Fast-acting current limiting device provides current overload protection for vulnerable circuit elements and then re-establishes conduction path within milliseconds. Fuse can also perform as fast-acting switch to clear transient circuit overloads. Fuse takes advantage of large increase in electrical resistivity that occurs when liquid metal vaporizes

Harris, L. P.

Jones, N. D.

Kinsinger, R. E.

English

NASA Technical Reports Server

MarCh 1974	 B74-10004 
NASA TECH BRIEF 
Lewis Research Center 
NASA Tech Briefs announce now 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 National Technical Information 
Service. Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington. D.C. 20546. 
Self-Healing Fuse 
The Problem: 
As electronic circuits become more sophisticated, there 
is a growing need for more sensitive circuit protection, 
especially for silicon controlled rectifiers (SCR's) used in 
certain inverter and bridge applications. Constantly in-
creasing system power levels also require improved circuit 
protective elements that will absorb the excess thermal 
energy due to high fault or short circuit currents. The 
"one shot" operation of conventional fuses make their use 
undesirable in most remote type applications such as 
space flight. 
The Solution: 
A new circuit protective device, called the self-healing 
fuse. The self-healing fuse is a very fast-acting current 
limiting device that provides current overload protection 
for vulnerable circuit elements and then re-establishes the 
conduction path within a few milliseconds. This fuse can 
also perform as a fast-acting switch to clear transient 
circuit overloads. The self-healing fuse takes advantage of 
the -large increase in electrical resistivity that occurs when 
a liquid metal vaporizes. 
How It's Done: 
The self-healing fuse has a metallic conductor of cross 
section and length appropriate to the current and voltage 
rating, much as is the case of any meltable-link fuse. The 
significant difference is that this conductor is a liquid 
metal confined in a small channel within a mechanically 
strong ceramic insulator. The liquid metal fills the fuse 
channel and the flexible, spring-restrained end reservoirs. 
The reservoirs provide the mechanical forces that return 
the liquid to the channel after the fuse fires. The liquid 
used is mercury; however, sodium, potassium, or a 
sodium-potassium eutectic (NaK) also have desirable 
properties for this application. 
When an overload occurs, the excess current rapidly 
generates a vapor bubble by heating the liquid metal in 
the fuse channel. The resultant vapor pressure expels the 
remaining liquid metal into the reservoirs as a mercury arc 
discharge is set up along the channel length. The metal 
vapor arc in the fuse channel dissipates the circuit energy 
at a low current level until the arc self-extinguishes or
until a circuit switch or relay opens the faulted circuit. If 
the fault is transient in nature, the fuse action should clear 
the fault without any further action by any other 
protective devices. Since the arc has an impedance. 
hundreds of times larger than that of the liquid, the fault 
current level is limited to a value less than rated current. 
The arc mode typically persists for a time on the order of 
one millisecond. When the arc extinguishes, the flexible 
spring-loaded reservoirs return the liquid metal into the 
conduction channel, thus healing the fuse. 
When the fuse fires, very large explosive forces are 
generated in the closely confined fuse channel. To resist 
these forces, the ceramic insulator is placed in heavy 
compression by the external metal structure of the fuse. 
In the design shown in the figure, the metal rings are 
shrunk-fit directly over the insulator ceramic. Other 
basically similar methods of obtaining pre-compression of 
the brittle insulator materials have been used, such as 
brazing or glass molding the metal members over the 
insulator. 
The flexible end reservoirs are designed to perform 
three vital functions: (1) to absorb the impact forces of 
the liquid mercury being ejected from the channels 
without permanent distortion; (2) to return liquid 
mercury to the conducting channel after the arc extin -
guishes; and (3) to allow for thermal expansion and 
contraction of the mercury. Therefore, a heavy spring and 
restraint strap to hold the spring almost fully compressed 
are added to prevent permanent distortion of the dia-
phragm and to insure the required restoring forces. 
The self-healing fuse uses commercially available 
sapphire, 998+% pure alumina, or high density beryllia of 
99.8+% purity for the fuse channels. The external rings 
are a nickel alloy with a thermal expansion coefficient 
slightly higher than the ceramic to allow the required 
precompression during assembly. The flexible end bellows 
and fill tube are stainless steel. They are arc welded to a 
lip on the end rings. The fill tube, which is used to 
evacuate and fill the fuse with mercury, is internally 
coated with silicon rubber to give a positive seal when 
flattened, folded, and crimped.
(Continued overleaf) 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	 information Contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United Stales
	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19740000004 2020-03-17T13:43:18+00:00Z
A 
End 
View 
Fuse 
channel
A 
-} 
Two 
channel 
design 
Arc-wel 
Section 
A-A
Tapered 
metal rings 
Mercury 
filled 
channel 
Tapered 
Feramic 
insulator Flexible 
diaphragm
Washer1
	 J( 
HV'J	 Restraint spring	
Pinchoff	 strap 
Design for One- or Two-Channel Devices
Heavy 
spring 
Restrain ibl 
straD
	
	
j/ 
- • 7j Q diaphragm ? A\ 
Notes: 
1. The response to overcurrent is directly proportional to 
12 t (current squared multiplied by a time interval) with 
a current-time curve almost identical to that for an 
SCR. Thus the self-healing fuse can be easily designed 
to protect an SCR, or possibly other semiconductor 
devices, over a wide range of overload conditions. 
2. Life tests were conducted demonstrating a capability 
of at least 500 operations with fuse ratings from 4 to 
40 amperes at 50 to 100 volts dc. 
3. Further information is available in the following 
report: 
NASA CR- 121244 (N73-3019 1), Self-Healing Fuse 
Development 
Copies may be obtained at cost from: 
Aerospace Research Applications Center 
Indiana University 
400 East Seventh Street 
Bloomington, Indiana 47401 
Telephone: 812-337-7833 
Reference: B74-10004 
4. Specific technical questions may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B74-10004 
Patent Status: 
NASA has decided not to apply for a patent. 
Source: N.D. Jones, R.E. Kinsinger 
and L.P. Harris
General Electric Co.
under contract to
Lewis Research Center 
(LEW-1 1964)
[I 
B74-10004	 Category 01,02 


Self-healing fuse

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