very small portable gensets often do have no NE bond - both ends of the windings come out to the socket but there is no specific sense  of live and neutral. The clue is that the earth to live voltage measures some intermediate thing that is very load dependent.  It does mean that having an RCD at that point is not much use, as those the in and out bound currents are always in balance. 
Such gensets are fine for one or two items and double inusulated power tools, and are sometimes seen throbbing away at the back of mobile burger vans and on small building sites without power etc.  For more permanent installations and when more than a few items are involved, so there is somthing like a CU somewhere, it becomes better to create a clear N-E link, so that ADS and RCDs all work to plan.

An earth rod at genset becomes  desirable at the time that transition is made.
Mike

mapj1 said:

It does mean that having an RCD at that point is not much use,

Second fault, IT system ???

mapj1 said:

It does mean that having an RCD at that point is not much use,

Second fault, IT system ???

Is only useful if one of the 2 faults is between the genset windings and the RCD, and the other fault is on the load side of the RCD. I'm pretty sure 2 pole  RCD right at the genset when the genset is truly floating is not much use, against any no of faults if they are all on the load side. A similar problem arises with fully floating isolation transformers. It will help if you get a fault between windings and chassis, of course, but there are better ways to detect that.
Mike

mapj1 said:

Is only useful if one of the 2 faults is between the genset windings and the RCD, and the other fault is on the load side of the RCD.

Disagree

1st fault to Earth (downstream of the RCD), nothing happens, but a TN system is formed.

2nd fault to Earth, the RCD will then operate.

This is the case I mean

Mike