"Rick Hughes" wrote
| I am, wiring up a load of new ccts .....I have 2 CU's and arranged
| to provide 3 discrimination classes - no RCD protection, 30mA RCD
| protection, and 100mA protection.

Why? If you have TT earthing then *everything* should be on 100mA and
sockets on 30mA.

| The question arises on Cookers & Immersion Heaters ?
| I know there is no requirement in the regs for protection,

There are requirements in the Regs for disconnection times of circuit
protective devices. If you cannot meet the disconnection times using circuit
protective devices then you have to use RCDs. In practice, this means using
100mA RCD on all TT installations unless you can prove by calculation and
measurement that they are not needed.

(For completeness, I will mention that if you have a split load CU with a
100mA RCD main switch and a 30mA RCD on socket circuits, the 100mA RCD
should be time-delayed to provide discrimination.)

Owain