1 AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION FOR AN INNOVATION PATENT Name of applicant(s): Pakton Developments Pty Ltd Actual Inventors(s): Paul Thompson Address for Service: PO Box 408 1 Helium St Narangba, Q 4504 Australia Invention Title: A SAFETY DEVICE FOR ELECTRIC FENCES Details of Associated Provisional Application: 2009903447- filed 23rd July 2009 This invention is described in the following statement: 2 Field of the Invention The present invention relates to electric fence energisers and electric fences and in particular the safety of electric fences in the presence of induced 50 or 60 Hertz AC voltage from the mains electrical power distribution grid. 5 Although the invention will be described with particular reference to electric fences that are used for security fencing applications, it will be appreciated that the invention may be deployed in other applications such as agricultural fencing. Brief Description of the Prior Art 10 Electric fences are often used for security purposes to restrict unauthorised entry to certain areas such as industrial premises. They are also used for containment in detention centres. Electric fences normally include a number of posts from which numerous non insulated wire conductors are strung such that the conductors are insulated 15 from the posts and therefore the ground. The conductors are coupled to an energiser that periodically outputs a high voltage pulse to energise the conductors so that intruders will receive a small electric shock if they contact the energised conductors. Although the voltage is very high-up to 10,000 Volts peak-the period of the pulse is very short in order to be safe and is 20 typically in the order of 100 microseconds. Energisers that operate in accordance with international safety standards such as IEC60335.2.76 limit the pulse rate to no more than one pulse per second. An intruder receives a shock if they touch the fence by completing the circuit from the energiser, via the live wires to ground and back to the energiser 25 ground terminal. The electric shock acts as a deterrent. The spacing and height of the wires is such that it is difficult to gain access to the protected area without contacting the wires. The live wires are often interleaved with grounded wires so as to make a circuit even if the intruder attempts to insulate him or herself from the ground but touches more than one 3 wire. If the wires are cut or shorted to ground a monitoring circuit connected to the electrical end of the live wires detects the change in voltage and can sound an alarm or trigger a call to a guard centre. This monitoring is typically achieved by measuring the peak voltage. If the peak voltage falls below a 5 predetermined threshold an alarm is generated. Another method for monitoring the live wires operates by applying a low voltage to the wires to detect a change in the continuity of the fence wire from one end to the other. Many security electric fences are used in situations where employees or the general public are allowed to access the area at 10 certain times, such as during the day. During such times, the use of high voltage pulses may be prohibited, leaving the low voltage system as the only way of monitoring the fence. Advanced low voltage monitoring systems can also detect a person touching the wires. These low voltage systems often require that a fence be well insulated from ground, i.e. there be high 15 impedance between the fence wire and ground. Electric fences are susceptible to 50 or 60 Hertz AC being induced onto the conductors by overhead power lines. This can cause a voltage to develop on the fence relative to ground which can lead to the fence giving a shock even when the electric fence energiser (also known as an energizer, controller, 20 fencer and charger) is turned off. The shock received from this induced voltage can range from a tingle which is barely perceptible through annoying to painful and dangerous. From a public health and workplace health and safety perspective this is clearly unacceptable. If the voltage exceeds the level defined as extra-low voltage (ELV) then it will probably contravene local wiring 25 codes. ELV is defined as a voltage lower than 50 Volts AC RMS or 120 Volts DC (ripple free). Electric fence design guides, wiring codes and safety standards contain recommendations about minimum distances between electric fences and overhead power lines. Even when these guidelines are followed it is still possible to find induced voltage on the fence wires. 30 In practice this induced voltage rarely occurs at all unless the electric fence runs under a high voltage power line for some distance. Even in this situation induced voltage may not be a perceived problem. This is due to the fact that 4 standard electric fence energiser outputs and monitor input circuits tend to present relatively low impedance loads which shunt the induced current to ground. These are described in the following conditions whereby a fence that appears to be free from induced voltage can still present a problem. 1) If the 5 fence is wired through a gate contact which when the gate is opened the contact opens the electrical circuit from the energiser to a portion of the fence. 2) If the energiser is disconnected from the fence for maintenance or repair. In any case, where the induced voltage is high enough to cause a problem the fence installer is obliged to mitigate it. To the authors knowledge there is 10 currently no product available which is designed specifically to do this. When faced with this problem an engineer might apply a high voltage resistor between the fence wire and ground such that the resistance is low enough to remove the induced voltage by shunting it to ground, but high enough not to adversely affect the high voltage pulses from the energiser. A value between 15 500 Ohms and 10,000 Ohms is typical. However, doing this renders inoperable some low voltage touch and continuity detection systems. Additionally, some energisers make use of a communications technique whereby signals are sent along the live fence wires. These may for example be used to allow communication with a handheld remote control. Such 20 communication signals can be adversely affected when the resistor is present. It is an object of the present invention to provide a device that overcomes, or at least ameliorates, one or more of the deficiencies of the prior art electric fences mentioned above, or that provides the consumer with a useful or commercial choice. 25 Other objects and advantages of the present invention will become apparent from the following description, taken in connection with the accompanying drawings, wherein, by way of illustration and example, a preferred embodiment of the present invention is disclosed.
5 Summary of the Invention According to a first aspect of the present invention there is provided a safety device for reducing induced continuous AC voltage from a plurality of electric fence conductors. The safety device includes a series circuit of a resistor and 5 a voltage dependent resistor (VDR). The safety device is characterised in that the values of the resistor and VDR are chosen to reduce induced continuous AC voltage on the fence conductors to a safe level. Also the value and type of the resistor and VDR are chosen so as to not affect the normal operation of the electric fence high voltage pulses or of low voltage continuity detection or 10 communications systems which use the fence conductors as a transmission line. The safety device may also include high voltage clamping devices to protect it from lightning surges. Preferably the safety device is housed in a weatherproof enclosure with two 15 high voltage and high current rated terminals. One terminal is to be connected to the fence conductors and another to a ground or earth. Alternatively the safety device may be included into the same housing as an electric fence energiser connected directly between the energiser fence output terminals or the monitor input terminals. 20 Brief Description of the Drawings In order that this invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings which illustrate typical preferred embodiments of the invention and wherein: 25 Figure 1 is a block diagram of a security electric fence according to an embodiment of the present invention. Figure 2 is a schematic diagram according to the preferred embodiment of the present invention.
6 A block diagram of a security electric fence according to an embodiment of the present invention is depicted in Figure 1. The electric fence 1 is constructed in proximity to an overhead high voltage electric transmission line 2, causing 50 or 60 Hertz AC to be induced on the live wires. The energiser 3 5 includes output terminals to conduct both high voltage and low voltage signals onto the electric fence live wires. The fence 1 contains live wires running from the feed terminal 4 to the fence 1, then down the fence in an alternating pattern with the ground wires. The live wires terminate back onto the live return terminal 5 whereas the ground wires are connected to the fence ground 10 6 rod before also returning to the energiser so that the fence can be monitored. The safety device 7 is connected between the live wires and ground to mitigate the induced voltage. A schematic diagram according to the preferred embodiment of the present invention is depicted in Figure 2. The safety device has two terminals to allow 15 it to be connected between the fence and ground. The fence terminal 8 is connected to a live wire of the electric fence and the ground terminal 9 is connected to ground directly or via the ground wires of the fence. The low voltage MOV 10 doesn't conduct if the voltage across it (V 10 ) is less than 20 Volts AC RMS. When subjected to more than 20 Volts AC RMS, the MOV 10 20 conducts and current (111) flows through the resistor 11 to ground via the ground terminal 9. The high voltage, high current MOV 12 protects components 10 and 11 from lightning surges without affecting the normal 10,000 Volt pulses at the fence terminal 8. The threshold voltage of the MOV 12 is chosen such that the current (112) flowing through it is zero when it has 25 up to 10,000 Volts across it (V 12 ), but that it conducts when V1 2 rises above 10,000 Volts, thus providing lightning protection to the more sensitive circuit elements 10 and 11. It follows that the current 111 is equal to 18 in normal operation. Since 18 is typically less than 10 milliamps and I11 is equal to l, resistor 11 30 needs to have a value such that V11 plus V 10 , i.e. the total induced AC voltage at 8, is less than the maximum allowed as ELV of 50 Volts AC RMS. Equation 1: V 11 + V 10 50 (Volts A C RMS) 7 Ohm's Law: V 11 = l 1 1.R1 (Volts A C RMS) Equation 2: R1 5 (50-V1o) / Is (Ohms) Solving Equation 2, a resistor 11 of less than 3000 Ohms will result in a maximum induced voltage between the fence terminal 8 and the ground 5 terminal 9 of less than 50 Volts AC RMS, and therefore within the safe ELV range. Since the effective output impedance of a modern energiser is approximately 100 Ohms, a 3000 Ohm load will have a negligible effect on the electric fence pulse voltage. 10 Since the MOV 10 causes the resistor 11 to have no effect when the voltage is less that the ELV limit, low voltage signals on the fence in the ELV range are free to operate as normal and aren't loaded down by the resistor.