This rule was adopted as a direct result of the death of 'Red' (his nic) -- at the time employed by ##### my old company.
I spent one-day, and only that one-day, working with Red.
He was an extremely popular apprentice that every foreman wanted on their jobs.
He died working on a Rayco NEMA3R Service box 800A 208Y120V -- IDENTICAL to one I worked on earlier -- for the same contractor and same developer-owner-customer.
When faced with the IDENTICAL requirement to land a neutral on that Rayco bus, I insisted that the panel be killed. The geometry and dimensions made it quite impossible to land a neutral with the hot bussing so close. (REALLY tight)
The Super on my job was the same man who was the super on Red's job. (!) In fact, the same developer-owner-customer was building a clone of 'my' warehouse when Red died.
I'm going to leave out the ugly specifics -- other than to say that Red should NEVER have been left to work a hot panel solo -- and that if requested, the Super had no qualms about killing all Service power -- when asked.
The job site was within one-hour's driving distance from Sacramento. So EVERY heavy weight Cal OSHA inspector flew down the freeway from state HQ.
Then my General Superintendent (and best buddy) and the owner (license holder) were absolutely grilled for a full day.
Cal OSHA came out with a safety directive within days.(!)
ALL of the foremen were given the new marching orders.
The owner-contractor hired on Red's widow -- as an office staffer -- for the rest of her days. She'd never held a job before in her life. (A very nice lady, BTW.)
This tragedy colored all of the other Sacramento based electrical contractors. None now permit 'hot' work around panels unless it can't be avoided. In which case, the tradesman has to go the full route.
The 'California rules' were adopted at the very next opportunity by the NPFA.
It's now accepted that 'hot' work -- as previously done -- is, in fact, unnecessary. It doesn't even pencil out as cheaper or quicker. No-one dares work around hot bussing at the same speed they would if it were dead cold.