API RP 552:1994(2007) pdf download
API RP 552:1994(2007) pdf download.Transmission Systems.
6.3 DIGITAL COMMUNICATiONS SIGNALS
Digital Communications Signals (DCS) are normally interconnected with data-highway cables. The control system manufacturer specifies the cable and recommends the installation details.
6.4 PROCESS CONTROL LOW ENERGY/
VOLTAGE SENSORS
Low energy/voltage sensors include thermocouples. Resistance temperature deiectors (RTDs) and strain gauges generate low DC voltages when energized with a DC power supply.
6.4.1 Grounding
Circuits that contain resistance temperature detectors and strain gauges should not be grounded at more than one point. The preferred location is near the source of power and on the lower voltage wire. Grounded thermocouples should have no other grounded point than that in the primary sensor. Sneak circuits can easily occur because the extent of isolation and grounding within instruments varies with their function and make. The selection of grounded-junction or ungrounded-junction is beyond the scope of this standards (also see Section 20).
6.4.2 Thermocouple Burnout Circuit
Frequently the electronics used with a thermocouple will include provision for upscale (downscale) burnout. This provision is typically accomplished by imposing a small circulating current on the thermocouple circuit. If the thermocouple or the connecting wiring is broken, the output signal is driven to the maximun (minimum). It is necessary to consider this current in any special thermocouple circuits which would put electronic devices in parallel, since the burnout circuits may interfere with each other.
6,5 PROCESS CONTROL WITH PULSE OUTPUT
METERS
Pulse output meters are usually connected to devices that have a relatively high input impedance.
The recommendations of the manufacturer concerning the preamplifier and the wiring should be carefully considered.
Power wires and other types of signal wires should not be mixed with pulse signal wiring. The wire type and the grounding of the signal circuit should follow the manufacturer’s recommendations.
6.6 LOW-IMPEDANCE SENSORS TO COMPUTERS
Low—minpcdaiicc scI1ors to colliputels are the Saflie as those listed in 6.4.
6.7 TURBINE METERS
The suggestions of the manufacturer concerning the preamplifier and the wiring should bc carefully considered.
Most turbine mcters are supplied with integral converters to amplify the pulses or to convert for transmission of 4—20 milliamperes DC. Wiring for integral or field converters may require more than two wires (sec 6.5).
6.8 MAGNETIC-FLOW TRANSMITTERS
The electric signal from magnetic flow electrodes is generally less than 50 millivolts. The low signal level requires thai the electrical interference be minimized. Minimizing is done by using a two-conductor, shielded, twisted- pair cable whose length is limited by the manufacturer’s recommendations. The recommended cable length is a function of the measured fluid’s conductivity and the grade of the cable.
The recommendations of the manufacturer should he followed. The transmitter, cable, and receiving instrument are designed and sold as a unit. The spacing requirements given in Table 3 should be followed. Alternatively, an integral amplifier/converter may be used.
7 Specifications For Wires And Cables In Electronic Systems
General wiring requirements, such as those for twistedpair. shielded, and other types of wire, arc given in Section 6.
The frequently used wires can be classified into one of the
six types that are described in Table 4.
7.1 WIRE SIZE
The smallest wire size that will not cause an excessive voltage drop and that has sufficient strength and workability should be selected. Normally, the size used for single conductor wire is 14 gauge. that for single twisted-pair wire is 16 gauge. and that for multipair cable is 20 gauge. Other wire gauges may be selected for reasons of economy, space. or application.
