venta

You need to fill the hole with something onto which the filler will grip; what you use depends on the size of the hole and, as there's wiring there should be non conductive and not flammable. I've filled large gaps with everything from crumpled chicken wire to Hessian with some lathes to hold it in place before applying plaster but for this I suggest something like Carlite Browning which is a rough cast plaster

Oh and stabilise any crumbly edges first PVA or you can get Stabaliser