The location: EU, industrial yard, pump station for crude oil... Ex zone
The application: control cabinet (installed outside EX zone) for heating cables for heating tanks of instrum. air:
Control cabinet data: 230V AC, TN-S, main cable max 4mm2, main CB C20A/1p/10kA

The question: Which SPD and a corresponding fuse should I install in a cabinet?
The fuse specially troubles me, since it seems ridiculous for instance to have a SPD fuse of 125A gG/gL (often recommended) and a main MCB of C20A (which is perfectly suitable for the application).

Sometimes it seems that I don't see the wood out of trees...
If you need any additional info, just write...

Thanks a lot

When a fuse is connected in series with a molded case circuit breaker, the protection is intended to perform two functions: (1) the fuse is intended to provide current limiting and fast acting response when the system fault current is very high; (2) the circuit breaker is intended to provide low-magnitude fault clearing and overload protection for the down stream cable and load.

Dear ;

For " SPD " :
1- As the Earthing System used is " TN-S ", the " SPD " should be " 1P + N ".
2- To define the " maximum Discharge Current - Imax " and the " Nominal Discharge Current - In ", it will better to know situation of the Switchboard in the Network, but refer to your information, it's a Sub-Distribution Switchboard, so, you can use " SPD type 2 ", Imax : 20kA, In : 5kA "

For " Fuses " :
1- The " Fuse Holder " : " 2P, 10 x 38 "
2- The " Fuses " : " gG or gl ", 25A, 10 x 38 "

For the " Installation " :
1- I'm sure that you know that the combination of the " Fuses + SPD " should be installed at the input of the main MCB ( before main CB C20A/1p/10kA )
2- The connection of the " SPD's Earth Point " in the Switchboard should be done by " 4mm2 ", and the length should be " max 50cm "