How does lightning protection in hydrostatic level sensors work

Many customer wonder why hydrostatic level sensors fail due to lightning strikes, even if they ordered an even sensor with integrated lightning protection. Understanding how the lightning protection works and how lightning strikes affect hydrostatic level sensors is important in resolving this confusion.
When submersible pressure sensors, a special type of hydrostatic level sensors, were first installed into reservoirs and lakes to monitor level, the sensor would become burned or destroyed following a lightning storm or bad weather periods. No Obligation prompted much research into the effect of lightning storms on the instrumentation in reservoirs and the cabling to and from the website.
What lightning protection really means
The first thing to understand is that the result of a lightning strike is considerably different depending on how close to the pressure sensor the lightning strikes into the ground or installation. It really is fair to say that no level sensor will survive a direct or very close nearby lightning strike, where often the whole cabin including all installed electronics will be incinerated by direct hits.
However, more distant hits is only going to improve the Voltage differential by for example 1500 Volts. In case a local lightning strike raises the electric potential of a reservoir, the level sensor may provide the shortest way for the raised voltage and current to earth. The power will then dissipate in to the ground via the level probe and therefore destroy or damage its electronics. This might equally be the case when overhead wires have a hit.
How does lightning protection in level sensors work?
However, sensors could be protected from these lightning strikes by installing or integrating a transient voltage protection in to the hydrostatic level sensor. Because of the short nature of the voltage pulse, an element may be suited to or integrated into the sensor that acts on rising differences in voltage potential. If the voltage goes above say 50 Volts, this lightning protection can short-circuit the electronic circuit to ground, allowing a path to ground for the surge and protecting the electronics around the maximum specified voltage potential.
The component would normally operate in a non-conductive state, but will be conductive for a voltage transient, allowing the voltage spike to flow harmlessly to ground. If the bond to earth is not sufficient or no lightning protection is integrated within the hydrostatic level sensor, then your electronics will take the entire level of energy of the voltage pulse and fail.Therefore the protection is only as good as the earth grounding supplied by the user.
In outdoor applications, where submersible pressure transmitters are generally used, WIKA offers an optionally integrated lightning protection inside the level sensors. The sensor electronics will then be protected from local power surges and transient high voltage. Lightning protection is a combination of protection within the instrument and an excellent low impedance earth grounding.
Check out the profiles of WIKAs submersible pressure transmitters LH-20 and LH-10.
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