Page 19 - Issue 2 2023

TECHNICAL

to determine which marked cables were
sold by criminals and which were sold
legitimately if scrap dealers are not
tracking inventory closely.

Mechanical protection devices
Mechanical protection systems consist
of collars attached to the cable, either
freestanding or anchored to posts in the
cable trench at random intervals, which
prevent the cable from being pulled out
of the trench by winching, either directly (a) cable collars (Aberdare) (b) Complete system (CableDNA)
or in reverse or lateral. A more advanced Figure 2: Mechanical cable protection systems
system provides additional protection
at cable entry and exit points as well as reflects the signal. In the case of AC loads, one must fit a DC-reflective device at the far end
anchoring along the cable run. of the cable, which will allow the signal to be properly reflected back to the control unit.

Cable alarms Optic fibre alarm
There are several types of cable theft In this system a plastic optical fibre cable is laid along the copper wires and clamped to them
alarm available. as well as being fixed to manhole lids or tray covers. An analyser senses when the plastic
fibre cable is moved and triggers an alarm. The system is only suitable for indoor cables or
Power failure detection cables in trunking.
Basic alarms detect failure of power at
the remote end of the cable but cannot Cable sensors
distinguish between power failure and Sensors fitted to the cable ends detect power interruption and distinguish between power
theft. Because South Africa uses load failure and tampering with cable and have radio connection to an alarm centre. The cable
shedding to manage electricity supply and theft monitoring unit uses Rogowski coils on the inputs and can be used to protect AC and
demand – and the schedules for these pulsed DC supply cables. Rogowski coils are ‘clipped’ around existing conductors. Conductors
planned outages are made public – thieves can be insulated or bare conductors and there is no need to undo existing terminations.
time their robberies down to the minute,
wait until the power goes off, steal the Motion and vibration detectors
cable and make their getaway before the These solutions use accelerometers buried on a platform above the cable being protected
power is restored. The typical alarms used and are connected wirelessly to the security network.
in these situations register the power cut When the device detects digging or similar movement in the vicinity, it sends an alarm
as a false-positive, meaning that the owner signal to the control room, providing the identity of the device and the location of the
doesn’t realise the cable has been stolen breach. The operators are then able to dispatch security response teams and prevent the
until much later. theft. Because of the sensitivity of the device, the alarm is raised before criminals reach the
cable and can cause any damage. More advanced types can recognise different patterns of
Continuity check vibration associated with sawing, hammering and digging. Accelerometer-based systems are
Other systems check the continuity of considered to be the most effective as they provide alarms before the cable is breached.
the cable, monitoring to see whether the The effectiveness of an alarm system depends on the ability to respond in a manner
cable circuit is open or closed, unlike alarm which would prevent further tampering with the cables. Effective alarms require effective
systems that simply look for the presence security backup.
or absence of power. In the event of a
power outage, the unit continues to run
on battery power, and sends an alert when
a break is detected.

Reflectometry
Reflectometry units allow both “live” or
“dead” cables to be monitored. The unit
injects a special DC signal into the cable
and verifies that it is reflected at the
far end of the cable. If the signal is not
reflected, the unit will activate the alarm. (a) (b)
In the case of DC loads, the load itself Figure 3: Vibration and motion sensors (a) Bidvest (b) Ntamotechnologies

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