[Lucien Nunes]

No, there is no obligation to do so, and testing a house installation takes many hours at minimum. However if a competent person spots something that could give rise to immediate danger, they might use their professional judgement to decide to isolate it. It would then be up to the owner to decide whether to leave isolated or re-energise.

Certain aspects of an installation are common to the whole. If a final circuit is being worked on, the adequacy of main equipotential bonding etc. will need to be checked even though this applies to more that just the circuit under test. Altering the characteristics of one circuit may require that circuit to be upgraded to comply with current regulations. If that involves adding earth-leakage protection etc, economics might dictate that the way forward is a CU upgrade, hence involving other circuits.

Replacement of a broken accessory like-for-like does not require detailed investigation, nor altering the circuit to comply with current regulations, provided certain essentials are complied with. If the cable, protection and main earthing are satisfactory, the most that is likely to be required here is a minor works certificate, which is less comprehensive than an electrical installation certificate.

FWIW gas regs are different both technically and legally, and one cannot extrapolate from gas practices to electrical. The microwave repair man would have tested that the earth was present in order to satisfy their own company safety policy and recognised safe practice, so that they do not leave customers unknowingly with lethal appliances even though they have no remit to interfere with the fixed wiring of the building. Some firms also look for minor non-compliances or legacy setups under the pretence of improving customer safety, when in fact they are looking for opportunities to quote for additional work.