Dave, Transformers of the same type can often be parallelled without
problems, but you need to be aware of the potential safety hazards in doing
this. If you isolate the primary of one, then it will still be live at 240
Volt, being backfed via the other. I'm not sure what the regs say about
this, maybe someone else can advise...? You shouldn't proceed until this
has been clarified, some other poor person working on such a system
unawares could be killed..
If the transformers are in the same enclosure - and BOTH primaries are
connected to the SAME mains power cord (permanently wired via a common
terminal block in the enclosure with the transformers etc) and both
have a common mains fuse/switch etc - then the setup should be just as
safe as with a single transformer.
I dont know about the regulations - but common sense would tend to
suggest that this method is fine. Actually on one occasion SC did
publish such a design for a car battery charger with 2 paralleled 12v
transformers (as 2 smaller ones fitted into the compact case used
where one large one of the same VA rating wouldnt) .
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If they are these "consumer" type transformers with a mains cord and
plug already fitted (as I would imagine to be the case with a garden
light transformer) - and you plan on using 2 separate mains plugs
into 2 power sockets etc - then DONT DO IT. It IS a serious safety
hazard as if one plug is plugged in and another isnt - then its pins
would be LIVE !