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[aprssig] slightly OT:recessed male twist and lock

joe at dellabarba.com joe at dellabarba.com
Tue Feb 7 18:14:07 UTC 2006


I think "phase" is used to mean more than one thing in these discussions. The USA, like everywhere else that has AC power, has 3 phase generating plants and 3 phase distribution (mostly - I have seen one wire HV distribution out in the country that must use the earth for half the circuit). Most houses do not require 3 phase power, so they are supplied single phase power from a transformer that is between one phase and ground on the primary side and with a 240 volt center-tapped secondary. This gives 120 volts between each hot lead and neutral and 240 volts between hot leads. Some people refer to these as "phases", but they are really one side or another of the center tap. You are correct about boats - I have a diode and capactitor arrangement in my ground lead to provide galvanic isolation on my boat. BTW, I think the original question was about a trailer. You can have LETHAL results if a trailer on rubber tires becomes hot and a person standing on the ground touches it. I would ground any trailer that had AC power if it was up to me.
73
Joe N3HGB



>
>The UK does indeed have much the same system.
>
>However, in domestic environments, there is always a 3 pole socket in
>use. Line/Live/Phase (whatever you want to call it) a Neutral/Zero line,
>and the safety Ground wire.  That last one makes first, and breaks last
>when the plug is inserted/removed from the socket.  The plugs too, have
>a fuse link in them in the Live side.  Switched sockets general only
>switch the Live side.  (There are exceptions)
>
>Many "Domestic" appliances are only 2 wire fed, being "Double Insulated"
>so not needing a safety ground wire.  TV's, sterio's etc...  Even Power
>Tools!
>
>The "Neutral" is usually tied to Ground at the sub-station, and often
>where the supply enters the premises.  The main circuit breakers (in the
>consumer unit or distribution panel) interrupt both Live and Neutral,
>the Ground is never switched.  Individual circuits are protected with
>fuses or current sensitive circuit breakers.  There is often a RCD
>(Residual Current Device, or as it used to be called "Earth Leakage
>Circuit Breaker) feeding a group of circuits, before the individual
>fuses/MCB's
>
>3Phase, can be 380/400/440V (Root3 times the Phase-Neutral voltage) and
>is usually supplied with the Neutral line, so you end up with up to 5
>conductors in some industrial equipment power leads!..  Switching,
>interrupts all 3 phases and the Neutral.  Equipment would usually be
>designed to balance the load round all 3 phases, though it's not
>strictly necessary...
>
>Before the great EU, Europe had 220/380V, the UK had 240/440V.  Now it's
>all be "Harmonised" to 230/400V or there abouts.  No actual change to
>the supply voltages, just that the specifications and tolerances were
>altered to cater for all the variations.
>
>The other big difference of course is over hear in the EU/UK we use
>50Hz, the USA/Canada use 60Hz.  Some parts of Japan have a right mix of
>100V, to 220V, 50 and 60Hz, often all present in the same street (if not
>the same building!)
>
>AFIK the USA has 3 phase distribution, but each of the 3 phases is split
>at the pole transformer to supply 110-0-110 to each house.  The "0"
>should be at the ground potential also I think.  What used to amaze me
>when I was there doing some non work (Hi Hi) was the pitiful reliability
>of the so called "Safety Ground" continuity, as it often relied on the
>metal trunking not a separate wire.
>
>There has been mention of boats.  Take care!  Many boats have a double
>diode device (anti-parallel) in the safety ground lead to shore, or a
>full galvanicly isolated transformer (with no ground continuity boat to
>shore) to prevent electrolytic corrosion of immersed metal parts.
>(Different metals, salt water, etc...)
>
>Take care All..
>
>Dave G0WBX.
>
>PS:	You do not know what pain is, until you step on an upturned 3
>pin UK plug in the middle of the night with bare feet!  In that respect,
>they are perhaps not so safe!.....
>
>
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