Fused plug assembly with push-in fuse unit

A fused electrical plug assembly is provided having a plug body with a fuse socket open to one side to hold a push-in fuse unit. The fuse unit has one pair of active fuse elements between which there is a spare pair which can be used by removing the fuse unit, turning it ninety degrees and pushing it back into the fuse socket.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to fused electrical plugs with replaceable 
fuses, and particularly to those for use with Christmas tree light sets 
and other decorative light sets. 
2. Description of the Prior Art 
Electrical add-on plugs having replaceable internal fuses for opening an 
electrical circuit responsive to excess current flow are in use on 
decorative light sets. Typical designs are shown in U.S. Pat. Nos. 
3,976,967; 4,080,039 and 4,178,061. 
Need has arisen for a safe, reliable, cheaper and simpler plug unit having 
a minimum of parts and in which a fuse is easy to change. There is also a 
need for a plug unit having a simpler means of providing a spare fuse. The 
present invention aims to meet these needs. 
SUMMARY OF THE INVENTION 
In accordance with the present invention the use of fuse carriers and fuse 
access covers at the sides of electrical plugs has been eliminated by 
providing a push-in fuse unit located in the plug body within the zone 
between the longitudinal axes of the prongs (blades). The fuse unit is 
preferably square in cross-section and has an active pair and a spare pair 
of fuse elements along its four sides, the active pair engaging two sets 
of contacts provided by the prong members and by the wire leads at 
opposite sides of a socket receiving the fuse unit. The spares can be put 
into use by removing the fuse unit from the socket and reinserting it 
after turning it ninety degrees.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the drawings it is seen that the fused plug assembly of the 
present invention comprises a plug body 10, a push-in fuse unit 12, a pair 
of prong members 14-15, and a pair of wire contacts 16-17 connected to a 
pair of leads 18-19. The plug body 10 is formed with a central square fuse 
receiving socket 10a at the front side with an enlarged upper rim portion 
and has a pair of standard prong sockets 10b, 10c at the outer end. 
The fuse unit 12 has a generally square body in transverse cross-section 
and is formed with an enlarged head 12b. Two pairs of fuse element strips 
20a-d extend along the four outer faces of the fuse body 12a, one opposite 
pair 20a, 20c of these strips electrically interconnecting the prong 
members 14--15 with the wire contacts 16-17 while the other pair 20b, 20d 
remain as spares. 
The prong members 14-15 are alike, but positioned with opposite 
orientation; hence, only prong member 14 will be described in detail. It 
is formed by doubling back the brass prong stock from the projecting tip 
to form an exposed prong 14b of double thickness for insertion into a wall 
socket or other plug. Within the plug body the prong member has two 
separated legs, one leg 14c being outwardly offset to extend along the 
outer side of the prong socket 10b to engage the respective prong 22 of an 
add-on plug when plugged into the plug body 10. The other leg is bent 
inwardly and then rearwardly to provide a contact portion 14d exposed to 
the fuse socket 10a. This contact portion 14d is opposed by a contact 
portion 15d on the opposite side of the fuse socket. The contacts 16-17 
are aligned with and spaced toward the outer end of the plug body from the 
contact portions 14d, 15d so that the fuse elements 20a, 20c will engage 
and electrically interconnect the contacts 16-17 with the contact portions 
14d, 15d of the prong members. The insulated leads 18-19 have bare wire 
terminal portions connected to the contacts 16-17 as by crimping 
extensions of the contacts therearound and extend from their connections 
out through the back of the plug body. The fuse elements 20a-d are formed 
from a suitable conductive foil such as, for example, zinc foil and are 
secured by a suitable adhesive to the fuse body. The exact thickness and 
width of the fuse elements, of course, depend upon the desired fuse 
rating. For a 3 ampere fuse, zinc foil strips 0.001 inch thick and 0.062 
inch wide can be used. 
It will be appreciated that the plug body 10 can be injection molded in one 
piece with the prong members 14-15 and contacts 16-17 and connected wires 
18-19 in place in the mold. The body of the fuse 12 can also be injection 
molded as one piece whereupon the fuse elements 20a-d are attached as by a 
suitable pressure sensitive adhesive. Hence the plug assembly need only 
have two plastic parts. 
To lock the fuse unit in place in the plug body the fuse body may be 
provided with a center through bore 30 which is aligned with a hole in the 
plug body exposed at the base of the socket 10a. A self-tapping screw 32 
passes through the bore 30 and is screwed into the hole in the plug body. 
If one of the fuse elements 20a, 20c should sever because of an overload 
or short circuit, after the difficulty is corrected, the spare elements 
20b, 20c can be placed in operation merely by removal of the fuse unit, 
turning it ninety degrees, and pushing it back into the fuse socket as 
before.