The output contacts of frequency relays are energized when the frequency exceeds the adjustable set point. Overfrequency and underfrequency relays are available in 50, 60 and 400Hz. Combination over/ underfrequency "band pass" relays are also available. These are energized at rated frequency and deenergized during overfrequency or underfrequency conditions. Frequency Differential relays are energized between the preset frequencies. The pick-up and drop-out frequency settings are independently adjustable.