First, a step back and a think about what's been posted so far ...

Most portable dual output 3.3 kVA site safety transformers have neither fuses nor RCDs at their output, but they do have:

• 13 A fuse at the input (in the BS 1363-1 plug); and
• a thermal overload trip

Does this mean they will not provide 5 s disconnection time under fault conditions? (Anyone wanting to think this through could assume an impedance voltage of, say, 5.6%.)

Does it mean they there is inadequate protection against overloading the socket-outlets?

Tom D said:

Can someone confirm if in place of FUSE 1 ca be used 25A RCBO Type D

If you are relying on the primary OCPD for ADS fault protection, you will need at least 100 A to operate a 25 A BS 88-2 fuse in 5 seconds ... a D25 needs 250 A (see Table in Fig 3A6 in BS 7671)

... but for general overcurrent protection  you'd need to compare adiabatic. If the D25 is operating in faults at less than 0.1 s, you would need to consider the I²t provided by the manufacturer.

There's not much benefit to this being RCBO as it won't pick up residual currents on the secondary (output socket-outlets).

Tom D said:

FUSE 2 16A MCB Type C  please?

Again, there are similar considerations of fuse vs mcb here ... although a C16 that could be reset by users (if in a DBO) might be advantageous, and provide selectivity.

I'm not clear what sort of overload if any needs protecting against assuming the thermal trip works and why that then needs to operate  within 5 seconds. Transformer heat up times at 100% overload are more likely to be closer to 5 minutes.

Note that the input current spec tells us that transformer is designed for more like 1500W steady state, and 3300 watts for short periods whatever that  'intermittent' spec really means. **

It may well be quite droopy at that full intermittent rated  load - could be more like 10%.  - so a dead short may not draw enough to prompt trip something on the primary either.
Mike.

** this one, is similar weight, and their the 3kVA rating is for  5 Mins on 15 Mins and half that indefinite.

I'm not clear what sort of overload if any needs protecting against assuming the thermal trip works and why that then needs to operate  within 5 seconds. Transformer heat up times at 100% overload are more likely to be closer to 5 minutes.

Note that the input current spec tells us that transformer is designed for more like 1500W steady state, and 3300 watts for short periods whatever that  'intermittent' spec really means. **

It may well be quite droopy at that full intermittent rated  load - could be more like 10%.  - so a dead short may not draw enough to prompt trip something on the primary either.
Mike.

** this one, is similar weight, and their the 3kVA rating is for  5 Mins on 15 Mins and half that indefinite.

mapj1 said:

I'm not clear what sort of overload if any needs protecting against assuming the thermal trip works and why that then needs to operate  within 5 seconds.

The thermal trip doesn't need to operate in 5 seconds ... nor does the OCPD providing overload protection.

The 5 seconds is for ADS.

For this particular size of transformer, only reason I can see for OCPD on the secondary side at all, is to provide selectivity in case of a fault L-L or L-PE.

The primary OCPD can protect primary and secondary against overload, and also theoretically the cables connecting to socket-outlets if we select things to meet adiabatic.

A primary-side OCPD will see faults L1-PE or L2-PE on the secondary, and may be suitable for ADS ... but calculation to prove it isn't as simple.