IEC 60255-27 establishes the product safety requirements that apply to measuring relays and protection equipment. Because such devices are permanently connected both to the power system and to the low-voltage auxiliary supply and communication network, they sit at the boundary between high and low voltage and must protect operators from electric shock while remaining reliable themselves. The standard therefore sets out the insulation coordination principles that govern the choice of clearances and creepage distances as a function of the rated impulse voltage, the pollution degree and the material group of the insulating parts. It specifies dielectric tests, impulse voltage tests and insulation resistance measurements, together with the rules for measuring these distances. Thermal safety is addressed through temperature rise limits for windings and terminals, and mechanical safety through enclosure strength and impact requirements. Requirements are also given for marking, for the documentation that must accompany the product and for the tests that must be repeated on every unit as routine checks. Devices complying with Part 27 are accepted across protection schemes built to the IEC 60255 family.