There-in lies the problem. Well-meaning technicians make undocumented modifications over several years (or decades) and you get to the point where what you have bears little or no resemblance to the "as-built" drawings.
I once was presented with a gas turbine compressor control system that bore no similarity to the drawings. So much so that systems that were expected to be powered had no power and vice-versa. The unit still ran (most of the time) and shut-down on demand (most of the time) but obviously needed to be sorted out
Having spent several weeks trying to trace wires in trunking without causing "accidental" trips I came to the conclusion that the only proper way to resolve the issue was to rip the whole system out and replace it.
I managed to get this approved, spent £2 million building a new system only to see it all moth-balled to avoid loss-of-revenue. At that stage I decided that early retirement was the better option.
Now here is the irony. I could work on massive gas powered gas turbines (on the basis of my professional qualifications and accreditation with HSE) but touch a piddling domestic boiler without a GS ticket and there'd be lots of wailing and gnashing of teeth.
A few loose wires need putting back in the trunking and some kind of management system needs to be built for all those SWAs at the bottom but other than that it honestly doesn't look too horrible to me.
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