TITLE: WIRE WOUND RESISTORS FOR SURGE PROTECTION IN
ENERGY METER
FIELD OF INVENTION:
The present invention provides wire wound resistors for surge protection. The present invention also provides a method of surge protection without using metal oxide varistors which are not reliable and degrade with time and exposure to transients.
PRIOR ART AND BACKGROUND OF THE INVENTION:
By way of description, a power surge is a sharp increase in voltage. Surges can occur on your electric, phone or even cable TV lines. They are often caused by lightning but can be associated with many other types of uncontrollable events such as hurricanes, tornadoes, ice storms, vehicle accidents involving utility equipment, etc. The most common type of surges are caused by the starting of motor-driven equipment such as electronic appliances like refrigerator, air-conditioner, furnace or garbage disposal.
In conventional meters, the problem or a surge on one of the live phases is often handled by using a varistor between the phase and neutral, the varistor being placed at the inlet to the meter so as to protect its electronic circuits. Below a predetermined threshold voltage, the varistor is normally non-conductive. If the voltage exceeds the threshold, then the varistor .becomes conductive and the surge is diverted to neutral.
That the power conditioning and backup is fast becoming a requirement for not only some very sensitive electronic devices including computers, audio processors, mission critical components, any device that relies on clean power for a CPU controlled device but also for Energy Meters. Without surge protection, excess voltage goes into the electric appliances or anything plugged in and may cause damage. Although the damage may not be noticeable at the time the surge occurred, it will shorten the life of appliances, or it could immediately make the equipment completely inoperable. It is therefore, every electric appliance requires a dependable surge protection device externally or internally.
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Conventionally used surge protection components are the. Metal Oxide Varistor (MOV). A MOV is a round disk made up of metal particles suspended in a ceramic compound. The MOV begins to conduct electricity and allows the excess voltage to flow into the disk to the ground while safe levels of voltage continue to flow into the appliance. The technology behind almost every suppressors, which is a device that reduces unwanted interference or current in a circuit, is that there are special components inside that get destroyed during a major lightning strike. The deadly force of the voltage is dissipated while components themselves are being destroyed and ideally not the equipment. Better surge suppressors have a longer useful life, but after so many lightning strikes they may fail.
In the Energy Meter Industry the trend is also to use the Metal Oxide Varistor (MOV) across the phase and neutral lines to meet the requirement. MOV are often used to protect circuits against excessive transient voltages by incorporating them into the circuit in such a way that, when triggered, they will shunt the current created by the high voltage away from the sensitive components. MOV trends to degrade with high voltage and reduce its life expectancy. If excessive current is conducted by a MOV, it may explode inside the case, keeping the load connected but now without any surge protection. Under the right conditions of over-voltage and line impedance, it may be possible to cause the MOV to burst into flames, the root cause of many fires hence it is also safety hazard.
Certain prior art document discussing attempts in surge protection of an energy meter.
WO 2011097538 deals with a system including an electronic device. The electronic device includes an electric power input configured to receive an electric power signal and has an input line terminal and an input neutral terminal. In some embodiments, the electric power input can also have an input ground terminal. The electronic device includes a metal oxide varistor protection module configured to protect at least one of the system or the electronic device. The electronic device includes a ground detection module configured to indicate the presence of an electric ground. The electronic device includes an electric power output having an output line terminal and an output neutral terminal. In some embodiments, the electric power output can also have an output ground terminal. The electric power input, the metal oxide varistor protection module, the ground detection module, and the electric power output are electrically coupled in series with each other. Other embodiments are disclosed.
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WO 2007 064960 relates to a surge protector includes a thermal fuse (62), a metal oxide varistor (64) electrically connected in series relation to the thermal fuse, and a detection network (60) electrically connected in parallel relation to the thermal fuse (62) and the metal oxide varistor (64). A first indicating means (LED1) indicates whether or not the thermal fuse is in operation and a second indicating means (LED2) indicates whether or not the metal oxide varistor is in operation
WO 2007/016346 deals with a system for managing electrical consumption includes a connecting means for connection to an incoming power supply of a facility, for connection in parallel, including a hot line and a neutral line, and at least one ground. The following components are connected between the hot line and the neutral line. They are connected -in the order of at least one front capacitor of predetermined capacitance, at least one front arc suppressor, at least one front metal oxide varistor line transient voltage surge suppressor having a predetermined number of joules capability to suppress undesired power spikes, at least two inductor/metal oxide varistor iterative transformers, at least a second capacitor of its own predetermined capacitance, at least one metal oxide varistor having a predetermined number of joules capability and at least two capacitors, each having its own predetermined capacitance different form one another.
WO 2007/002751 relates to a system for coupling communications signals (10) onto a medium-voltage power line (12) includes a metal oxide varistor, a disconnect device and a capacitor. The metal oxide varistor is electrically coupled to the medium voltage power line. The disconnect device includes a resistor in parallel with an air gap and is electrically coupled in series between the metal oxide varistor and a communications device (18). The capacitor is electrically coupled to the metal oxide varistor and the communications device (18) in parallel with the disconnect device.
WO 2005/091312 teaches a station class surge arrester includes a module assembly. The module assembly includes at least one metal oxide varistor (MOV) disk and a pre-impregnated composite that is applied around the at least one MOV disk. The pre-impregnated composite is capable of withstanding ah 80 kA fault current for 12 cycles. The station class surge arrester also includes contacts on opposite ends of the module assembly
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with which the module assembly is connected to electrical equipment to be protected and to . electrical ground.
WO 2005/072317 deals with an electrical module assembly (Fig. 5, 210) used in a surge arrester is manufactured by wrapping an electrical module assembly including at least one metal oxide varistor (MOV) disk (Fig. 8, 115) to which a reinforcing structure including a pre-impregnated epoxy/glass-fiber composite has been applied with shrink film (Fig, 8, 305) and compacting the wrapped electrical module assembly b heating the shrink film such that the shrink film shrinks and applies a radially compressive force to the electrical module assembly. The wrapped electrical module assembly then is cured at a temperature at which the shrink film no longer applies a compressive force.
WO 2004/034545 teaches an invention relating to an overvoltage protection circuit comprising an MOV (Metal Oxide Varistor) voltage limiting device (3) which is disposed in series with a GDT (Gas Discharge Tube) voltage limiting device (4). The invention is characterized in that a resistor (5) is disposed in parallel with the aforementioned GDT device (4), the value of said resistor being such that the voltage supported by the GDT device (4) is less than the holdover voltage thereof. In another embodiment of the invention, a second resistor (6) is disposed in parallel with the MOV device (3) and, together with the above-mentioned reSistor (5), forms a resistive divider such that the voltage applied the GDT device
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(4) under steady-state conditions is less than the holdover voltage of the GDT. The invention is suitable for communication systems and, preferably, for applications involving communication via electrical network, since the protection effectiveness is improved owing to the fact that the holdover voltage of the GDT device (4) must be less than that normally required.
US 4,736,269 teaches a protector module (30) for protecting the conductors of a telephone loop includes a pair of protector assemblies (40-40') which are supported within a common housing (32). A gas tube voltage protection subassembly (42) of each protector assembly is connected electrically to a grounding subassembly (44) for causing current associated with excessive voltage surges to be conducted to ground. A first electrode includes a portion which extends through annular dielectric and metallic members and an opening of an open-ended metallic container (93) to engage a shunting element (62) of a current protection subassembly (41). The superimposed annular members are held in engagement with the first electrode at a substantially constant pressure by axial forces applied by turned-in portions
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(109—109) of a side wall of the metallic container. The shunting element is supported at one end of the line pin in an initial position by a fusible material. A spring (43) between the container of the voltage protection subassembly and the housing maintains the voltage protection subassembly in engagement with the shunting element. When current flow exceeds a predetermined level that is sufficient to melt the fusible material, the spring is effective to cause the shunting element to be moved to engage the grounding subassembly to establish a fault current path to ground. The excess voltage arcs across the wide gap to ground, or in the event that a gas in the gap has vented, the vojtage arcs across openings in the dielectric member to ground in a fail-safe mode.
US 4,074,221 teaches a lightning protection is provided by an electronic valve molded mainly of silicon carbide powder. The manufacturing cost and high reject rate of previous ceramic bonding is greatly reduced by bonding with epoxy resin, the mixture being pressure molded. The surface-tracking characteristic of epoxy resin is rendered harmless by covering the periphery of the molded piece with insulation and by inclusion of alumina powder in the. mix. The insulating sleeve is extended to provide a hood preventing harmful migration of an ionized arcing path.
US 4,656,555 deals,0 with an electrical assembly comprising a plurality of electrical
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components aligned in a row under an axial load and wrapped with filament windings. The wrapped components are then preferably enclosed in a housing. MOV blocks can be used as the electrical components, resulting in a surge arrester having excellent heat transfer properties as well as improved tensile and cantilever strengths. The electrical components can be varistors, resistors, capacitors and insulators or any combination thereof.
IN patent application 1425/MAS/1998, titled an electricity meter provided with a system for protection against surges, teaches an electricity meter comprising at least one live conductor and one neutral strap, the respective inlets and outlets of the live conductor and of the neutral strap being disposed inside the meter housing parallel to one another and being coplanar. A metal bar connected to neutral is disposed inside the meter housing in the vicinity of and at a short distance from the inlet and/or the outlet of the live conductor to define a gap in which a controlled arc between the live conductor and the metal bar can propagate in the event of a surge.
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Similarly, in protection circuits provided with one or more varistors, varistor frging can make the circuits ineffective so that a violent surge can strike an arc between one of the phases and neutral, or even between two of the phases, with consequences described above.
In view of the above drawbacks and the shortcomings of the above features of surge protection in convention energy meters, the Inventors have attempted to develop the state of the art towards an improved surge protection device.
The present invention is an attempt by the Inventors to improvise upon the technology of existing surge protection devices/ circuits and provide best surge protection to Energy Meters internally and to protect the electronic components against the effects of voltage surges.
The instant invention is explained in the description as illustrated herein supported by necessary illustrations and working protocols.
OBJECTIVE OF THE INVENTION:
The principal object of the present invention is to provide special wire wound resistors for
surge protection.
Another object of the present invention i£ to provide a method of surge protection without
using metal oxide varistors which are not reliable and degrade with time and exposure to
transients.
DESCRIPTION OF THE INVENTION:
A surge protector (or surge suppressor) is an appliance designed to protect electrical devices from voltage spikes. A surge protector attempts to limit the voltage supplied to an electric device by either blocking or by shorting to ground any unwanted voltages above a safe threshold. The present invention discusses specifications and components relevant to the type of protector that diverts (shorts) a voltage spike to ground.
The terms surge protection device (SPD), or the obsolescent term transient voltage surge suppressor (TVSS), are used to describe electrical devices typically installed in power distribution panels, process control systems, communications systems, and other heavy-duty
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industrial systems, for the purpose of protecting against electrical surges and spikes, including those caused by lightning. Scaled-down versions of these devices are sometimes installed in residential service entrance electrical panels, to protect equipment in a household from similar hazards.
Metal Oxide Varistors or MOVs are commonly and conventionally used components towards surge protection. A metal oxide varistor consists of a bulk semiconductor material, such materials including but not limited to sintered granular zinc oxide among others, that can conduct large currents (effectively short-circuits) when presented with a voltage above its rated voltage. MOVs typically limit voltages to about 3 to 4 times the normal circuit voltage by diverting surge current elsewhere than the protected load.
MOVs come with their set of disadvantages as well. MOVs have finite life expectancy and "degrade" when exposed to a few large transients, or many smaller transients. As a MOV degrades, it's triggering voltage falls lower and lower. If the MOV is being used to protect a low-power signal line, the ultimate failure mode typically is a partial or complete short circuit of the line, terminating normal circuit operation.
If used in a power filtering application, eventually the MOV behaves as a part-time effective short circuit on an AC (or DC) power line, which will cause it to heat up, starting a process called thermal runaway. As the MOV heats up, it may degrade further, causing a catastrophic failure that can result in a small explosion or fire, if the line current is not otherwise limited.
MOVs are often connected in series with a thermal fuse, which eventually would disconnect. before catastrophic failure can happen. A failing MOV is a possible fire risk.
When used in power applications, MOVs usually are thermal fused or otherwise protected to avoid persistent short circuits and other fire hazards.
Looking at the conventional usage of MOVs and the problems associated with the same, the inventors of the present invention has attempted to introduce better and distinct components towards surge protection, which would be free from the disadvantages of MOVs.
Conventionally, wire wound resistors are commonly made by winding a metal wire, usually nichrome, around a ceramic, plastic, or fiberglass core. The ends of the wire are soldered or welded to two caps or rings, attached to the ends of the core. The assembly is protected with
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various materials like a layer of paint, molded plastic, or an enamel coating baked at high temperature. Owing to the very high surface temperature these resistors can withstand temperatures of up to +450 °C. Wire leads in low power wire wound resistors are usually between 0.6 and 0.8 mm in diameter and tinned for ease of soldering. For higher power wire wound resistors, either a ceramic outer case or an aluminum outer case on top of an insulating layer is used. The aluminum-cased types are designed to be attached to a heat sink to dissipate the heat; the rated power is dependent "on being used with a suitable heat sink, e.g., a 50 W power rated resistor will overheat at a fraction of the power dissipation if not used with a heat sink. Large wire wound resistors may be rated for 1,000 watts or more. In special circumstances these resistors come with Ayrton Perry winding.
These wire wound resistors are connected in series of all three phase voltages and neutral. The core of wire wound resistor is consisted of a ceramic rod, with Nichrome wire (NiCr 8020) wrapped around the substrate. The material used to construct the resistance becomes highly resistive at high frequency range and the induced current inside the resistor is dissipated as heat.
The wire is wrapped in Aryton Perry fashion to reduce the resistor's inductance and to increase the resistance wire mass, which in turn increases the resistance's surge handling capability.
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Conventionally, Ayrton-Perry winding is a type of bifilar winding pattern used in winding wire on forms to make electronic components. Its advantage is that the resulting coil of wire has low values of parasitic inductance and parasitic capacitance. Ayrton-Perry windings of resistance wire are used to make wire-wound RF resistors that are used at high frequencies, where inductance and capacitance are unwanted.
The winding is made of two separate wires wound in opposing directions along an insulating form and connected in parallel at the ends. Since there are the same number of turns of wire in either direction, the magnetic fields of the two wires cancel each other out, so the coil has little inductance. And since adjacent turns of the two wires are at approximately the same voltage, there is little parasitic capacitance between the turns.
The present invention provides wire wound resistors for surge protection.

The present claimed invention relates to the wire wound resistors for handling overvoltage . surges. These wire wound resistors may be used in a wide variety of applications as a current limiter and are used in the concept of the present invention, for the first time, for the purpose of surge immunity.
The inventors of the present invention have attempted to introduce Ayrton Perry winding type in case of winding Nichrome wire onto the ceramic blocks.
Due to high electrical resistivity of Nichrome at high frequency, all high surge current passed through the resistance produces heat. The Ceramic core acts as a heat sink and releases all the heat through its surface. After the resistance wire is wound on the fiber or ceramic core, it is encapsulated to protect it from moisture and physical damage. A high temperature silicon coating is used for the encapsulation.
The wire wound resistors in the present invention is commercially procured. Significant characteristics desired in a "wire wound resistor would be robust construction, non-flammable coating, ceramic core and high pulse loading capability.
In a principal embodiment of the present invention relates to an energy meter comprising of a wire wound resistor for surge protection.
In another embodiment of the present invention relates to a method of surge protection in three phase energy meters as per standard IEC61000-4-5 and 10KV/2ohm using wire wound resistors.
In yet another embodiment, the present invention relates to an energy meter wherein the energy meter comprises of a multiplicity of 0.25 Watt metal film resistors in series of voltage measuring circuit.
The present invention attempts to introduce reliable (>10 years) and failsafe requirement of 10KV, 2ohm surge immunity in energy meters, preferably three phase energy meters with an objective to overcome the limitation of the metal oxide varistors. An arrangement of five metal films, 0.25 Watt resistors is used in series of voltage measuring circuit. These resistors limit the surge current and additional protection is given with the transorbs at each signal line.
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The present invention is further illustrated with the help of working example in order to evaluate the wire wound resistors of the present invention towards surge protection capability, as given herebelow:
Working Example 1:
Resistor rating calculation:
The value and power rating of the wire wound resistor is selected so that it can dissipate 10KV, 20. surge waveform energy. Surge 1.2/50 us waveform shape can be approximated to triangular waveform by these resistors, and then the equivalent rectangular pulse width will behalf i.e. 25 us.
l.R = 470Q
2. Peak pulse voltage, Upeak = 10000 V
3. Effective pulse width, T = 25 us
4. Energy (Joule) = (Upeak) 2/R x T = 5.3 J or 0.0053 Ws/Q
5. In case of repetitive pulse train, the average power shall be less than actual wattage of; resistor.
As per IS 13 779 the repetition time between to impulses is minimum 3 s, the average power
Pavg = (U2/R)x(T/T)
= (10000 x 10000/470) x (25 us/3 s) = 1.77W
The rated power of the selected resistor at P70 is 5W, which is more than surge average power.
Working Example 2:
l.R = 560Q
2. Peak pulse voltage, Upeak = 12000 V
3. Effective pulse width, T = 25 us

4. Energy (Joule) = (Upeak) 2/R x T = 6.4 J or 0.0064 Ws/Q
5. In case of repetitive pulse train, the average power shall be less than actual wattage of resistor.
As per IS 13779 the repetition time between two impulses is minimum 3 s, the average power
Pavg = (U2/R) x (T/T)
= (12000 x 12000/560) x (25 us/3 s)
= 2.13W
The rated power of the selected resistor at P70 is 5W, which is more than surge average power.
- The selection of resistor value depends on power consumption of energy meter as it comes in series path. Value of the resistor can further be increased if meter power consumption is lower than the allowed limit in IEC and IS metering standards.
IEC 62053-11 standard allows 1W power/phase consumption in voltage circuits for Class 1 energy meter.
Resistor PPM (Part Per Million)
Resistor PPM selection is important as it comes in series of voltage divider and would impact the voltage accuracy with temperature variation. Resistor PPM selection should be done in a way that voltage drifts with temperature should not be more than 0.5% for Class 1 meter. In the present invention, resistors of 50 ppm are selected which satisfies the Class 1 energy meter requirement
The present invention will be further clear from the comparison of diagrams wherein the layout of a conventional energy meter and an indicative illustration of the present invention using wire wound resistors for the purpose of surge protection is shown below.
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WE CLAIM:
1. An energy meter comprising of a wire wound resistor for surge protection.
2. A method of surge protection in energy meters as per standard IEC61000-4-5 and 10KV/2ohm using wire wound resistors.
3. An energy meter as claimed in claim 1, wherein the energy meter comprises of a multiplicity of 0.25 Watt metal film resistors in series of voltage measuring circuit.
4. An energy meter and a method of surge protection substantially as herein described with reference to examples and figures.