Generator Ground

lazza · February 2012

We are installing a new off-grid PV system here. I have made sure all the protective ground requirements have been adhered to.

However, we have included a simple switch between the PV system and a small petrol generator (currently being used by the client) just in case of a failure of the PV system, so that he can switch to temporary generator power. The two systems are completely separate.

However, his generator doesnt seem to have a ground/earth connection. Am I to understand therefore that this will compromise the whole protection system in the eventuality of having to use the generator at any given time? If there is no earth reference for the neutral lead, then I presume that the RCD (GFCI) will not function correctly?

RCinFLA · February 2012

Re: Generator Ground

GFCI systems rely on the principle that all the current going down the hot wire must come back the return wire. The return wire (neutral) is grounded back at the main entrance, outside the local service outlets area.

If there is a leakage path within the local service outlets area from hot to earth ground then all the current will not be flowing back on the neutral and the minute difference (as little as ten microamperes) will be detected by the GFCI and cause it to trip its breaker open.

GFCI's have a small toroid (donut) transformer with a couple of counter wound turns for hot and neutral wires so their generated magnetic fields cancel out within the toroid core when their currents are exactly equal, as they should be when all is well. A third multi-turn pickup winding senses any residual current when the hot and neutral currents are not exactly equal. This sense winding output is amplified and used to run the GFCI breaker trip trigger.

Grounding of the generator casing is for safety around the generator, not GFCI functionality. You do need a grounded neutral at the service entrance breaker box or at the generator for GFCI's to work properly.

Cariboocoot · February 2012

Re: Generator Ground

RC has done a fine job of explaining the GFCI function, so I'll address the mundane aspect of the wiring.
"Floating neutral" on generators is common. Normally they're used for portable power so there is no ground available. They often have ground lugs on the frame so that they can be "safety grounded". The neutral-ground bond is purposefully left off so that if the frame is not grounded the neutral line cannot energize the generator frame.
Your inverter/household wiring should already have one neutral-ground bond at the service box. Additional bonds are not warranted or desirable. The generator should be wired so that it either feeds hot & neutral through the inverter (pass-through type) or with an external transfer switch that replaces the inverter feed to the service box with the generator feed. In either case, when the generator is supplying power it will do so to the service box where the neutral-ground bond occurs.
If the inverter is one of those "portable" types with the built-in GFCI outlets and power is switched from inverter source to generator source by transferring the plug from one outlet to another, there is no GFCI on the generator output. Don't worry about it as GFCI is not really necessary.

As always, the devil is in the details.

lazza · February 2012

Re: Generator Ground

The inverter is where the neutral-ground bond is done. In this system we are by-passing completely the inverter when switched to generator. Service panels in Spain do not have neutral to ground connections. In grid connected systems I think the neutral to ground bond is done at the transformer (correct me if i'm wrong). This system is off-grid and hence doesnt have that set-up.

Cariboocoot · February 2012

Re: Generator Ground

lazza wrote: »

The inverter is where the neutral-ground bond is done. In this system we are by-passing completely the inverter when switched to generator. Service panels in Spain do not have neutral to ground connections. In grid connected systems I think the neutral to ground bond is done at the transformer (correct me if i'm wrong). This system is off-grid and hence doesnt have that set-up.

Short form: no neutral-ground bond on the AC = no functioning GFCI.

Normal household wiring has the neutral-ground bond at the service panel. You probably don't see this in Spain because they use 230 VAC, si? On such a system there is no neutral: GFI looks for current imbalance between what we would call the two "hot" leads and shuts them both off if a discrepancy is found.

North American split-phase has a center tap on the 240 VAC supply transformer which is "neutral". This is bonded to ground at the service (breaker) box and creates the 240/120 system. In that case the GFI looks for current imbalance between one "hot" lead and neutral and shuts down the hot if a discrepancy is found.

So just to check, are we talking about a 120 VAC off-grid system or a 230 VAC off-grid system?

RCinFLA · February 2012

Re: Generator Ground

Portable generator's have their ground prong of their outlet sockets connected to their chassis. Their neutral connections are usually floating but you should check with an ohmmeter. It is fine to connect that ground prong of the plug to the ground (service box chassis) of the house. It does not have to be switched via the transfer switch and can be left connected even when grid power is available and transfer switch is on grid power.

Non-portable generators on permanent fixed installations should have a ground rod connected to their chassis.

BB. · February 2012

Re: Generator Ground

Cariboocoot wrote: »

Short form: no neutral-ground bond on the AC = no functioning GFCI.

Sort of yes and no... A truly floating power supply (such as a 12 volt car battery connected to nothing else but the floating load) will not drive current to a person or connection to earth ground.

However, with larger systems and AC voltages--There tends to be enough capacitive/inductive coupling to other stuff (sheet metal, wire runs in parallel with metal/green wire grounds/EMI and RF filters/etc.) that you can start getting milliamps (0.001 amps) or more flowing through somebody touching both "hot" and some sort of metal structure/earth grounding.

A GFI will then "trip" if the current differential is >~5 mAmps (0.005 amps) (for US/North American GFI devices). I guess people will feel ~10 uAmps (0.00010 amps) or about 1/200th the current that can risk stopping a heart (if properly applied).

Regarding generators in the US--Think I read somewhere that generators >3.5kW rated output (or was it 5kW) will have "Neutral" bonded to the chassis. Smaller gensets will have the "Neutral" floating.

BB. wrote: »

A couple threads about Lightning:

Off Grid Grounding Technique?
Another Question, this time about Lightning

Note, the above are discussions, not a do A, B, and C--and you will be "safe". There probably is no such thing with lightning. Several different techniques are discussed--and a few of those posters even have experience with lightning. :cool:

And our host's consolidated FAQ page:

www.windsun.com
Lightning Protection for PV Systems

From other past posts here, Windsun (admin/owner of NAWS), he said that most of lighting induced failures he saw were in the Inverters' AC output section.

Towards the end of this thread is a very nice discussion of proper generator grounding.

-Bill

BB. · February 2012

Re: Generator Ground

By the way--I am not a fan of using a GFI outlet on a generator to supply multiple circuits (such as a home or cabin for main/backup power). Also same thing for inverters that include a GFI outlet on their main output.

It is pretty easy to trip a GFI breaker/outlet--And if that one circuit supplies all of your loads--You will not only turn off the power to your mixer that fell into the sink, you will also trip the power to all of the lighting circuits in your cabin/home/RV too...

If you use GFI breakers/outlets--They should be installed where they are needed (near water, sinks, outside, etc.) and there should not be one that trips if any of the "branch circuits" gets a ground fault. This will leave you with lights and other power. plus quickly point to the culprit that got the ground fault.

-Bill

Cariboocoot · February 2012

Re: Generator Ground

Something else Bill and I agree on: it's better to have the GFCI outlet at the load that needs to be protected this way. Using them for everything is a waste, and running multiple lines off one GFCI seems to beg trouble. I'm not certain, but I suspect some inductance in additional wiring occasionally "fools" the GFI causing it to trip. GFCI breakers on dedicated lines are no trouble, providing that line doesn't run everywhere and do everything. Like he said, you don't want the room going dark when it trips or else you could have further trouble.

I've never seen GFI on a generator, but wouldn't be surprised if someone thought it was a good idea. It is often found on "portable" inverters, the idea being that the loads are an unknown quantity and an extra measure of protection is in order. More than once people have posted on this forum because they were having trouble with such units.

I'm not a big fan of GFI either. :roll:

lazza · February 2012

Re: Generator Ground

Errr my head is spinning a bit now

Maybe i'll try rephrasing.. hope this may be a bit clearer:

It's a simple 230VAC system. Completely off-grid. The Inverter of the renewable energy system has a neutral to ground connection... however, the switch to the generator isolates the renewable system (ie the generator goes directly to the service panel). Thus when the renewable system is engaged... no problem we have a neutral to ground connection... but if the generator is connected we have a floating neutral.

As the generator has a floating neutral (230VAC)... does this mean that the GFCI will not work properly if there is a short to ground on a toaster for example? and why wont it work?

Cariboocoot · February 2012

Re: Generator Ground

Must be a nomenclature problem. To my knowledge, 230 VAC systems don't have a neutral (center tap of supply transformer, giving 1/2 Voltage between each hot leg and the center tap). In such a system, the current flow between the two legs is compared and both are shut down if an imbalance is detected (like tripping a double breaker on NA's 240 VAC). Grounding and neutral-ground bond doesn't enter in to it. I could be wrong about this, as it's been a while since I've seen European systems and they have changed a few things over there to bring all the various set-ups together (some of which was done by fudging the tolerance numbers).

So long as the GFI device is comparing the current on the two sides of the line, whether 'Hot' and 'Neutral' or 'L1' and 'L2', it should function; if neutral isn't used for the circuit it isn't needed. Many 240 VAC applications here (electric heating) have no neutral line because the 1/2 Voltage isn't required.

What really won't work without the neutral-ground bond is the 1/2 Voltage circuit; the Voltage between L1 & N and L2 & N may drift up and down. One half going high while the other goes low. Tie neutral to ground and the two halves will stabilize.

As a general rule, when you swap one supply source for another their functionality should be identical. This means any neutral-ground bond or GFI should be "downstream" of the power source. We see this mostly with off-grid systems & back-up gens: most gens have floating neutral but work fine because they connect to the service panel where the bonding occurs.

RCinFLA · February 2012

Re: Generator Ground

lazza wrote: »

Errr my head is spinning a bit now Maybe i'll try rephrasing.. hope this may be a bit clearer:

It's a simple 230VAC system. Completely off-grid. The Inverter of the renewable energy system has a neutral to ground connection... however, the switch to the generator isolates the renewable system (ie the generator goes directly to the service panel). Thus when the renewable system is engaged... no problem we have a neutral to ground connection... but if the generator is connected we have a floating neutral.

As the generator has a floating neutral (230VAC)... does this mean that the GFCI will not work properly if there is a short to ground on a toaster for example? and why wont it work?

Yes the GFCI will work. If the hot wire current does not match the return neutral current from the protected outlet the GFCI opens the circuit.

If you want to check it, put a 100k to 500k resistor from hot line on outlet socket to ground prong on socket. GFCI should immediately pop open.

(in Europe, they call the 230vac blue return wire the neutral wire, which is grounded at service box. Yellow-green is safety ground wire. Brown, black, or grey wire is hot line. per IEC code)

stephendv · February 2012

Re: Generator Ground

Why not make the neutral to ground bond in the service panel instead of in the inverter? That way it's always there whether running on gen or inverter.

lazza · February 2012

Re: Generator Ground

As far as I understand it- as the neutral for the generator will not be ground-referenced, the GFCI will not trip if you happen to have a bad day and touch the live wire ?¨Likewise if the metal chassis of a an appliance becomes "hot"- it wont trip either.... am i missing something?

ps Yes neutral in Europe is the name given to the blue return wire. Grounding the neutral is normally done at transfomer stations in Europe not at the "service panel" if this is what you mean by service panel http://desenchufados.net/wp-content/uploads/2009/05/caja-general-de-proteccion.jpg

Cariboocoot · February 2012

Re: Generator Ground

Yes, that's a service panel: the thing with all the breakers in it where the power feed and loads connect up.

Just to be clear, GFI looks for an imbalance between the current going through the two wires to the load. If one is detected, it trips. This works on our NA split phase 120 circuits (where the CT neutral is grounded) as well as 240 VAC circuits (where there is no neutral) that trip both sides (because both are "live"). Your situation is similar to the latter: it is not necessary to have one of the "hot legs" actually connected to ground for this. The GFI acts after the power source in the circuit, looking at current flow to and from the load. Should either wire come in contact with the case of something or water or otherwise create an alternate path for the current to flow through the GFI will trip and interrupt power.

BB. · February 2012

Re: Generator Ground

lazza wrote: »

As far as I understand it- as the neutral for the generator will not be ground-referenced, the GFCI will not trip if you happen to have a bad day and touch the live wire ?¨Likewise if the metal chassis of a an appliance becomes "hot"- it wont trip either.... am i missing something?

To get current flow if you touch a "hot" wire, you need to touch something else that is the "return" wire/path.

So, if you have a "floating" power source, like a genset. If you touch one of the hot wires, there may not be any current flow through you--because there is no return path.

If the "neutral"/Blue wire for the genset was earth grounded, then if you touched "hot" and something grounded (another appliance with a grounded plug, a kitchen faucet, step in a mud puddle), then you would have current flow from "hot" to you to puddle and back to "neutral"--A complete current path.

ps Yes neutral in Europe is the name given to the blue return wire. Grounding the neutral is normally done at transfomer stations in Europe not at the "service panel" if this is what you mean by service panel http://desenchufados.net/wp-content/uploads/2009/05/caja-general-de-proteccion.jpg

In the US, the Neutral is bonded both at the pole/local transformer AND at the home. So--by definition, our NEC does require multi-point grounding. And, there have been (at least once) somebody noticed the snow melting around their home ground rod because of a shorted/improper connection made somewhere.

Which is better (grounding neutral at both at the transformer and at the home service panel)--I am not sure. I can understand that earth grounding at the home could be "more reliable" than relying on the pole grounding. But the multi-point grounding could hide problems too (probably very rare problems).

What a GFI will not do is detect you touching both Hot and Return at the appliance/end of the extension cord. In this case, you look like a "normal" load--the current out the Hot equals the current coming back on the Return. So, you will still be nicely shocked/electrocuted because the GFI will not open (no "missing" current between Hot and Return).

-Bill

Generator Ground

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