I have a really old electrical installation which never had an earth leakage and still doesnt have an earth leakage. The owner has been in the flat for many many many many years and now wants to sell the flat.
The plugs in the building have been replaced with new plugs many years ago but no extension or addition have been done to the electrical installation. Aircons have been installed directly from the DB using an isolator.
My advice is to install an earth leakage unit, but the owner feels the electrical installation has worked fine for all these years and now upgrades are required.