Monitoring relays are essential safety devices with a wide range of industrial applications. Although many different types are available, they all work by monitoring an input signal and taking appropriate action, usually involving an alarm (relay). Most devices rely on predetermined upper and lower measurement limits and some offer visual indications of status, such as LEDs or an LCD display. Depending on the type, resetting may be manual or automatic and some units feature memory and fault storage. Some of the most widely used are for monitoring current, voltage and other supply features, such as frequency and phase. Machinery employing windings, such as electric motors and transformers, is susceptible to damage from overheating, and thermistor equipped monitoring relays are able to ensure that safe operating temperatures are not exceeded. Level indicator monitoring relays are frequently used to keep check of the levels of conductive liquids. Most monitoring relays are of compact dimensions and are normally designed for fitting to a standard 35mm DIN rail, but other types are also to be found, including plug-in versions. Monitoring relays are available for DC and AC supplies, including single and three-phase.