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That is very droopy!  20V -25V drop for 50A load is  about 0,4- 0.5 of an ohm.  You would have trouble convincing me that any large trips (cooker circuit, shower)  will  be sure to meet the prompt disconnection requirement if there is a fault,  without help from an RCD unless the indoor wiring is very short. (we like to see Zs low enough to be sure of a fault current of rather more than 5 times the breaker rating to  ensure a rapid break, in this case most of the Zs allowance seems to be used up outside.)

Still it means you will be able to easily see the kettle being plugged in, as the light will dim visibly. ?

An additional test, if you have the patience, would be to look at the L-E drop and then the N-E rise when a known load is added and removed (that kettle would do).  That will tell you if N and L are both undersized but the same resistance, suggesting a thin cable that has always been wrong, or if there is a higher resistance in only one or the other - more likely to be a fault. (And if you are not sure if it is really TNS or PME in the street presenting as TNS, which is quite common after repairs or additions to the mains, then the CPC earth to true terra-firma earth voltge will bounce with load if the NE link is on your side of the neutral voltage drop.

Are other buildings on the same substation affected or is it just this one ?


Older buildings may have originally  had a 'lighting only' supply that has been modernised with only a new cut out fuse, rather than a new cable.

Welcome.


That is very droopy!  20V -25V drop for 50A load is  about 0,4- 0.5 of an ohm.  You would have trouble convincing me that any large trips (cooker circuit, shower)  will  be sure to meet the prompt disconnection requirement if there is a fault,  without help from an RCD unless the indoor wiring is very short. (we like to see Zs low enough to be sure of a fault current of rather more than 5 times the breaker rating to  ensure a rapid break, in this case most of the Zs allowance seems to be used up outside.)

Still it means you will be able to easily see the kettle being plugged in, as the light will dim visibly. ?

An additional test, if you have the patience, would be to look at the L-E drop and then the N-E rise when a known load is added and removed (that kettle would do).  That will tell you if N and L are both undersized but the same resistance, suggesting a thin cable that has always been wrong, or if there is a higher resistance in only one or the other - more likely to be a fault. (And if you are not sure if it is really TNS or PME in the street presenting as TNS, which is quite common after repairs or additions to the mains, then the CPC earth to true terra-firma earth voltge will bounce with load if the NE link is on your side of the neutral voltage drop.

Are other buildings on the same substation affected or is it just this one ?


Older buildings may have originally  had a 'lighting only' supply that has been modernised with only a new cut out fuse, rather than a new cable.