Thanks for the reply.
The plan is to use a 12V power supply to power several items: (a) 12V PIR detectors; (b) waterproof LED strip lights; (c) CCTV cameras. No extra earth connections.
The complication is that they may be outdoors, but all connections will be made in IP55 boxes.
The alternative power supply is something like this:
Buy LED Drivers LED Driver, 75W 24V 3.15A IP65 PowerLED PCV2475E online from RS for next day delivery. which is SELV.
So, the question really is - is a computer ATX power supply used for this type of circuit PELV?
Thanks for your help.