The same manufacturer who likes to stencil "No User Serviceable Parts Inside, Do Not Open" just to get you to throw it out and buy another?
Some days you can do it exactly like they say and go home. But there are the times you have to read the instructions and understand what they want and why they want it - then ignore parts of the instructions and do your work in a Safe and Workmanlike Manner. And be prepared to defend your logic and decisions.
Often because the customer isn't going to pay you to rip out the entire panel and start over just to make it look like the ideal pretty picture on the cover of the instructions.
That instruction sheet can be a valuable resource - but a lot of the stuff they print is in there solely for Lawsuit Immunity and nothing else.
Sunshade instructions printed on the back side: "Remove from your car windshield before driving the car." You say "Well, Duh!" but they have been sued for far stupider stuff before - and lost.
Example: Do you always shut off power to the entire building just to work on a branch circuit? The first paragraphs of the instruction sheet are inevitably a whole laundry list of Shall Nots, and the top of the list is "Never Work On Any Electrical Equipment Hot" - and the customer is running an office with 50 computers.
If you shut off the main or start killing branch circuits at random during the day without a Real Good Reason (usually involving flames) they're going to be carrying you out the front door in a pine box, toes first.
And your customer is not paying extra for night work either, so you'll just have to eat it when you come back. IF you can even get access after hours at all - the customer doesn't want to pay an employee or a security guard to let you in and babysit the site.
What color is the sun on your homeworld? Theoretically, you are correct. Practically...
A proper installation with a fused disconnect switch should not have the Line and Load leads crossing inside the can, just in case things get hot and they short against each other. Practically, you aren't going to rebuild an existing old power system just to avoid this.
You call it "Grandfathered" and move on. You do not have to bring the entire1953 installation up to 2009 standards just because you change a bad breaker.
Theoretically, you aren't allowed to modify any pieces inside the panel, you have to replace with all new - but try to order piece parts. For new panels they are not easily identified by part numbers, prohibitively expensive and have a real long lead time. For old panels they are flat out not going to be available, especially if the maker has been out of business for thirty years.
Practically, you get a chunk of 1/8" x 3/4" radius edge copper bar stock and make new main buss bars for that Zinsco panel. Done.
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