Hi James.

RCD is not necessary but good ground is.

I would disconnect all the motors and start checking them one by one ie. if it runs and draw the right currant connect the next and so on, compressor first.

As these units should run with suction temp above 0, there must be a leak some where.
As I recall, the beer in the pubs isn't too cold.

If you are going to flush clean the system, replace the receiver if any present, thats where all the oil and dirt accumulate. That will save you running back to change oil.

POE is bad news. Make sure the unit was not operating in vacuum, if it did the oil need to be replaced. disconnect the compressor tip it over and wait till all the oil comes out. That will also give you a chance to see if it is clean or have metal particles in it.
You did not say if it is a semi or hermetic ( don't tip over semi hermetic )

My guess is that one of the motors is leaking and trips the RCD or burnt

Chemi