Unfused spur.

Andy, you are making a possible mistake, it could be worse than I expect, but unlikely. The energy let through you are quoting is at the DB with 6kA PSCC (or 3kA) which you will notice is not much more than the 1mm2 rating to only get to 155C. The other points to consider are the PSCC at the point the 1.0 Earth conductor joins, which will not be 6kA or even 3kA, it might be as high as 575A if the ring is short and the DB PSSC high. The by the book calculation is not a theoretical possible maximum value, it is an actual value. The resistance of 2.5 copper cable is 18mOhms/m. The drop /m at 6ka = 108 volts per metre, so for the live only 54 volts per metre and the 1.5 Earth 87 volts per metre. The mains source impedance at 6kA PSCC is 38 mOhms, which is more than doubled for a metre of 2.5 cable, so with 1 metre of 2.5 will be halved approximately. More realistically the supply will probably be 2kA or less, and the cable a few metres. The actual PSCC to be faced will therefore probably be well safe. To cause a 1mm cable to actually melt (which is the danger condition) will need roughly 4 times the energy let through to get to 155C, although this may cause insulation failure. This will be detected by the RCD if it happens. I suggest this is the real scenario, one needs to examine the actual situation (PSCC present at the reduced cable size) not the possible maximum which could occur. A PSCC measurement at the source DB and the socket connecting to the 1.5 cable would also be instructive. You will notice that for design purposes we reduce the PSCC value between the main switchboard and sub-DBs in line with the cable impedance, otherwise a large supply would need impossible sized switchgear everywhere. This is the same situation.

Naughty question to Z, I presume you have the loop impedence figure at your new socket or the R1+R2 value and the Board PSCC so that we can estimate the possible maximum fault current?

Andy, you are making a possible mistake, it could be worse than I expect, but unlikely. The energy let through you are quoting is at the DB with 6kA PSCC (or 3kA) which you will notice is not much more than the 1mm2 rating to only get to 155C. The other points to consider are the PSCC at the point the 1.0 Earth conductor joins, which will not be 6kA or even 3kA, it might be as high as 575A if the ring is short and the DB PSSC high. The by the book calculation is not a theoretical possible maximum value, it is an actual value. The resistance of 2.5 copper cable is 18mOhms/m. The drop /m at 6ka = 108 volts per metre, so for the live only 54 volts per metre and the 1.5 Earth 87 volts per metre. The mains source impedance at 6kA PSCC is 38 mOhms, which is more than doubled for a metre of 2.5 cable, so with 1 metre of 2.5 will be halved approximately. More realistically the supply will probably be 2kA or less, and the cable a few metres. The actual PSCC to be faced will therefore probably be well safe. To cause a 1mm cable to actually melt (which is the danger condition) will need roughly 4 times the energy let through to get to 155C, although this may cause insulation failure. This will be detected by the RCD if it happens. I suggest this is the real scenario, one needs to examine the actual situation (PSCC present at the reduced cable size) not the possible maximum which could occur. A PSCC measurement at the source DB and the socket connecting to the 1.5 cable would also be instructive. You will notice that for design purposes we reduce the PSCC value between the main switchboard and sub-DBs in line with the cable impedance, otherwise a large supply would need impossible sized switchgear everywhere. This is the same situation.

Naughty question to Z, I presume you have the loop impedence figure at your new socket or the R1+R2 value and the Board PSCC so that we can estimate the possible maximum fault current?