inside the attached image,3 types of surge arresters are connected to the system in three different zones(LPZ1,LPZ2,LPZ3) , but I can't find out why F1,F2 and F3 fuses must be connected upstream of the surge arresters? are they to protect the surge arreters? how?
Consider the working of a surge arrestor. If the arrestor commes into action it can be destroyed and form a short circuit after it has completed the protective action. Then the fuses are there to disconnect the then faulty surge arrestor from the network.
F1 is the main fuse. While F2 and F3 are fuses that protect the connecting wire from bus to the arrester. If the arrester fails and remains in short circuit, then some fuse has to disconnect the arrester.
A line transient may be suppressed by a metal oxide surge arrestor however a arc gap suppressing overvoltage protector acts as a negative resistance clamp and MUSt be followed by a blown fuse upstream to prevent further damage.
