For what does PELV require protective conductor? And also what is differences between PELV and FELV?

There is a general principle with all electrics since about 1970, that equipment and installations must remain safe until two independent things have failed.


There are basically 2 common approaches that achieve this 'double fault to danger' level of protection.


For earthed mains equipment and PELV, the two things that might fail are the earth connection, which is needed to blow the fuse or trip if the mains gets out, and the basic insulation, which is the mechanism by which the mains does get somewhere where it may be accessed - remember PELV wire itself does not need insulation to withstand mains voltages, so  in principle if mains is on the bell wire or whatever, it is considered to be already accessible.


For double insulated mains equipment, and SELV systems, we have basic and reinforced insulation, so two things have to fail - for example in a split bobbin transformer, an insulation fault that makes the transformer core live still does not make the secondary live, unless there is a second fault connecting the secondary circuit to the core as well, as the secondary bobbin is also insulated from the core to the same standard as the primary.

There is a general principle with all electrics since about 1970, that equipment and installations must remain safe until two independent things have failed.


There are basically 2 common approaches that achieve this 'double fault to danger' level of protection.


For earthed mains equipment and PELV, the two things that might fail are the earth connection, which is needed to blow the fuse or trip if the mains gets out, and the basic insulation, which is the mechanism by which the mains does get somewhere where it may be accessed - remember PELV wire itself does not need insulation to withstand mains voltages, so  in principle if mains is on the bell wire or whatever, it is considered to be already accessible.


For double insulated mains equipment, and SELV systems, we have basic and reinforced insulation, so two things have to fail - for example in a split bobbin transformer, an insulation fault that makes the transformer core live still does not make the secondary live, unless there is a second fault connecting the secondary circuit to the core as well, as the secondary bobbin is also insulated from the core to the same standard as the primary.