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.
An earth bonding point using an M4 screw, two washers, two toothed washers and nut. The earth wire
uses a crimped 4mm ring terminal.
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 0VB screw
terminal, either directly or via the distribution system, either back to the panel's earth bonding
point using at least 24/0.2 wire, or by making a second bonding point on the panel. For the
latter, you have one bonding point solely for the mains earth which can be located near the
mains IEC inlet socket, and another, located next to the PSU2 board, for the PSU's 0VB and
optional 4mm banana socket connections. The metal panel serves as a low resistance
connection between the two bonding points – 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. 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.
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.
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