It really does not matter whether it’s ac or dc, the earth-wire concept is to provide local reference ..
Nowadays, we don’t use dc to transmit electricity and any electrical circuit that uses voltages below 48Vdc – 24Vac – or so – is recognized as safe, so in those low voltage situations there is no real need for an extra earth wire ..
However, when you go above the values mentioned above, the earth reference becomes a legal issue and therefore .. bla .. bla .. bla .. you have to have it ..
IanP
:wink: