Hydrogen-Fuel Cell

Base on my short experiences when execute several automation project, I would like share how to make the instrumentation wiring diagram. The main concept that needed to be understand when make instrumentation wiring diagram is we should ensure there is no noise that disturb the signal from field transmitter and to field actuator. Control loop’s actual performance of automation system will affected when signal transmission from/to actual field instruments are disturbed by noises. Several sources of noise that needed to be considering are:

1. High AC Voltage Cable → when instrument cables adjacent to strong AC fields which are present near large motors, generators, electric furnaces, and transformers, then instrument cables will act as an antenna that picking up the magnetically noise that generated by that devices.
2. Radio Frequency (RF) radiation → these noise generate by walkie-talkie signal interference near to instrument cables.
3. Electrostatic or capacitive coupling with other circuits system.
4. Direct coupling with other circuits system, like leakage current paths, ground current loops, or a common return lead for more than one circuit system.

Minimizing the effect of electromagnetic coupling :

• Don’t mixed instrument cables and high AC voltage cables in same cable trays. We should have several distance for segregation instrument cables and high voltage cables. These is the wires separation table base on API RP 552 – Section 5.2.6 :

• Grouping wires bearing signal which have same magnitude to one route, and other difference signal magnitude to other route. Don’t mixed the difference signal magnitude to one route to minimize the interference disturbance. These are the standard of grouping wire bearing signals base on API RP 552 – Section 6.1.2 :

Might be these top to bottom tray arrangement could be considering :

1. Light-capacity power circuits.
2. Signal wiring.
3. Medium voltage AC and DC feeders.
4. High-voltage feeders.

• Transposing or twising the AC power conductors to cancel out or reduce the AC field that normally generated by parallel AC power wires.
• Installing instrument cable into steel conduit or covered trays.
• Shielding the high AC voltage cable or installing high AC voltage cable into steel conduit if this is known to be the source of signal interference.
• Choosing the best twisted-pair wires for instrument cable to reject most of electromagnetic interference. With the twisted-pair wires, electromagnetic induced will be very nearly same in each wire. Since the current flow is indentical in each wire except that it travels in opposite directions, the noise is mostly canceled out.
  

Minimizing the effect of electrostatic or capacitive coupling :

The most effective method is using a grounded braid or foil shield around the signal wires. With a grounded shield around the signal wires, the external voltage will couples strongly with the shield and only weakly with the signal wires. The shield should never be grounded in more than one place to avoid a ground loops. I recommend to grounding the shield at the marshalling panel. It is easier to verify, manage and ensure a good ground when they are in one place in control room.

Because instrument signals are always low voltage, usually 24VDC, we should consider the circuit protection from short circuit and high voltage expose potential. Several vendor/manufacturer has integrated the circuit protection to protect their electronic module, so please confirm to vendor when design the instrumentation wiring diagram. These decision very important to saving more money for your automation project.

Fuse terminal generally used to protect most of electronic devices in marshalling cabinet. I recommend to use fuse terminal between DC power supply and electronic devices. It will be ensure safety when installation and maintenance in marshalling cabinet. Then to protect all of electronic devices in marshalling cabinet from external short circuit and high voltage expose potential, please provided the fuse terminal for digital signal cables and surge protection for analog signal cables.

Aboveground conduit cables

We could use aboveground conduit cable to handling wires and cables from junction boxes to marshalling cabinet when the exiting cable tray not appropriate. Several conduit material that commonly used for aboveground are :

1. Galvanized steel.
2. Aluminum coated by polyethylene or PVC.
3. Steel coated by polyethylene or PVC.

Nonmetallic conduit not recommended for instrument cables, since its not provide an electrical shielding. There are any recommendations for conduit installation base on API RP 552 – Section 14.2.3 :

1. Do not use piping to support conduit.
2. provide for thermal expansion and other instrument equipment movement.
3. Fasten conduit to support with pipe clamps or bolts.
4. Install conduit runs with a minimum number of bends and offsets.
5. Provide conduit drain at low points position.
6. Provide fireproofing for 10 minutes circuit integrity or more. Several fireproofing material are calcium silicate, fiberglass, etc.
7. Provide junction boxes for pulling cables, places where type of wire or insulation changes, or places where single pairs change to multipair cable. Junction boxes also provide an access to testing and repairing instrument circuits and wiring.
8. Provide solid connection between the conduit and tray for electrical ground continuity.

Underground conduit cables

Conduit material that commonly used for underground are galvanized steel or plastic conduit arranged in banks that have a protective red-dyed concrete cover envelope. Aluminum conduit not recommended. Because these conduit will bury permanent by civil structure, we should have spare minimum 50 % conduit for future connection.

Junction boxes

There are several consideration when involved in junction boxes selection and engineering design :

• Indoor versus outdoor location.
• The electrical area classification.
• Instrinsically safe versus non-Instrinsically safe installation.
• The presence of a corrosive atmosphere or dripping liquids.
• The need to exclude nest-building insects, rodents, and other wildlife.
• The materials conposition of construction for the box, which may be :
  1. Painted, galvanized, or aluminized carbon stell.
  2. Painted, galvanized, or allumnized cast iron.
  3. Aluminum.
  4. Stainless steel.
  5. Fiberglass.
• Security key locks.
• The size of the box determine based on :
  1. Number of terminal stripps involved in junction box.
  2. Wire space between, behind, above, and below the terminal strips.
  3. The side or bottom area required for the entry or exit of cables, ducts, or conduits.
• The need for access to the box, and the number and type of doors or covers plates.
• The need for fire and blast protection.
• The minimum cost that is consistent with the service requirements.
• allowance of adequate terminals for the shields.

After you have complete reading on above short explanation that summaried from API RP 552, please see the example of actual experience on junction box instalation and wiring termination.

Tags: Instrumentation