Fuswitch provides both switch and fuse functions within a single housing. A mercury capillary is used to alternately vaporize and condense the mercury within a reservoir. The housing is impervious to mercury and the fuse portion of the device operates on the principle of the self-healing mercury fuse

Cohn, E. M.

Maxwell, P. T.

English

NASA Technical Reports Server

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Figure 1. "ON Position (Fuse Conducting) S
No Hg in Capillary 
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IS-D0C-12Ls 
RM. 1313 
September 1970
	
KSC 1QS.	 Brief 70-10003 
NASA TECH BRIEF 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage commercial 
application. Tech Briefs are available on a subscription basis from the Clearinghouse for Federal Scientific and Technical 
Information, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Fuse and Switch Functions Combined Within a Single Housing 
The problem: 
To design a switch and fuse in a single housing. A 
mercury (Hg) capillary would be used to alternately 
vaporize • and condense the Hg within a reservoir. 
The solution: 
A device, known as a "fuswitch," that provides 
both switch and fuse functions within a single housing. 
This silent, fast-acting device in the "on" position 
protects the equipment to which it is connected (Figure 
1). When the device is in the position shown in Figure 
3, the current is turned off as in a conventional Hg 
switch. If an overload condition arises, the Hg in the 
capillary is heated to the point where it vaporizes and 
breaks the electrical connection (Figure 2). After the 
fuse "opens," the Hg condenses . in the storage space 
located below the capillary, but gravity prevents the 
circuit from being closed again (Figure 3), until the 
switch is moved to the "off" (reset) position. The Hg 
then reenters the capillary, aided by gravity, but circuit 
continuity is reestablished only after the device is 
moved to the "on" position. The ampere rating and 
time constants of the fuswitch are determined by the
length and diameter of the capillary. The housing 
must be impervious to mercury and the connecting 
wires must not be adversely affected by reaction with 
Hg (amalgamation). The fuse portion of the device 
operates on the principle of the self-healing Hg fuse, 
except that its cyclic action is prevented, a significant 
improvement. 
How it's done: 
The housing may be made of two plastic disks 
containing identical half channels with a groove 
and tong fit, and appropriate half channels for the 
electrical conductors. Hg is metered into one side 
of the cavity, the conductors are laid into the bottom 
disk, the top disk is pressed on, and the assembly is 
evacuated and sealed by heating the thermoplastic 
material to itself and to the coated wire. The housing, 
which may be either clear, colored transparent, or 
colored translucent, will be so designed that the user 
can see the position of the Hg. Integral plastic stops 
would limit the movement of the fuswitch, which may 
be grooved around its periphery so that it can be 
pressed into its retaining plate.. A grip such as a knob 
(continued overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately ownedrights.
https://ntrs.nasa.gov/search.jsp?R=19700000003 2020-03-17T00:01:54+00:00Z
I	 - 
or blade can be molded into the disks, for which space 
must be provided at the rear of the retaining plate. 
The plate would have corresponding stops. In addi-
tion, an optional external indication of fuse condi-
tion may be provided by an integral lamp connected 
across points A and B shown in Figure 3. 
This innovation would be useful because switching 
contacts are available, thus providing internal indica-
tion and alarm for the open circuit condition. 
Figure 3. "OFF-Reset" Position
Note:	 - - 
No further documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Headquarters 
National Aeronautics 
and Space Administration 
Washington, D.C. 20546 
Reference: B70-10003 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546. 
Source: Preston T. Maxwell and Ernst M. Cohn
NASA Headquarters 
(HQN- 10497) 
Brief 70-10003	 Category ol


Fuse and switch functions combined within a single housing

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