I would say full day, however I got a house buyers survey done, this included a report on the condition of the electrics, and I am sure he was not at this house that long, and I know my son use to sign off the jobs some years ago, the firm he worked for it seemed did not trust their electricians so employed inspectors who would visit the houses being wired, and my son said he got to know which gangs were good and only needed to take readings and which gangs tried to cut corners and he had to be careful with.
Not sure of the system now with scheme providers, I know in England third party inspections are possible, although not in Wales, so what does "electrical installation signed off with a safety certificate" mean? I know I did some wiring in parents wet room, and I submitted the installation certificate to the LABC and I got a completion certificate and no one even visited the house. So clearly the LABC inspector trusted me, and I could see the same with any other inspector, he would get to know who he could trust.
