Attached is a picture of an early draft of a relay board I am designing just for non-commercial use in our home, to work around limitations in an existing central heating controller.
The (hard to see) grid overlaid on the image has 5mm squares.
I am mainly interested to see if any other users have comments about potential mains-isolation / safety issues in my design, as I am not experienced in designing for mains voltages.
The board itself will live inside a box with no user interface or buttons (just a closed box with air-vents, etc) as it will be controlled from outside wirelessly. My main concerns are just the passive safety of the board wrt itself (i.e. that it shouldn't set itself alight, etc, and that if it fails it does so gracefully) through reasonable compliance with sensible guidelines.
The board has a HV side 80% of which is just relays to turn on and off 240VAC based central heating zoning valves, etc. The remaining bits are a fuse and an (optional) off-the-shelf encapsulated PS that might be used to power the LV side of the board if that side doesn't get battery powered.
The main place where I have my doubts as to regulatory compliance concerns the pads under the POWER-IN connector near the 3A fuse. It has 2mm-wide pads with a 5mm pitch which puts 3mm creepage between the live pads. Nearly every ebay-sold optocoupler relay board has these connectors for the mains side of the relays, so I imagine they are pretty safe in practice, however that does not necessarily mean they meet regulations. I am not too worried about the space between the pads on the other connectors as they come AFTER the fuse, where I believe 3mm is OK?
However, is the Live-Earth separation OK between the two pads at the extreme bottom left?
[ Aside 1: Although I have put a 3A fuse on the board, I should note that the power that supplies it will have come from a permanent wiring to the existing CH controller which has it's own 3A fuse. So perhaps the "pre fuse" part of my board counts as "post fuse" ? ]
[ Aside 2: The cyan thing above the fuse is a slot cut to increase the creepage between its pad and the LV side of the board. The board will be coated (except on its pads, of course) with insulating solder-resist, and the components are all encapsulated except on their legs, so I think the problem is confined to assessing pad safety, and board to enclosure safety -- not worries about track-to-track safety on the HV side. But again, please comment if you think there are big problems I've not spotted. ]