Development of in-line fuse device for protection of electric circuits from excessive currents and voltage

Casey, L. O.

English

NASA Technical Reports Server

TO e 
Supplbmentary Corporate 
Sotarc6 (if applicable) o 
SA Patent Case ?Soe 
- %E $his patent covers an invention made by a 
pf a EASA Contract 
4 Yes 
Pursuant t r i  Sectfon 385 
Space Act, the name of the A 
the first page of the 
inventor (author) L the heading of C o l  
the Specification, foPEowimng 
https://ntrs.nasa.gov/search.jsp?R=19710003051 2020-03-17T02:31:52+00:00Z
. 0. CASEY 
ELECTRICAL LOAD PROTECTION DEVICE 
Rf 17 
+ D  1 u+ TO SOURCE TO LOAD 
15 
f n - TIME (MILLISECONDS) 
I 20 
19; -- 
Lewis 0. Casey 
INVENTOR. 
BY 
atente 
3,448,341 
U.S. CI. 31'9-31 
A 
An in-line electrical load protection device is dis- 
closed which includes a hollow, hermetically sealed, elec- 
trically non-conductive body portion filled with an inert 
gas, such as neon, an electrically conductive fuse link 
and a cathode element. The fuse link is connected at o p  
posite ends to terminals that extend from the body por- 
tion for connecting the fuse link in a circuit between a 
source of electrical power and an electrical load. The fuse 
link will melt and open the circuit when the current 
through it exceeds a predetermined amount. The cathode 
element i s  spaced from the load side of the link and con- 
nected to a ground or other electrical common between 
the source and load. When the source voltage exceeds a 
predetermined value, current flows from the fuse link to 
the cathode through the gas. When this current, in com- 
bination with the load current, exceeds said predetermined 
amount, the fuse link melts. A resistive element may be 
provided in series with the cathode link conduction path 
to provide a range of constant voltage at the load side of 
the device when the source voltage is above said prede- 
termined value and said combined currents are below 
said predetermined current. 
~- 
The invention described herein was made by an em- 
ployee of the United States Government and may be 
manufactured and used by or for the Government for gov- 
ernmental purposes without the payment of any royalties 
thereon or therefor. 
Backgroltnd of the invention 
This invention relates to in-line fuse devices for pro- 
tecting an electrical load from excessive currents and/or 
voltages. 
In-line fuses are commonly used to protect electrical 
apparatus from excessive currents, however these fuses 
do not protect against excessive voltages. Protection 
against excessive voltage is of particular importance in 
fields such as aerospace and medicine, where astronauts 
and other human subjects are connected to electronic 
equipment which monitors various human functions. Elec- 
tric shocks from voltage surges and faulty equipment are 
always a possibility when connected to such equipment. 
While voltage regulator circuits can be provided in the 
power source or the electrical apparatus connected there- 
to, they generally are relatively expensive, consume power, 
and provide no protection when they fail. Furthermore, 
they are not readily adapted for connection in the power 
line between the power source and any load, such as a 
fuse, so that positive protection is provided close to the 
Ioad and irrespective of other protection devices that may 
fail. 
Summary of the invention 
It is thus an object of this invention to provide an in- 
h e  fuse device which will open the circuit when sub- 
jected to a predetermined source voItage and/or load 
current. 
Another obiect is to provide a fuse device having a fuse 
Iink that wilcmelt when the source voltage between the gated strip of electrically conductive metal, which will 
fuse link and the common or ground side of the circuit 
exceeds a given amount. 
It is another object to provide a fuse device which will 
provide a conductive path for the current having a lower 
impedance than the load to protect the load from the 
shock of an excessive current when the source voltage ex- 
ceeds a predetermined amount. 
Another object is to provide such a fuse device that i s  
of relatively simple and inexpensive construction. 
Another object is to provide such a fuse device that 
provides for a load power fusing threshold, Le., a com- 
bination of source voltage and load currents that would 
open the fuse link providing load power protection. 
Another object is to provide such a fuse device that may 
Another object is to provide such a device which pro- 
vides a constant voltage to the load when the source volt- 
age is within a predetermined range of voltages, and an 
open circuit in the power line when the source voltage 
These and other objects, advantages, and features of 
this invention will be apparent to those skilled in the art 
from a consideration of the specification, claims and ap- 
pended drawings. 
In accordance with the invention, a voltage sensitive 
fuse device is provided by using the dielectric capacity of 
a gas. At a given source voltage, current will flow through 
the gas and shunt the load. This causes an increase in the 
flow of current which melts a fuse link at a given cur- 
10 
15 be incorporated in a standard in-line fuse housing. 
20 and/or load current exceeds this range. 
25 
30 rentflow. 
Brief description of the drawing 
The invention will now be described in detail in con- 
nection with the drawings in which, 
FIGURE 1 is a perspective view of the preferred em- 
bodiment of the load protection device of this invention 
with a portion of the body cut away to expose the inner 
construction, 
FIGURE 2 is a vertical sectional view through the de- 
FIGURE 3 is a bottom plan view of another embodi- 
ment of the load protection device of this invention with 
a portion of the body broken away; 
FIGURE 4 is a sectional view taken along line 4-4 
FIGURE 5 is the equivalent electrical circuit of the em- 
FIGURE 6 is a graph illustrating the voltage regula- 
FIGURE 7 is a partial sectional view taken along the 
35 
40 vice of FIGURE l; 
45 of FIGURE 2; 
bodiment of the invention illustrated in FIGURE 3; 
tion characteristics of said embodiment; and 
line 7-7 of FIGURE 2. 
50 
Description of the preferred embodiments 
The electrical load protection device of FIGURE 1 
65 includes a gas tight cylindrical body IO of electrically non- 
conductive material. The body includes hollow, tubular 
member or shell 11 which is closed at each end by plugs 
12. Thermosetting epoxy resin is one material that can be 
6o used to form plugs 12 and hermetically seal the open ends 
of tubular member 11 to form gas tight chamber 13. Metal 
caps I4 and 15 are attached to the ends of body 10 by a 
suitable adhesive to provide electrically conductive ma- 
terial at each end of the body. 
The device may be placed in series between a source 
of electrical power A and a load B by insertion into a 
standard in-line fuse holder 16 including clips 16a and 
I6b, or other suitable connecting means. 
A fuse link 17 electrically connected to metal caps 
14 and 15 forms the electrical connection between the 
electrical source and the load. The fuse link is an elon- 
65 
3,448,341 
melt at a relatively low perature, such as one of 
the alloys of lead and tin commonly used as fuel solder. 
Body shell 11 is constructed of material that is elec- 
trically non-conductive so that any current conducted from 
the source to the lo will have to pass through fuse 
link 17. Body shell is preferably made of ceramic 
or an opaque glass, such as smoked glass, for reasons 
which will be explained below. 
The apparatus described so far is similar to an ordinary 
fuse connected in line between a power source and a 
load. Such fuses have a fuse link that will melt and open 
the circuit when the current flowing through the link 
exceeds a predetermined amount. The present invention 
includes in addition to such protection against excessive 
current, means for preventing the load from being 
exposed to excessive source voltage thereby preventing 
severe electrical shock to a person or sensitive electrical 
equipment. In the embodiment shown such means includes 
an inert gas, such as neon, which fills chamber 13 of 
the body and a cathode element 18. A circular ground 
ring 19 in chamber 14 supports one end of the cathode 
element. The other end of cathode 18 is embedded in 
and supported by one of plugs 12. Ring 19 is connected 
to the electrical common (not shown) between the volt- 
age and current source and the load, which is usually 
ground, by wire 19a. Ground ring 19 may be the same 
diameter as tubular member 11 in which case tubular 
member 11 would be in two sections and connected to 
ground ring 19 by epoxy or other adhesive. If desired 
ground ring 19 may #be indented as shown in detail in 
FIGURE 7 to provide a projection 19b against which 
ends of the two sections of tubular member 11 are abutted 
and bonded, and shoulders 19c and 19d on which these 
ends sit and are bonded. Alternatively, ground ring 19 
may be of a smaller diameter so that it may be inserted 
into tubular member 11. As can be seen in FIGURE 1, 
ground ring 19 is provided with cross bars 20 and 211 
connected to form an inverted T for supporting one end 
of cathode element 18. 
Ground ring 19 supports cathode element 18 in spaced, 
parallel relationship with the load side of link 17. When 
the electrical voltage between the load side of the link 
and the cathode exceeds a predetermined value, the 
inert gas will fire and current will conduct from link 
17 through the gas to cathode element 18. This increases 
the flow of current through link 17. By adjusting the 
spacing of cathode element 18 from link $7, the melting 
temperature of link 17 and the neon gas content in 
chamber 13, the conduction of current between link 17 
and cathode element 18 can be made to occur at any 
desired voltage and the fuse link 17 can be caused to 
melt open instantaneously at that voltage or at some 
higher voltage, as desired. Further, control of the voltage 
at which current conducts between the link and the 
cathode element can be provided by sprinkling the neon 
gas or cathode element 18 with small radioactive par- 
ticles to increase the conductivity between fuse link 17 
and cathode element 18. Use of such radioactive parti- 
cles would allow this conduction at relatively lower volt- 
ages than otherwise would be possible. 
Link 17 has enlarged end portions 22 and 23 and an 
intermediate, narrow portion 24, wherein the fusionin 
response to excess current takes place. While it is not 
necessary that current pass completely through link 17, 
but only the intermediate portion 24, it is preferred that 
one of the enlarged ends, such as end 22, be connected 
to the load through cap 15 and that cathode element 18 
be spaced from and parallel to this end. This insures 
that the current to the load and the current flowing to 
the cathode both pass through the fusible portion 24. As 
shown, cathode element 18 is of the same shape and size 
as end portion 22 so that current may conduct to the 
cathode from any point on end 22. 
An alternate embodiment of the invention is shown 
in FIGURE 3. FIGURE 5 i s  the equivalent circuit of 
such embodiment. Since the same structure shown in 
FIGURES 1, 2, and 4 is used, the same reference char- 
acters will be used to indicate like parts. This embodi- 
ment not only protects against undesired high voltages 
but also serves as a voltage regulator when the source 
voltage is within a preselected range. To accomplish this, 
a resistance element is placed in series with the cathode 
element link current conduction path. Accordingly, there 
is an equivalent resistance Rf located between fusible 
portion 22 and cathode 18. In operation, current will 
conduct through the gas to cathode element 88, allowing 
the voltage between them to reach a predetermined 
amount. This current will cause a given voltage drop 
across the equivalent resistance Rf. In this embodiment, 
15 the cathode resistor is selected so that the current is 
not sufficient to melt the fuse link. As the source voltage 
is increased, the anode voltage will remain steady. The 
voltage regulation is a feature of the inert gas used 
in the embodiment to regulate voltage and limit power. 
20 At a given higher voltage, however, the cathode current 
plus the load current will exceed the predetermined 
power necessary to melt link 17 and open the circuit. 
FIGURE 6 is a graph of time versus the voltage at  the 
source and load side of the fuse device of FIGURE 3, 
25 when the source voltage is increased. For example, a 
voltage surge may be received at point a. The increased 
voltage will fire the gas between the cathode and the fuse 
at  point b and current will flow to the cathode. The 
source voltage can then continue to increase to the fusing 
30 voltage d, yet voltage c to the load will remain constant. 
If the source voltage is increased ibeyond point d, fuse 
link 17 will melt open and load voltage drop to 0, so that 
the required protection from excess voltages (or power) 
is provided. 
If the fuse device is to be used in the presence of 
light, it is preferred that the body shell be opaque because 
light will effect the point at  which current will flow in 
the cathode if the inert gas is doped with a radioactive 
element. 
From the foregoing it will be seen that this invention 
is one well adapted to attain all of the ends and objects 
hereinabove set forth, together with other advantages 
which are obvious and which are inherent to the appa- 
ratus and structure. 
It will be understood that certain features and sub- 
45 combinations are of utility and may be employed with 
reference to other features and subcombinations. This 
is contemplated by and is within the scope of the claims. 
As many possible embodiments may be made of the 
invention without departing from the scope thereof, it 
50 is to be understood that all matter herein set forth or 
shown in the accompanying drawings is to $be interpreted 
as illustrative and not in a limiting sense. 
The invention having been described, what is claimed is: 
1. An in-line electrical load protection device compris- 
55 ing, in combination: a hollow, gas tight body containing 
an inert gas and made of substantially electrically non- 
conductive material; an electrically conductive fuse link 
in said body which will melt and stop the flow of elec- 
trical current therethrough when such current exceeds a 
60 predetermined amount; means for connecting said link in 
series in an electrical circuit between a source of electrical 
current and voltage and an electrical load so that current 
passing from the source to the load will pass through said 
link; and a cathode element supported in said body for 
65 connecting to an electrical common between the source 
and load and spaced from the link so that when the elec- 
trical voltage between the link and cathode exceeds a 
predetermined value current will flow from the link 
through the inert gas to the cathode element to increase 
70 the flow of current through the link to melt the link when 
the current reaches said predetermined amount. 
2. The device of claim 1 wherein said gas is neon. 
3. The device of claim B wherein the current flow 
through the link when the voltage between the link and 
75 cathode reaches the value required to start current to flow 
35 
40 
0 
3,448.hl 
therebetween is greater than the predetermined current 
amount required to fuse the link. 
4. The device of claim 1 in which the fuse will melt 
when the voltage between the fuse and cathode reaches 
a second and higher value than said predetermined value 
and the device is further provided with a resistive element 
in series with the cathode link current conduction path 
to provide a voltage drop across said link in response to 
conduction of current through the cathode element when 
the source voltage is above said predetermined value and 
below said higher value whereby the voltage supplied to 
the load will be at a substantially constant value when 
the voltage between the fuse and cathode between said 
predetermined value and said higher value. 
5. The device of claim 1 wherein the link is adapted 
to be connected at opposite ends through said connecting 
means to said source and load, and said cathode is located 
adjacent and spaced from the end of said link adapted 
to be connected to said load. 
6. The device of claim P wherein said body material is 
opaque. 
7. In an in-line fuse including a hollow, cylindrical 
body of electrically non-conductive material having a 
duction of current throug aid link to melt the metal 
thereof when said increased conduction causes the current 
conducted through said link to exceed said predetermined 
amount; and means for connecting said cathode element 
to an electrical common between said source and load. . The fuse of claim 7 wherein said gas is neon. 
. The device of claim 7 in which the fuse will melt 
when the voltage between the fuse and cathode reaches a 
second and higher value than said predetermined value 
and the device is further provided with a resistive element 
lo in series with the cathode link current conduction path 
to provide a voltage drop across said link in response to 
conduction of current through the cathode element when 
the source voltage is above said predetermined value and 
15 below said higher value whereby the voltage supplied to 
the load will be at a substantially constant value when 
the voltage between the fuse and cathode is between said 
predetermined value and said higher value. 
10. The device of claim 7 wherein the means for con- 
2o necting the cathode element to an electrical ground in- 
cludes a cylindrical body having a ground ring which is 
positioned at one end of the cathode element. 
References Cited sealed chamber therein, a metal cap enclosing each end 
UNITED STATES PATENTS of said body to provide electrically conductive terminals 25 at each end thereof, a fuse link of electrically conductive, 
fusible metal disposed in said body chamber and elec- 2,400,408 5/1946 HaefeEnger _ _ _ _ _ _ _ _  317-40X 
trically connecting said metal caps to provide a path for 2,504,804 4/1950 Clarke _ _ _ - _ _ _ _ _ _ _ _  317-40 X 
electrical current to flow between the caps to a load, said 2,886,744 5/1959 McNatt _ _ _ _ _ - _ _ _ _ _  337-32 X 
fuse link melting open to stop such conduction when it 3,255,330 6/1966 MacKenzie et al. _ _ _ _  337-34 X 
exceeds a predetermined amount, the improvement in 
combination therewith of means for limiting the voltage 
conducted to such a load, said means including an inert 
gas filling said chamber, a cathode element disposed in 
said chamber so that voltage between the link and cathode 35 
greater than a certain value will cause current to flow be- 
tween said link and cathode, thereby increasing the con- 
JOHN F. COUCH, Prinznry Examiner. 
J. D. TRAMMELL, ~~~i~~~~~ ~ ~ i ~ ~ ~ .  
U.S. C1. X.R. 
3L7 -16, 40; 337-311 


Electrical Load Protection Device-Patent

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server