Building a Power Supply in a metal case

[Voltz]

May sound like an extremely noobish question (sorry) but does it need to be earthed or what?

 

[Hero999]

Yes the metal case does need to be earthed, unless it's double insulated which basically means that you've designed it in such a way that no failure can result in the metal case becoming live.

In short it's easier to just earth the case rather than making it double insulated.

 

[bychon]

Just in case you need reassurance, Hero is right. The general rule is: Any circuit that is connected to the mains gets its box grounded. That is the safe way to do it.

 

[Hero999]

Note that just earthing the case is not the only thing you have to do to ensure electrical safety, although it is perhaps the most important thing you should do.

For protection against fire:

• There should be a suitable fuse on the primary side of the transformer: slow blow and double the primary current rating is normally good enough, for example, if you're using a 50VA transformer with a 230V primary, I = 50/230 = 217mA so a 500mA fuse will do.
  
• Ideally the transformer should have an inbuiilt thermal fuse: generally smaller transformers do (<25VA or so) as a short circuit won't be enough to blow a fuse before the transformer catches fire. For large transformers, it's less important as the mains fuse should kick in.
  
• And all cables and PCB tracks should be large enough to carry the required current.

There needs to be adequate insulation and separation between the primary and secondary sides of the circuit:

• Use a transformer with adequate insulation between the primary and secondary - don't worry, if you bought it from a good supplier recently enough, this should be the case as long as it's not an autotransformer which shouldn't be used.
  
• And make sure there's at least 7mm of clearance between the mains and secondary tracks on the PCB.
  
• The enclosure should be well made enough to ensure it's not possible to come into contact with and parts of the circuit at mains potential.