Tuesday, October 20, 2009

For a class-1 device you need a mains earth!

We had a monitor that was giving people mains shocks off the SDi BNC connectors. I opened it up and there was no earth from the mains inlet to the metal chassis. No then - if you want to have no internal earth you have to have a plastic case and one other layer of insulation between the user and any current-carrying conductors (that can be a layer of air - but the device must be 'double-insulated' AKA 'class-2'). I hope this was a manufacturing fault and not a design oversight!
It's not unusual for equipment (particularly with a resonant or switch-mode power-supply) to have the internal earth float at half-mains (with a very high-impedance to the power-source, no real ability to deliver any current), but that's why you need a safety earth connection if you can touch the chassis-earth (on the BNCs, for example) and not get a little belt!

before - oh dear! - after - much better!

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