IEC 60364 Table 48A

TN-C-S, Ze = 0.35 Ohm TN-S Ze = 0.8 Ohm Both supplies have the same line conductor and same transformer, so the difference, > 50 %, must be the distributor's protective conductor.

I wonder if there might be an historical factor in there too. Back in the days when TN-S was the norm the wirings regs didn't demand specific disconnection times - but simply that an fault of negligible impedance should produce an earth fault current of at least 3x the protective device's rating - which 0.8Ω does very nicely for a 240V nominal supply and a 100A fuse.


Later on we started demanding 5s disconnection times - which for a typical 100A fuse would need around 0.36Ω together with a v.d. limit implying no more than 0.368Ω L-N for a 100A supply, so (rounding off slightly) 0.35Ω seems sensible.


So I wonder if it's less to do with the physical differences between TN-S and TN-C-S systems, but more to do with what was acceptable at the time - having existing TN-S supplies that might still comply with the older numbers, the current guidance can't simply be revised.


   - Andy.

TN-C-S, Ze = 0.35 Ohm TN-S Ze = 0.8 Ohm Both supplies have the same line conductor and same transformer, so the difference, > 50 %, must be the distributor's protective conductor.

I wonder if there might be an historical factor in there too. Back in the days when TN-S was the norm the wirings regs didn't demand specific disconnection times - but simply that an fault of negligible impedance should produce an earth fault current of at least 3x the protective device's rating - which 0.8Ω does very nicely for a 240V nominal supply and a 100A fuse.


Later on we started demanding 5s disconnection times - which for a typical 100A fuse would need around 0.36Ω together with a v.d. limit implying no more than 0.368Ω L-N for a 100A supply, so (rounding off slightly) 0.35Ω seems sensible.


So I wonder if it's less to do with the physical differences between TN-S and TN-C-S systems, but more to do with what was acceptable at the time - having existing TN-S supplies that might still comply with the older numbers, the current guidance can't simply be revised.


   - Andy.