While the most common application is the over-current protection relay, transformer protection and other power system protection applications are also very common. For example, protection relays are used in substations. So just as motor protection relay is common, so are phase monitoring relay applications. Those new to this subject may be reminded there is also the under voltage relay, the generator protection relay, etc. that offer much more protection than just a breaker would do. Monitoring current, voltage, phase, temperature, differential, etc., Especially the solid state relays and microprocessor relays with complex time current curves (trip curves) and combinations of monitoring power.