grid power - grounding the neutral wire

[jack11]

I've been wondering the purpose of grounding the neutral wire from the grid.
This relates to renewable power thru the issues such as: the use of GFCI-equipped inverter in a house with grounded neutral, grid-tie system wiring, etc.

I believe the grid power to my house comes on 3 wires: 120vac 0 deg phase hot wire, 120vac 180 deg phase hot wire, and the neutral wire. Right off the bat, the neutral wire from the grid is grounded at my circuit breaker box.
Then, each house circuit's hot wire is protected by its CB, and goes to a load in the house. The neutral wires of each of these house circuits come back to the CB box, and are also grounded there.

For single phase 120vac circuits, the purpose of grounding neutral could be to prevent the negative (reactive) power from coming back to the grid (in case of a reactive load in the house), right? Or, for some other safety-related issues???

It seems that the grid and house 120vac wiring could be made without grounding the neutral, floating the house loads between hot and neutral, and grounding only the load chassis to protect against internal shorts. But then the reactive power would flow back to the grid, is this the reason for not doing it this way?

However, for dual phase 240vac circuits, there is no neutral wire, and the load floats between the two hot wires (0 and 180 deg phase). The load chassis is connected to the ground. So, unlike the 120vac single phase circuit, a reactive 240vac house load always sends the reactive power back to the grid, right??? Or, does the dual phase nature of the 240vac circuit provide some immunity against reactive power flowing back to the grid?

Can anyone comment on the purpose of the neutral wire grounding, and the options for connecting a GFCI inverter to such house wiring when the grid fails (after disconnecting the grid wires of course)?

[mab]

I'm not sure how relevant this is to you as grid power where I am (UK) is 240v single phase (which has 'hot' live and grounded neutral), but my understanding of the grounded neutral system is that you can use single pole switches, circuit breakers, etc and when a  circuit is 'off' it is at ground potential.

If using a floating supply, then all switches/ circuit breakers should be double pole to isolate all 'live' conductors.

I don't think reactive power has anything to do with it.

[joestue]

no the reason the neutral is grounded (you could ground one of the hot legs if you wanted, but the neutral is grounded at the transformer as well.. so you can't...

the reason is so that you can't accidentally put 7000 volts between your 120vac outlet and the earth.
its limited to whatever the transformer puts out..

[SparWeb]

Well at least I can say your house is wired up correctly.  Mine wasn't when I moved in.  Every once in a while something weird happened and I traced it back to a bad connection in the CB panel or backward wiring in the outlet boxes.
Joe summed it up, it's to prevent high voltages from either the hot or the neutral wires in your house.  It also prevents fault currents from also attaining much voltage in combination with anything that might be in contact with the ground.  Things like lightning strikes, equipment faults, and broken wire insulation can cause fires when they produce heat, arcing and the like.  The electrical code was developed in large part to define ways of wiring the house that reduce the likelihood of these problems becoming destructive (as well as reducing the risk that they can happen at all).  There are many sections on grounding, bonding, and earthing, and when they get specific about it all, they make all of these terms mean very particular things.  Chapter 250 is a most valuable read (if you're into that sort of thing!)

One way that the ground-bonded neutral wire affects people on this forum is the use of portable or mobile-home inverters.  I have one, too, but I'm not currently using it, and this is one of the reasons.  These little inverters make AC from batteries, but they often do not have the provisions built into them to bond one AC wire to earth.  If you try to do so you can cause problems!  They tend to have an internal ground reference and split one phase of 60V 180deg to the other 60V phase, to get 120VAC out of it.    It makes it potentially dangerous to interface these things to house wiring unless you take extra precautions, and usually the best is to use an isolation transformer with a 1:1 ratio and take power from the secondary into the house.

[SparWeb]

One more thing: I once measured 7 VAC from the "neutral" to the ground in an outlet in my house.  I was perplexed, until I opened the box and discovered that it was not only wired backwards (hot and neutral) but the neutral wasn't bonded to ground, AND there was another large load on the circuit at the time, meaning that the 7VAC turned out to be the voltage drop across the current-carrying wires in that load!  Totally insane.  If you didn't follow that, don't feel bad, it took me some study to get it, too.  I just knew I had a big problem and had to shut the base-board heaters off in that room until it got sorted out.

[birdhouse]

i feel like it's important to mention that there should only be ONE ground to neutral bond in a residential system.  this happens near the meter. 

if you have a "back to back" with the meter being close to the main house panel, then this bond happens in the panel.

then lets say you have what is known as a meter/main.  this is where the panel with most of the circuits is many feet from the meter, thus having a main breaker in the meter/main box.  again, the bond is in the meter/main box in this situation.  this is where you need four wire to tie the panel to the meter/main box, thus isolating the neutrals and grounds and the only neutral to ground bond being in the meter/main box. 

most panels have two "ground" buses tied together with a bolt on strap, and a grounding screw on each bus.  by removing the strap and one grounding screw, you can isolate one of these buses to use as a floating neutral bus.  by floating, i mean floating within the panel, but bonded near the meter.  this is why you need 4-conductor to go between the panel and the meter main.  2 for hots, one for ground, and one for neutral. 

end grounding/neutral rant...

adam

[OperaHouse]

"GFCI-equipped inverter" doesn't say very much.  As said earlier code only allows bonding neutral and ground at ONLY ONE point.   A GFCI can not have neutral  and ground connected together after the device.  That will cause it to trip.  Finally see what the inverter manufacturer says about grounding  the neutral.  Most low cost inverters have a H bridge output which means neutral is also hot and must be kept floating.  There is quire a premium for being able to ground the neutral.  I've been looking at buying a blown up Xantrex Freedom 2000W inverter.  This has a transformer output.  New retail these are like $1400.  Even at the under $100 range I can't justify it for my needs.   You can always buy a  used 1kw transformer and solve the problem that way.

[jack11]

After the joestue post, I looked closer at my transformer. Seems like the grid high voltage (HV) comes in on one hot line and one neutral line. The hot line is connect to the primary of the transformer. The neutral line is grounded at the pole, and also connected to the transformer case, the neutral wire that comes to my house, and the center tap of the transformer's low voltage (LV) secondary (all these are grounded at the pole). Then the LV phases L1 (0deg) and L2 (180deg) come down to my house, and also go to the neighbors.
So, the neutral that comes to my house is grounded because the HV neutral to which it is connected at the pole (and also the transformer's LV center tap) is also grounded.

Also, a GFCI-equipped inverter, simply the GFCI function is built into the circuit board (see for ex. Samlex PST-200S-24A), you cannot bypass or disable this function. I believe using this type of inverter you have to “unground” the house, and have all the circuits inside float between the hot and the neutral (which may also be hot going back to the inverter – with respect to ground). This may be consistent with what SparWeb and OperaHouse were saying about the inverter connections.

I've swapped grid for inverter once in our cabin, fully disconnected the grid, connected a non-GFCI inverter to the house hot and neutral, and connected the house CB panel ground to the inverter ground and the earth ground (I don't believe the neutrals are connected to the CB panel in our cabin). It worked fine, no problems for over a month we were there.
Has anyone else connected the existing house wiring to power off an inverter (with or without built-in GFCI), and what mods did you have to do?

[SparWeb]

Has anyone else connected the existing house wiring to power off an inverter (with or without built-in GFCI), and what mods did you have to do?

I'm not sure if you're asking about permanent installation, temporary installation, or emergency installation.  Each comes with different levels of sophistication.  You aren't the first person to try either combination so look up the recommended practice for installing back-up power supplies to residential electrical panels first.  Usually it's done with a sub-panel to hold breakers for the essential loads and a feed-in from the inverter or generator (or both if you have them).  One of the breakers is used as a "tie" between the two panels.  It stays on as long as it's not over-loaded, so normally when the grid is up, you can use power from all circuits in the house without thinking about which panel the breaker is mounted in.  On the day the grid goes down, shut off the main grid service entry breaker, also break the essenial/non-essential tie-breaker, then engage the inverter and/or fire up the diesel generator, to get the essential loads going again.  When the grid comes back on-line, reverse the process to get back to normal. 

This requires a number of manual steps to be taken.  In this day and age you can automate that.  You pay for that convenience, of cour$$e.

I would discuss the inverter with a licensed electrician who has experience installing inverters such as the kind you have.  There's a lot at stake on that one.  It sounds to me like it's not the right type for your house, but maybe they're making them differently these days.  The only inverter I use now is a type designed for interconnection with residential/commercial building wiring.  I feel it's the only safe way to go.

Oh apart from the grounding issue, the Samlex may be a square-wave inverter, or a mod-sine, which probably will but maybe won't get along with all of the devices in your house.  Another thing to check by trial and error.

[jack11]

that Samlex I quoted is a true sine wave inverter, its THD is < 3% as far as I recall. I'd buy one but for the fact that the GFCI function is integrated and cannot be bypassed.

you said "The only inverter I use now is a type designed for interconnection with residential/commercial building wiring.  I feel it's the only safe way to go."
Can you recommend a couple of brand names/model numbers of inverters you use that meet this standard, 24 or 48vdc, about 3kW capacity?

[Mary B]

I run a 1kw Samlex pure sine inverter. Put in a 35 amp transfer switch that switches hot and neutral. Runs my computer/ham gear/TV/surround system. Turn inverter on transfer switch goes to the solar panels.

[SparWeb]

Hi Jack,

I have a 1990's "vintage" Xantrex SW 4024 true-sine transformer-based inverter.  It's grid-interactive and can manage the operation of a stand-by generator (though I haven't started using it for that purpose yet, and I bought it used in 2006).  This type of inverter has been a standard in RE systems that rely on batteries for energy storage.  A few years ago, Xantrex was bought by Schneider Electric, so they're swallowed up in a megaglomerate and it's harder to get through to them (not impossible) when you need something.  Case in point, checking myself as I write I just opened up a browser tab to see Schneider Electric- Solar, and to give you a link.  Yea, it won't load today.  Typical.  On the other hand, the technology has continued to move forward at these companies, so they have a lot more to offer now.  You can find XW and TR series of inverters from Schneider that may suit you, depending on the size of system you want to be operating.  Due to the ownership changes at Schneider, you can open the current machines and find "Xantrex" printed on the inside of their equipment, and digging down to the circuit board level you find "Trace".  The lesson is that some products are 30 years old, and others are fresh off the sweatshop floor.

Ah finally after 3 tries the website opens.  http://www.schneider-electric.ca/canada/en/products-services/solar/solar-intermediate.page?f=F13%3ASolar~!NNM1:Solar+Backup+and+Off-Grid+Systems&p_function_id=5099

Another member of the forum, Chris Olson, said recently that there's some new product on offer from Xantrex/Schneider.  Don't ask me to find them though.  Hunting around all I see are more boat/RV power inverters for 12V batteries.

Splitting out of Xantrex was Outback, in the 1990's.  The story goes, it's the same band of engineers taking another crack at the problem, and doing it better.  Again, Outback has the size, battery, and generator management capabilities to operate for a long time in many system configurations, including probably yours.  Better, because you get excellent warranty support from them.  They would get my highest recommendation:

http://www.outbackpower.com/products/sinewave_inverter/

The proliferation of grid-tied battery-less systems has changed the market.  Many inverters are transformer-less, will manage solar panels with MPPT routines that operate at very high voltages, and do not provide any back-up power or generator start-up.  If you're out in the countryside looking for some off-grid living, then this isn't the choice for you.  Even if you want to keep the grid, but find it unreliable (like I do) then you want to have backup power capabilities that limit your choices of inverter.

Also on the market, advertised for RE systems with batteries, are brands like Magnum, but the service, warranty stories are mixed there.
New to the market is Apollo Solar.  I only know their advertising at this point.

I must also mention the Sunny Boy from SMA.  They have a much better reputation.  They aren't cheap!  I think they only sell to 48V battery systems.

My knowledge of Samlex hasn't been refreshed for a long time, so you're right to correct me there.

[jack11]

thanks SparWeb, I and surely others will benefit from your experience with these companies.

this built-in GFCI function can sure throw a monkey wrench in one's plans if your house neutral is grounded. The inverter will not work, and they won't take it back once it's been used. You have to be careful what you are buying.

You seem to have quite a lot of experience with the grid and grid-tie devices, maybe you can also comment on the issue the fellow mab from the UK brought up earlier in this thread (feedback from others also appreciated).

How about the US-UK difference in delivering 240vac mab brought up, dual-phase v. single-phase.

As mab was saying, the grounding of the neutral may have nothing to do with cutting off the flow of reactive power. The reactive power may still flow back to the grid even with the neutral grounded, is that possible? Maybe someone that took ac circuits theory can answer this?

However, the dual-phase delivery method resembles to me some sort of differential setup where common-mode errors can be mitigated/rejected by differencing the signals on L1 and L2 (for example surges, if they go in opposite directions on L1 and L2), something the single-phase system in the UK could not do.
Also, maybe this dual-phase power delivery has some inherent power factor compensation characteristic built into it by its differential nature, unlike the single-phase system, can you comment?

[jack11]

incorrectly used the word "differencing" above.

what I ment is that the surges/spikes would subtract (or add to 0) if they occur in opposite directions on L1 and L2, and one of the Ls is used as the reference, and the other one as the signal.

[SparWeb]

European mains electricity standards are very different from the ones in North America.  I'm speaking from a position of profound ignorance when it comes to the standards in the UK, but I can't take the suggestion from Mab at face value without research.

Residential wiring voltage in the US and Canada:  240V AC split-phase with neutral return.  60Hz frequency.  Neutrals bonded to earth at selected locations.  The step-down transformer that converts the highline (or buried line) voltage has one grounded phase, or a grounded center tap on the transformer (I think).
Residential wiring voltage in the UK and Europe:  220V single phase taken from 1 legs of a 3-phase service and a neutral (center of the 3-phase star).  50 Hz frequency.  Enclosures bonded in daisy chain to earth ground.  The step-down transformer that converts the highline (or buried line) voltage has no grounded phases (I think).  The neutral from the house service 3-phases (or just one phase) to the transformer 3-phases, and along the high-voltage lines, all the way back to the power station, does not ever see the ground.  Maybe grounded at the power station or at a substation.  The ground at the sub-station may deliberately have some resistance in it so that fault currents to not jump too high.

One of the up-shots of this is that touching the hot wire in North America while standing in your bathtub exposes you to 120 VAC.  Doing the same thing in the UK exposes you to an open circuit and you may not experience any voltage at all.  Confirm this with some research, but I believe that even if voltage can be experienced from the hot wire to ground, the current may be limited.

All of the assumptions about grounding, bonding, and earthing of certain conductors must be questioned when comparing the systems in other countries.

Your question about inherent "error rejection" in the split-phase AC service is an interesting perspective.  Not exactly the point but in a way similar.  You sound as if you have an engineering background but never took the cover off your circuit breaker panel.    The split-phase design is to simplify wiring, by eliminating one conductor from a large number of circuits and service delivery.  If there is a current in the neutral conductor then it's because the separate phases are not loaded "in balance" with each other.  A bit of imbalance is unavoidable, but the circuits in a house are usually set up so that this can't or probably won't ever happen.

There's an awful lot to explain in this subject.  I can explain a bit more, or correct here and there, but lesson after lesson isn't necessary.  EVERYTHING I'm writing is better explained elsewhere on the internet.  Not hard to search either.

[Simen]

As i understand it, we got a non-standard here in Norway (240VAC, 50Hz). We have no 'Live' and 'Neutral' here. Measuring between ground and any of the phases gives 120VAC; a single-phase are taken from any 2 of the three wires in a 3-phase.
A very sensitive earth-fault braker are mandatory here though...

But i'm no expert here.

[mab]

it's possible i misunderstood the original question so I hope I didn't add to the confusion.

The UK system is 240v* 50Hz single phase / or 3phase - 415v between phases*. the neutral (star point is grounded at the transformer and sometimes (depending on age of the installation) at multiple points along the lines to the supplied property. If the property uses the power network to provide an earth, then this can be supplied along a separate conductor from the substation (common where the old lead-sheathed cables are still in use), or is 'split off' from the neutral before the meter (more common on newer installations). If the network does not provide an earth then an earthrod is sunk at the property, and there is no connection between earth and neutral at the property (though the neutral is still earthed at the transformer of course).

* actually 230v/400v nominal now, to standardize with Europe, but a lot of existing supplies are still 240v.

One of the up-shots of this is that touching the hot wire in North America while standing in your bathtub exposes you to 120 VAC.  Doing the same thing in the UK exposes you to an open circuit and you may not experience any voltage at all.  Confirm this with some research, but I believe that even if voltage can be experienced from the hot wire to ground, the current may be limited.

well the bathtub test would give you 230v in the UK (although given the plastic plumbing and nonconductive tubs today you might not feel anything), but all domestic circuits should have a 30mA residual-current-circuit-breaker nowadays which would disconnect in a few mS, so hopefully you wouldn't feel it for long.

[dnix71]

I live in the US. When in college I did maintenance in the dorm I lived in (a co-op). I disliked doing ballast replacements because there was sometimes as much as 16vac on the neutral wire with the light switch off. There were 2 or three rooms overhead copper-ballasted flourescent lights wired to the same breaker. That meant if the neutral was not securely grounded there would be stray currents from less than unity power factor in the ballasts and you would get a jolt if you touched the neutral and the junction box supporting the light.

Grounding the neutral securely gives stray current a safe place to go. It is also required to get some flourescent light styles to fully light because those fixtures use capacitance between a metal reflector or clip to start the tube.

[kevbo]

There are a lot of different ways you could wire things that will allow it to work when everything is in good condition.  Some are simpler and cheaper than the way it is done.

The electrical code is designed, however, with the assumption that conductors will sometimes open, and insulation will sometimes fail.  Wiring to code reduces the danger when these abnormal events happen.  Sometimes there are tradeoffs, in which case they allow for the most likely or most dangerous problem to have the safest outcome, and sometimes accommodating a rare failure is very expensive, so they don't require it.

In the case you ask about, it is based on the assumption that the insulation in the pole-pig transformer will occasionally fail, placing 7.2KV or whatever the local distribution voltage is onto the nominal 230V circuit.  If that circuit is grounded somewhere then the insulation fault will cause a breaker to blow upstream, shutting things down.

The neutral is grounded so that the other two legs are only 115V off ground, reducing the shock hazard when you touch one. 

The grounding connection is done only at one location, the service entrance.  If there were multiple connections, then the ground wire could/would carry current if the neutral failed open, and the ground conductors are only intended to carry load current long enough to trip a breaker, do not have heat resistant insulation, etc.  Ground connection is a safety backup, so the design is that it never carries current unless something else goes wrong. 

Also, by requiring that the only possible return path for current flowing in the hot conductor is the neutral conductor, it becomes possible to detect insulation faults my comparing the two currents and making sure they are exactly equal...that is how GFCIs work. 

Requiring the neutral and hot to take the same path also minimizes the loop area, and that reduces radio interference. If the neutral were grounded at various points, you could end up with some very large loops.  This is a common problem in instrumentation, where signal grounds are often connected to earth ground at various points.  This creates a "ground loop" that picks up lots of radio interference.  If you allow this on the power side, then every arcing commutator becomes a fairly high power radio transmitter.

[OperaHouse]

Jack.....I've really been trying to follow your train of thought but all this phase talk sounds more like mystisism than engineering.

In a GFI system the hot and neutral must pass through a current source transformer.  If 15A goes out 15A must come back.  Those two out of phase scurrents cancel each other out magnetically.  Even juat 1mA of difference creates a voltage in the sense coil and circuitry trips out the load.   If the neutral after the GFI is grounded there will be an alternative path for the current and it takes less than 1mA to bypass  neutral to ground to cause a trip.

Short of modifying the inverter the beat option would be to add an isolation transformer.  Size it for you maximum load.  Not everything would have to be powered through the transformer.   It would be acceptable to run major loads directly from the inverter.  The furnace could be powered through a short electrical plug stub.  Durring powerout periods it can be plugged into the inverter. 

[SparWeb]

Dnix,

I've felt exactly that same jolt myself a few times!  You think the darn thing is off, and suddenly you're at the bottom of the ladder wonder where the F*** that just came from!

[OperaHouse]

I highly recomend thos little non contact wands tht sense AC for about $5. I helped a frend demolish office space and turn it into wharehouse.  The wiring was crazy.  In the same conduit power fron the left going right and power from right going left. Don't like getting hit with 277 when on a ladder and grabbing onto metal conduit for support.

[jack11]

So, we all now know HOW the 240vac delivery systems work in the US and the UK, and HOW the neutrals are grounded.

The question from the beginning was: WHY did they choose to design the power delivery system that way, and ground the neutrals the way they did?

I like the reasons that kevbo and joestue gave, to prevent up to 7kV from coming into your house when the HV transformer fails – this to me seems to be the primary reason WHY.

But also, there may be other benefits, like preventing the reactive power from going back to the grid (the experience dnix71 had may support this), and possibly auto-rejecting some common-mode voltage spikes or surges (I'd need to do some more thinking about this). These questions needed to be asked, or we'd have never had an interesting discussion that we did.

As far as the “phase talk”, there is nothing un-engineering about this, people in communications, power engineering, etc use it all the time (see for example basic term “phasor”, or call the power signals split-phase, dual-phase, bi-phase, etc, it's still the same thing). This is apparent if you look at electric power from the signal processing perspective, as I try to do. Then some additional benefits like power line communications, common-mode error cancellation, etc may become obvious and feasible.

[OperaHouse]

I believe the normal street HV is 1400V.  You will see a lot more spikes from that stuff that comes from the sky.

The normal arc over voltage of an electrical outlet is about 6000V.  I have an Applied Research arc cabinet that can do up to 35,000V.  Kinda neat, has these clear plastic doors to keep things frpm flying at you and a fan to duct away the smoke.  For more normal testing I have one that goes up to 6000V.  Also got a Hypatia 309 for doing bond testing.  That can produce up to 100A, normal appliance testing is about 30A for 30 seconds.  With an old cord you can pop that ground connection open.

The ground is to produce a local ground.  If the utility neutral should become elevated, you will be standing on ground at the same potential.

Phase.  I have always believed the truth should never get in the way of a good story.