"Site transformers" are UN-known in the USA (can't even be shipped to the USA). So you should discount anything from anybody in the USA (even me).
The actual specs for site transformers sold in England (or Mid Wales) are very obscure. Is the 110V "centre-point-earthed" or "floating"? BS7671 seems to require centre-point-earth; a bloke who opened his site-transformer (why?) could not confirm that.
So is it PELV or SELV? If you do not understand that question, you maybe should not be re-wiring specialty electric supply devices to unapproved purposes.
The customary US power is 120V with one side grounded at the fusebox. The White wire shows low voltage (<3V) and low resistance <1 ohm) to the green wire.
This is *NOT* the same as 110V earthed-center. As you found, you have 55V from either White or Black to green.
In fact this is not illegal in the US. And 99% of US gear will tolerate 120V center-tap. There's audiophiles who do this. There's also (on this forum) a report of a mishap where a small fault in such a system BLEW-UP thousands of dollars of gear. Power schemes where neither current-carrying conductor is "near" the earth/ground conductor are so rare (world-wide) that equipment is not tested for faults which would only appear in such unusual systems.
I think you need a REAL electrician versed in the depths of BS7671. This is not a situation to rely on idle net-chatter.
Or---
http://www.airlinktransformers.com/american-transformers.asp
http://www.airlinktransformers.com/transformer/us10002-american-120volt-transformer.asp
US1000/2 1kVA American transformer 230V to 2 x 120V (3 pin US plug outlets) British built £64.00
This can be used with standard US 6 way power strips (if you didn't cut-off the US plugs) to power a couple of guitar amps or a whole lot of small stuff.
I _REALLY_ think you want an "American Transformer" plus some power strips. It is made for the purpose. It should be cheaper in a thin tin case than the drop-proof housing required on a site transformer. (Though I don't see a big difference in price.... maybe low demand?)
You STILL have the issue that much US gear, especially older pre-world-market gear, is wound for 60Hz power. A 50Hz power transformer must be a bit larger. Some 60Hz gear does not have enough safety-factor to survive 50Hz for long. (Also any very old clock or record-player will run 50/60th speed.)
For US readers:
"site transformer" means "Construction Site". Power tools used in mud. These are dangerous on US 120V power, more so on 230V power. BS7671 is similar to our NEC Code and specifies special wiring and supply to such situations. The average builder does not need the details: in the UK the tools, cables, and transformers are made to meet the requirements.
> to make this safe...
The kit as sold is "safe" for the intended purpose. I believe it is safe in a dry environment.... except that the US-market loads are not certain to be expecting a center-tap supply (even if Equi=Tech sells such gear).
> RCD will only be effective if
If the 120V winding is truly floating, the RCD can never trip. If it is centre-tapped, then neither current-carrying conductor is "neutral" and bonding one to earth makes a dead-short.