I wonder if anyone can help. I'm having my kitchen refurbished including the electrics but when my partner tried to insert and secure the mounting boxes into the wall the plaster around it has crumbled away. There is now not enough plaster remaining to secure the mounting boxes in place. All the trunking and wires are in place now but we just have this one mounting box that we can't secure. Is there a way to fix this issue? Any advice would be greatly appreciated. Thank you
At this point, I don't think an old-work box can be used. As bib mentions, you need a sturdy surface to attach these. You also have a plaster repair in your future, so if you're going to patch that, you may as well go with a new-work box. You have two styles to chose from, one with a tab that attaches to the face of the stud and another that nails into the stud:
I'd recommend the version that nails into the side of the stud, but I'd also recommend swapping out the nails for screws. The process of nailing this box in will disturb a lot more of the plaster.
Go ahead and open the wall up enough to install the new-work box, get the cables inside, screw it to the stud, and secure the wiring with staples to the center of the stud. Then patch your plaster by installing some backing material, applying the plaster with a wide knife, and smoothing the joints so they can't be seen with a light shining at a sharp angle.
An electrical box is subject to strain as plugs are inserted and removed. It needs to be firmly mounted for safety.
In old-work (retrofitting), you need to either mount a box securely to a framing member, such as a stud, or mount it securely in a solid wall structure.
If the box is near a stud, and the wires can reach, it can be screwed directly into the stud.
If not, you need to be able to reach sound wall material.
Some old-work boxes have wings that grip the back of plaster or drywall as the flanges of the box grip the front.
Also, there are outlet brackets that slip into the wall cavity and then bend over the edges of the box, again using the flanges on the front to hold the box in place.
In both of these cases, you need a fairly solid wall surface, both near the flanges and where the brackets grip the inside surface of the wall cavity. if you do not have solid wall in both those places, you need to rebuild the wall surface to attach the box. This can be done with a drywall patch. If you need to do this, you will be cutting away the crumbled areas, and you may have room to insert a small framing piece for a more solid mounting.