T2 changing channels, volume and going into standby by itself

[Black Hole]

'Shorted turn' springs to mind BH. Not good.

Correct... but not until you connect the unit (assuming earth = chassis = 0V) to another unit that is also 0V to earth!

I have seen this happen!!!

 

[MikeSh]

toroidal transformer

Wow. Been a few years since I thought about one of them

 

[Trev]

Ah. I thought you meant the shorted turn within the unit, not via inter unit earthing. Although the result would be the same.

@MikeSh Finest transformers known to man. But winding them by hand is a bit of a bugger.

 

[Black Hole]

The shorted turn isn't complete until there is another link to earth - that's why it catches novices out. Everything is fine to start with, then all hell breaks loose when the system is connected together.

 

[/df]

...
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?

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.

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. ...

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 :-(((

 

[cdmackay]

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.

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.

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.

But I suppose the metal Humax isn't really big enough, or likely enough to be made live by something else, for that to be an issue.

Entirely separately from bonding, I think I'd also got it into my head that double-insulated things, with no earth connection, were likely to be plastic-cased rather than metal, but clearly not.

thanks…

 

[MikeSh]

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, 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.

Similar, even more convoluted, rules are developing to deal with electric vehicle charging outside, where there's again a mix of electricity, metal and water.

 

[Black Hole]

The overriding philosophy is that a single fault must not present a hazard, and preferably be detectable (although this is often not the case). Thus: with an earthed chassis, an insulation failure to the live wire results in the hazard current being shorted to ground and the fuse blows. Protection by double insulation means there is no hazard if one layer of insulation fails... but the fault is not detected until the second layer fails and presents a hazard!

For my analyser example, using the protective earth as a part of the circuit meant that the single failure of a broken earth wire resulted in a hazard! If the safety audit did its job properly (instead of being subservient to commercial considerations), Y mains input filters would be outlawed.

 

[/df]

Bonding, earthing, the electrons don't care.

As far as the Humax HDx products are concerned the claim of double insulation seems to rest on the power supply PCB and transformer design and mounting. You can see the layout in this picture from BH's disassembly guide, in the area of the pink and green arrows.

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.

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.

Nowadays a double insulation failure probably just throws the RCD so there's no coroner's report to indict the manufacturer, but I suspect there are real electrical engineers in this forum who might comment on whether this is an adequate design. Naively you might have expected something like a plastic box around the mains voltage side

 

[Black Hole]

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.

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).

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.

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.

Nowadays a double insulation failure probably just throws the RCD

Good point, I had forgotten that... but as a last resort.

 

[cdmackay]

thanks much for the details, all.