[sdiy] Quick Question about Grounding Power Supplies

[Richie Burnett]

Yes, the mains protective earth should be connected to the 0v output on the low-voltage side for safety, (and the enclosure if it is metal!) Otherwise one of the following scenarios can result in dangerous mains voltages appearing at the low voltage output:

1. A high-voltage mains wire becomes disconnected and comes into contact with one of the low-voltage wires on the secondary side of the transformer.
2. The transformer's primary/secondary insulation fails and they windings come into contact. 
3. There is a high-voltage transient (potentially 1kv+) on the mains supply that could arc over to the secondary side wiring if creapage and clearance distances are not sufficient. 

If the secondary side is earthed (grounded) any of these scenarios should result in either a blown mains fuse or a tripped circuit breaker. Either way the mains power is cut off due to the large fault current flowing to earth and nobody receives a nasty shock.

Only exception is if the transformer is Class 2 (double insulated.)  Many small wall wart power supplies are made this way. Often potted to ensure the high voltage and low voltage sides cannot come into contact, and prevent flashover etc.

-Richie,

Sent from my Xperia SP on O2

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