The 501 level relay with electrodes are most commonly used as a simple safety device providing level alarms in case of water leaking on floors with no constant monitoring.
501 provides an alarm signal based on the conductivity in the liquid thereby securing the important electronics or non-submersible pumps.
Some waste water plant also uses 501 as an additional security device providing alarm signals, or starting pumps based on the 2-4 electrodes in individually adapted lengths
The 501 level relay with electrode is the simple safety device. 501 controller is most commonly used to provide alarm signal in case of water on floor, in decentral drinking water wells with no daily workforce.
501 is a simple way to get an alarm, when important electronic control are in the risk of getting water damage from for instance heavy rainfall or a pipe slowly leaking.