Resumen en español

Numerical protection devices are much more than just relays. Actually they are small computers with protective functions complemented with others like control, metering, logic, logging and communication. These functions allow a big flexibility that enhances even more the functions built in by the manufacturers. This paper illustrates how protective functions that are not originally incorporated can be emulated and in particular the over/under reach feature associated to over current protection. The logic of implementation is well illustrated as well as the procedure to test the implementation as it was done in the LAPEM laboratories of Mexico.