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Paul_L Guru Joined: 03/03/2016 Location: United StatesPosts: 769
Posted: 02:54pm 03 May 2017
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In the states the insurance companies reasoned that the white wire was most easily identified when things got dirty so it should always be neutral. Green seemed like it had something to do with grass, (whoops, I refuse to go off on that tangent), so it should be ground and should not carry current. Any other color could be hot.
The next tangent should probably be the places where armored cable (/BX/AC), flexible metallic conduit (FMC / Greenfield), galvanized threaded pipe (GRC), EMT (Electrical Metallic Tubing), IMT (Intermediate), RMC (Rigid), rigid nonmetallic tubing (RNC) and corrugated nonmetallic tubing (ENT) should be used to protect exposed wire runs.
Us crazy people in the states don't want electrons to escape from the cables so we cage them up in pipes!
https://en.wikipedia.org/wiki/Electrical_conduit
@Bob .... I think Violet is a very interesting young lady.
Paul in NY
Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814
Posted: 04:44pm 03 May 2017
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PauL @Bob .... I think Violet is a very interesting young lady.
@ PaulL Similar,version but in the one I learned, Violet was very uninteresting, almost pure you might say
BobD Guru Joined: 07/12/2011 Location: AustraliaPosts: 935
Posted: 05:45pm 03 May 2017
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was she like pure olive oil?
HankR Senior Member Joined: 02/01/2015 Location: United StatesPosts: 209
Posted: 07:43pm 03 May 2017
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You're referring not to UL but to the (US) National Fire Protection Association, which originated and maintains the National Electrical Code. The NEC safety standards are the ones adopted by many US government agencies as part of their building codes.
HankR Senior Member Joined: 02/01/2015 Location: United StatesPosts: 209
Posted: 07:55pm 03 May 2017
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In fact it's the difference in current between the hot and neutral wires that is sensed, not at all the ground wire current.
It's possible for leakage (fault) currents to flow in the outlet ground wire, but that's not always the case. So it's a critical requirement for the difference current to be sensed to guard against all possible ground fault situations.
HankR Senior Member Joined: 02/01/2015 Location: United StatesPosts: 209
Posted: 08:04pm 03 May 2017
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Better wording:
The NEC safety standards are the ones adopted (often verbatim) by many government agencies in the US, typically local, as part of their building codes.
Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814
Posted: 08:40pm 03 May 2017
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AFAIK these days its the imbalance between neutral and active thats the most ideal way of monitoring for safety, any imbalance is reason to shut down(if current is going out the active and not coming back in the neutral, that means its going somewhere else it not supposed to be going! (yes I know its AC and doesnt go out or come in, trying to keep it simpler))
Plus many (most??) appliances these days dont even have a ground/earth wire ie are double insulated- a quick look around this room has 14 doubled insulated with 2 pin plugs, 2 antique lights with 3pins and earthed for safety and 1 fixed appliance with 3 pin 15a plug/gpo (window rattler A/C)
Paul_L Guru Joined: 03/03/2016 Location: United StatesPosts: 769
Posted: 12:09am 04 May 2017
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There's another tangent for you. Now we're talking about Popeye's girlfriend.
https://en.wikipedia.org/wiki/Olive_Oyl
Paul in NY
Paul_L Guru Joined: 03/03/2016 Location: United StatesPosts: 769
Posted: 12:48am 04 May 2017
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HankR & Boppa
You're right, the NFPA publishes the NEC. But they don't really write it, or at least they didn't in the past. One of my first jobs after I finished up my last degree back in 1961 was working for UL at their old headquarters in Melville NY. I was part of a project to rewrite the code for UL. It hadn't been modernized for over thirty years. WW II got in the way. I removed the knob and tube wiring standards, added some stuff about gas tight conduit and boxes in hazardous areas, and got the idea for the U ground in the duplex receptacle and the green ground wire. I spent about seven months on that project. I enjoyed rooting out all of the ancient wiring methods from 1915 or so.
You're also right about the GPF circuitry comparing the current in the hot and neutral wires. They don't monitor the ground conductor at all. I really should have worded that more accurately.
I worked on similar circuitry at Pan Am. All of the Lockheed Constellations were grounded for a while after several of them had their wings separated from the fuselage when generator feed through bolts shorted to the fuselage, did some welding and blew up the gasoline tanks. The Connie was the first fully pressurized aircraft and Lockheed missed the fact that the fuselage expands and contracts with the pressure cycles. The flexing cut through the bakelite step washers that centered the feed through bolts in the fuselage holes.
Pan Am engineering solved the problem in 1950 by routing the generator leads through two current transformers, one near the generator and the other behind the circuit breaker panel in the cockpit and through bakelite feed through tubes mounted in glands in the fuselage instead of using bolts. The CT secondaries were wired to buck each other's voltage. If there was a difference in the current in the generator wire from one end to the other the resultant voltage would pull in a sensitive relay which would interrupt the field current to the generator and shut it down.
The Connie was a DC only aircraft but the CTs were able to sense the ripple voltage from the generators.
I got involved in calibrating the sensitivity of the CT circuits on the 208 volt 400 Hz generators on the 747 during 1969 and 1970 when the 747 was introduced. It had a 40 Kva generator on each engine and two 60 Kva generators on the APU, so we had a total of 20 CT pairs on all of those 3 phase windings. Switching transients were causing unnecessary in flight generator shutdowns which tended to upset the flight crews when everything went dark. I was blessed with the job of figuring out how sensitive the whole mess really had to be and how to sense when transients upset the load sharing among paralleled generators. The problem was not as vital because the kerosene in the tanks was not as explosive as the gasoline had been.
That was another fun project.
Paul in NY
HankR Senior Member Joined: 02/01/2015 Location: United StatesPosts: 209
Posted: 09:40am 04 May 2017
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Paul,
What exactly do you mean by "got the idea for the U ground in the duplex receptacle"?
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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