There are 6 tests for a RCD done in pairs, so three on positive half cycle and three on negative half cycle.
1) Test it does not trip at half rated current.
2) Test tips a rated current in 300 mS.
3 Test it trips at 5 times rated current in 40 mS.
Clearly if 2 trips below 40 mS no real reason to do third test.
Also one should test that the combined circuit leakage for the 30 mA RCD does not exceed 9 mA, this is how it is decided if one RCD for many circuits or use RCBO's. Also the items being supplied by the RCD need checking to ensure the right type of RCD is fitted.
The question is when a RCD fails, what then, we have two options Code C2 or Code C3. Some are clear, no RCD with a TT supply would be Code C2, and tripping at half rated current code C3, but with for example a 100 mA RCD instead of 30 mA, or type AC instead of type A or B, or no RCD with a TN supply if bonding is OK, then it is a grey area if code C2 or C3.
Big question is what is the difference between code C2 and C3? If a owner is told the wrong or no RCD is fitted, he has been made aware, but with a C2 there is some urgency getting it corrected. So if an item is given a C3 where should be a C2 what does it mean for the inspector? It means should some one be injured he would need to explain in court why he only coded it as C3, clearly all 230 volt electricity is potentially dangerous, so the wording means very little, so if he can find any official report which supports his coding then he can claim he was following accepted standards.
This is from the electrical safety council best practice guide, and if the electrician says he was following that guide when awarding a C3 it would be hard for a court to say he got it wrong. So as long as he picks up on lack of RCD with a code C3 and it is not a TT supply or had bathroom bonding removed, then really he has no problem.
The FI and LIM codes is another argument, if you can't get access to a garage that could be a code LIM, but not getting access to a bedroom due to say lodger keeping door locked, then a code FI. However I forgot my loop impedance meter can not really be awarded a code FI, it is clear he needed to return home and get his loop impedance meter. There has been questions over terraced houses where only one house has access to the loft, is it the same as no access to wall, or should access be made?
However when one plugs in a RCD tester it will give some report, as with any meter, even if it says OL which basically means out of range rather than over load, it will say some thing. So we have to look at “qualified person” means a person competent to undertake the inspection and testing required under regulation 3(1) and any further investigative or remedial work in accordance with the electrical safety standards; if he can't do the RCD test, is he a qualified person? or is the whole inspection invalid?
You should test RCDs in isolation. I have had many an RCD (esp. older ones) fail while connected to a circuit chock full of appliances.
I must agree, yes I will test from a socket and if all passes all well and good, but if it fails then there is no short cut I have to test the RCD in isolation, yes I know today there is a question as to if there is a DC current stopping the RCD working then we want to know, so maybe it should be tested under load? But the C&G 2391 training I did said it should be tested in isolation from circuits it supplies, so no real option.
As to if one thinks some thing like solar panels, inverter washing machine, or electric vehicle charging is stopping a RCD from tripping and want to get details of the suspect device, I suppose that would come under code FI, however I would think it would be a phone call can you let me view the paperwork on the solar panel etc, not simply write down code FI. I have considered if an inspector should have a clamp on ammeter able to measure 0.001 amps and select AC or DC? I have not got one, however now retired so don't do EICR using my own equipment, if I did do one then I would need to use some one else's test gear, only place I am likely to do an EICR is at the heritage railway where I volunteer and would use their test gear.
two RCD's feeding two Wylex boxes, so yes the meter shouldn't cause them to trip, but it did, back in around 1992 when they were fitted there was no electronics in the RCD and they were 4 modules wide each, and there was no 
or any other sign to say what type, however it is still serviceable, my son does intend to change it, but really no need.
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