenfuss, University City, Mo., assignor m Wagner Electric Corporation, St. Louis, Mo., a corporation of Delaware Application April 30, 1951, Serial No."223,762

1 Claim. (01. 174-51 This invention relates to transformers and is more particularly directed to an improved ground connection therefor. I

The use of transformers in modern power and distribution circuits requires that the casing of the transformer besolidlygrounded as a protection against failures in the power system which would otherwise cause dangerous voltages to be built up between the transformer case and ground. Unless a low resistance ground connection can be made to the transformer case, an arcing condition could be produced which, by reason of the inherent inductance and capacitance of thecore and coil construction, could cause voltages. to be produced across'parts of the windings far greater than the voltage for which it is designed, thus causing failure in the insulation of the transformer. To overcome the possibility of an arcing ground on a transformer, it is necessary to provide a low resistance connection to the casing of the transformer which will maintain its characteristics without attention over many years of service.

One of the objects of the invention is to provide a grounding pad which, when applied to a transformer casing will eliminate previous objections, such as cor- 2,831,913 Patented Apr. 22, 1958 ICC , accidental damage in handling the transformer, as well as providing for as short a grounding cable 6 as feasible.

Since the grounding cabled and its associated lug 7 are usually made of copper in order to secure high conductivity and long life, it is necessary to provide a contact surface on the grounding pad 4 which will produce a low resistance junction at all times after connection has been made therebetween. It is a wellknown fact that when, any ferrous material is exposed to the atmosphere, it will rust. The rust so formed consists principally of iron oxide which is a poor conductor of electricity and, therefore, it is necessary to face the ground pad which will provide the requisite conductivity and, at the same time, a strong assembly that can be readily.

, assembled on the transformer casing.

Referring to Figs. 3- and 4, the grounding pad 4 consists of a face plate 9,'preferably made of copper, and

bondedto the steel block 10 in'such a manner that there is no chance of atmosphere reaching the face ofthe block 10 and causing oxidation thereof. The edgesand opposite face of block 10 are painted with rustresisting paint after assembly on the transformer casing, unless the grounding pad-4 is assembled on the transformer casing rosion in the contacts, loosening of joints, insufiicient Widely different physical properties into a bonded struc- I configuration of grounding pads for transformers.

In the drawings: 7 Fig. 1 is a transformer casing with the grounding pad attached thereto, a

Figure 2 is a side view of the casing of Fig. 1, Fig. 3 is a detail drawing showing a preferred method I of attaching the grounding pad to the transformer casing,

Fig. 4 is a detail drawing showing a modification of the method of attaching the grounding pad to the transformer casing,

Fig. 5 is a perspective view of the fabricated transformer grounding pads prior to separation into individual units; and a Fig. 6 is a detail view of the assembly of the necessary parts prior to bonding.

Referring to Figs. 1' and 2, the transformer casing 1 has a bottom plate 2 and a base 3. Welded to the edge of the bottom plate 2 is a grounding pad 4 which serves as a means for making connection between an earth ground 5 through the grounding cable 6 provided with a lug 7 secured thereto which, in turn, is removably secured to the grounding pad 4 by cap screws 8. 'Ihe grounding pad may be attached to the transformer casing, as shown in detail in Fig. 3, or may be alternately welded directly ture that can be readily machined to form the desired 7 as shown in Fig. 4 with the opposite face of the block 10 in surface abutment with the casing. The lug 7 of the grounding cable 6 is secured to the face of the grounding pad by cap screws 8 (Fig. I) screwed into the tapped holes 11 projecting through both the copper face 9 and the steel block 10 of the grounding pad 4.

The grounding pad 4 is fabricated by pre-cutting blocks of steel to the required dimensions and the copper 12 to such a dimension that several blocks 10 may be placed thereon, as shown in Fig. 5. Between each steel block and the copper slab 12 there is placed lengths of low melting alloy or other suitable bonding material, such as a silver bearing alloy solder known as easy fio, shown at 13 in Fig. 6. The assembly is then heated in a controlled atmosphere furnace with the copper slab on the bottom, to a temperature suchthat the alloy fuses and, with the aid of flux previously applied, wets the adjacent surfaces of the copper and steel. The weight of the individual steel blocks and capillary attraction causes the molten alloy to flow over the entire adjacent surfaces of the steel blocks and copper slab. The assembly is then slowly cooled to a point below freezing of the alloy and the assembly removed from thefurnace. After the bonded steel blocks and copper slab have cooled to room temperature, the copper slab is cut by a saw, or other convenient means which will not distort the exposed surface of the copper, to the lateral dimensions of the steel block. The individual pads 4 then have holes 11 (Fig. 3) drilled and tapped therein, the copper face being sanded to remove any irregularities and oxides which might have formed during the fabricating process. 7 I

The assembly of the ground pad on the transformer casing is accomplished by welding it to the bottom plate 2 of the casing, as at 14 (Fig. 3), or welding it to the side wall of the casing 1, as at 15 (Fig. 4). The advantage of producingground pads for transformer casings in the manner described is that bypre-machining the steel block 10 to dimensions and bonding it to the copper slab, the diificulty in economically machining a combination of'copper and steel having such widely varying machining characteristics as to require entirely different tools, is removed. This method requires that only the copper be out after the bonding process. Due to the to drill and tap the holes in the assembly.

Another advantage of the grounding pad above described, is that the fabricated pad may be welded on the transformer casing, whereas should a solid copper pad be used, it would be necessary to braze the joint which is both costly and difficult. When the grounding pad 4 is utilized as in Fig. 3, it is an advantage to have the steel block backing up the copper face 9 to supply the necessary strength for preventing damage by accidentally hitting it in any manner.

While the grounding pad has been described in detail in connection with its use on transformer casings, it is evident that no modification whatever would be required for its use on any steel structure to which a ground must be attached.

What I claim is:

In a device of the type described including an electrically conductive metallic casing for a transformer and a non-corrosive metallic electrical grounding connector for the casing, the improvement which comprises a grounding pad for providing a low resistance non-corrosive connection between the casing and the grounding connector, said grounding pad comprising a relatively thick ferrous metal block with faces having an area substantially larger than the cross-sectional area of the grounding connector, one of the faces being secured in contiguous abutting relationship with the casing to provide an intimate electrical contact therebetween, a relatively thin non-ferrous non-corrosive metal facing having an area similar to the area of the faces of said block, said facing being bonded to the other face of said block in impervious abutting relationship, and an internally I 2,831,913 H r f threaded aperture extending through said facing andihto the block for securing the grounding connector to said grounding pad, the grounding connector having a lug with a substantially flat surface in abutment with said facing over an area larger than the cross-sectional area of the grounding connector whereby a low resistance non-corrosive electrical connection having an area larger than the cross-sectional area of the grounding connector is provided between the grounding connector and the casing.

References Cited in the tile of this patent- UNITED STATES PATENTS 970,693 Donald Sept. 20, 1910 1,792,552 Siegmund Feb. 17, 1931 1,813,448 Horton July 7, 1931 1,837,238 Siegmund Dec. 22, 1931 1,907,932 Pugh May 9, 1933 2,137,617 Imes et a1. Nov. 22, 1938 2,181,083 Payette Nov. 21,1939 2,264,271 Barnsteiner Dec. 2, 1941 2,347,897 Febrey May 2, 1944 2,373,861 Van Inwagen Apr. 17, 1945 2,427,517 Wilson et al. Sept. 16, 1947 2,513,147 Chorpening June 27, 1950 2,520,981 Thomson Sept. 5, 1951 FOREIGN PATENTS 359,346 Great Britain Oct. 22, 1931 709,210 France Aug. 4, 1931