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CROSS REFERENCES TO RELATED APPLICATIONS
The Applicants hereby make reference to their French Pat. Application Nos.
PV 76 04 005 of Feb. 13, 1976 and PV 76 37 698 of Dec. 15, 1976 and claim
priority thereunder, following the provisions of 35 U.S.C. 119.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns a miniature lightning protector, i.e. a miniaturized
lightning protection device of the type which absorbs, by a
short-circuiting electrical discharge, overloading caused by phenomena of
atmospheric or other origins (lightning, inductions, accidental contact,
etc..) and which may occur across the terminals of units, circuits or
electrical or electronic installations, normally operating on low current,
for example, subscriber telephone installations.
2. Description of Prior Art
The miniature lightning protectors, especially those intended for the
protection of subscriber telephone installations, must conform relatively
severe requirments concerning their operating characteristics and
dimensions. Those most commonly used comprise two discharge electrodes, in
a sealed box, which contains a radioactive gaseous atmosphere. The
discharge is therefore absorbed in a short time, because of the high
probability of the presence of at least one germ electron in the gap
between the electrodes, at the time when an overload wave occurs.
These current types of miniature lightning protectors are normally mounted,
when intended to protect subscriber telephone installations, in junction
boxes which contain, integral with a case or a panel, five screw
terminals, two removable fuses and two removable lightning protectors. Two
of the terminals connect the box with the two line conductors connected to
the network. Two others are connected to the two conductors of the
subscriber installation and the last one is connected to a local ground.
The two fuses are respectively connected between one of the network
terminals and one of the installation terminals. The two lightning
protectors, which are respectively connected between one of the line
terminals and the ground terminal, are intended to shunt the line current
to ground, in the case of a sudden overload, due, for instance, to a
lightning strike. This type of junction box has several disadvantages. Its
structure is heterogeneous and its arrangement is consequently
complicated, its installation and removal take a relatively long time, and
the protection which it provides is uncertain.
The structure of the said type of junction box is heterogeneous because it
is made from an assembly of parts which must undergo very different
mechanical or electrical stresses and which are consequently made of
different materials. Without even mentioning insulating materials, when a
box is installed, up to seven combinations of different types of alloy or
metal could be mentioned (for example nickel-plated copper-brass,
nickel-plated brass and cadmium-plated steel, cadmium-plated steel and
stainless steel, etc..) resulting from contacts being made by clamping or
brazing. Production is all the more complicated that the fuses and
lightning protectors are separate interchangeable components, which are
produced separately. The result of all this, among other unfavorable
consequences, is an irregular resistance to corrosion and expensive
manufacture.
The installation and removal are long, as they involve many different
operations, including the fixing of the base of the case by screws and
plugs on a building wall, the trimming and uncovering of the five
conductors, their clamping by terminal screwing, installation of the fuses
and lightning protectors and the fixing of the box cover. In addition, as
a precautionary measure and especially in rural areas, the box should
preferably be placed high on an outside wall, above the ground, so that it
is impossible for a person to touch the box accidentally. This does not
facilitate operations.
The protection provided is not sure for several reasons. The geometry of
the components is unsuited to pulse operation. For example, a fuse wire
and the plugs which are soldered onto it form a turn of a coil which,
during high overload pulses, may be subjected to violent internal
mechanical reactions, which may break the soldered joint, i.e. destroy the
fuse, while the pulse itself was too short to be dangerous.
Generally, a correctly calculated fuse is also fragile due to the fact that
the wire is very fine and made of silver-tin alloy, which is mechanically
very weak. Therefore, in order to limit the service engineers' traveling,
there is an unfortunate tendency to use oversize wire, especially in rural
areas, i.e. exactly where the lines are the most exposed to lightning
strikes. Thus, the fuses no longer fulfil their protective role.
In addition, it should be noted that the failure of a fuse is not shown on
the switchboard. The failure can only be detected when the subscriber
calls from another telephone, which may be far from his own.
SUMMARY OF THE INVENTION
The object of the present invention is a miniature lightning protector,
which:
does not require the use of a radio-active atmosphere and consequently does
not cause any safety problems during manufacture, storage or replacement,
or any ageing problems;
can also be used as a circuit-breaker fuse, in order to provide protection
against both voltage overload and current overlaod;
protects an installation with several conductors, thus serving as a
junction box;
may be quickly and easily installed and removed, thus offering an
improvement, compared with conventional junction boxes.
In its simplest form, the lightning protector of the invention comprises,
in a case enclosing an atmosphere, an insulating plate and an electrical
circuit including the following components:
a first discharge electrode connected to first connection means across the
case,
a second discharge electrode connected to second means of connection across
the case, and separated from the first discharge electrode by a discharge
gap,
a first discharge ignition electrode, penetrating into the discharge gap,
and
an electrical resistor, connecting the ignition electrode to the first
discharge electrode.
The lightning protector of the invention is further characterized by the
fact that the said components are metal blades secured flat on one side of
the plate.
In this type of embodiment, the invention device may be connected between
one input terminal of the component or circuit to be protected and a
ground terminal, to provide protection against overloads of a given
polarity.
The invention further offers the advantage of the possibility of adding a
second ignition electrode to the electrical circuit, which also penetrates
into the discharge gap, and a second electrical resistor coupling the
second ignition electrode to the second discharge electrode. Thus, the
lightning protector provides protection against overloading of the
polarity opposite to said given polarity.
In addition, the invention still further offers the possibility of adding
to the electrical circuit a circuit-breaker fuse which also comprises a
metal blade fixed flat on the same side of the insulating plate and
connected between one of the discharge electrodes and third means of
connection. The invention device is then capable, when connected in
series, using the first and second means of connection, with the component
or circuit to be protected, and if the third means of connection are
grounded, of providing protection against both voltage overloads and
current overloads.
The invention also offers the possibility of placing two lightning
protector electrical circuits on the same plate, thus providing protection
for a two conductor installation, for example a subscriber telephone.
It also offers the advantage that all blades forming the components may be
made of the same metallic material and have the same thicknesss, thus
allowing the circuits to be produced using a known process and with a very
small number of operations, for example by metallic plating of the plate
followed by electro-erosion and finishing or selective chemical machining,
or even by vacuum blasting with masking.
Other advantageous features, notably concerning the placing of lightning
protector of the invention inside a plastic case while clearing the quick
clamp connection terminals fixed to the connection blades will be
described, together with the resulting advantages, in the following
description, given as an example of the embodiment forms of the said
device, referring to the drawing in which:
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is an elarged cross-section, in the X--X plane in FIG. 2, of an
intial embodiment designed for replacing conventional types of miniature
lightning protectors in the junction boxes of subscriber telephone
installations;
FIG. 2 is a plan view, to the same scale, of the miniature lightning
protector of FIG. 1, certain elements of which have been omitted;
FIG. 3 is an enlarged detail view of FIG. 2;
FIG. 4 is a large-scale view of another embodiment form comprising a two
pole lightning protector with fuses and connections allowing it to
advantageously replace conventional types of junction box;
FIG. 5 is a longitudinal section of the same lightning protector, in the
I--I cross-section plane of FIG. 4;
FIG. 6 is a half cross-section of the same lightning protector in the
II--II plane of FIG. 5;
FIG. 7 is a larger scale section of a connection terminal of the same
lightning protector, in the III--III plane of FIG. 1; and
FIG. 8 is a larger scale and more detailed view of a part of FIG. 4.
DESCRIPTION OF PREFERRED EMBODIMENTS
Firstly, consider FIGS. 1 and 2. Elements 62 and 64, the purpose of which
will be defined later, in FIG. 1, are not shown in FIG. 2. A supporting
plate, 1, of insulating material, carries metal blades 2, 3, 4 and 5 on
one of its sides (top side in FIG. 1). Blades 2 and 3 are generally
rectangular in shape and their respective edges 21 and 31 are opposite,
and approximately parallel. Blade 4 penetrates into gap 11 between edges
21 and 31, by a thin strip, 41. The shape of this strip includes, opposite
edge 31, an edge 42 which is substantially straight. Considering FIG. 3,
it will be seen that edge 42 is so positioned, in relation to edge 31, as
to make an acute angle, a, opening towards gap 11. Edge 42 is connected,
at its end opposite gap 11, to a rounded apex, 43, with an average radius,
r, separated from edge 31 by a distance, d, which is noticeably less than
the average width, D, of gap 11.
Blade 5 penetrates into gap 11, through the opposite end of that into which
strip 41 is inserted, by a thin strip, 51, arranged and fitted in relation
to side 21 of blade 2, in the same way as strip 41 fits in relation to
side 31 of blade 3.
Blades 4 and 5 are respectively connected to blades 2 and 3, by electrical
resistance couplings 44 and 54, the role of which will be defined later
on.
Against the bottom of insulating support 1, a blade or metallic layer, 12,
is applied in electrical contact with blade 2, by stud 22, inserted into a
hole through the insulating support. Conductor 32 (wire or metallic tape)
is brazed onto blade 3.
The assembly described above is fitted into a metal cover formed by case
61, upon the base of which support 1 is fixed by brazing the metal layer
12, and by a cover, 62. A ceramic frame, 64, is inserted and brazed
between the folded edge, 63, of case 61, and the cover 62. Conductor 32 is
clipped and brazed between cover 62 and the frame, 64. Preferably, the
closed cover should contain a low pressurized atmosphere, more details of
which will be described later on.
After closing, the miniature lightning protector thus comprises two
conductor sub-assemblies, insulated from each other, in a controlled
atmosphere. These are, firstly, electro-blades 2 and 4 coupled through
resistor 44 and connected to case 61, and secondly, electrode-blades 3 and
5 coupled through resistor 54 and connected to the cover, 62.
If a negative voltage is applied to case 61, with respect to the cover 62,
and if this voltage exceeds a threshold which depends upon factors already
mentioned, then electrode 4, acting as a cold cathode, outputs electrons
through the apex 43, towards edge 31 (FIG. 3). These electrons ionize
closely located gaseous molecules and the discharge propagates by next to
next ionization between edges 42 and 31 and settles between edges 21 and
31. The purpose of resistor 44 is to limit the current in electrode 4, in
order to avoid damaging it when the discharge takes place. Thus, a
miniature lightning protector according to the invention, only comprising
electrodes 2, 3 and 4 would act as a single-polarity protection device. A
two-polarity device could be produced by connecting two similar miniature
lightning protectors, head to tail. However, the same result is obtained
much more simply by adding electrode 5.
As mentioned above, the miniature lightning protector in FIGS. 1 and 2 is
designed to be used in the blade contact systems used in subscriber
installations, but can easily be modified for plugging-in, by cutting the
cover or by brazing pins onto the cover.
The invention also allows overload protection circuits to be produced at
the same time as the components or circuits to be protected. Firstly, it
is possible to produce and apply the conductor plates by many known
processes, for example by cutting and sticking, or also be chemical
electrolytic or cathode plating, etc., with the shape of the plates being
obtained during the deposit, by masking, or after the deposit, by
selective chemical attack, electro-erosion, etc.. Secondly, the insulating
support material may be selected from a wide range (epoxy resin and glass
fibre laminates, glass, silica, ceramic, alumina, etc.).
Also, it is possible for example, to produce a circuit element, according
to the invention, on a printed circuit, at the same time as the
connections of the components to be protected. It is even possible to
produce "in situ" a circuit element, using the invention, within an
integrated circuit. These possibilities, together with the fact that the
device dimensions may be extremely small, allow telephone cable heads to
be produced, for example, in which the protection circuits are
incorporated, thus substantially reducing the number of mechanical
contacts which are known to be an almost permanent cause of operating
faults.
Although it is not prohibited, in principle, to design circuit elements
according to the invention, operating in the open air, i.e. using the
ionization of the air molecules close to the electrodes, it is generally
preferable, to obtain a reaction to relatively weak overloads in
conditions which can be sufficiently reproducible, to use a low pressure
single-atom gas as an atmosphere. In this case, a protection cover is
provided covering, where applicable at the same time, the protection
circuit element and the elements or components to be protected.
It is also of interest that:
(a) to avoid premature damage to the electrodes, they should be made from
an alloy or metal having a very low evaporation rate in the atmosphere
selected,
(b) to avoid having to reduce the radius r and the distance d (FIG. 3) to
values which are difficult to obtain, an alloy or metal should be selected
with a low electron output rate, which should be less than 5 eV.
Some test results obtained using a circuit as shown in FIGS. 1, 2 and 3 are
shown below, in which electrodes 2, 3, 4 and 5 are of an alloy, the
electron output rate of which is approximately 4.5 eV (iron - nickel -
cobalt - molybdenum alloy or nickel over a sub-layer of chromium) with a
vitrified alumina support. The atmosphere used is pure argon under an
absolute pressure of 80 torrs. Distance D is 400 micrometers, distance d
is 10 micrometers and radius r is 40 micrometers. Angle a is between
30.degree. and 60.degree.. Using such a circuit, the static ignition
voltage is 230 V and the arc voltage reaches approximately 20 V. If a
voltage wave is applied, the slope of which is 5 kV/.mu.s, then the
ignition time is less than 150 nanoseconds, i.e. the dynamic ignition
voltage is less than 750 V. For a wave with a slope of 1 kV/.mu.s, the
dynamic ignition voltage drops to the level of the static ignition
voltage. The charge flows which the circuit can support depend upon the
area of the main electrodes 2 and 3. If each electrode has a surface area
of 0.24 cm.sup.2, then the circuit will support a current of 10 kA,
without damage, for a pulse of 8 microseconds increase time and 20
microseconds wide at half-amplitude.
The invention thus allows a symmetrical miniature lightning protector to be
produced, on a ceramic sub-layer of 1 cm.sup.2 area, supporting an energy
level of approximately 350 joules, like the miniature lightning protectors
now in use.
For the electrodes, the use of a low output rate metal, such as magnesium,
would allow arc voltages of less than 10 V to be obtained in argon.
Now consider simulataneously FIGS. 4, 5 and 6, on one of the sides of a
rectangular plate 110, made of insulating material such as vitrified
ceramic or reinforced resin; there are five connection terminals 121, 122,
123, 124 and 125. Terminals 121 and 122 are each intended to take a line
conductor, and both point towards one of the shorter sides of the
rectangle, both being close to one of the longer sides of the rectangle.
Terminals 123 and 124 are each intended to take an installation conductor.
These are close to the other shorter side, with terminal 123 close to the
same longer side as terminal 121 and terminal 124 close to the same longer
side as terminal 122. Terminal 125, which is intended to take the ground
conductor, is located between terminals 123 and 124. The composition and
fixing method of these terminals will be examined, later on.
All electrical components in the box which are connected to these terminals
are deposited or secured on the same side of plate 110, in the form of
small plates or blades of conducting alloy. These components include:
two fuses, 131 and 132, the first of which couples terminal 121 to terminal
123 through connecting strips 141 and 143, and the second of which couples
terminal 122 to terminal 124, through connecting strips 142 and 144;
two rectangular discharge electrodes 151 and 152, on each side of the
center-line of plate 110, together covering an area between the
half-lengths of fuses 131 and 132, located close to strips 141 and 142,
and respectively connected to the said strips;
one rectangular discharge electrode 155, covering approximately the area
between the other half-lengths of fuses 131 and 132, connected to ground
terminal 125 by a strip 145, and the edge of which opposite the edges of
electrodes 151 and 152, delimits, with these electrodes, a discharge gap
of constant running width;
two ignition electrodes 161 and 162, connected respectively to electrodes
151 and 152, by resistor strips which run in the space between the said
electrodes, the first one terminating in a triangular strip which is
inserted between electrodes 151 and 155, and the second one terminating in
a triangular strip which is inserted between electrodes 152 and 155;
finally, two ignition electrodes 171 and 172, which are respectively
connected to electrode 155 by resistor strips which run on each side of
the said electrode between this electrode and fuses 131 and 132, the first
one of which terminates in a triangular strip which is inserted between
electrodes 151 and 155 and the second of which terminates in a triangular
strip which is inserted between electrodes 152 and 155.
Referring to FIG. 8, more details will be given later on, on the
composition and layout of the electrodes, fuses and their connections.
A frame, 111, of insulating material stuck or soldered onto plate 110 and
to the connecting strips, encloses the fuses and electrodes, while leaving
the terminal clear. A cover, 112, (not shown in FIG. 5 for reasons of
clarity) stuck or sealed to frame 111, thus delimits, with the frame and
plate 110, an enclosed space into which, when the cover is fixed, an
atmosphere of controlled composition and pressure can be introduced (dry
air, argon, mixture of these two gases, etc..) in order to give the
electrodes a determined level of static ignition voltage.
Plate 110 is inserted, for example by moulding, against the bottom, 181, of
case 180, which is of elastomer or plastomer insulating material. A cover,
182, made of the same material and which can also be moulded, fits around
frame 111 and cover 112. It is shaped and dimensioned so that, together
with case 180, it forms modules, 183, with parallel opposite sides, i.e.
recesses, thus leaving terminals 121, 122, . . . 125, clear. These may be
conventional type screw clamp terminals, being clamped by the insertion of
a screwdriver into the recesses after inserting wires through the passages
provided in the case 180. However, the invention allows a conductor fixing
method which is much faster and more economical.
For this purpose, the terminals must be produced as shown in FIG. 7.
Terminal 121, shown as an example, is a double square, of which FIG. 7
shows the base, 191, and one of the sides, 192. Base 191 is brazed to the
corresponding connecting strip (in this case 141). Each branch is hollowed
out, at 193, to an acute V shape, 194. The hollowing out is extended
towards the base 191, by a rectangular slot, 195, with rounded-off edges
and a round base dimensioned to accept the slightly tight conductor of the
wire to be connected.
Concerning the recesses leaving the terminals clear, these may be blocked
by the pistons, 184, the sides of which have notches fitting the openings
in the sides of the recesses, to allow piston entry (for example using
pliers) and preventing their removal. Then, to connect the insulated
wires, all that is required is to insert them, without their having to be
stripped, through the passages 185 in the sides of the end of the case
(see, for example, conductor C, FIG. 5) into the openings, 193, of the
terminals. Sufficient pressure exerted by the pistons, 184, will, firstly,
force the wires to locate in the openings, 193, breaking the insulator on
the sharp edges 194, and secondly, the wire conductors will locate with
auto-tightening, into the slots 195.
It should be mentioned here that a complete protection device, using the
invention, only weighs a few grams and that the wires to be connected are
normally fixed to a wall. Advantage can be taken of this to provide, at
the same time as the connections, the fixing of the device without any
extra operations. To do this, the ends of case 180 are sufficiently thick
for other recesses, 186, to be made, thus clearing part of the insulated
conductor, and receiving other pistons, 187, which can be coupled to
pistons 184 by the bridges 188, and the ends of which will grip the
insulated wires against the bottom of the recesses, 186. Each assembly
184-187-188 then forms an integral clip, 189. The clips, 189, are not
shown in FIG. 4.
Almost complete protection can be provided for the terminals and the
external frame connections, 111, against the ambient atmosphere (humidity,
salt fog, acid vapours, etc..) by inserting, before inserting the clips
189 in the waiting or pre-connection position (i.e. in the position shown
in the right-hand part of FIG. 5), a determined quantity of
water-resistant grease (for example a silicon compound) part of which will
be forced into the gaps between cables and the passages, 185, during the
final closing of the box, by pressure on the clips.
Examples of material selection for the various components are given below:
Plate 110 and frame 111 : vitrified ceramic
Electrode strips, fuses and connections : iron-nickel-cobalt alloy
Cover 112 : stainless steel
Connection terminals 121, 122 . . . 125 : cupro-glucinium (if the
conductors are copper) or stainless steel (if conductors are aluminum)
Case 180, cover 182, clips 189 : injection-moulded resin (polyamide or
polycarbonate).
The list of materials to be used in the manufacture of the invention
junction box is therefore much shorter than that required by previous
junction boxes. In addition, the said manufacture does not require any
machining operations (apart from the cleaning up of the moulded elements
and the cutting and bending of the terminals, 121, 122 . . . 125). In
particular, it will be noted that none of the component elements is
threaded. In fact, most of the manufacturing phases can be automatized. It
will also be noticed that the list of connecting and fixing operations is
reduced to setting the insulated wires to the correct length, inserting
them through passages 185 and pressing clips 189, using pliers.
Now let us consider FIG. 8, in which the components are referenced by the
same numbers as those used in FIGS. 4, 5 and 6. FIG. 8 represents the part
of the area of plate 110 located inside frame 111. Concerning the
electrode functions and characteristics, indications already given in this
description will not be repeated.
Resistors 611 and 621 which respectively couple ignition electrodes 161 and
162 to the discharge electrodes 151 and 152 comprise narrow winding strips
(for example, as shown in FIG. 8, or in zig-zag), like resistors 711 and
721, which couple ignition electrodes 171 and 172. This gives them a
sufficiently high value, without increasing their size. Remember that
their purpose is to limit the current in the ignition electrodes, in order
to avoid damaging the electrodes when a discharge begins.
Concerning strips 131 and 132, which are used as fuses, these form an
alternating succession of sections like 311, of given width, and sections
like 312, which are narrower. If these widths are suitably calculated, one
may say, taking into account the type of alloy and the thickness of the
strips, that:
the current overload pulse sufficiently high and sufficiently long to
damage the installation causes at least one of the narrow sections to
melt. On the contrary, a high value pulse, but with a duration too short
to damage the installation (for example, less than 20 nanoseconds), is
damped by the impedance distribution resulting from jumping the widths
between consecutive sections. The wider sections also play a mechanical
part. They reinforce the link between plate 110 and the fuse assembly,
thus raising the threshold of the resistance to the mechanical effects of
the pulses.
In addition, the melting of a fuse element may cause sublayer
metallization, favoring a short-circuit, notably between the overloaded
conductor and the ground electrode 155, which is very close to it, thus
causing an automatic indication of the fault to appear at the telephone
exchange.
As already pointed out, the above-described embodiments, referring to FIGS.
4, 5, 6, 7 and 8 are by no ways limitative and other variants could be
added.
Thus, for example, the bottom, 181, of case 180 may be made of metal, if
the box is required to be mechanically stronger. The overall case may also
be pressed from metal sheet and used to reinforce a polymer cover.
The resistance of the device to electrical overloading may be improved by
increasing the ground connection conductor cross-sections. For example,
the ground terminal could be a nut and bolt terminal secured to the end of
the case and connected to the cover 112, made of metal, and itself
connected to the ground electrode 155. The ground connection could also be
made by means of a metal plate positioned between plate 110 and the
bottom, 181, connected to a ground terminal outside the case, and also to
a ground electrode, 155, by a conductor passing through plate 110.
* * * * *
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