I have turned it into a bit of a rambling conversation, as i am wandering about another thought came into my head.

Thirty five years ago i was working on the one site as a carpenter, the companies H&S guy told me i had to have an earth rod when I was using my petrol generator, so I got a road form pin off the groundworkers, along with a pipe earth clamp and some 10 mm green/yellow off the electricians, who thought it was hilarious, and earthed my generator whilst I was working, I still have that rod somewhere in my garage.

Roll on a few years, after I had gained electrical qualifications and had my own test equipment, I loop tested one of my 110 volt transformers on both the primary and secondary sides, with and without the installation earth connected and actually confirmed for myself that the earth connection on the secondary side is not reliant on having a good installation earth connection, it is a complete fresh start from the transformer onwards.

So the rod on my generator was only a reference to earth and as i did not have an RCD, was not going to ensure any circuit protection operated, but all my tools were double insulated anyway.

I loop tested one of my 110 volt transformers on both the primary and secondary sides, with and without the installation earth connected and actually confirmed for myself that the earth connection on the secondary side is not reliant on having a good installation earth connection, it is a complete fresh start from the transformer onwards.

Although a loop test only shows the loop impedance, rather than a connection to true Earth. So while ADS might work fine, there may be other problems. A floating system, if supplied through an ordinary transformer, can be capacitively coupled to the supply windings - so might in fact "float" at say half mains voltage (a lot of variables there, but as a for instance) - so 55-0-55 might in fact end up as 60-115-170 and while the available current will be low, it might still be enough for an unexpected tingle or jolt from anything with exposed-conductive-parts. Normally the secondary centre tap is connected to the primary c.p.c. in lieu of its own electrode - so ideally you wouldn't want to connect it to a PME system and then use it outside.

Don't forget that the secondary loop impedance also depends on the primary L-N loop too (the power to drive the fault current has to come from somewhere), so perhaps not entirely a fresh start.

    - Andy.

For construction sites generators are covered under BS7375 + BS7340, not sure if they were around 35yrs ago

much debate remains especially with smaller gensets, stuff like - less than 5kva doesn’t need any earthing, is what you’ll often hear.
This is only partially correct, the code states the task should be short duration with only 1 item of class II equipment in use - but this is a pretty broad statement, and doesn’t say if the generator output should have a RCD or not - often they don’t. 
like most things these days it will come down to the risk assessment, because if there’s an incident the first thing they do is look at the paper work. 

much debate remains especially with smaller gensets, stuff like - less than 5kva doesn’t need any earthing, is what you’ll often hear.
This is only partially correct, the code states the task should be short duration with only 1 item of class II equipment in use - but this is a pretty broad statement, and doesn’t say if the generator output should have a RCD or not - often they don’t. 

Unearthed, the generator would usually meet the requirements for a separated system - when feeding just one item of current using equipment (possibly better read as a system having at most one exposed-conductive-part), the principles are covered by BS 7671 section 413 - generally the system remains perfectly safe on faults and ADS of any kind isn't required. RCDs would be pretty pointless as the system isn't referenced to Earth. Feeding two or more items of current using equipment (or rather having two or more exposed-conductive-parts) is slightly riskier and is covered by BS 7671 418.3 - while it remains safe on 1st faults, a 2nd fault to a different exposed-conductive-part from a different line, raises the possibility of a shock between the two exposed-conductive-parts. Again RCDs wouldn't help, unless the two exposed-conductive-parts were fed from two separate RCDs - a single common RCD wouldn't see any imbalance. Generally ADS for 2nd faults is handled by connecting all the exposed-conductive-parts together (not to Earth) and using overcurrent devices to disconnect.

  - Andy.

For an interesting read up on the topic, consider consulting ‘Temporary Power Systems’ by James Eade.

It is a subject that causes a lot of confusion, mythbusters #6 has some good breakdowns in it, for construction sites though it’s BS 7375 that causes the contention. Then add in that the majority if not all manufacturers tend to write in their instructions - ‘must be earthed’ often referring to a multitude of different international standards. 

It is a subject that causes a lot of confusion, mythbusters #6 has some good breakdowns in it, for construction sites though it’s BS 7375 that causes the contention. Then add in that the majority if not all manufacturers tend to write in their instructions - ‘must be earthed’ often referring to a multitude of different international standards.