Cooper MAR724 Heavy Duty Conventional Relay The heavy duty relay unit (MAR724) is designed for interfacing heavy loads such as door release units or plant shut down equipment with fire alarm systems. This unit is simple to fix and install and the neat unobtrusive design makes it suitable for use in a wide range of areas. The heavy duty relay unit is powered by 24V dc external supply and is provided with 2 separate sets of changeover contacts. The front mounted status LED illuminates when relay is energized.