PEC Earthing
A detached structure fed from the main building raises earthing questions that a single-building installation never has to answer.
A detached structure — a garage, workshop, or outbuilding fed by a sub-feeder from the main building — raises earthing questions that a single, self-contained installation doesn't.
The earth fault loop for a detached structure's circuits runs back through the sub-feeder's own protective conductor to the main earthing point — a longer loop than a circuit originating directly at the main switchboard. That extra length adds resistance to R2 in the Zs calculation, which can push the loop impedance closer to the limit for a given protective device's disconnection time requirement.
Depending on the installation and local requirements, a detached structure may also need or benefit from its own supplementary earth electrode, bonded back to the main earthing system rather than relied upon as an independent earth reference — the details depend on the specific configuration and should follow the applicable PEC clauses for the installation type rather than being assumed.
Don't assume a detached structure's circuits automatically pass Zs just because the main building's circuits do — the extra feeder length is a real addition to the loop, and it's worth checking explicitly.
Check the loop impedance for your specific run in the PEC Earthing calculator.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks