Delta supplies are still popular afloat which adds complexity when speccing certain types of kit, e.g. entertainment lighting dimmers that take 3-phase input but control single-phase outgoing circuits. One of my colleagues recently succeeded in altering the specs for a number of forthcoming cruise ships, to include extra star-connected substation transformers for this application.
Ungrounded Delta (IT earthing) does not offer any advantage (IMO) in typical applications- at best, at worst it leads to insulation break down. An intermediate or arcing fault can cause phase to earth voltages to rise 6-8x their normal values damaging the electrical system and the equipment its connected to. MOVs and electronics in particular have a hard time coping.
To remedy this you would go with either resonant or high resistance earthing, however unless you're dealing with critical process I don't see it being worth the complexity. Especially when ground fault indicators are typically ignored outside of Hospitals and industrial.
Of course you could add 30ma RCDs on circuits and a time delayed 500ma RCD on the main board nullifying the continuity of service. But one is left to argue, is it still worth the effort?
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Do you have any feel for how many (what proportion) of appliances aimed at such markets use 2 pole switching internally ? I've only a limited sample, but a significant proportion of those I've ever had to look inside (especially things like drills and other hand-held tools) have had double pole switching. But as mentioned, it should be routine to pull the plug before opening anything up or poking things in - as a farmer I once worked for did, start clearing the air vents on an angle grinder with a metal pocket knife (he did quickly unplug it when I pointed out it was still plugged in).
Honestly, small 230 volt appliances seem to have single pole switching, while larger 230 volt appliances tend to have a double pole isolation main switch some place.
I still think schuko sockets have successfully shown that polarity and earth reference of the second line conductor should and does not dictate protection against electric shock.
Generally not - as attested to by news reports of household fires when some scrote nicks ÂŁ20 of neutral link from the substation.
And right here is one of the biggest dangers of connecting loads phase to neutral.
In the US shared neutrals are common exiting the panel, if not right up to the light fitting or sockets. You have cases where a disconnected neutral results in hundreds of light fixtures being smoked in a warehouse or office building.
But it's not just the phase-neutral voltage that's an issue - and that could be dealt with by a voltage monitoring relay to disconnect the supply. There's an electrical safety issue in having the MET, and hence everything attached to it, at a potential that's very different to local earth. Not too bad inside where there's equipotential bonding, but bad news for someone leaning on their car and touching a metal bollard/lamp post/whatever nearby (and hence the extra requirements for charging points).
Yup, a very real danger with PENs.
I was under the impression that the idea sparking off the original question was along the lines of "if people switched to 400V for single phase, we (or more correctly, the DNOs) could ditch the neutral and use the current combined N&E as an earth only for a TN-S supply".
Correct. If all single phase equipment moved to 400 volts the DNO's PEN would become just a CPC.
I worked very very briefly aboard a ship that used the same system - 3 wire 3 phase, 220V phase-phase and everything connected across 2 phases. Well to be more accurate, some things were like that, most of the ship was still 220V DC - they'd converted one generator to AC to allow all those AC appliances people like to have to work, while most large loads were still DC. I noted while doing a little electrical job in the engine room, that there was an air compressor driven by a motor with a higher power rating than my car engine !
Fortunately for them, all the circuit breakers (which must have been exceedingly modern for something built in the early 50s) were AC or DC so they only had to add a third wire to the main distribution boards and distribute the sub mains to the local boards across the 3 wires. So getting single phase AC to the hotel loads was fairly easy. The breakers were also 2 pole - I think all the system was floating and nominally +/-120V. I know the emergency lighting was as they had a pair of indicator lights by the battery tank to indicate if there was an earth fault - both lights dim and equal = nothing dragging one side of the DC to the hull.
And everything was originally labelled in Italian. And on push button starters, the red button was for start and the green button for stop - which seems much more sensible to me than pressing a green button to make something dangerous and a red button to make it safe
I know IEEE and IEC equipment seem to treat red and green differently. Protection relays for the IEEE market have green as open and red as closed, while those for IEC market have green as closed and red as open. Could be wrong but thats what I've personally seen.
