Instrumentation Systems Installation: Precautions to Take

Sep 23, 2024

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Analog systems are normally based on low voltages and as a result, they are vulnerable to electrical noise. In most plants, a PLC may be controlling 415 V high power motors at 100 A, and at the same time, reading thermocouple signals of a few mA. Great care must therefore be taken to avoid interference from the high voltage signals.

The first precaution is to adopt a sensible earthing layout. For example, a badly laid out system illustrated below will have common return paths, and currents from the high powered load like motors returning through the common impedance Z_e will induce error voltages into the low level analog circuit.

Figure 1.0: A poorly installed earthing system which leads to interference between signals.

It should be realized that that there are at least three distinct ‘earths’ in a system, namely:

A safety earth (usually used for doors, frames, and so on).
A dirty earth (used for high voltage/high current signals).
A clean earth (for low voltage analog signals).

These should meet at one point and one point only, which means that, all analog signals should return, and hence be referenced to, the same point.

Screened cable is required for all analog signals, with foil screening to be used in preference to braided screen. The screen should not be earthed at both ends as any difference in earth potential between the two points will cause current to flow in the screen as demonstrated in the figure 1.1 below and induce noise onto the signal lines. A screen must be earthed at one point only, ideally the receiving end. When a screened cable goes through intermediate junction boxes, screen continuity must be maintained, and the screen must be sleeved to prevent it touching the frame of the junction boxes. Earthing faults in screened cables can cause very elusive faults.

Figure 1.1: An earth loop formation by connecting both ends of a screened cable to the local earth. Screens should be earthed at one point only.

High voltage and low voltage cables should be well separated, most manufacturers suggest at least one meter between 415 V and low voltage cables such as multicore thermocouple cable; however, this can be difficult to achieve in practice. It is a good practice to use trunking or conduit for low voltage signals as a way of identifying low voltage cables especially for future installers who may as a result of not knowing the system well, for instance strap 415 V cable to the same cable tray as a multicore thermocouple cable. We may also achieve the same result by using cables with different colored PVC sleeves. Inevitably high voltage and low voltage cables will have to cross at some points. If spacing can’t be achieved, the crossings should always be at 90°.

In an ideal world, separate cubicles should be provided for 110 V/high current devices, and low voltage signals, but this is often not cost effective. Where both types of signals have to share a cubicle, the cables should take separate, well separated routes, and high and low voltage devices separated as far as possible.

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