Electrical circuit interrupter for dry metallized film capacitors

An electrical dry metallized film capacitor includes a capacitor roll section, formed from a metallized film having connecting leads attached to the top and bottom thereof. The capacitor roll section is then coated with a non-stick coating and then inserted into a closed-end container, the connecting lead attached to the bottom of the capacitor roll section being dressed along the side thereof to the open end of the container. Potting compound is then inserted into the container up to the top of the capacitor roll section. Upon electrical failure of the capacitor, the non-stick coating allows gases, formed in the capacitor roll section, to axially shift the capacitor roll section thereby breaking the connection to the bottom connecting lead which is securely held by the potting compound.

BACKGROUND OF THE INVENTION 
The invention relates to electrical circuit interrupters for capacitors, in 
particular, for dry capacitors of the metallized film type. 
In a typical capacitor, a capacitor roll section is assembled in a 
container having a cover sealed thereto. Electrical connecting leads pass 
from the capacitor roll section in the container through the cover. In 
such a capacitor, failures result from internal arcing and thermal 
runaway, which cause gases to be formed in the capacitor roll section. 
These gases escape from the capacitor roll section and cause a pressure 
buildup in the container causing, in turn, the container, or the cover, to 
bulge to such an extent that the seal, or the container itself, fails. If 
such a capacitor is filled with a dielectric fluid, the fluid can then 
leak from the container causing damage to surrounding components and, if 
the fluid is flammable, a potential fire hazard. 
In U.S. Pat. Nos. 3,377,510 to Rayno and 4,106,068 to Flannagan, there is 
disclosed a pressure-sensitive circuit interrupter combined with a 
fluid-filled electrical capacitor. The pressure buildup within the 
capacitor roll section is transmitted through the fluid to a deformable 
cover carrying connecting terminals. The deformation of this cover due to 
the transmitted pressure moves at least one of the terminals to such an 
extent that the electrical connection between this terminal and the 
capacitor roll section is broken. This removes the electrical potential on 
the capacitor roll section, which then ceases to produce gases therein 
thereby terminating the pressure buildup. The interrupter is dimensioned 
such that the electrical connection is broken by the deformation before 
the seal between the cover and the container, or the container itself, 
fails. 
It has been suggested that this same type interrupter be used with dry 
metallized film capacitors. This has proven to be impractical since the 
air captured within the sealed container, unlike fluids, is compressible 
and does not as readily transmit the pressure buildup in the capacitor 
roll section. This leads to an inordinately long delay between the failure 
of the capacitor roll section and the breaking of the electrical 
connection, and the resulting generation of an increasing pressure 
differential between the interior of the capacitor roll section and the 
surrounding air in the container. This pressure differential may then be 
sufficient to rupture the container even though the electrical connection 
to the capacitor roll section is broken. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide an electrical circuit interrupter 
for a dry metallized film capacitor which rapidly responds to failure of 
the capacitor. This object is achieved in a method for manufacturing an 
electrical dry metallized film capacitor with an electrical circuit 
interrupter, the method including the steps: 
forming a capacitor roll section from the metallized film; 
attaching connecting leads to a top and a bottom end of the capacitor roll 
section, the capacitance of the capacitor being established between the 
connecting leads; 
applying a non-stick coating to the capacitor roll section; 
inserting the capacitor roll section into a container closed at one end 
thereof such that the bottom end of the capacitor roll section is adjacent 
the closed end of the container, the connecting lead attached to the 
bottom end of the capacitor roll section extending along a side thereof to 
an open end of the container; and 
filling any space between the capacitor roll section and the container with 
a potting compound up to the top end of the capacitor roll section, 
whereby, upon failure of the capacitor due to an electrical potential 
applied to the connecting leads, the non-stick coating will enable gases 
formed in the capacitor roll section, due to the failure, to axially shift 
the capacitor roll section away from the closed end of the container 
thereby breaking the electrical connection between the bottom end of the 
capacitor roll section and the connecting lead attached thereto.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, a capacitor roll section 1 is formed in a customary manner from 
a metallized film, for example polypropylene. A first connecting lead 3 is 
attached to the capacitor roll section 1 at one end thereof, electrically 
connecting this lead 3 to one conducting layer on the film. A second 
connecting lead 5 is attached to the opposite end of the capacitor roll 
section 1, electrically connecting this lead 5 to the other conducting 
layer on the film. 
A non-stick coating 7 is then applied to the outside of the capacitor roll 
section 1. This non-stick coating 7 is permeable to gases and may be, for 
example, mold release, "peel strip", etc. The coated capacitor roll 
section 1 is then inserted into a container 9, the connecting lead 5 
extending along a side of the capacitor roll section 1 to the open end of 
the container 9. The open space 11 between the capacitor roll section 1 
and the container 9 is then filled with a potting compound 13, for example 
epoxy resin, up to the end of the capacitor roll section 1 adjacent to the 
open end of the container 9. A plug 15 is shown at the bottom end of the 
capacitor roll section 1 in a central hole 17 therein to prevent the 
potting compound 13 from travelling up the central hole 17 from the bottom 
of the container 9, which potting compound 13 would otherwise prevent the 
capacitor roll section 1 from moving, and also to prevent gases formed at 
the bottom of the capacitor roll section 1 from escaping through the 
central hole 17. This central hole 17 may then also be filled from the top 
end thereof with the potting compound 13. 
Finally, a top cover 19 is mounted in the container 9 and is located by a 
shoulder 21 formed around the inside periphery of the container 9 near the 
open end thereof. The cover 19 includes terminals 23 mounted thereon to 
which the first and second connecting leads 3 and 5 are electrically 
attached, respectively. The terminals 23 provide means by which the 
capacitor is connected in a circuit. 
In operation, an electrical potential is applied across the terminals 23. 
Upon failure of the capacitor, gases are generated in the capacitor roll 
section 1 and escape therefrom through the ends thereof. The gases 
escaping through the top end of the capacitor roll section 1 have no 
effect on the operation of the capacitor due to the relatively large 
volume of compressible air at the top of the capacitor roll section 1. 
However, the gases escaping from the bottom end of the capacitor roll 
section 1 rapidly build up in pressure due to the potting compound 13 
surrounding this end. Since the capacitor roll section 1 was first coated 
with the non-stick coating 7 before being potted, the capacitor roll 
section 1 shifts axially in an upward direction. This shifting breaks the 
connection between the capacitor roll section 1 and the connecting lead 5, 
since the connecting lead 5 is securely held in the potting compound 13, 
thereby interrupting the electrical circuit (see FIG. 2). 
Numerous alterations of the structure herein disclosed will suggest 
themselves to those skilled in the art. However, it is to be understood 
that the present embodiment is for purposes of illustration only and not 
to be construed as a limitation of the invention. All such modifications 
which do not depart from the spirit of the invention are intended to be 
included within the scope of the appended claims.