Protective sheath

There is disclosed a tubular protective sheath for containing a power transmission conduit, the sheath comprising: first and second flexible elements each helically arranged to form respective inner and outer tubular members. The first element is in the form of a first electrically insulating film having respective first and second layers of electrically conductive low melt material adherent to its opposite surfaces. The second element is in the form of a second electrically insulating film having a third layer of electrically conductive low melt material adherent to one surface. The first element is configured in overlapping arrangement with the respective first and second low melt layers in contact and the second element is configured in overlapping arrangement with the third low melt layer spaced from the adjacent first or second low melt layer of the first element by the second insulating film, a break in the power transmission conduit resulting in heating of the sheath and melting of one or more of the low melt layers and flow of low melt material to form an electrical connection between the respective low melt layers of the elements.

FIELD OF THE INVENTION 
This invention relates to a protective sheath including means for providing 
warning of the formation of a breach in the sheath. The present invention 
has particular application in providing a safety barrier around a power 
conduit such as an optical fibre serving as a power conduit for a high 
power laser. 
BACKGROUND OF THE INVENTION 
Power conduits may be used to transfer power from a power source to a power 
consumer. In many cases, safe transmission of the particular form of 
energy will require the conduit to be sheathed in protective material. If 
the power conduit is broken or damaged a breach in the sheathing material 
may result in damage to adjacent equipment or injury to personnel. In the 
case of an optical fibre which acts as a power conduit between a high 
power laser and one or more work stations, a fracture of the optical fibre 
could result in immediate burning of the conduit and a consequential 
safety hazard to equipment and personnel. 
It is an object of the present invention to provide an improved safety 
barrier for use in sheathing such power conduits. 
SUMMARY OF THE INVENTION 
According to one aspect of the present invention there is provided a 
tubular protective sheath for containing a power transmission conduit, the 
sheath comprising: 
first and second flexible elements each helically arranged to form one of 
an inner and outer tubular member, the first element in the form of a 
first electrically insulating film having respective first and second 
layers of electrically conductive low melt material adherent to its 
opposite surfaces, the second element in the form of a second electrically 
insulating film having a third layer of electrically conductive low melt 
material adherent to one surface, the first element configured in 
overlapping arrangement with the respective first and second low melt 
layers in contact, the second element configured in overlapping 
arrangement with the third low melt layer spaced from the adjacent first 
or second low melt layer of the first element by the second insulating 
film, a break in the power transmission conduit resulting in heating of 
the sheath and melting of one or more of the low melt layers and flow of 
low melt material to form an electrical connection between the respective 
low melt layers of the elements; and 
monitoring means connected to the low melt layers for detecting such 
electrical connection between the layers. 
In use, a break in the power transmission conduit resulting in heating of 
the sheath will also result in formation of breaks in the insulating 
films, through which the low melt material may flow. On detection of an 
electrical connection between the low melt layers by the monitoring means 
an alarm may be activated and the power supply to the power transmission 
conduit shut off. Thus, damage to the power transmission conduit will be 
detected quickly and the damage caused by release of energy from the 
damaged conduit will be minimised. 
Preferably, the connection between the monitoring means and the low melt 
layers is in the form of respective relatively high conductivity drain 
members. The drain members provide a low resistance path between the low 
melt layers and the monitoring means to facilitate detection of a 
connection between the low melt layers, and use of the members is 
particularly advantageous if the sheath is of any significant length. Most 
preferably, one drain member is in contact with the outer surface of the 
outer tubular member and the other drain member is in contact with the 
inner surface of the inner tubular member. The high conductivity drain 
members may be in the form of round or flat metal conductors, or may be in 
the form of flexible conductive strips and such strips may be helically 
wound. 
Preferably also, the third low melt layer of the second element does not 
extend to the edges of the insulating film to assist in preventing stray 
connections between the third low melt layer and the first and second low 
melt layers of the first element. 
Preferably also, the sheath further comprises an internal tubular member of 
low friction material to facilitate location of the power transmission 
conduit in the sheath. 
Preferably also, the sheath includes an outer protective jacket. 
Conveniently, each low melt material layer is 8-12 microns in thickness and 
the insulating film is about 23 microns in thickness. The low melt 
material preferably fluidises at about 100.degree. C. 
Preferably, the electrically insulating film is of polyester, such as Mylar 
(trade mark) and the low melt material is carbon filled polyester, the 
latter being applied to the insulating films by ink wheel printing. 
Typically the carbon filled polyester is 50% filled with graphitic carbon 
to provide a resistivity of the order of 0.3 ohms-cm. Other materials may 
be used to form the electrically conductive low melt material for example 
certain thermoplastic resins such as polyamides, polyethers, 
polyurethanes, polyvinylacetate and certain uncured silicones loaded with 
carbon or conductive salts such as caesium iodide. Primarily these 
materials are of low molecular weight and fluidise at temperatures in the 
range 70.degree.-130.degree. C. 
According to a further aspect of the present invention there is provided a 
tubular sheath comprising: 
first and second flexible elements each arranged to form one of an inner 
and outer tubular member, the first element of electrically conductive low 
melt material, the second element in the form of an electrically 
insulating film having a layer of electrically conductive low melt 
material adherent to one surface, the second element configured in an 
overlapping helical arrangement with the low melt layer spaced from the 
first element by the insulating film, a break in the sheath involving 
elevated temperature resulting in heating of the sheath and melting of the 
low melt material and flow of low melt material to form an electrical 
connection between the first element and the low melt material layer; and 
monitoring means connected to the first element and the low melt layer for 
detecting such electrical connection therebetween. 
In use, the elevated temperature will also result in the formation of a 
hole in the insulating film, such that the low melt material may flow 
through the film to form an electrical connection between the respective 
low melt material layers. The source of elevated temperature creating the 
break in the sheath may be a damaged power conduit contained within the 
sheath, or may be an external source of heat, such as a cutting tool. On 
detection of the break in the sheath, the monitoring means may activate an 
alarm or, if the sheath contains a power transmission conduit, cut off the 
power supply to the conduit.

DETAILED DESCRIPTION OF DRAWINGS 
Reference is first made to FIG. 1 of the drawings which shows a tubular 
protective sheath 10 in accordance with a preferred embodiment of the 
present invention, the sheath 10 providing a safety barrier around an 
optical fibre 12 (FIG. 2) which acts as a power conduit between a high 
power laser 14 and one or more workstations 16 (only one shown). The 
energy carried by the optical fibre 12 is such that a fracture of the 
fibre 12 would result in immediate burning of the conduit and sheath and a 
consequential safety hazard to equipment and people. As will be described, 
the present invention is intended to obviate or mitigate this hazard by 
providing means which permit the condition of the conduit to be monitored 
such that the supply of energy through the fibre 12 may be cut off if the 
integrity of the sheath 10 is compromised. 
Reference is now made also to FIG. 2 of the drawings, which shows a portion 
of the fibre and sheath, in section. The sheath 10 comprises: an outer 
jacket 24; a detection layer 18 having inner and outer tubular members 
18a, 18b, formed of first and second respective elements in the form of 
tapes 20, 22 (shown in more detail in FIG. 3 of the drawings) which are 
helically wound in overlapping relationship to form the inner and outer 
tubular members 18a, 18b; an inner tube 26; and conductive members in the 
form of drain wires 28, 30, which are connected to a monitoring circuit 
32. The optical fibre 12 passes through the inner tube 26 and is of 
considerably smaller diameter than the sheath. 
The fibre may be retro-fitted in the sheath and the inner tube 26 is 
therefore of a low friction material, such as polytetrafluoroethylene 
(PTFE). In this example the inner tube 26 has a diameter of 3 mm, whereas 
the optical fibre 12 has a diameter of 600 microns. 
As mentioned above, the tubular members 18a, 18b formed by the tapes 20, 22 
form a detection layer 18, each of the tapes 20, 22 being wound in 
overlapping relation. It is necessary for the tapes to overlap, as 
otherwise bends in the sheath would result in unacceptable gaps at the 
outer radius of the sheath. 
The first tape 20 is in the form of an electrically insulating film 34 
having respective layers of electrically conductive low melt material 36, 
38 adherent to its opposite surfaces. The second tape 22 is in the form of 
an electrically insulating film 40 having a layer 42 of electrically 
conductive low melt material adherent to one surface. The insulating films 
34, 40 are preferably formed of Mylar and the low melt material forming 
the layers 36, 38, 42 is preferably carbon filled polyester, with a carbon 
loading of about 50% by weight to give a resistivity of approximately 
0.3.gtoreq.0.4 ohms-cm. 
The second tape 22 overlies the first tape 20 with the low melt layer 42 
spaced from the adjacent low melt layer 38 of the first tape by the 
insulating film 40. A fracture in the optical fibre 12 would lead to a 
hole being burned through the sheath, and when a hole is burned through 
the detection layer 18, the low melt material forming the layers 36, 38, 
42 is fluidised and may thus flow to form an electrical connection between 
the respective layers 36, 38, 42 of the tapes 20, 22 and thus form a 
connection between the drain wires 28, 30, which connection is detected by 
the monitoring circuit 32 which in turn shuts off the laser 14. A 
fractured optical fibre would normally result in one or more holes being 
formed through the insulating film 34 in the first tape, and more 
importantly through the insulating film 40 on the second tape through 
which the low melt material may flow. 
As will be seen more clearly in FIG. 3, the first tape 20 is overlapped 
such that the low melt material layers 36, 38 are in contact with one 
another and thus effectively form a single conductive member. It will also 
be noted that a gap X of 0.5 to 1 mm is provided between the edges of the 
low melt material layer 42 and the adjacent edges of the insulating film 
40 to prevent stray conduction between the tapes 20, 22. Layers 36, 38 may 
be similarly spaced from the edges of film 34. 
The sheath is likely to be of some length and thus it is in many cases 
necessary to provide drain wires 28, 30 to provide a low resistance path 
to the monitoring circuit 32. It is preferred to locate the wires 28, 30 
on either side of the detection layer 18, and the inner low melt material 
layer 36 provides a conductive inner coating for the layer to form a 
connection with the wire 28. Although the wires 28, 30 are illustrated in 
the form of stranded metal, preferably copper, other configurations of 
conductor may be used such as a flat copper conductor, aluminium coated 
Mylar or a carbon filled PTFE strip. Where the conduit will be subject to 
flexure, it is necessary to provide flexible conductors which will not 
abrade or otherwise damage the detection layer 18. In such applications, 
helically wound carbon filled PFTE may be used. 
FIG. 4 of the drawings illustrates the monitoring circuit 32 in more 
detail. In this example the circuit includes a relay 46 with normally open 
contacts 48 which, in normal operation, are held closed to complete a 
laser power control circuit (not shown). The relay coil 50 is supplied 
from a voltage source 52, through a current limiter 54, set slightly 
higher than the threshold to hold the contacts 48 closed. The coil 50 is 
also connected to the tapes 20, 22 via the drain wires 28, 30. In normal 
operation there is very high resistance between the wires 28, 30 (megohms 
per meter length) and an electrical connection between the tapes 20, 22 
through flow of the low melt material 36, 38, 42 provides an electrical 
loop with a resistance of less than 1 kohm. In the event of such a break 
in the sheath the current from the source 52 is divided between the loop 
created by the fault and the relay coil 50 such that the current in the 
coil 50 is reduced below its threshold level and the contacts 48 open, 
switching off power to the circuit. 
Thus it will be seen that the sheath described above provides means which 
will detect a break in the sheath due to the localised heating which 
results from a fracture in the optical fibre 12. The sheath will also 
similarly detect a hole being burned through the sheath from the outside 
of the sheath. 
It will be clear to those of skill in the art that the above description is 
merely exemplary of the present invention and that various improvements 
and modifications may be made to the example without departing from the 
scope of the present invention: for example, the first and second tapes 
may be arranged in different configurations, and the first tape could 
equally well form the outer tubular member and the second tape form the 
inner tubular member. In other embodiments, the first tape may be solely 
of electrically conductive low melt material, and thus be in the form of a 
continuous tube.