Yes common sense is a large part of what we do but essentially your statement that a single RCD as the incomer meets the 17th edition of BS7671 is just plain wrong.
It meets the requirement to protect certain circuits with 30mA RCD fault protection but it is agaings BSs7671 in many other respects as pointed out above.
This chestnut illustrates why:
"You are 79 years old. The time is 9.30 p.m. It is December, it's dark and the weather is cold.
You come from the bathroom on the first floor, just on top of the stairs. The thermostat of the central heating comes on, a neutral to earth fault on the pump trips the residual current device which is the incoming device on the distribution board and all supply is lost in the home.
No lights.
What happens next..."
It meets the requirement to protect certain circuits with 30mA RCD fault protection but it is agaings BSs7671 in many other respects as pointed out above.
This chestnut illustrates why:
"You are 79 years old. The time is 9.30 p.m. It is December, it's dark and the weather is cold.
You come from the bathroom on the first floor, just on top of the stairs. The thermostat of the central heating comes on, a neutral to earth fault on the pump trips the residual current device which is the incoming device on the distribution board and all supply is lost in the home.
No lights.
What happens next..."