When monitoring filling levels, make sure that the level sensors are sufficiently grounded in order to prevent serious damage. Should this be inadequate or absent altogether can lead to three serious effects.
- Because of insufficient potential equalization in conductive media such as water, corrosion can occur. This is a gradual process, which can be observed in long-term applications. The voltage differences between the sensor and its surrounding fluid lead to electrolytic corrosion. The metal housing becomes gradually perforated and liquid then penetrates into the housing itself. Damage to the electronics will then be the consequence here. This process can be observed both in open waters and in fill level monitoring within vessels, where the potential difference between the level sensor, medium and vessel wall can cause electrochemical corrosion.
- Filling level sensors are connected to the control system by cables or plugged into telemetric systems. Through these connections, atmospheric voltages can be passed on to the sensor. Overstrain to the electronics will be the end result in this case.
- If lightning strikes near the level probe, a very high voltage difference will exist over the shorter term. The increased voltage in the water will then seek the shortest path to earth here via the level sensor.
Grounding and lightning protection of level sensors
To protect level sensors from these effects, they can be equipped with lightning protection. For this purpose, a transient overvoltage protection is integrated into the level probe, which will react to rapidly rising voltage differences. Should a sudden voltage surge occur, the lightning arrester will trigger a short circuit within the electrical circuitry to channel that overvoltage to ground. This surge protector normally operates in a non-conductive state, but does conduct voltage transients so that they can flow to ground without causing any damage. It should be noted, however, that with a direct lightning strike to the immersion probe, even overvoltage protection cannot prevent damage.
Additionally, an earth connection that should have a resistance of less than 100 ohms is used for grounding. For fill level monitoring in liquid-carrying tanks made of metal or even plastic, care must be taken that all of the isolated metallic components are connected together to ground. In open waters, a greater effort is generally required to create a low resistance to ground. For this reason, an earthing grid is often set into the ground for these applications.
Users are generally advised to discuss a grounding concept with the manufacturers in regard to their respective application.
