Hi
@gmdaly!
As an industrial/commercial inspector having dealt with a few assessors, I concur with several comments made above and summarize for you what your registration body is likely to consider the "correct" usage of the model form.
An FCU is merely another accessory whose purpose is to provide a form of additional (derating) protection for a cable feeding a piece of equipment (the fact that the "equipment" may be a string of socket-outlets is not relevant in this context).
So: no it does not become considered the DB in this context, it is not considered different to any other accessory on this circuit that you have broken into, the Z(db) is at the DB supplying the circuit.
You have the mindset that the form wants to record the "circuit" you are "adding" from the point at which you are adding it. Adjust your mindset because this is not what the form was intended for: you should realize that BS7671 is only interested in the safety of the installation, and these tests you are doing are to prove,
a) that the circuit was safe enough for continued use in the first place for you to work on in (hence its interest in the means of earthing and bonding, frequently making you go and look under Mrs Miggins's kitchen sink for the stopcock);
and b) that you have not reduced its safety or changed the performance of any safety device by your working on it, by cutting the cable to break in to the circuit and insert your new accessory.
(Remember that, if you were actually adding a new circuit with its new OCPD, which you are suggesting is "effectively" what you are doing, then you in fact need to use a full EIC with its schedules. Same if you change the operating characteristics of a circuit's OCDP, e.g. by changing the rating of an MCB: full EIC required.)
So, in this context: your OCPD is that of the circuit you have altered but, because the spur protected by the FCU supplies one or more socket-outlets, the Z(s) is taken at the furthest point of the effectively extended circuit, the last socket. (Note the term: it is an altered and extended circuit, not a new or additional cct.) This enables you to test both that the circuit protection is adequate and that the BS1362 will operate thus adequately derating to protect the spur. (Normally, of course, you take the Z(s) at the FCU in that horrible method of leaving the accessory hanging off the wall live while using all three hands to hold your probes at the very convenient angles that every manufacturer designs for the supply terminals to be readily accessible for testing, while using your toes to hold the faceplate stable. But in this context your spur isn't supplying fixed equipment, so you get to use a plug box instead, yay!)
Maintaining the correct attitude to what BS7671 actually wants from your inspections and tests frequently makes uncertain situations far simpler to resolve. Many questions of the nature "do I need to test this or can I get away with that" are resolved by simply remembering "what is the real purpose of this test, what test do I actually need to do to determine that, is this reading satisfactory" and not going any further than that for fear of making unnecessary work for yourself. Extra testing and record keeping beyond ensuring safety of property and life, is only required for fault finding investigations.
