If you fit metal light switches, then for safety (your safety!) they need to be earthed. The problem is that on older installations, you may find that that their does not appear to be an earth-wire in the back of the box….
Or if there is – you could have another problem…. Is it actually connected to earth – or does it “just go somewhere” ? Again in older property (and modern installations that have not been installed correctly) it was not unusual for the earths on the lighting circuits to be left un-connected.
I experienced this recently at a house. The existing lighting circuit had recently been extended by “another electrician”. He had not confirmed that the existing circuit earth was actually connected to anywhere… actually he didn’t really connect the new earths together either – at least not in compliance with current regulations – which he should have been working to…
The customer wanted to fit a nice chrome metal light switch. The earth wire was in the switch box – Good… NO NOT GOOD…. I tested the earth wire before connecting it – it had around 120v ac on it! – this would have been “leakage” (actually capacitance) from the live conductors in the cable. If I had just connected this as it was – someone could have had a nasty surprise when they turned the light on or off – especially in bare feet!
Fix: I had to check all the lighting circuit connections back to the consumer unit, ensuring that every earth wire was correctly connected together at each joint. I did also check that the consumer did actually have an earth on it – I’ve come across that problem in the past also!