Relays are parts that output electrical signals according to input electricity amounts or physical amounts in order to control electrical circuits. These are used for monitoring and protecting power systems, power supply circuits and machines such as pumps. When used in a machine or device, the amount of power used can be understood in detail, which is useful for energy-saving management or countermeasures. Relays come in a notable variety of types. Typical examples include overcurrent protection relays that operate after detecting overcurrent due to shorting or excessive loads in an electrical circuit; ground relays that detect ground fault accidents when an electrical circuit contacts the ground; and voltage relays that detect voltage increases caused by malfunctioning power generators or voltage drops caused by power failures. Some types are not easily affected by high harmonics, while display functions in a variety of designs are available to make checking easier.