Consumer Installations and Service Equipment
Core concepts
This domain covers the intake to a premises: the supply authority's service cable, the meter, the consumer unit or distribution board, main switch, and the earthing arrangement. Understand the difference between the supplier's responsibility and the consumer's installation, and where the demarcation point lies. Know the components of a consumer unit - main switch, RCDs, MCBs - and the need for correct labelling of circuits. Understand diversity, maximum demand estimation, and the sizing of tails and main protective devices.
Traps
Questions often ask who is responsible for a fault on the supply side of the meter (the supplier) versus the installation side (the consumer). Another trap is assuming a consumer unit must have a main RCD for every circuit, or misidentifying the main switch as an isolator for individual circuits. Watch for diversity questions where adding all loads gives an unrealistically high maximum demand.
Worked example
A dwelling has a 10 kW shower, a 3 kW immersion heater, and general socket load estimated at 5 kW. Applying diversity, the maximum demand is not simply 18 kW; the shower is taken at full rating, the immersion at a reduced factor, and socket outlets at a diversity allowance. The result determines the size of the main fuse and tails.
Rule of thumb
Add the largest load at full value, then apply diversity to the rest, and size tails and main protective device from that figure.