Protection classes

Transformers are subdivided into 3 protection classes according to the constructive nature of their protection against dangerous currents for the human body:

Protection class I (protection by protective conductor)
Device with protective conductor connection and basic insulation

Protection class II (protection by protective insulation)
Device without protective earth connection with double or reinforced insulation

Protection class III (protection by extra-low voltage)
Device in which the protection against electric shock is based on the supply with SELV and in which no higher voltages than the SELV are generated.

SELV is a voltage that not exceeding <50V AC or <120V DC smoothed between the conductors or between a conductor and ground.