I have seen it referred to by several people who are more technically competent than you about GFCIs.
But you just post smug, pompous crap.
The pile of crap I posted, which you replied to, was: "There is a second current transformer that tries to put a small common mode current in both the H and N wire."
The second current transformer is plainly seen in both the data sheet I posted and the data sheet Ed referred to, that I posted a link to.
But some people have a problem admitting they are wrong.
:-) AFAIK that's the only way N-G short detection is done in GFI's.
Maybe you remember and can comment on this: I seem to recall that a while back, when GFCI's were first introduced, they were not designed to detect N-G shorts. I'm not sure.
BTW - you can make a nice AC load sensing switch using a modified GFCI receptacle. I've done it with both the 1851 and 4141 IC GFI's.
I presume in the US the Neutral and Earth are linked in the house or building.
In the UK the Neutral enters the property and is generally kept separate from the house or building Earth, so there is always a small voltage difference between Neutral and Earth. Here, I've not seen the need to introduce a small common mode voltage into the load Live-Neutral to check for Neutral-Earth conduction paths.
--
Mike Perkins
Video Solutions Ltd
www.videosolutions.ltd.uk
Utility supplies line & neutral to customer. *Customer* provides earth. In the past, a cold water pipe (as long as it was metal for at least 10 ft through soil) was used for earth. Code was revised to *add* the need for a separate "earth" (metal stake driven into the soil near the service entrance -- can you spell "thermite"?).
Neutral is bonded to earth IN the electrical panel.
Neutral and earth (avoiding the use of "ground", temporarily) are supposed to be at (roughly) the same potential. The key difference is earth is never *intended* to carry current (in normal/non-fault circumstances).
E.g., you can EARTH the case of a device but can't tie the case of a device to "neutral" (notable exception is the main electric panel and some large appliances). I.e., the earth conductor on a three-wire outlet has to be connected to *earth*, not "neutral". Ditto for metal Jboxes, etc. (this is one of the reasons I *prefer* metal Jboxes over plastic -- miswired fixtures in plastic boxes aren't obvious and can have potential on surfaces that are exposed to users!)
As many rely on water pipes (possibly mistakenly!) for "earth", anything that can interrupt that continuity must include provisions to ensure earth's integrity even when it would otherwise be interrupted. E.g., our outdoor plumbing (irrigation, bibbs, etc.) are all immediately fed from copper pipe -- but, just upstream from that is *PVC* so you can't rely on the hose bibb to be "earthed".
E.g., if you install a water softener in your supply line, you add a jumper bonding the "inlet" pipe to the "outlet" pipe so that continuity is maintained even if the water softener is removed. (ditto for water heater, filtration unit, etc.).
Perhaps the biggest single section/subject in the Code book is concerned with grounding! :-/
For a very long time, although incoming services must be bonded to Earth, they cannot be relied up to provide an Earth. We also have Earth Spikes.
That's a total no-no for us in the UK!
I also thought that many US homes and premises had their own transformers, such that a local Neutral and Earth were inherently at the same potential?
They are for us, but the separation of Neutral and Earth mean that under Live-Neutral fault conditions, any fault Neutral potential doesn't find its way to the Earth. Under normal conditions they are up to a few volts different.
Our Neutral is Earthed at the local substation which may be quite a distance from the property.
We have the same principle and call it equipotential bonding. Though many rules regarding Earthing arrangements become relaxed if Residual Current Devices are used.
--
Mike Perkins
Video Solutions Ltd
www.videosolutions.ltd.uk
** The hazard with linking neutral and earth at the supply entrance to each premises arises if ever the incoming neutral is lost. Load current in the premises then returns via the local grounding system - which can include plumbing.
Joe plumber then comes along, digs a hole, hops in and disconnects a pipe.
** Lack of reply here shows this smug dude's total dishonesty. --------------------------------------------------------------
** More smug, pompous crap.
** Not seen outside the USA and not seen in all GFCIs there either.
And you failed to mention that it is not needed to produce tripping in 99% of practical situations.
You aren't supposed to rely on earth to be the "return" for the supply. E.g., you can't use the metal frame of an all steel building to "eliminate the need for a neutral connector" (like you would in an automobile)
"Earth" never carries current.
Businesses (high power demands) may have their own transformers, etc. But, individual residences don't.
Here, for example, four homes will be fed off one transformer (which is located at grade level as our utilities are below grade).
I *assume* there is an earthing conductor there (it's under a half cubic yard/meter "box")
At the panel, they are equivalent. In the residence, neutral may differ instantaneously due to IR drops in the neutral conductor. Because no current flows through "earth", it is always *at* the same potential as inside the electrical panel.
There are rules governing how "good" the earth must be. Along with the wire gauge to be used (based on the rating of the circuit).
Depends on what you consider a "substation".
E.g., here, there is probably one main "substation" for the neighborhood (~100 residences). This feeds all of the "4-home transformers" in the nieghbnorhood.
[Actually, there must be at least one other such "feeder" as I know when this "main transformer" I'm thinking of died, there were some homes in the neighborhood fed from a different circuit]
I am guessing this "main transformer" also is earthed (?)
RCD/GFCI is in *addition* to these requirements. They are mandated for use in wet locations (outdoors, bathrooms), garages, kitchens, etc. There are some exceptions that *could* allow non GFCI protection in these locations -- usually if the service outlets are ~8 ft above the floor (so a user can't reach them!).
And, other caveats that affect how they are deployed. E.g., some places will run a "220" circuit into the kitchen (to satisfy the "two small-appliance counter-top circuits" requirement). This is neutral plus the two "legs" of the supply (think of our service as a 220V center-tapped transformer where neutral is the center tap). This cuts down on wire costs (as the current flowing through the neutral should be *less* than what it might otherwise be -- if there are active loads on both "legs").
As both of these circuits are to be GFCI protected, if a ground fault is detected on *either*, BOTH are disabled. Kinda inconvenient if you lose *all* your kitchen counter outlets at the same time :-/
**Correction** GFI's trip the breaker based on a differential current between Line and Neu tral. ( with the difference assumed to be flowing thru ground )
Actual construction of a GFI uses a current transformer with BOTH Line & Ne utral thru the primary path so that the difference is detected.
Trip Levels for GFI are set to 5mA. I agree this designed so that Ul/CSA/IE C approved line filters in products which have Y rated caps to ground for E MI purpose will not trip the breaker. I recall that ground leakage current (from line filter) was set to 0.5mA max per device or 2.5mA max per system to meet safety requirements and thus not trip the GFI.
If you are getting false trigger GFI, look for any pulse currents are NOT b alanced on Line and Neutral ( bad leaky caps in circuit appliance, dust & moisture on exposed conductors or in high humidity zones)
250V / 5mA = 50KOhm leakage which could be the resistance used by the GF I for self test. (20k perhaps for 120/240 V rated device)
The latter. "Earth the plumbing" not "rely on the plumbing for earth".
The house I grew up in relied on water main for earth. And, the water *meter* being metalic (I don't recollect a bonding strap
*across* the meter -- which was highly visible in the basement workshop)
Here, I was stunned to see this same *apparent* practice (this house being 30 years NEWER than my childhood home) as I could see the clamp to the hose bibb located near the panel. On closer inspection, however, I noticed another earthing conductor embedded in the stucco heading off to a ground rod.
That's what I'd remembered when I had overhead service. But, the below grade service is covered by a large rectangular "box". One would think the box itself would be earthed if just for safety.
Looking inside doesn't leave you with a nice warm feeling! Lots of *big* conductors that are "exposed" (relying on the box to cover them). Makes you wonder how good that bonding strap is in the event of a car driving into it! :-/
[I was going to upgrade our service, here. Much of the EXPENSIVE work can be done by the homeowner: trenching, installing plastic conduit, shading, etc. But, the sight of those big conductors left me squeamish: "Maybe we can just take this as an opportunity to CONSERVE!" :> ]
(the half of that sentence unsaid is "... thereby allowing the hazard to persist")
Sort of like designing a pinout for a power cable in a device such that installing it backwards, *against* the mechanical keying's wishes causes the power supply to see a short -- instead of taking out the electronics.
No doubt a way of dealing with grandfathering?
Some were silly (e.g., tying the panel to neutral given that neutral was bonded to earth *at* the panel).
I'm not fond of handle tie's (though realize they are an economical way of doing things) as they are too easily severed. When we moved in, here, I immediately noticed the tie for the 220V cooler was split. Someone seeking to be able to turn off the *furnace* blower (that shared the circuit) while leaving *half* the cooler circuit live??
:-(
Ah, I didn't realize AFCI's were now Code requirements. Bedrooms?
Problem with all building codes is they are written requiring a "language lawyer" for proper interpretation. This, IMO, is counterproductive (code wars, etc.). The code should go to great pains to explain what it's intent is so folks don't have to reconcile "conflicting" sections. It's purpose is to make things *safer*. Imagine if "STOP" signs said,
"GRADUALLY DECELERATE UNTIL FORWARD MOTION HAS BEEN HALTED FOR A PERIOD SUFFICIENT TO ENSURE ALL FORWARD MOMENTUM HAS BEEN ABSORBED" (WTF?)
I can recall looking for clarification for a code section that implied the need for a "divided Jbox" (other than mixing line and low voltage conductors in the same box). I was never able to find anyone who could clarify the requirement and ended up just using *two* Jboxes (instead of drawing attention to a single *divided* box)
Not sure either. It has been a feature for a long time.
In the US, the N and G are at the same potential at the service due to bonding. That means you need significant load current to get "several amps" ground current for a N-G short. You would, in general, need 2x "several amps" to a downstream load using Romex wiring.
A metal water service pipe (10 ft...) is still required to be part of the earthing system. Not obvious you were saying water service pipe is not to be used as part of the earthing system, but a lot of people think that. An additional electrode is now required for new construction.
A stake (ground rod) is easy to install but is the worst of the commonly used earthing electrodes. For new construction that has footings or foundations a "concrete encased electrode" is required. It is a good earthing electrode and a ground rod is not used.
I have overhead distribution, which is 8kV to distribution neutral (13.8kV phase to phase). The 240/120V secondary neutral is bonded to the transformer can and earthed at the pole. It may be earthed at poles without transformers. The secondary neutral could be interrupted, as the hot wires are, where power is supplied by another transformer, but the neutral is continuous. It has to be continuous because it is also used as the distribution neutral, and it connects to the distribution neutral where the 8kV is tapped of the 3-phase supply.
I expect underground would at least connect the secondary neutral to the transformer can and earth them at the transformer.
In the US, a major function of the N-G bond required at the panel is to carry fault current back to the transformer. If you have a H-G short, the fault current takes the ground wire to the N-G bond at the service and then the service neutral to the transformer. That results in a high current to trip a breaker. With no N-G bond there would not be a reliably high current.
My understanding is some of the UK does not have a "ground" wire in the utility supply and the building ground is just earthed at the building. And a RCD main in the UK is to provide a trip in the absence of the high current above.
Most, if not all, of the exceptions were removed maybe 5 years ago.
There are a couple changes in the US code that make a common neutral ("multiwire branch circuit") less common.
One is that the supply breaker must now simultaneously disconnect all the circuits associated with the neutral. (It does not have to be a common-trip, a listed handle tie can be used.) In a business it can be a real disadvantage to turn off 2 or 3 circuits to work on only one of them.
The other is that in a house most of the circuits now have to be (new construction) AFCI protected. Like a GFCI breaker, AFCIs require a separate neutral that goes through the breaker. You could use a 2-pole breaker but they are expensive. AFCI receptacles are now available, but in general you can't use them as an alternative to a breaker like you can with a GFCI receptacle.
That is true if you use a 2-pole GFCI breaker. It is cheaper to make the first receptacle on each circuit a GFCI and wire through it. GFCI breakers are expensive, and 2-pole GFCIs are a lot more expensive.
IMHO it is the most confusing of the commonly used chapters.
Well, you could conceivably get GOBS of amps on a neutral-ground short (flowing in the ground conductor) if the neutral had a high resistance connection.
E.g., imagine a 10 ohm load from line to neutral with "a couple" of ohms in the (faulty!) neutral return -- caused by one or more splices that have degraded/failed over time. The "normal" neutral current would be V/(10+2) while the "shorted neutral-ground" current would be V/(10+e) -- e being much smaller than "2".
This is one reason why I never "daisy chain" outlets prefering instead, to tie the downstream conductor to the upstream conductor
*and* a pigtail (for the local outlet) under a single wirenut. I've found that relying on the "second set of screws" to carry the current for the "downstream" loads is a problem waiting to happen. Whenever you remove the receptacle from the Jbox (even just to examine it), those screws see some stress as you try to manhandle the receptacle (esp with 12AWG conductors).
And, the "push in" sort of connections are for folks who opt for ease of wiring vs. minimizing fire department calls... :<
My house has a water service pipe as the only earthing electrode. It was code compliant when installed and is compliant until the electrical service is replaced.
A metal municipal water system (what I have) will be the best earthing electrode available at a house. A bond across the water meter has been required for a long time. Not quite as long - the connection to the water pipe has to be withing 5 feet of where it enters the house.
A "supplemental electrode" is only required because water service pipes may be replaced with plastic.
Ground rods are crap. The code requires a resistance of 25 ohms to earth for a single ground rod, but for 2 rods there is no requirement. What is the fault current for a hot connected to a 25 ohm rod?
The 2002 NEC required AFCIs for bedroom circuits.
The 2008 NEC required AFCIs in residences generally where GFCI protection was not required. The AFCIs are more sensitive than in the
2002 NEC.
All of this is for new circuits.
The 2011 NEC requires AFCI protection for extensions to circuits where the extension is in an area where AFCI protection is required. Also requires AFCI protection for replacement receptacles that are in areas where AFCI protection is required. (Some of this protection can be provided with AFCI receptacles.)
I have read that the 2014 NEC will require AFCI protection for kitchen and laundry circuits, some of which also need GFCI protection.
The NEC is only effective when adopted (and sometimes modified) by an enforcement agency - like a state.
The current NEC is 8.5 x 11" and 860 pages. It is not intended to explain how to design or wire or (usually) intent. But there are a couple good handbooks and endless "how to" books.
"Lawyer language" is necessary to make the code enforceable. If you read the code much you get used to it. Took me a while to get used to "grounded conductor" (neutral) and "grounding conductor" (ground wire). For a few codes back there has been a push to make the NEC more readable. For instance, the grounding chapter was reorganized.
There shouldn't be "conflicting" sections (I can offhand think of one). But general rules can be modified by rules for specific instances, and the rules for a simple task may be found in many different sections.
There are metal dividers to separate sections in a metal box. The box manufacturer is likely to make dividers that are attached to the box with screws.
alt.home.repair can usually answer electrical wiring questions.
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