Instrument cabling usually runs from the control room to the plant area (either below or above ground) in multicore cables and then from field junction boxes in single pairs to field measuring or actuating devices.
- They should be kept as short as possible.
- They should not cause any obstruction to prohibit access to personnel or traffic.
- They should not interfere with other equipment items accessibility for maintenance.
- They should avoid potential fire hazard areas or hot environments.
- They should avoid areas where spillage is likely to occur or where a hazard could result from escaping vapors or gases.
There are three types of signal cabling generally:
- Instrument power supplies (above 50 V).
- High-level signals (between 6 and 50 V). This includes digital signals, alarm signals, and high-level analog signals (e.g., 4–20 mAdc).
- Low-level signals (below 5V). This generally includes thermocouples that compensate leads and leads elements of resistance.
Only signals of the same type should be contained within any one multicore cable. They should be separated into groups according to the signal level when installing cables above or below ground and segregated with positive spacing between the cables.
As a general rule, low – level signals with the high – level signal cables in between should be installed furthest apart from instrument power supply cables.
Cables used for high – integrity systems such as emergency shutdown systems or data roads should be completely independent or positively separated from other cables.
Instrument cables should be operated without electrical power cables and should also avoid noise – generating equipment such as motors as far as possible. Always make cable crossings at the right angles