| Today I was asked to provide comment on an invention that uses the | existing wiring (active+neutral+ground/earth) in a house with the | following modifications...(a) an isolation transformer at the incoming | circuit-breaker box, (b) float the ground/earth wire inside the house | (wires are there, but not connected to earth/ground), (c) a monitor on | the (now-floating) ground/earth line looks for small voltage | excursions, (d) a relay disconnects active (and possibly also neutral | - whatever that means on an isolation tranformer - as well) when the | voltage on the ground/earth line exceeds some pre-set voltage (which | is small).
This is an ungrounded system.
| I declined to provide comment because I was at the same time intrigued | and skeptical and felt I couldn't provide the "expert" opinion they | were after (hence my posting - I'd like to learn about this).
It is prohibited by the National Electrical Code in the USA, except for special circumstances involving industrial requirements and expert maintenance and monitoring.
| The idea of appliances left un-earthed worried me although the more I | think about it, the less worried I am given the way they had it wired. | The setup and demo was quite convincing...a toaster with a metal knife | poked into it did nothing but when touched to the chassis, immediately | tripped the relay...a bath full of water had live light globes tossed | into it while the guy providing the demo had his hand in the water. | Again it tripped, no shock, or cardiac arrest. A metal nail file | shoved in to the active pin (by exposed fingers) did nothing (because | of the isolatoin transformer). Water poured on a desk lamp tripped the | relay.
These are examples of a _single_ fault. Single faults do not pose such a hazard on an ungrounded system. However, once one fault is in effect, and second fault can be very dangerous. The first fault must be cleared immediately.
In UK, it is common, and I believe required, to have a "shaver outlet" that is fully isolated. This can be safe by having no more than one such outlet per transformer. It's a small load so a small transformer.
In the USA, the NEC allows (or requires) low voltage lighting systems to be isolated from ground. This then permits open conductors, such as a pair of metal bars to hang lights on. At no more than 30 volts they pose very little shock hazard. By being ungrounded, they pose no "first fault" hazard, either. A "second fault" would not be much of an issue at these low voltages.
| Curious what people think of this - it seems interesting, and they | might be on to something (they've already patented the technology). A | few observations/concerns (comments welcome) are below:
I would consider it to be very dangerous. It's even more dangerous because of the false sense of safety it seems to provide. People might more readily do things that can more easily kill them when theu _think_ it is safer.
| * the voltage in the house is undefined relative to the earth and is | likely to vary with humidity, ground moisture, etc. On a very dry day, | a rather large voltage differentital can be established between | someone entering the house and any appliance they contact. I'm | thinking that this stablizes as soon as the person enters the house, | and the worst-case scenario is a static electricity-style zap that | dissipates the charge. Sound right?
This is not the hazard. The hazard is that one conductor faults to ground in a way that does not carry a lot of current ... yet.
| * Touching active and neutral simultaneously would not trip the relay | (unless the victim were touching an appliance chassis at the same | time) - I'm not sure if an RCD/GFI would effectively detect this fault | either (it might, but I'm not prepared to try it). I believe a RCD/GFI | would trip unless the two wires were touched within a few ms of one | another. The "specially" wired house would allow the user to take | their time without risking tripping the relay while they first touched | active, then neutral. Sound plausible? I'd imagine that this would | only occur when someone intentionally wanted to do this...or some kid | who didn't know better.
And RCD/GFI would detect any fault that represents current diverted from the normal return path. If you have a plug with 2 open wires for the current conductors, and plug it in, and touch both wires, you will be shocked. Because these points represent the normal return path, an RCD/GFI will NOT detect this. However, this represents only a rare case of people getting shocked. The common cases involve some kind of fault failure between wiring inside an appliance and the handler, such as via water or the appliance enclosure. When a person touches that water or enclosure (metal), and is also touching ground, they get shocked. This is a diverted path, and the RCD/GFI can detect it because there is a difference in current between the current conductors on the order of
2 to 6 milliamps. The RCD/GFI is designed as a detector of current difference.
| * Metallic pipes not connected to the house's ground circuit offer an | alternate shunting path in the event of (e.g.) the active being | exposed. I think this doesn't matter in an isolated house since | touching active and something attached to ground/earth simply | equalises their potential. It's an unfathomably long-shot but someone | else doing the same thing with the "neutral" wire elsewhere in the | house would experience a shock (since "active" is now at the same | potential, and a reference is now established). | | Dangerous or better than what RCD/GFI can provide?
The RCD/GFI route should be used to protect against the "casual" human to ground electrocution hazard.
I have considered using a whole house isolation transformer for a different issue. That issue is the "stray current" from the utility distribution. It is not a big issue in most cases. Still, the secondary of the transformer would be grounded (at the center tap for USA split-phase Edison systems). I've discussed this before and issues that get raised include the voltage potential across the windings on the transformer could be as high as the MV distribution system or even higher in a lightning strike. Note that such voltages would sink to ground in a normal system so these do not represent any significant shock hazard. But it can represent a grounding electrode deterioration hazard, as well as stray ground currents. For me it remains an undecided issue. But it will still be a fully grounded LV system on the secondary side.
I would not even consider an ungrounded system. No way.