Balanced Mains Transformer

The RCD being 63A is completely irrelevant. The rcd is NOT an over current device and will NOT trip out should current exceed 63A. The rcd will detect an imbalance between phase and neutral and trip if current leakage exceeds 30mA. To protect the cable feeding the airline you'll need the correct rating of MCB after the RCD. I should have thought the airline would have its own over current device. 

There seems to be some confusion here: if it is a plain RCD, capable of switching 61A when the earth leakage exceeds 30mA, then it would not be a problem (and the electrician wouldn't say it is) feeding a device needing 13A protection, however there should be a fuse or MCB (miniature circuit breaker) In series with it to protect the transformer.

Alternatively if it is an RCCB - a residual current circuit breaker - that combines functions of RCD and MCB then the electrician's advice would be correct (alternatively adding a fuse or MCB between it and the transformer).

i suggest doublecheck with the electrician as to what it is and what he meant, just in case something has got lost in translation. Meanwhile maybe people's experience re any sonic effects would prepare  you to make a decision at that point.

Ok -  thanks, obviously some confusion on my part but in essence airlink suggest putting either an mcb or a RCBO either side of the transformer. Are there any views as to whether using these in a dedicated mains supply has any impact on sound quality?

The answers given are electrically correct, however an RCD on a balanced supply should never trip since the balanced power lines are isolated from Line and Neutral of the mains, so there should be no significant residual current flowing even if a low impedance path is created between a balanced line and protective ground (which is still needed btw).

Overcurrent protection of 13A (max) is still needed to protect the transformer.  Where this is placed is another question - before and after the transformer is certainly the safest option.  However personally I'd very much prefer to have the overcurrent device rated at 0.8x the current rating of the transformer.

Huge posted:

The answers given are electrically correct, however an RCD on a balanced supply should never trip since the balanced power lines are isolated from Line and Neutral of the mains, so there should be no significant residual current flowing even if a low impedance path is created between a balanced line and protective ground (which is still needed btw).

Overcurrent protection of 13A (max) is still needed to protect the transformer.  Where this is placed is another question - before and after the transformer is certainly the safest option.  However personally I'd very much prefer to have the overcurrent device rated at 0.8x the current rating of the transformer.

An RCD trips when an earth leakage occurs, the current in live & neutral should normally be equal. An earth on a live or neutral causes an inballance in the wheatstone bridge or current transformer monitoring current flow & tripping at preset current leakage - 30mA. So if you get your pinkies in the wrong place you'll just feel a tingle before the RCD clicks off - without such protection the electric shock would be more severe.

An RCD does not provide over-current protection hence the requirement for one. I dont think an RCD placed before the transformer on primary supply would work for a fault on the secondary side.

A smaller rated RCD will probably have smaller surface contact area in the switch - uncertain if that would affect sound & I'd follow the electricians advice for ratings & where to fit protection as the regulations & requirements differ, but 80% of the transformer rating sounds about right.