Practical SCADA for Industry David Bailey - FER-a

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2.3.8 Mixed analog and digital modules SCADA systems, hardware and firmware 33 As many RTUs have only modest requirements, as far as the analog and digital signals are concerned, a typical solution would be to use a mixed analog and digital module. This would typically have: • 4 analog inputs (8-bit resolution) • 2 digital inputs • 1 digital output • 2 analog output (8-bit resolution) 2.3.9 Communication interfaces The modern RTU should be flexible enough to handle multiple communication media such as: • RS-232/RS-442/RS-485 • Dialup telephone lines/dedicated landlines • Microwave/MUX • Satellite • X.25 packet protocols • Radio via trunked/VHF/UHF/900 MHz Interestingly enough, the more challenging design for RTUs is the radio communication interface. The landline interface is considered to be an easier design problem. These standards will be discussed in a later section. 2.3.10 Power supply module for RTU The RTU should be able to operate from 110/240 V AC ± 10% 50 Hz or 12/24/48 V DC ± 10% typically. Batteries that should be provided are lead acid or nickel cadmium. Typical requirements here are for 20-hour standby operation and a recharging time of 12 hours for a fully discharged battery at 25°C. The power supply, battery and associated charger are normally contained in the RTU housing. Other important monitoring parameters, which should be transmitted back to the central site/master station, are: • Analog battery reading • Alarm for battery voltage outside normal range Cabinets for batteries are normally rated to IP 52 for internal mounting and IP 56 for external mounting. 2.3.11 RTU environmental enclosures Typically, the printed circuit boards are plugged into a backplane in the RTU cabinet. The RTU cabinet usually accommodates inside an environmental enclosure which protects it from extremes of temperature/weather etc. Typical considerations in the installations are: • Circulating air fans and filters: This should be installed at the base of the RTU enclosure to avoid heat buildup. Hot spot areas on the electronic circuitry should be avoided by uniform air circulation. It is important to have a heat soak test too.
34 Practical SCADA for Industry • Hazardous areas: RTUs must be installed in explosion proof enclosures (e.g. oil and gas environment). Typical operating temperatures of RTUs are variable when the RM is located outside the building in a weatherproof enclosure. These temperature specifications can be relaxed if the RTU is situated inside a building, where the temperature variations are not as extreme (provided consideration is given to the situation, where there may be failure of the ventilators or air-conditioning systems). Typical humidity ranges are 10–95%. Ensure at the high humidity level that there is no possibility of condensation on the circuit boards or there may be contact corrosion or short-circuiting. Lacquering of the printed circuit boards may be an option in these cases. Be aware of the other extreme where low humidity air (5%) can generate static electricity on the circuit boards due to stray capacitance. CMOS based electronics is particularly susceptible to problems in these circumstances. Only screening and grounding the affected electronic areas can reduce static voltages. All maintenance personnel should wear a ground strap on the wrist to minimize the risk of creating and transferring static voltages. If excessive electromagnetic interference (EMI) and radio frequency interference (RFI) is anticipated in the vicinity of the RTU, special screening and earthing should be used. Some manufacturers warn against using handheld transceivers in the neighborhood of their RTUs. Continuous vibration from vibrating plant and equipment can also have an unfavorable impact on an RTU, in some cases. Vibration shock mounts should be specified for such RTUs. Other areas which should be considered with RTUs are lightning (or protection from electrical surges) and earthquakes (which is equivalent to vibrations at frequencies of 0.1 to 10 Hz). 2.3.12 Testing and maintenance Many manufacturers provide a test box to test the communications between the RTU and master stations, and also to simulate a master station or RTU in the system. The three typical configurations are indicated below in Figure 2.18. Figure 2.18 SCADA test box operating mode
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Page 5 and 6: Contents Preface xiii 1 Background
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4.1 Introduction 4 Landlines Landli
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Figure 4.21 Telemetry installation
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5 Local area network systems 5.1 In
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7 Central site computer facilities
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9.1 Introduction 9 Specification of
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PIN NO 1 2 3 4 5 6 7 8 9 Appendix B
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Objective Appendix C CITECT practic
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Accumulator 29 counter specificatio
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COM port 7 Common mode rejection ra
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enclosure 213 power connections 214
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ead-only access 173 standard web br
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token ring 147 token-passing proces
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sinusoidal signal 193 technique 181
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multi-master mode 88 physical layer
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data communication system 6 disadva
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