Download Fence Detection and Control

The Philosophy of Fence
Intrusion Detection Energisers
Functional Specification
The ability to deliver an effective non-lethal shock and to detect an intruder depends on both the
fence configuration and energizer characteristics.
Fence configuration
A well-designed security fence should meet the following functional criteria:

The fence configuration must allow for the non-lethal high voltage to be present between any
adjacent elements that can be separated to enable penetration of the fence. In the case of a
conventional multi-strand fence, this means that a voltage must be present between all
adjacent wires as well as between any wire and elements (strain and intermediate posts and
ground) at earth potential.

The voltage applied across fence insulators must be minimized, while retaining a sufficient
working voltage between adjacent fence wires. This reduces the electrical stresses on the
plastic insulators normally used on security fences.

The fence configuration must allow for both the non-lethal pulsing and continuous low voltage
detection signals to be applied to all insulated elements, and also for the fence to be
monitored for open and short circuit as well as earth conditions.
Electrical considerations

Inter-connections between the fence and the energizer as well as between fence wire must
be kept simple for easy installation.

The energizer must be able to facilitate continuous low voltage detection and non-lethal
pulsing as totally independent measures.

The system must conform to statutory safety requirements.
Design Specification
To meet specific user requirements, the design specification should prescribe the methods to be
used for detection and non-lethal high voltage shock. The following discusses briefly the
principles.
Non Lethal Pulsing
Non lethal pulsing is commonly done with either single loop or dual loop energisers.
Single loop systems
Except for fence configurations that make extensive use of elements at earth potential, like
barbed tape coils, a system based on single loop pulsing and detection does not normally
satisfy the requirements of an ideal detection fence.
Dual loop systems
All the above mentioned functional requirements are met by the use of a dual loop energizer
linked to a correctly designed fence configuration.
A dual loop energizer provides two independent synchronized outputs of opposite detection
and pulsing potentials. For instance, loop A will generate a positive pulse at half the voltage
while loop B will generate a negative pulse at half the voltage. The low voltage detection
signal will be a alternating voltage with a phase difference of 180 degree between loop A and
B. Alternating detection signals are normally used to prevent galvanic corrosion of fence
components.
The fence is electrically looped in such a way to form to independent, insulated electrical
circuits to be connected to the two energizer outputs. Alternate fence wires are therefore
common to the specified loop.
Fence Detection and Control
There are numerous products offered in today's market. The user, not normally being an expert in
perimeter intrusion detection or electronics, often installs products not suited to the application or
not offering the full range of benefits. The following is a brief discussion of detection methods
normally used and a discussion to a proper approach to the choice of a detection control system.
Detection methods
Fence intrusion detection is commonly done in conjunction with a high voltage fence energiser
system because the detection system normally uses the same fence conductors as the high
voltage system. There are mainly two methods used for detection namely


Presence of Pulse Detection
Continuous Low Voltage Detection
These two methods quite often become the argument from the various energiser suppliers as
to why the system that they prefer or use is superior. In fact these two methods are rather
complimentary to one another than supplementary as their individual pros and cons combined
allow for a better detection system. However very few manufacturers supply a system using
both methods.
Presence of Pulse Detection
Presence of pulse detection looks for the presence of the high voltage pulse during
energiser pulsing. It therefore only does detection at the moment when the energiser
pulses. This is done by measuring the output current or voltage from the energiser during
pulsing. A higher that normal current or lower return voltage will indicate a short circuit
while zero current or voltage might indicate an open circuit. Typically, In a situation where
flash over on a insulator due to surface pollution or arcing between two closely spaced
fence conductors or intermittent arcing due to debris (papers, vegetation etc) in the fence
occurs, the system might interpret this as a short circuit. In practice this is one of the most
common causes of nuisance alarms with this type of system.
Continuous Low Voltage Detection
Low voltage detection is a system that does open short circuit detection by monitoring the
current (milliamps) when applying a low voltage to the fence between pulses. Normally a
fence is only pulsed with high voltage every second and the duration of the pulse is in the
order of 100 to 500 microseconds. The time between pulses is then used to supply a safe
voltage (typically under 50-Volt) to the fence. It therefore allows for a must larger time
window of detection than the pulse detection method and has the major advantage that
fence detection can be done while the high voltage pulsing is inhibited to allow for
maintenance and cleaning of the fence. It also allows for a higher probability of detection
as it does take into account any changes that may occur between pulses. With this type of
fence detection care must be taken that the total fence insulator leakage is compatible to
all environmental conditions for the specific area.
Intelligent Process Control
In simple installations the typical system will indicate open or short circuit conditions when the
detection criteria is outside general factory set hardware limits. With the use of
microprocessors today the analogue value of current can be sampled and intelligent decisions
can be made by comparing minimums, maximums, trends and rate of change. This type of
approach is similar to a typical PID process control algorithm. There are enormous
advantages to implement detection systems that take a process control attitude rather than
elementary logic control.
For example, a fence that is persistently wet in the morning due to fog or dew will have a
global change in environmental inputs that must be recognised by the programmed
microprocessors. The processors can then analyse the full-digitised analogue value of the
detection loops and their behaviour. This approach not only allows for a better detection
probability and lower nuisance alarm rate, but also can be used as a maintenance tool. A
constant high current might indicate that vegetation in the fence requires weed killer of that a
certain energiser’s high voltage is below acceptable limits and needs replacement of service.
The Ultimate User Specification
The only use of installing a detection fence system that is not based on proper process control
principles, is for controlling the movement of live stock. The capital layout required for a
fencing structure is normally large comparing to the system cost. The user must therefore
choose a system to allow for the maximum effective use of his perimeter fence. Systems that
does not have a process control backbone and make use of the full concept of loop detection
by using both detection methods, should be regarded as inferior.
The optimal Perimeter Fence Intrusion Detection System must comply with the
following specification.



Dual loop non-lethal Energiser complying to the local statutory safety
requirements
Intrusion detection based on both Presence of Pulse and Continuous
Low Voltage Detection
Analogue detection interface to allow for algorithm based
microprocessor detection control