Confusion over table 7.1(i) OSG

maybe a good idea to go for 1st principles .

A B breaker at the least sensitive edge of the permitted tolerance  needs to be hit with 5 times its nominal rating to guarantee to instant trip. That would be 200A.

The mains may be as low as 230V- 10% or 207V. So

loop resistance of 207/200 = 1.035 Ohm. 

now neither the OSG nor BS7671 actually work like that, but prefer to give tables so we need to guess what has been done.

The OSG, I think has taken the 0.8 ohms of the external network, and decided that 

1.035 for the whole loop, minus the 0.8 ohms outdoors, leaves  a mere 0.235 ohms for the indoors bit, and asked how much cable is that resistance.

1m of 6mm on the way out ~ 3 milliohms and 1m of 2.5mm core on the way back, 7 milliohms so 10milliohms per metre, and  0.235/0.010 = 23.5 metres

I suspect by rounding the numbers in various places, like the core resistances you can get closer to 23m exactly.

Its a hack as not almost no TNS systems have exactly 0.8 ohms external loop impedance and almost no B type breakers need the full 5 times nominal to fire, so in reality a longer cct is usually fine.

Mike.

maybe a good idea to go for 1st principles .

A B breaker at the least sensitive edge of the permitted tolerance  needs to be hit with 5 times its nominal rating to guarantee to instant trip. That would be 200A.

The mains may be as low as 230V- 10% or 207V. So

loop resistance of 207/200 = 1.035 Ohm. 

now neither the OSG nor BS7671 actually work like that, but prefer to give tables so we need to guess what has been done.

The OSG, I think has taken the 0.8 ohms of the external network, and decided that 

1.035 for the whole loop, minus the 0.8 ohms outdoors, leaves  a mere 0.235 ohms for the indoors bit, and asked how much cable is that resistance.

1m of 6mm on the way out ~ 3 milliohms and 1m of 2.5mm core on the way back, 7 milliohms so 10milliohms per metre, and  0.235/0.010 = 23.5 metres

I suspect by rounding the numbers in various places, like the core resistances you can get closer to 23m exactly.

Its a hack as not almost no TNS systems have exactly 0.8 ohms external loop impedance and almost no B type breakers need the full 5 times nominal to fire, so in reality a longer cct is usually fine.

Mike.