One of the standard 3-phase systems here in the U.S. is 120/208V, but as you appear to be in Northern Ireland, you'd have 230/400V, i.e. the voltage from any phase to neutral/earth is 230V but the voltage between any two phases is 400V.All control (mixing desks, radio racks, 1ph amp racks (for monitors), backline (instruments, amps) etc are ALL on the same phase (all the single phase outlets are on the same phase on the distros) and generally all the larger amps for PA are then wired evenly across all 3 phases. (most of the distros have a ton of 13/16/32a 1ph and then 4 or so 32a3ph and usually 63a1ph x 3 on each phase and also usually a 63a3ph through) So as a result you end up with a little more load on the first phase. The reason being, which I have been told, is that if there was a fault between an instrument and the sound equipment on different phases you would then have a potential difference 208v.
But you'd have to have a pretty unlucky set of multiple faults in order to end up with 230V mains on two separate pieces of equipment such that you get 400V between them. And even though it could happen (however unlikely), it could just as easily happen between your low-power preamps, mixing desk etc. and one of the power amplifiers.
That's not to say that this unlikely sort of set of circumstances has never been considered.....
No, if you have a standard 3-phase supply, you'll have 400V between phases. That's just the way the system works: The phase-to-phase voltage will be equal to the phase-to-neutral voltage multiplied by the square root of 3.If you have a 3phase supply, what prevents potential differences? Perhaps modern day MCBs etc prevent this, and its become just habit for some of the longer lasting companies around.
There used to be rules in the IEE Wiring Regs. which attempted to minimize the chances of this higher voltage appearing between two pieces of portable equipment in close proximity even in the event of the unlikely combination of faults all occurring at the same time. They specified that sockets within one room of domestic premises must never be on different phases, and that even in commercial environments there could be more than one phase on sockets within the same room only if it was impractical not to do so and then only if sockets within each area of the room were grouped together by phase and at no point were two sockets on different phases within 6 ft. of each other.
The same rules applied to the single-phase 3-wire supplies we're discussing here (and to the old 3-wire d.c.supplies), in which the voltage between sockets on different phases was generally even higher than that between phases on a 3-phase system (up to 500V in some cases).