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Microprocessor Based Installation Practices --
Application Precautions

1. Microprocessor based digital instruments are more sensitive to poor installation practices than are analog or older-type solid state instruments. Standard electrical engineering practices must be rigidly observed if the advantages of microprocessor based instrumentation are to be realized.

2. Instrumentation power: The preferred method is the use of a constant voltage transformer to establish a 3-wire, 117 VAC power circuit consisting of safety ground, neutral, and hot legs. The neutral should be tied to ground at the transformer only. This circuit should have restricted use for instrumentation, computers and other low current draw equipment with steady-state amperage usage throughout the plant. If this is not possible, power instrumentation at each location with a separate isolation transformer. Anti-surge protection is recommended.

3. Signal Transmission: Physical electrical wiring considerations include the mechanical isolation in separate conduit of low voltage DC signals and power-supplying higher voltage AC power wires. Shielded thermocouple extension wire or extension wire in a separate, well-grounded conduit should be used (you may run several pairs of T/C, RTD, or low-current DC (4-20 mA, etc.) signal lines in the same conduit, but never with power lines). Shielded and twisted pairs may also be used, but the shield must be grounded and grounded at one point only.

4. Earth Grounds: The measured resistance between earth grounds must be less than 5 OHMs.

5. Avoid mounting instruments in the same panel with high energy switching devices such as contactors, motor starters or other line-spike inducing devices.

6. Avoid ground loops on 4-20 mA DC control: In a 4-20 mA DC current loop, only one ground point can exist (all points must be "floating", also known as "Isolated", "Differential", Push-Pull", I.E. ungrounded). That single ground point is usually at the control. Do not assume that the mAchine is properly grounded; check it.

7. Inductive loads (I.E. relays, solenoid valves, load contactors etc.) operated by the instrument should be neutralized by using a resistor-capacitor (RC) network across the load if the steady state current draw is greater than 40 milliamps.
   The values:
   • Up to 70 mA, C: 0.047/0.05 Microfarad R: 22 OHMs/2W
   • Up to 150 mA: C: 0.1, R: 22 OHMs/2W
   • Up to .5 A, C: 0.22/0.33, R: 47 OHMs/2W
   • Up to 1 A, C: 0.47/0-5, R: 47 OHMs/2W
   CPS should be 250 VAC operation, resistors should be carbon.

8. Signals from relatively rapidly transmitting sensors may sometimes be delayed by slow moving responses from control devices such as control valves. Rate action can then cause instability. A general rule is if rate causes instability at 30 seconds or less, remove the rate action completely. A large proportional band may be necessary for adequate control (400%).