| I understand that there are countries where the primaries of the
| distribution transformer are not connected to the ground/earth in any
| way (from the perspective of the customers ground). This is not
| necessarily a 'better' or safer system. There are complex trade-offs
| involved and there are a lot of good reasons (for increased and other
| reasons) for making sure that at least one primary lead of that
| transformer is earthed.
Being earthed is one thing. Connecting primary and secondary neutrals
| In the USA, there is a special case exception in rural areas where
| there are problems with stray voltage traveling through the earth in
| farm situations. One solution is to use a special neutral isolator to
| keep the secondary neutral/ground/earth disconnected from the primary
| ground unless the voltage difference reaches a certain threshold, in
| which case the bond is re-established.
So are you saying stray voltage never happens anywhere else by in rural
areas? How clever of those electrons to recognize that.
| Not everybody does it that way. France is one such country with the
| TT Earthing Scheme. In other words, the power company does not supply
| any grounds to the customer on their distribution lines. The customer
| is responsible for providing their own local earth connection that is
| not connected to the larger distribution network. For safety, an RCD
| (Residual Current Device) is required for such installations. An RCD
| is similar to what we know in America as a GFI or ground-fault
| interrupter. However it is not exactly the same. Typically, an RCD
| is set to trip on a ground fault with a greater current than the
| typical GFI 5ma threshold and also it contains complex timing circuits
| and often covers more than just one circuit.
In many countries, an RCD main is required. I've seen the requirements
of such for Japan.
| In my opinion, the North American scheme gives several advantages when
| safety is concerned as you have the following unique features when
| compared to the typical Euro Systems that use the RCDs.
| 1. Individual circuits can be protected by inexpensive outlet-type
| GFI's or slightly more expensive breaker GFI's. As mentioned
| previously, RCD's are expensive and complex.
| 2. Unlike a typical RCD installation, the whole house is not plunged
| into darkness should a single GFI trip due to a real fault or from
| 3. In the North American System, 240 V. is available (via the split
| phase wiring scheme) for the larger appliances that need it (range,
| dryer, large air conditioner). However, at no point is any voltage
| inside the house higher than the nominal 125 volts to ground.
... except when the effects of MV distribution systems are factored in.
| 4. Bathrooms are arguably safer and more convenient to wire
| electrically. You don't need to worry about 240V wiring near the sink.
| Light switches can be conveniently mounted on the wall (instead of
| using pull chains), you don't need to install isolation transformers
| as they do in the UK for shavers. Outlets don't need to have switches
| on them and because of the lower voltage, you have less paranoia about
| bonding everything together in the bathroom. Hair dryers can be
| safely plugged into convenient bathroom outlets protected by GFI's,
| I've seen some of the wiring in France and I'm not sure what they did
| before RCD's became widely used. Did they just not care if anything
| was grounded? I'll bet it wasn't all that safe.
| I'm old enough to remember the days in the USA (late 1950s and early
| 1960s) when we switched to plugs and outlets with 2 prongs to 3
| prongs). The electrical system became a lot safer, as a result,
| particularly for users of portable power tools. Double insulated
| appliances and the later GFI requirements provided further safety
Safer, yes. But I still think the safety can be improved even more by
better isolation from the MV distribution.
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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