Some amd3 boards hinge from the bottom with little catches that keep them shut. Not all are top hinged with gravity doing the rest.
Those manufacturers are going to make a killing selling CU's for being mounted sideways then. Only kidding
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Below is not aimed at TheGayOwl, I can't double quote, must brush up on my skills there!
However my two penneth; if you have to mount a CU vertically because of lack of space to mount it horizontally (if horizontal is the way that the product has been designed to be mounted), by definition isn't that suggesting insufficient accessibility, reg. 132.1.
Reg. 537.2.2.2, 510.3, 511 reading of labels for isolation and circuit identification etc. To have to turn your head on it's side to read the labels, not all people are able to do so (e.g. those physically impaired), non compliance with those regs.
Reg. 510.3, 511. If the manufacturer has diagrams showing CU mounted horizontally or adds devices (level bubble) to assist in that, surely 'documentation would be required from the CU manufacturer that the specific orientation is suitable and comlies with BS EN 60439-3' (citation ESF).
Obviously, if you have considered those things and carried out a risk assessment & documented it, it's down to the individual.
I've read a lot of posts of disparaging the mounting of sockets in kitchen cupboards for example, but somehow its seems acceptable to mount a CU in a vertical plane, because of limited space?
