Equipotential bonding

Sorry, terminology slip-up there. In the past, neutral was sometimes relied on to provide an earth for the house (and still is in some US set- ups), but that practice was stopped many years back, as it could produce the fault described, or even give 415V as once happened in Gosport with spectacular results.

Cheers Richard

I can assure you that 0V specifically means a true reference earth in panel and control wiring - indeed for testing of electrical switch gear the factory will have a line to each test station very visibly marked as such, and testing is not valid unless it is connected.

However, 0V has also crept in automotive applications as meaning the same as battey -ve, but that is not strictly correct, though I suppose common usage overrules all, hence the confusion.

Cheers Richard

On Fri, 02 Jan 2009 06:31:15 -0600, beamends said in :

Ha! And the falsity of that is easily demonstrated on an old enough Mini - not at all uncommon to have half a volt between the notionally grounded frame of a light cluster and battery -

Guy

I don't know what you mean by `panel and control wiring'. If you mean the low-voltage (non-mains) wiring in model trains then I'm happy to believe that some confused model train people use it that way - the same kind of people who think they should earth their track, presumably.

Also I don't think the phrase `true reference earth' is meaningful. Did you mean a safety earth connection or an earth potential reference ? If the latter, why ? Why won't a potentially floating system ground do just as well (if not better) ?

As an example of why a floating supply can be better: if you want to use grounded test equipment (eg most CRT oscillosopes, which hobbyists are often still using), a floating power supply for the railway avoids creating a ground loop.

Nearly all of the low voltage circuit diagrams I have ever seen have `0V' or a little earth symbol on one of the power supply rails (usually the most-negative one) but this does not mean that that is supposed to be connected to actual earth. Look at any analogue integrated circuit data sheet, in the example application circuits.

Oh! no it hasn't stopped, in fact more and more installations are using the neutral as 'earth'. That is what PME earthing systems do and why all exposed metal has to be bonded. The local protective earth is bonded to neutral at the consumer unit, and there is no connection to 'real' earth at the premises. Under fault conditions (such as a rupture of the neutral somewhere between the sub station and the consumer) the 'earth' could float up to line potential, but since all the metal is bonded there is no path to a 'real' earth so no shock risk. Well that's the theory!!

Regards Jeff

Something is needed to liven the place up.

Actually it has happened to me when an electrician got a shore cable wired wrong once. I noticed because the fridge suddenly got louder for a bit. It survived. His predecessor was just as bad put 220 DC down some AC circuits which upset my telly and HIfi somewhat.

G.Harman

Was shown around a submarine and visited the escape training tower once - that was exciting.

Cheers, Simon

I know of an instance were the electricity company, whilst renewing a substation on an industrial estate, managed to swap two phases, went totally un-noticed at many companies, only those with 3ph machines started to wonder what the F**k was going on - it reached critical when someone started up a 3ph belt driven grinder - BANG - he survived shaken but complete, unlike the grinding wheel...

Here in Ontario, our mains power supply is two-phase, to give 120V for domestic circuits and 240V for items such as electric stoves, dryers, high-wattage heaters and air conditioners. The neutral bus in the service panel is bonded to earth by a clamp on the water supply pipe.

Plugs for stoves and dryers have 4 pins - ground/earth, neutral, and 2 live at 110V to neutral and 220V to each other. Most small appliances, including double-insulated power tools, hairdryers, electronics (TV, stereo, etc.) and model train transformers, have only a 2-pin plug. Desktop computers are always earthed.

Firstly I am not a professional electrician so I'll admit that I am possibly overreaching my knowledge but.

There is a danger of some difference in the way that two phase is sometimes used to describe the North American system within N America and two phase as is understood in the UK and a large part of the rest of the World. ISTR that what you have over the pond for domestic premises is a supply of 240 V single phase, much the same as we do give or take a few Volts and Hertz. However in N America at the connection to the premises you then split this single phase 240 V into a +120V neutral

-120V via a center tapped transformer. To get the 240 V for high wattage items you use the two 120 V legs which gives you 240 V between them. Your 120 V items use one leg and the neutral and ideally such items are balanced around the premises to use each 120 V leg as evenly as possible. Two Hots or two Lives and a Neutral I think describes it better .

This is not 2 phase as we would refer to it in the UK, it is just a two halves of a single phase.

We use the system with 230 V to 110 V transformers on construction sites etc. The transformer is again center tapped so each Hot leg is only 55 V to ground. This means that if a tool operator gets a belt in most circumstances this would be a live to ground path and 55 V is a lot safer as dry skin will almost resist this. Even in the UK a lot don't realise this thinking it is safer because site tools are 110 V. It isn't, 110 V just kills you slower it's the

55 V to ground that is the life saver.

Again this does not suddenly become two phases ,just a split single phase.

Two phase supply has probably completely died out in the UK by now, A long retired electricity worker told be about some he knew some back in the 1950's and it was rare then. He worked at Dartford ISTR.

G.Harman

Systems differ across North America, but under the Ontario Electrical Code the supply arrives at the meter as two live cables and a neutral. Perhaps the phase split is done at the transformer box on the street.

On 04/01/2009 19:01, MartinS said,

I think there's some UK/Canada terminology confusion here! What you're describing sounds like a split single-phase system, rather than a two-phase system. You still have two lives and a neutral, but the lives are in phase with each other rather than out of phase.

seems to describe it, particularly the phrase "it is sometimes incorrectly referred to as two phase" :-) Two-phase proper is described at

It's as described just under Connections in Split Phase. The distribution transformer is in a box on the street, or up a pole.