Method and apparatus for monitoring anti-lightning protection equipment

The invention relates to a method of monitoring anti-lightning protection equipment that includes a grounding line (1), the method being characterized in that a "lightning" electrical current travelling along the grounding line is measured. The measurement may be obtained, in particular, by means of a magnetic circuit (2) constituting a closed loop around the grounding line, and electrical circuit that includes a winding (5) disposed around the magnetic circuit so that a lightning current travelling along the grounding line induces current in said winding, the magnetic circuit including a core (3) of solid magnetic material that has at least one gap in the magnetic circuit, said gap being occupied by material that is not magnetic.

The present invention relates to methods and apparatuses for monitoring 
anti-lightning protection equipment, in particular lightning rods and 
dischargers or lightning arresters. 
Lightning rods comprise a metal rod or spike secured vertically on the roof 
top of a structure to be protected and connected to the ground via a 
grounding line. Because it is at ground potential, the metal spike 
captures lightning, and the grounding line conveys lightning to the ground 
where it is dissipated. 
Since the amount of lightning energy captured by a lightning rod can be 
considerable, this energy may possibly damage the grounding line, or more 
commonly it may dry out the ground at the interface between the ground and 
the grounding line. When this happens, the resistance between the ground 
and the spike of the lightning rod becomes greater than normal. 
Consequently, the grounding line that runs up the height of the structure 
to be protected can then be raised to electrical potentials that are very 
high when lightning strikes the lightning rod. These potentials can in 
turn propagate through the structure of the building and may cause fire to 
break out, or may at least severely disturb the operation of electrical 
equipment or electronic equipment contained in the structure to be 
protected. 
To avoid such accidents, the quality of the electrical connection between 
ground and the lightning rod is therefore checked from time to time. 
Such checking may be performed either at regular intervals, however that 
runs the rise of checking being pointless if lightning has not struck the 
lightning rod since the previous check, or of being too late if lightning 
happened to damage the lightning rod shortly after said previous check. 
That is why lightning rods are sometimes monitored by counting the number 
of times they have been struck by lightning since the preceding check. 
When the number reaches a predetermined value, then the quality of the 
electrical connection between ground and the lightning rod is checked. 
Usually, such counting is performed by means of a device that comprises a 
magnetic circuit that forms a closed loop around the grounding line of the 
lightning rod, and an electrical circuit including at least one turn wound 
around the magnetic circuit so that lightning current passing along the 
grounding line induces current in said turn, the turn being connected to a 
relay which causes a mechanical counter to advance by one step on each 
occasion that it closes. 
Nevertheless, that method of monitoring a lightning rod suffers from the 
drawback of not taking into account the magnitude of each lightning strike 
received by the lightning rod. 
A predetermined number of low energy lightning strikes will not produce any 
damage to the grounding line, and will cause relatively little drying out 
of the ground at the interface between the ground and the grounding line, 
whereas a single lightning strike of exceptional energy runs the risk on 
its own either of destroying or damaging the grounding line or else of 
severely drying out the ground at the interface between the ground and the 
grounding line. 
The same drawbacks apply to methods of monitoring other anti-lightning 
protection equipment, in particular dischargers or lightning arrestors, if 
they operate merely by counting lightning strikes. With dischargers or 
lightning arrestors, a single high-energy lightning strike runs the risk 
of damaging not only the connection to ground, but also, and above all, 
the discharger or arrestor itself. 
A particular object of the present invention is therefore to propose a 
method of monitoring equipment for providing protection against lightning, 
which method avoids the above-mentioned drawbacks and makes it possible to 
trigger checking of said equipment as soon as it is liable to have been 
damaged or to have suffered any deterioration in its electrical 
characteristics. 
To this end, the invention provides a method of monitoring anti-lightning 
protection equipment that includes a grounding line, the method being 
essentially characterized in that a "lightning" electrical current is 
measured travelling along the grounding line on each occasion that it 
dissipates a lightning strike. 
Preferably, at least one value of the lightning current is recorded 
together with the data of said lightning current on each occasion that the 
grounding line dissipates a lightning strike. 
The invention also provides an apparatus for monitoring anti-lightning 
protection equipment that includes a grounding line, the apparatus being 
characterized in that it includes measurement means for measuring a 
"lightning" electrical current travelling along the grounding line on each 
occasion that it dissipates a lightning strike. 
In perferred embodiments of the device of the invention, use is also made 
of one or more of the following dispositions: 
recording means are provided for recording at least one lightning current 
value and a date of said lightning current on each occasion that the 
grounding line dissipates a lightning strike; 
the record means are designed to record a maximum value of the lightning 
current on each occasion that the grounding line dissipates a lightning 
strike; 
the recording means are designed to record a succession of values of the 
lightning current as a function of time, on each occasion that the 
grounding line dissipates a lightning strike; 
the measurement means include: 
a magnetic circuit which forms a closed loop and which is designed to be 
disposed around the grounding line so that the lightning current 
travelling along the grounding line generates a magnetic field in said 
magnetic circuit, said magnetic circuit including a core made of a solid 
magnetic material that extends along the magnetic circuit and that 
includes at least one complete gap, said gap being occupied by a 
non-magnetic material; and 
an electrical circuit including a winding disposed around the magnetic 
circuit so that the magnetic field generated in said magnetic circuit 
passes axially along said winding, thereby generating an induced current 
in said winding representative of the lightning current, the gap in the 
magnetic core serving firstly to enable the current induced in the winding 
to be a substantially linear function of the lightning current in spite of 
the very wide range of possible values for the lightning current, and 
secondly enabling said induced current to be substantially insensitive to 
the frequency spectrum of the lightning current; 
the, or each, gap in the magnetic core has a width, and the magnetic core 
has a mean section such that the total width of the gap or gaps in the 
magnetic core lies in the range half to twice the square root of the 
section of the magnetic core; 
the gap in the magnetic core is constituted by an air gap; 
the gap in the magnetic core is constituted by a lenght of non-magnetic 
solid material which is fitted to the magnetic core; 
said length of non-magnetic material is removable; 
a capacitor is connected in series with the winding to block high frequency 
components of the current induced in said winding; 
the measurement means deliver a signal representative of the lightning 
current, said measurement means being connected to a microprocessor to 
transmit said signal thereto, the microprocessor being connected to a 
clock, a memory, and an interface, and said microprocessor being 
programmed to store in the memory the lightning current measurements that 
are supplied thereto by the measurement means together with the 
corresponding date (e.g.: day, month, year, hours, minutes, seconds) of 
the lightning current, the microprocessor also being programmed to play 
back the contents of the memory to the interface circuit; 
the interface circuit is connected to telephone interface via means for 
remote transmission of information without making electrical contact; and 
the microprocessor is programmed to play back the contents of the memory to 
the interface only after it has previously received a determined code 
signal. 
The invention also provides anti-lightning protection equipment provided 
with monitoring apparatus as defined above. 
Other characteristics and advantages of the invention appear from the 
following detailed description of an embodiment thereof, given by way of 
non-limiting example and with reference to the accompanying drawing.

FIG. 1 shows an outside wall B of a building that has a conductive 
grounding line 1 running vertically therealong, connected at one end to a 
lightning rod spike (not shown), and at its other end to ground. In 
general, the grounding line is in the form of a copper tape. 
At least a portion of the grounding line 1 is spaced apart from the wall B 
by supports (not shown), sa as to enable a magnetic circuit 2 to be 
disposed in a loop around the grounding line 1, but without coming into 
electrical contact with said line. 
In the example shown in FIG. 1, the magnetic circuit 2 comprises a core 3 
of magnetic material, e.g. soft iron or preferably ferrite, which core 
extends around the grounding line 1 but does not provide a fully closed 
loop since the core 3 has two axial ends 3.sub.1 and 3.sub.2 which are 
spaced apart from each other by a gap 4 that is filled with air and that 
serves to complete the magnetic circuit. Thus, the magnetic circuit 2 
comprises both the core 3 and also the gap 4 which is occupied by a 
non-magnetic material, in this case air. 
In particular example shown, the core 3 is in the form of a substantially 
rectangular frame of section S that is also rectangular (see FIG. 2) and 
that may be two centimeters by two centimeters, for example. 
One of the sides of the frame constituted by the core 3 is adjacent to the 
wall B and is fixed thereto by tabs 3.sub.3 having screws 3.sub.4 passing 
therethrough and penetrating into the wall B. Nevertheless, the core 3 
could be of any other shape that extends around the grounding line 1, so 
long as it has at least one gap; and it could be fixed to the wall B by 
any other means. 
In a variant, the gap 4 could be occupied by a length 6 of a solid material 
that is not magnetic, e.g. copper or aluminum. 
Because of the gap 4, the magnetic field induced in the magnetic circuit 2 
by lightning current travelling along the grounding line 1 is a 
substantially linear function of the lightning current, and secondly the 
overall permeability of the magnetic circuit is substantially independent 
of the frequency spectrum of the lightning current, unlike a magnetic core 
which is in the form of a fully closed loop. 
Preferably, to achieve optimum efficiency of the magnetic circuit 2, the 
width e of the gap 4 lies in the range 1/2.sqroot.S to 2.sqroot.S. 
In addition, when the gap 4 is left empty, it provides the advantage of 
making it easier to install the magnetic circuit 2 on a grounding line 1 
that is already in place, since the grounding line can be passed through 
the gap into the frame that is formed by the magnetic circuit. 
Similarily, if the gap 4 is occupied by a solid length 6 of non-magnetic 
material, it is advantageous for said length to be removable. 
The magnetic core 3 may optionally be interrupted at a plurality of 
locations. As shown in FIG. 1, it may be interrupted, for example, not 
only by the above-mentioned gap 4, but also by an additional gap 4' that 
is occupied by a non-magnetic solid material 6' which is fixed to the core 
3 on either side of the gap 4'. 
In this case, it is preferable for the sum of the widths e+e' of the gaps 4 
and 4' still to lie in the range 1/2.sqroot.S to 2.sqroot.S. 
An electric circuit that includes a winding 5 having at least one turn and 
maybe as many several tens of turns is wound round the magnetic core 3, or 
possibly around one of the gaps 4 or 4', particularly if the gap is 
occupied by a solid non-magnetic material 6 or 6'. In this manner, a 
magnetic field induced in the core by the passage of lightning current 
along the grounding line 1 passes axially through the winding 5 and 
induces current in said winding that is representative of the lightning 
current, and that is a substantially linear function of said lightning 
current. This makes it possible to measure the lightning current to an 
accuracy of within about 10%. 
The circuit including the winding 5 is connected to two input terminals 
8.sub.2 and 8.sub.3 of an analog-to-digital converter 8, a resistor 
8.sub.1 being connected in parallel between the two inputs 8.sub.2 and 
8.sub.3 to deliver a voltage signal across said inputs that is 
proportional to the current conveyed by the winding 5. The resistor 
8.sub.1 may optionally be included in the analog-to-digital converter 8. 
Advantageously, a capacitor 7 is connected in series with the winding 5 so 
as to block the highest frequency components of the current induced in the 
winding 5. 
The analog-to-digital converter 8 is powered by a power supply circuit 9 
which be of any known type, and which may possibly use energy from the 
electromagnetic wave is generated during the formation of a lightning 
flash, as taught by document FR-A-2 624 319, for example. 
The analog-to-digital converter 8 also includes an output terminal 8, which 
is connected to a microprocessor 10 in order to deliver thereto a signal 
that is representative of the lightning current. 
The microprocessor 10 is also powered by the power supply circuit 9, and it 
is connected to a clock 11, to a memory 12, and to an interface circuit 
13, with the three circuits 11, 12, 13 likewise being powered by the power 
supply 9. 
The microprocessor 10 may be programmed to store in the memory 12, on each 
occasion that lightning strikes the lightning rod, the maximum value of 
the lightning current together with the date on which said current 
occurred (e.g.: day, month, year, hours, minutes, seconds), said date 
being provided by the clock 11. 
The microprocessor 10 may optionally also be programmed to store in the 
memory 12, on each occasion that lightning strikes the lightning rod, a 
succession of lightning current values as a function of time, i.e. the 
curve of the lightning current as a function of time, together with the 
date of the lightning strike in question. 
The interface circuit 13 may optionally include a keypad 13.sub.1 and 
screen 13.sub.2 for interrogating the memory 12 via the microprocessor 10 
to make it possible to check the intensity of lightning currents that have 
travelled along the grounding line 1 on each occasion that lightning has 
struck the lightning rod in a given period. 
The interface circuit 13 need not necessarily include a keypad and a 
screen, but may merely have a connector 13.sub.3 to which a portable unit 
14 can be connected, which unit has a keypad and a screen, and serves to 
check the lightning currents stored in the memory 12. The interface 
circuit 13 may alternatively include not only a keypad and a screen, but 
also a connector 13.sub.3. 
It would also be possible to connect a printer to the interface circuit 13 
so as to print out the measured values of lightning currents together with 
their dates, or indeed to print out curves of lightning current, together 
with the corresponding dates. 
As shown in FIG. 1, the interface circuit 13 may also be connected to a 
telephone transmitter 16 via optical couplers or possibly acoustic 
couplers 15 (to provide electrical isolation between the circuits 13 and 
16), thereby enabling the contents of the memory 12 to be remotely 
interrogated by means of the microprocessor 10. 
The microprocessor is preferably programmed to give the contents of the 
memory 12 to the interface 13 only if it has previously received a 
specified encoded signal so as to preserve the confidentiality of the data 
contained in the memory 12. 
As shown in FIGS. 3 and 4, apparatus similar to that shown in FIGS. 1 and 2 
can be used to monitor one or more dischargers or lightning arrestors 20 
protecting electricity lines 21 of low, medium, or high tension, by 
measuring the lightning current carried by the grounding lines 22 of said 
dischargers or lightning arrestors by means of respective magnetic 
circuits 2 similar to that described above. 
When a plurality of dischargers or lightning arrestors 20 are to be 
monitored, it is possible to provide a single grounding line 22 and 
monitoring apparatus for each of them (FIG. 3) or else to provide a ground 
line 22 that is common to a plurality of dischargers or lightning 
arrestors, together with single monitoring apparatus that is likewise 
common and that is fitted to the common grounding line (FIG. 4).