Design, Manufacture and Supply of Process Instrumentation

GROUND OR EARTH LOOPS

What are they and how can they be removed?

Most analogue loops are grounded at a single point, indeed wiring standards often specify that loops should be grounded at a single point. Problems occur when there is more than one grounding point because earth potentials are never the same, and earth currents flow between the two points causing errors, noise or erratic signals.

 

Eliminating multiple grounds can be difficult or even impossible because of system requirements or if the particular instrumentation to be used has multiple non-isolated inputs. In other applications, there may be a safety requirement to earth through, say, zenner barriers. Even when the system has been designed to have a single earthing point, sometimes leakage, through, for instance, mineral insulated thermocouple probes can give an unexpected path to earth and cause unwanted errors.

A simple way to ensure the removal of ground loops is to use signal isolators. These isolators accurately transfer the analogue signal whilst at the same time effectively blocking the DC continuity between the input and the output of the isolator. Isolators can be loop powered, i.e. they do not require any external power source, or externally powered.

Loop power obviously has an advantage in that no external power supply is required, however in order to work they require some power which is supplied in the form of a loop voltage drop i.e. there is some voltage required for the isolator and hence less voltage available for other devices in the loop. Some isolators also double as signal convertors such that a voltage signal can be isolated and converted to a 4 to 20 mA signal in the same device. The choice of isolator often comes down to whether power is available on the input side or the output side of the isolator, and also if there is sufficient voltage available in the loop to power a loop powered isolator. The SEM1000 series provides a complete family of small high accuracy isolators to suit almost every application.

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