In my point of view, facing that situation will delay the project. But if you are working with not stable persons, they should produce enough detailed information and documentation and work in short period tasks, that could allow others to continue with the project
Right now I'm facing that situation I got a entire 2 years old project with out, any documentation, so I have to start from -10, but the contractor started making foundations. And I have to try to understand what they where trying to do, and explain my boss why it is wrong before doing it right. I just brace myself and do my best.
One thing is divide the project in "zones" (you put the design criteria, "the rules") if is possible, like a Lego and finally you put all pieces together if someone goes away other could absorb the work. (That could work for small projects). Better to put an extra work to the project guys than getting a new one ( but you need to pay them to make things work) like the mythical Man-Month said:
"Adding manpower to a late software project makes it later" but I think that if the project needs a lot of communication it can apply.
If the project is big enough you need a good team and specialists (protections,cables, "software analysis", like a dedicated "ETAP" engineer to test the system for example) and that just for the power part. Then maybe others guys from mechanical or civil engineering. So try at least to find 1 experienced engineer per branch, with good commitment. Big mistakes occur when bad communication occur between the civil,mechanical and electrical engineer ( Is like "the tale of the ground electrode and the bulldozer").