That's the point of "double" insulation, surely? No single insulation failure can cause the case to become live. Otherwise one's premises would have to be festooned with earth wiring. Metal pipework is a special case because of its historical use as a common earthing point as well as typically being close to water....
You'd think with all that metalwork, there'd be an earth simply for "metal parts" protection, e.g. the chassis becoming live because of something external to it, i.e. bonding?
Indeed. Example from our kitchen: the dishwasher with a live back panel and the TV with a live aerial after suspected rodent rewiring of the ring into several disconnected pieces, different for each conductor :-(((There is a potential (excuse the pun) problem with earthing the case. If the shared earth wiring, or the neutral/earth in a pme system, becomes faulty then all the earthed equipment can become live. ...
sorry, no I was talking about the possibility of a large piece of metal becoming live from something else, not from within the Humax itself.That's the point of "double" insulation, surely? No single insulation failure can cause the case to become live. Otherwise one's premises would have to be festooned with earth wiring. Metal pipework is a special case because of its historical use as a common earthing point as well as typically being close to water.
Yes, bonding is (was anyway, the regs change periodically) required for exposed metal in high risk areas such as bathrooms and kitchens. This is partly because the presence of taps, etc, which are already bonded and a person with wet hands and maybe feet presents a particularly high chance of lethal shock.I think that protection against that is called bonding, rather than earthing, and that's why for example my steel trunking in the kitchen is "earthed". The point being that if it's accidentally made live by a fault in something else, e.g. the extractor fan it's close to, it's not a danger.
Yes, that's the broad principle of a switch-mode power supply (there is not always feedback to the HT [high-tension] side, when there is it isn't always isolated optically [can be isolated by other means], and [depending on intended application] the HT side might not be isolated at all).Typically, and I think in this case, the mains input is rectified to generate a higher frequency waveform that can be down-converted more efficiently, with optical feedback from the low voltage side controlling the mains voltage oscillator.
I am not familiar with transformer safety requirements. With regard to creep distances, it's a "pass" as long as a certain minimum separation exists between immobile conductors.The insulation has to be achieved by wide separation (even slots) in the PCB between the mains and low voltage sides and by design of the transformer.
Good point, I had forgotten that... but as a last resort.Nowadays a double insulation failure probably just throws the RCD