On our course it was stated that it was 100% of circuits, 20% of accessories on each circuit. I would do an EFL test on every socket incase there were contact problems (I have often found 1 socket giving 7,8,9 ohms ) How would you feel if a week after you did your test there was an electrical fire? - its happened where a socket overheated and caused a fire!
Yes 100% test for new installation, but periodic are slightly different, Inspect all the sockets for damage, I prove continuity using the R2 method, then prove conductor form a ring if circuit ring final, only if result are not satisfactary will I perform full ring continuity, then insulation test between Live-Earth then live and neutral on 250V, if ok the 500V, if circuit has neon in etc do not do Live to Neutral, then make live and use martindale or ELZ tester to prove polarity, Yes ZS every socket, also check switch working at the same time, Yes test every RCD.
Lighting R2 method, insulation between LIVE and earth only if all the lights were working before I did the test, also if dimmer in place, (alway test dimmer working on inspection before starting any test, (you be amazed how many faulty dimmer i have come across)
Domestic installation I ElZ furthest point only, commerical I may do more than 1.
Finall all CCU distribution boards are inspected Ze Zdb etc plus Ipfe and I pscc are measured
I never do as must inspection or testing on periodic as that are required for new installation, I do end up doing a lot of faultfinding etc
As I stated in previous blogs, I worked on preventive maintenance programme where an installation was tested over 5 year period testing only 25% of the circuits each year, 5% of the pervious year and 20% new circuits, so it is possible that you could be testing only 20% of and installation each year.