This invention relates to electric plugs and is especially, but not exclusively, concerned with electric plugs for use in domestic and similar wiring systems.
It has been proposed to provide an electric plug with electrically conductive pins which are substantially hollow and have been formed by folding or otherwise shaping a sheet metal or metal alloy preform, for example as shown in UK Patent 618141. It has also been proposed to provide such a preform with a gold inlay prior to folding or otherwise shaping (see, for example, Machine Design Vol. 47, No. 2, January 1975, Page 18).
When an electric plug is electrically connected in a socket outlet but the faces of the plug and outlet are not in interfacial engagement, there is a substantial risk that a child or other person will introduce a finger or other object between these faces and receive an electric shock. To reduce this risk, it has been proposed and is sometimes the practice to provide on at least each line pin of the plug an insulating protective covering which extends around and along a substantial part of the length of the pin projecting from the insulating plug body. By "substantial part" is meant that the pin is covered to the extent necessary to avoid the risk of a person touching a live pin. Such insulating protective pin-coverings may be integral parts of the plug body but more recently it has been proposed to use, for each insulating pin-covering, a separate sleeve of insulating material which fit over a part of a length of a pin or a separate body of insulating material which is moulded in situ on a pin. Examples of these known arrangements are described in UK Patent No. 1181370 and US Patent No. 3710287.
These methods of providing an insulating covering on pins are not readily adaptable to pins formed from folded sheet metal or metal alloy preforms.
According to the present invention, a hollow electrically conductive pin, for use in an electric plug, is formed from sheet metal or metal alloy which has been folded or otherwise shaped to form the pin, characterised in that a substantial part of that surface of the folded sheet metal or metal alloy forming the outer surface of the pin carries a covering of electrically insulating material which extends over a substantial part of that length of the pin which will project from the plug body, said covering having been applied to the sheet metal or metal alloy before the sheet has been folded or otherwise shaped to form the pin, and in that the sheet metal or metal alloy is folded or otherwise shaped so that the edges of that part of the folded sheet metal or metal alloy carrying the covering of electrically insulating material are turned inwardly.
This arrangement has the advantage that if the electrically insulating material becomes detached from these edges the metal or metal alloy underneath is not exposed. Also the distance of these edges of the metal or metal from the surface of the pin is greatly increased, increasing the possible tracking distance and so making the pin less susceptible to high voltage breakdown.
The covering of insulating material preferably has a thickness lying in the range of 0.025 mm to 0.1 mm.
The pin is preferably formed from sheet metal or metal alloy which has a groove or other recess in the surface, and electrically insulating material at least partially fills the recess, preferably so as to fill it. In this way limitations on the thickness of the insulating material are considerably eased.
Preferably, the insulating material is a hardenable thermosetting synthetic resin, for example an epoxy resin. Other suitable hardenable insulating materials that may be employed include systems based on polyurethane.
Alternatively the insulating material may be a linear polymer such as PVC, polythene, nylon or PTFE.
The invention also includes a method of forming a hollow electrically conductive pin for use in an electric plug, which method comprises punching or otherwise cutting a sheet of metal or metal alloy to form a multiplicity of integral pin preforms and, before or after detaching a pin preform from the sheet, folding or otherwise shaping the preform to form a hollow pin, characterised in that before or after the sheet has been punched or otherwise cut but before the preform is folded or otherwise shaped, electrically insulating material is applied to a selected part of a surface of the sheet to cause a layer of electrically insulating material to adhere to said selected part of the surface to form a covering of electrically insulating material, the covered part of the sheet being such that, when each of said pin preforms is detached from the sheet before or after it has been folded or otherwise shaped to form a hollow pin, the pin has on the outer surface of a substantial part of the length of the pin that will project from a plug body, a covering of electrically insulating material, and characterised in that the pin is so folded or otherwise shaped that the edges of that part of the folded sheet metal or metal alloy carrying the covering of electrically insulating material are turned inwardly.
Preferably, a groove or other recess is formed in the said selected part of the surface of the sheet metal or metal alloy before the sheet is folded, the said selected part of the surface of the sheet being the surface of the groove or other recess.
In one preferred method, before or after the sheet has been punched or otherwise cut but before the preform is folded or otherwise shaped, the sheet is heated and electrically insulating material is applied to the said selected part of a surface of the heated sheet in powdered form and is permitted or caused to coalesce, with or without pressure.
Any convenient method of applying powdered insulating material may be employed. For example, a stream of powdered insulating material may be directed on to the selected part of the surface by at least one appropriately shaped nozzle, if necessary the part of the surface of the sheet that is not to be so coated being suitably masked. Where the sheet is in the form of a strip with the pin preform side by side, the stream of powdered plastics material may be directed on to the selected part of a surface of the strip as the strip is advancing in the direction of its length.
Other methods of applying electrically insulating material to a selected part of a surface of the sheet include electrostatic spraying of powdered material, painting, spraying or brushing a layer of a hardenable liquid material, silk screen techniques, and applying coated tapes.
Alternatively, where a groove or other recess is formed in the strip of metal or metal alloy, a preformed strip or other insert of insulating material may be laid in the groove or other recess and secured by the application of heat and pressure, or by an adhesive as appropriate for the particular material.
The invention further includes an electric plug which has at least one hollow electrically conductive pin substantially as hereinbefore described.
The invention is further illustrated, by way of example, by the accompanying drawings, in which:
The hollow electrically conductive pin 1 shown in Figure 1 has a covering 2 of PVC over the outer surface of a substantial part of its length. The pin 1 is formed from one of a plurality of metal strip preforms 3 as shown in Figure 2 which have a recess 4. A preformed strip 5 (Figure 3) of PVC is secured in the recess 4. The metal sheet 3 is then cut into preforms 6 as shown in Figure 3, in which PVC strip 5 is inlaid in the recess 4 on the underside of each preform 6. Each preform 6 is then folded into the hollow conductive pin 1 with the edges 7 and 8 of the preform 6 abutting, and with the edges 9, 10, (Figure 4) of that part of the folded metal sheet 11 carrying the covering of insulating material 2 turned inwardly, before it is detached from the metal sheet 3. An alternative arrangement is shown in Figure 5 where the inturned edges 91, 10' of that part of the folded metal sheet 11' carrying the covering of insulating material 2' of the pin 11 are contiguous.
A method of attaching an electrical conductor to the pin 1 in Figure 1 is by crimping. The fingers 12, 13 are cut and formed as an integral extension of the pin 1 when the pin is cut and folded. These fingers 12, 13 can be folded around a conductor to make a crimped connection.
One alternative method of attaching an electrical conductor to a hollow conductive pin is described and claimed in the specification of BICC-Burndy Limited's UK Patent Application No. 7901361.