Author Topic: How do you deal with inverter grounding on an off grid building?  (Read 13380 times)

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Offline mtdoc

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #25 on: June 01, 2018, 10:12:11 pm »
Here is a nice concise diagram of off grid inverter wiring from Morningstar.   Perhaps this helps.

I think it is floating before you choose a ground reference. After you ground it is not floating any more. What you cannot do is ground it in two places at once, for example you cannot tie together the terminal on the inverter marked G and the terminal on the inverter marked N as this will create a short circuit and destroy the inverter. You either have to pick G or N as your ground point and use it consistently.

G and output N of the inverter normally should be bonded - it should not destroy the inverter. Once you bond G and N together to earth ground they are electrically the same point.

Tying ground to battery negative will only work if there is complete isolation between the battery side and the AC side of the inverter. Otherwise there could also be a damaging short circuit.

Well if there is no isolation there you will have problems regrardless of whether or not battery negative is earth bonded.

« Last Edit: June 01, 2018, 10:29:51 pm by mtdoc »
 

Offline Red SquirrelTopic starter

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #26 on: June 01, 2018, 10:54:33 pm »
I think there may be a bit of misunderstanding.  The ground output of the inverter (the ground pin of the socket) has 60 volts potential between neutral or hot.  In a normal electrical system neutral and ground should be bonded, but if I do this, I will create a 60 volt short and damage the inverter.

The charge controller has some odd circuitry which causes the negative of the PV side and the negative of the battery to not be isolated from each other either and are at a different potential (I forget what) so I cannot bond those either. 

I'd have to measure battery and AC output to see if those are isolated or not but I have a feeling they arn't either.

So for now I will leave the PV and battery side float, but I'm more concerned about the AC grounding, so I'm proposing to ignore the inverter's ground, and supply my own from the ground rod which will tie to the AC ground and the neutral at the main junction box that feeds everything else.  This will ensure that the AC ground, wiring boxes, and neutral are all referenced to earth ground as it should be in a normal AC system. 

This would be a somewhat temporary solution though as I do plan to buy a better inverter, so if I'm lucky the new inverter will have proper isolation between input/outputs so that I can bond everything correctly without causing issues.  But that's not something I'll know until I pick an inverter and measure it.  We are very limited here in Canada as far as PV related products so I have slim pickings. 
 

Offline IanB

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #27 on: June 01, 2018, 11:11:57 pm »
So for now I will leave the PV and battery side float, but I'm more concerned about the AC grounding, so I'm proposing to ignore the inverter's ground, and supply my own from the ground rod which will tie to the AC ground and the neutral at the main junction box that feeds everything else.  This will ensure that the AC ground, wiring boxes, and neutral are all referenced to earth ground as it should be in a normal AC system.

I see what you are saying, but I don't understand why you want to install a ground rod and tie things to it. This is not a normal AC system, it is more like a portable gasoline generator. With a portable generator you just run it and then plug things into it.

So I still think you are overthinking this and trying to make it much more than it is. It's just a small power supply in your shed. That's all. Like any other power supply, just plug things into it and use it.
 

Offline mtdoc

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #28 on: June 01, 2018, 11:24:21 pm »
The ground output of the inverter (the ground pin of the socket) has 60 volts potential between neutral or hot.  In a normal electrical system neutral and ground should be bonded, but if I do this, I will create a 60 volt short and damage the inverter.

Hmm, ok I guess I see the confusion now.  I have not seen that - even on the cheap inverters I've looked at.  I'm still not sure that bonding negative to ground on the output of the inverter would cause an issue - it would depend on the inverters internal wiring. 

Quote
The charge controller has some odd circuitry which causes the negative of the PV side and the negative of the battery to not be isolated from each other either and are at a different potential (I forget what) so I cannot bond those either. 
OK, but I never said you should bond the CC input negative to your earth ground with battery negative.  CC inputs should be left floating. (see diagram in link I posted)



 

Offline Red SquirrelTopic starter

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #29 on: June 01, 2018, 11:25:36 pm »
I was thinking more from a safety standpoint.  If power accidentally contacts metal work like junction boxes or an appliance I am plugging in.   With a proper ground it will short out and trip the breaker (which I don't have now...suppose I should, but inverter probably would shut down anyway) I guess it's not the end of the world, but I'm just thinking there's potential for power to be used in a wet location too so risk is a bit greater.   Though I guess a GFCI could work too.   It's all connected up now and works fine though so maybe you're right I can just leave it.
 

Offline f4eru

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #30 on: June 01, 2018, 11:28:01 pm »
Quote
This is not a normal AC system, it is more like a portable gasoline generator. With a portable generator you just run it and then plug things into it.
Nope.
You have to ground it also.
And not like this :


It's a safety issue to not ground a generator, especially when plugging class I devices in it, which need an earth to be safe.

Offline mtdoc

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #31 on: June 01, 2018, 11:35:52 pm »
So I still think you are overthinking this and trying to make it much more than it is. It's just a small power supply in your shed. That's all. Like any other power supply, just plug things into it and use it.

Yes, I agree.  If he leaves the output of the inverter floating, it is essentially the same as the output of the typical portable generator (though technically you should ground reference those - few people do). There will be some shock risk if any non double insulated appliances, etc are plugged into it since there is no GFCI outlet.

RV inverter set ups are typically floating relative to earth but still, battery negative, neutral and ground are bonded to the frame of the vehicle which acts as the "earth" while inside it.
 

Offline IanB

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #32 on: June 02, 2018, 12:41:53 am »
I was thinking more from a safety standpoint.  If power accidentally contacts metal work like junction boxes or an appliance I am plugging in.   With a proper ground it will short out and trip the breaker

This is true. Which is why you should wire the terminal on the inverter marked G or ground to the ground wire in the appliance cables or receptacles.
 

Offline Red SquirrelTopic starter

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #33 on: June 02, 2018, 01:00:11 am »
I was thinking more from a safety standpoint.  If power accidentally contacts metal work like junction boxes or an appliance I am plugging in.   With a proper ground it will short out and trip the breaker

This is true. Which is why you should wire the terminal on the inverter marked G or ground to the ground wire in the appliance cables or receptacles.

This will create 60 volt potential between ground and either hot/neutral though (the inverter has no ground screw, just the ground outlet output).   Would this be an issue?  Right now this is how it's connected but I feel that's not right.  The neutral and hot are not bonded given it would cause a 60 volt short.  Do I just run my ground to a rod and call it a day and not worry about the neutral and ground bonding?   I doubt it would be to code but I'm not too worried about that, I just want it to be safe. 
 

Offline IanB

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #34 on: June 02, 2018, 01:04:24 am »
It's a safety issue to not ground a generator, especially when plugging class I devices in it, which need an earth to be safe.

That's not true according to this article: https://c.ymcdn.com/sites/www.csda.org/resource/resmgr/imported/Sept%2007%20Safety%20Counts.pdf

Quote
In my mind, there are two different ways to use a generator. The first is as an isolated system in which the generator is powering a specific piece of equipment like concrete saw through the receptacles on the generator. I call this the cord and plug system.

If you are using the cord and plug system, typically you do not need to supply an earthgrounding rod with the generator. There are no other sources of electricity, lightning surges, or fault currents that you would need to contend with. The housing/frame of the generator will serve as an adequate ground for the fault currents generated by that particular generator.

Broadly speaking, the purpose of grounding and bonding is to prevent any metal parts of the system or structure getting raised to higher voltages than the real earth ground caused by external current or voltage sources (like lightning, fallen power cables, other mains power cables in the vicinity, and so forth).

If you have a completely isolated and portable power supply that is far away from any other electrical systems, then there are no other stray currents to need protection against. In this instance the portable power supply can be left floating. However, the ground terminal in receptacles should still be solidly connected to the ground terminal and metal frame of the generator.

I would consider a simple floating and isolated PV generator on a shed far away from mains wiring to be like a portable generator. (I'm thinking a box on the wall, a couple of receptacles and a light fixture.) It is not the same as running a mains feed to a sub-panel in the shed. In any case, 300 W hardly even counts a generator. It won't run more than a few lamps and it is barely worth connecting it to plug sockets. All you could power would be some battery chargers for cordless tools.
« Last Edit: June 02, 2018, 03:31:26 am by IanB »
 

Offline IanB

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #35 on: June 02, 2018, 01:08:15 am »
This will create 60 volt potential between ground and either hot/neutral though (the inverter has no ground screw, just the ground outlet output).   Would this be an issue?

No, it's not an issue. As previously mentioned this is the standard system used on UK construction sites where they use 55-0-55 V step down transformers for power tools (where the 0 V center tap is grounded). It limits the maximum electric shock from an insulation fault to 55 V, which is much less damaging than the normal mains voltage.
 

Offline Red SquirrelTopic starter

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #36 on: June 02, 2018, 01:11:50 am »
This will create 60 volt potential between ground and either hot/neutral though (the inverter has no ground screw, just the ground outlet output).   Would this be an issue?

No, it's not an issue. As previously mentioned this is the standard system used on UK construction sites where they use 55-0-55 V step down transformers for power tools (where the 0 V center tap is grounded). It limits the maximum electric shock from an insulation fault to 55 V, which is much less damaging than the normal mains voltage.

Good to know, guess I'll just keep it as is then.  I'll put in a grounding rod or two and tie it in to that ground then.   I'll just solder the wire straight to it and drive it in the ground.  My main water valve is not that far, I might tie to that too.
 

Offline NorthGuy

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #37 on: June 02, 2018, 01:35:25 am »
... the battery ground is a few volts from the solar negative (it changes based on solar input).

This is because one (or more) of the wires you used to connect them is too thin (and thus has high resistance). When current flows through it, the wire's resistance produces voltage drop - Ohm's law at work.

Unless you have extremely unusual solar charger, solar negative is directly connected to the battery negative.
 

Offline Red SquirrelTopic starter

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #38 on: June 02, 2018, 01:54:49 am »
... the battery ground is a few volts from the solar negative (it changes based on solar input).

This is because one (or more) of the wires you used to connect them is too thin (and thus has high resistance). When current flows through it, the wire's resistance produces voltage drop - Ohm's law at work.

Unless you have extremely unusual solar charger, solar negative is directly connected to the battery negative.

It's just a cheap PWM controller, so I do plan to replace it eventually.  Would love to be able to reference everything to ground, as it would enable me to actually easily measure voltages via an MCU.  When I originally built this portable system I wanted to be able to measure PV voltage and battery voltage but could not do both as they are not referenced to ground.    This is the PCB:



I may eventually design my own too, would make a fun project and I'd be able to design it with proper isolation/bonding in mind.  It's hard to source this kind of stuff anyway outside of cheap Ebay/Amazon jobbies straight form China.  So if that stuff has not caught on fire (yet) my own stuff might be just as safe. :P   Actually I'll probably want smoke detectors that alert me in the house/phone in that shed considering it has electrical. 
 

Offline KE5FX

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #39 on: June 02, 2018, 02:02:46 am »
Every portable generator instruction manual I've ever seen warns that it should be grounded.  No instruction manual ever says why, or cites the relevant electrical code.  From my point of view, that's a green light to ignore the warning.   >:D   

The only reason I can think of to ground a generator is to prevent static electricity buildup that could prove hazardous during refueling.  You have rotating parts, rubber belts in some cases, that sort of thing.  There is likely to be a good counterpoise in the form of the connected cord(s) and device(s).  I only use propane generators, never gasoline, so I don't see that as an issue, but it's worth keeping in mind.
 

Offline NorthGuy

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #40 on: June 02, 2018, 03:17:29 am »
This is the PCB:

A very small one, probably designed for car batteries. Looks like it has N-channel FET and switches the low side. So, the positives of the battery and the panel must be connected, perhaps through the big diodes we see over there. You can reverse engineer the power connections just by looking at the PCB traces.
 

Offline f4eru

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #41 on: June 02, 2018, 07:47:39 am »
Quote
As previously mentioned this is the standard system used on UK construction sites where they use 55-0-55 V step down transformers for power tools
They use 110V in UK for power tools ?
That's strange.

Offline IanB

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #42 on: June 02, 2018, 07:56:34 am »
They use 110V in UK for power tools ?
That's strange.

For commercial use on construction sites. Ordinary consumer tools at home run off the standard mains supply.
 

Offline f4eru

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #43 on: June 03, 2018, 10:25:02 am »
Quite a strange thing.
Probably it has historic reasons.
doesn't really make much sense today, coz it tries to protect against touching one conductor, which RCCDs are available now, and it doesn't really do anything for touching both conductors, and it complicated construction...

Are US style plugs used in this setup ?

Offline b_force

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #44 on: June 03, 2018, 12:14:49 pm »
Meanwell has some very good solar inverters for a fair price (around 1500-3000W)
They also provide a lot of information and support, so I would just give them a call or send an e-mail.

Offline Seekonk

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #45 on: June 08, 2018, 03:35:50 pm »
Everyone keeps saying 60-0-60.  While that is RMS, it is a H bridge output and the output should be about 140V to battery common if it is not electrically isolated. Many inverters do not even connect the ground pin.
 

Offline NiHaoMike

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Re: How do you deal with inverter grounding on an off grid building?
« Reply #46 on: June 08, 2018, 04:28:30 pm »
Some standalone inverters are isolated and some are not. I have torn down a CAT 1kW unit that is isolated and has ground fault sensing. Funny thing is that because it's isolated, grounding either side of the output will not trip the ground fault protection. My best guess is it's a backup in case the isolation gets bridged somehow.
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