It is essential that everything you build, that has both live mains inside and a metal case or
panels, has a safety earth fitted. UK legislation says that any metal panelling should be
adequately insulated, ie. double insulated, or connected to earth. Since making a double
insulated case is not practical you should ensure that any exposed metal parts be properly
earthed.
Firstly you need to ensure that the heatsink panel is earthed. Given its size and required air
flow this will certainly be an exposed piece of metal and thus should be earthed well. It should
be bonded to earth via a short thick piece of wire back to the earth tang of the IEC mains inlet
socket.
The PSU board should be securely mounted (using all four mounting holes) onto the earthed
heatsink using appropriate screws and star or spring washers. Connect the dedicated 0V screw
terminal on the PSU board back to the panel's earth bonding point using at least 24/0.2 wire. It
may be useful to make a two bonding points on the panel. One solely for the mains earth
which can be located near the mains IEC inlet socket. And another, located next to the PSU,
for the PSU 0V and optional 4mm banana socket connections. The panel serves as a low
resistance connection between the two – although you must ensure that any paint or anodising
is scraped off around the bonding hardware. By using two bonding points in this way keeps
the 0V wiring as short as possible and reduces the risk of the grounding wires coupling with
the transformer and introducing mains hum on the local 0V.
None of this, however, may be sufficient for a solid safety earth bond as required by your local
regulations. Ideally all modular panels should be earthed directly, either with their own direct
connection to the earth bond near the power inlet, or via the modular's earthed metal mounting
rails and suitable toothed washer and screw. This isn't always practical though so is not often
done.
You will also need to provide earthing to any exposed transformer core. This does not apply
normally to toroidal types but EI types should have their metal frame earthed.
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