Field of the Invention
[0001] This invention relates to a overhead cable protection system for use in worksites
where vehicles are required to pass under cables.
Background to the Invention
[0002] Conventional protection for overhead cables at a worksite where vehicles, for example
excavators having elevatable arms, are required to pass underneath the cables, is
afforded by hanging some form of alerting line or chain between a pair of posts at
either side of the track by which vehicles pass under the cables. The line or chain
is set at a height from the ground less than that of the cables to provide a safety
margin. This is particularly important where the cables carry high voltages, to avoid
the risk of flashover from the cable to the vehicle. Flags or bells may assist in
alerting the driver of a vehicle to check that the highest point of the vehicle passes
freely beneath the line or chain, and will thus not risk fouling the cables or a dangerous
passage of current through the vehicle.
[0003] Even with these aids, there is a risk that the driver of a vehicle of excess height
might still not notice that he is at risk of striking the cables, with the result
that cables could become damaged and an electrical discharge could cause injury or
death. Contractors working on a site with overhead cables therefore need to be able
to eliminate even the small risk that the conventional warning systems might prove
insufficient.
[0004] Systems have been proposed for detecting the passage of vehicles which are in excess
of a safe height for, say, a bridge or tunnel. For example,
GB2137005 discloses a system in which a light beam is sent across the road at the height limit
to a detector on the other side. A vehicle breaking the beam triggers audible and
visible warnings. Because this is to be applied to a permanent construction, the light
source and detector can be mounted and precisely aligned to ensure continued accurate
detection. A problem with worksites is that they are temporary, and so the mounting
of stable structures to carry a light source and a detector, and the accurate alignment
of these, would be unreasonably costly. The mounting of such a system on the temporary
structures normally used (for example wooden poles set directly in the earth), and
by untrained personnel, could well result in unreliability of detection, rendering
the system unfit for purpose.
Summary of the Invention
[0005] The present invention provides an overhead cable protection system comprising a detector
and signalling unit adapted to be temporarily mounted on a first pole and comprising
a detector switch having a detachable member, separation of which from the switch
causes the switch to operate, control means being provided responsive to operation
of the switch to trigger operation of audible and/or visible warning signals, the
detachable member having a cord secured thereto, a free end of the cord being securable
to a second post spaced from the first so that the cord stretches between the first
and second posts at a desired safety height limit.
[0006] Preferably, the control means comprises a radio transmitter arranged to transmit
a signal, in response to operation of the switch, to a remote receiver, the remote
receiver, the remote receiver comprising audible and/or visible warning signal means
to provide a warning in response to receipt of the signal. There may be more than
one remote receiver, for example one such receiver being mounted on the pole at a
height readily visible by the driver of the vehicle, while another receiver may be
suitable for mounting in the cab of a vehicle to be operated on the worksite, so that
the driver of the vehicle is given immediate warning of his vehicle being at risk
of fouling the cables before any such fouling can occur.
[0007] The control means can also be configured to transmit a predetermined SMS message
by means of the cellular telephone network to a designated mobile telephone.
[0008] The detector switch is suitably a reed switch, the detachable member being magnetic
so as to change the state of the reed switch when in proximity with it, or during
the action of detaching the detachable member from the switch.
[0009] Alternatively, the detector switch may be a mechanical switch in which the contacts
are held apart by a detachable member in the form of a pin inserted into the switch,
and are allowed to close when the pin is pulled out of the switch. Alternatively,
the contacts may be held closed by the pin, being allowed to separate when the pin
is removed. Yet another alternative is the use of an electrically-conductive pin to
complete the circuit when inserted in the switch and breaking the circuit when it
is pulled out by the vehicle snagging the cord. Other forms of switch may be possible,
for example relying on a change of inductance in a switching circuit when the detachable
member is removed from the switch. Another possibility would be to arrange for the
detachable member to operate as a shutter between a light source and a light detector
within the switch. When the detachable member is removed as a result of snagging of
the cord, the light path is restored, and this constitutes operation of the switch.
[0010] The detector and signalling unit may be attachable to the pole in the desired position
by simply providing the casing of the unit with an external loop through which a fixing
strap may be passed. The fixing strap may, for example, be a plastics cable tie of
the type including a ratchet fixing to hold the strap firmly in place and prevent
unauthorised or accidental removal.
[0011] By providing a visible and/or audible warning as soon as a vehicle snags the cord
stretched across poles at the maximum safe height for vehicles passing under cables,
the system of the invention provides a simple and reliable addition to safety provisions,
substantially reducing the risk of damage to cables or dangerous electrical discharge.
Brief Description of the Drawings
[0012] In the drawings, which illustrate an exemplary embodiment of the system:
Figure 1 is a diagrammatic view of the system installed on a pair of poles marking
the access to the path under cables on a worksite;
Figure 2 is a diagrammatic view of the detector and signalling unit;
Figure 3 is a diagrammatic view of the pole-mounted receiver; and
Figure 4 is a diagrammatic view of a remote vehicle cab-mounted receiver.
Detailed Description of the Illustrated Embodiment
[0013] Referring first to Figure 1, the system is installed on the pair of temporary poles
1, 2 conventionally set up at either side of the access path beneath overhead cables
on a worksite, for example beneath overhead electricity power lines forming part of
the national grid, or overhead line equipment on an electrified railway. Such poles
are usually provided with a cord 3 stretched between them at a height equal to the
maximum working height beneath the cables, the cord typically carrying warning bunting
4. Engagement of the vehicle with the cord 3 breaks the cord and is intended to alert
the vehicle's driver by the falling bunting. In accordance with the invention, the
detector and signalling unit 5 is attached to the first pole 1, at a position just
beneath the bunting cord 3, using a plastics cable tie 6 passed through a mounting
ring 7 on the casing of the unit 5. A cord 8 is tied to a ring 9 extending from a
pin 10, described hereinafter in more detail with reference to Figure 2, and is stretched
across the gap between the poles to be tied to the second pole 2 so that the cord
8 is substantially horizontal. The tension in the cord 8 is sufficient to hold the
cord straight, but not such as to place significant tension on the pin 10. A receiver
unit 11 is attached to the first pole 1 below the unit 5 and at a position such that
it is in the eyeline of the vehicle driver in use. The receiver unit 11 has a pair
of mounting arms 12 extending from it, by which it may be attached to the pole by,
for example, plastics cable ties passed round the pole and the respective arm. The
receiver unit, described hereinafter with reference to Figure 3, has a light 13 mounted
thereon which is caused to flash on and off to warn the driver of a vehicle that the
cord 8 has been snagged and therefore that his vehicle is too tall to pass under the
overhead cables, for example because an excavator arm has not been lowered sufficiently.
Snagging of the cord 8 causes the pin 10 to be pulled from the unit 5, activating
the alarm transmitter.
[0014] Referring now to Figure 2, the detector and signalling unit is shown with one side
of the casing 14 removed for the sake of clarity. The casing contains a battery 15,
a controller 16, and a radio transmitter 17. It may also contain a cellular telephone
transmitter device 18. A plastics block 19 is mounted on the casing 14, and has a
tubular bore 20 therethrough provided with a short section 21 of slightly larger diameter
bore adjacent to one end thereof. A transverse bore 22 extends through the block 19
and intersects with the wider section 21, and the mounting ring 7 is passed through
the transverse bore 22 and the wider section 21. The mounting ring 7 is suitably of
the split type typically used as a key ring. The pin 10 has adjacent to the end thereof
remote from the ring 9 a spring-loaded ball 23 which projects outwardly of the cylindrical
wall of the pin and which engages the shoulder at the intersection between the bore
20 and the wider section 21 to resist removal of the pin from the bore under normal
conditions, but allows the pin to be released if tension on the cord 8 increases above
a predetermined value as a result of snagging by a vehicle, for example.
[0015] The upper part of the casing 14 contains a resin-encapsulated reed switch 24 connected
to the controller 16 and operable by the withdrawal of the pin from the bore, the
pin being magnetised and so causing a change of state of the reed switch 24 as it
is withdrawn. This in turn changes the state of a detector circuit in the controller
16 to which the reed switch is electrically connected. This causes the controller
to go to an alarm state, causing transmission of an alerting radio signal by the radio
transmitter 17. It can also trigger the transmission of an alerting text or voice
message by the cellular telephone transmitter to a predetermined remote telephone,
for example to alert the site manager of an incident. The alerting radio signal is
received by the receiver unit 11, which comprises, referring to Figure 3, a radio
receiver and controller 25, powered by a battery 26 within the unit, and triggering,
in response to receipt of the signal, operation of the flashing warning beacon 13
and an audible alarm device 28, for example an electronic sounder.
[0016] In addition, a secondary receiver unit 29, illustrated in Figure 4, can be mounted
in the vehicle's cab to provide the driver with a more immediate warning. The secondary
unit 29 similarly has a radio receiver 30 activating a warning lamp 32 on the exterior
of the unit when an alerting signal is received. The secondary unit may be powered
from the vehicle's electrical supply through a suitable electrical lead 31 and plug
(not shown). The unit may also include an audible sounder 33 to reinforce the alerting
signal and ensure that the driver stops his vehicle well clear of the overhead cables.
This unit is mountable within the vehicle and could be provided with temporary attachment
means such as a sucker pad for attachment to the vehicle's window or other smooth
surface.
1. An overhead cable protection system comprising a detector and signalling unit adapted
to be temporarily mounted on a first pole and comprising a detector switch having
a detachable member, separation of which from the switch causes the switch to operate,
control means being provided responsive to operation of the switch to trigger operation
of audible and/or visible warning signals, the detachable member having a cord secured
thereto, a free end of the cord being securable to a second post spaced from the first
so that the cord stretches between the first and second posts at a desired safety
height limit.
2. An overhead cable protection system according to Claim 1, wherein the control means
comprises a radio transmitter arranged to transmit a signal, in response to operation
of the switch, to a remote receiver, the remote receiver comprising audible and/or
visible warning signal means to provide a warning in response to receipt of the signal.
3. An overhead cable protection system according to Claim 1 or 2, wherein additional
remote receivers are provided to provide additional warnings in other locations.
4. An overhead cable protection system according to Claim 1, 2 or 3, wherein the control
means also comprises a cellular telephone transmitter arranged to send an audible
message or SMS text message to a predetermined cellular telephone number in response
to operation of the switch.
5. An overhead cable protection system according to any preceding claim, wherein the
detector switch is a mechanical switch in which the contacts are held apart when a
detachable member in the form of a pin is inserted into the switch and allowed to
close when the pin is pulled out of the switch.
6. An overhead cable protection system according to any of Claims 1 to 4, wherein the
detector switch is a mechanical switch in which the contacts are held closed when
a detachable member in the form of a pin is inserted into the switch and allowed to
separate when the pin is pulled out of the switch.
7. An overhead cable protection system according to any of Claims 1 to 4, wherein the
detector switch is a mechanical switch in which the contacts are electrically connected
by a detachable member in the form of an electrically-conductive pin inserted into
the switch, the connection being broken when the pin is pulled out of the switch.
8. An overhead cable protection system according to any of Claims 1 to 4, wherein the
detector switch is a reed switch and the detachable member comprises a magnet, whereby
separation of the detachable member from the unit influences the reed switch to change
the state thereof.
9. An overhead cable protection system according to any preceding claim, wherein the
detachable member is held in the switch by a latching arrangement which releases the
member when the force exerted on the member by the cord exceeds a predetermined value.
10. An overhead cable protection system according to any preceding claim, wherein the
detector and signalling unit is provided with an external loop through which a fixing
strap may be passed to secure the unit to the pole.