Usually when 'oversize' wire is specified it is to allow a longer length, for reasons of voltage drop or because the location is hot and ventilation is poor. I suspect if this is all co-located and well ventilated. it could easily be reduced a bit.

The MCB vs RCBO thing is more interesting, and depends how reliable earthing might be and if the designer is assuming it may be a TT supply or a higher risk location, like supplying sockets outdoors. A primary side RCBO offers no protection to the socket users of course- that would need one on the secondary side - so really we are looking at detecting damage or damp in side the transformer case causeing a fault to the case that is not enough to trip an MCB.

It looks like a 'one size fits all' diagram that is safe when kit is installed almost anywhere - it may well be that in very specific situations that can be relaxed quite a lot without any risk.

Mike.

Tom D said:

I am also questioning the wiring from that snapshot.

I am too - the secondary wiring looks distinctly odd for a site transformer - usually they're wired CTE (i.e. 55-0-55 with the centre tap earthed, in effect a RLV TN-S system) - this one looks to be wired as a 110V LV IT system.

   - Andy.

Correct. Should have been wired that way.

I think I know why external cable are 4mm2. According to Emergency disconnector datasheet the terminals can accommodate wires min 6mm2/10mm2 depending on the wire type so it's not perfect but I understand the idea of the size increase. This 110V circuit is going to be fitted in PRR so it maybe worth stick to RCBO then however shouldn't it be something higher than 16A? 

Indeed - unless there is a good reason, then 'normal' site practice is a single split phase and earthed in the centre.

Then its not strictly  L1 and N1 secondary, as both are equally live - think more like L1 and L2.

Also one might expect some sort of breakers on the load side if one transformer is feeding many sockets as there is scope for over loading one socket.
16A at the primary side would equate to  about 30 something amp on the 110 side of things so if the secondady sockets are 16A ones, then that too needs a  bit of thought.

Mike

edited for grammar/clarity.

mapj1 said:

Then its not strictly  L1 and N1 secondary, as both are equally live - more L1 and L2.

Yes, this secondary arrangement is properly called mid-point earthing, and definitely L1 and L2. The 'midpoint' or 'neutral' is not distributed as a live conductor (but it could be - they do this in houses in USA, where they provide 220 V single-phase, mid-point earthed, to a dwelling, and lighting and small power uses the 110 V from L1 to N and L2 to N, whereas larger appliances such as laundry appliances, use the 220 V L1 to L2 single-phase supply, with 'ground' connected to N at the transformer.

After speaking with electricians in my place and finding this info below I am going to use MCB type D as a protection of this isolation transformer not RCBO previously considerated.

"Will and RCD work with an isolation transformer"...

But note that they're talking about RCDs on the secondary side of the transformer - not the primary (feed) - and if yours ends up CTE (as above) the bit about No Earth isn't quite the case either. RCD/RCBO on the primary side would have to be decided by the usual rules (e.g. soft cables concealed in walls, run in some special locations, fed via a plug & socket, or high Zs (e.g. TT systems)) for the primary circuit.

  - Andy.

True. So should I treat this circuit same as regular 230V socket circuit and fit RCBO for this purpose? I am confused now. There 2 x 16A sockets on the output of this TX with no MCBs dedicated.

Below is the unit I am thinking to use.

I don't suppose they bother with MCBs on the output, because there's one on the supply.  For a 230V to 110V step down, the input current will be pretty close to half the output current.  So if the correct MCB is specified for the input, it will protect the output too.