Switch from Grid To Inverter ?

[Jim Mac]

Hi, I have a question:

After blowing the circuitry in my Outback inverter 2 times this year from lightning, I decided it's probably best to start powering down the inverter during lightning storms..

I currently have my outback inverter AC out powering a sub-panel in HBX charger off mode. When the battery bank power is low, it switches to Grid AC.. When the batteries are full, it switches to inverter.. Pretty standard..

This is exactly how I had it when the circuitry blew up 2 times..

So what I am thinking is to put a 3 way switch before the sub-panel and feeding one leg of the three way with a direct Grid powered line, the second leg of the three way with the inverter power, and the 3rd leg to the sub panel.. This way I can keep the sub-panel running from the inverter as usual, but flip the switch when a thunderstorm comes - causing the sub-panel to now be powered by the grid..

But a regular 3 way switch is not designed for this.. it is designed to have 1 power input and 2 outs.. Would there be any harm running 2 power sources through a 3 way switch like this? Is there another kind of 3 way switching device that is better for this application?

Thanks!

[Ralph Day]

Could there be a grounding problem/possible improvement with your system? Nothing will save you from a direct strike, but surges/emp will cause problems. I have Delta Lightning Arrestors on all the DC and AC equipment in my house system and not problems after 7 years off grid. That might be because of good grounding too.

Ralph

[Chris Olson]

Hi, I have a question:
But a regular 3 way switch is not designed for this.. it is designed to have 1 power input and 2 outs..

Hi Jim,

Why not use a standard manual throw generator transfer switch (see attached photo). The Ronk switch I have is rated at 200 amps and it wasn't real expensive about $250 if I remember right.

I have Xantrex inverters and they have auto gen start and auto transfer switches in them. But I put in the manual throw switch so I can switch my house over to gen power without going thru the inverters. With the handle up in the "on line" position the power comes from the inverters (or gen if the gen is running) and then to the load. If I pull the handle down to the "on aux" position, then the gen power is disconnected from the inverters and is fed directly to the house loads. It's only there in the event I would want to disconnect the battery bank for service, or have a problem with an inverter.

The generator transfer switch is designed for two inputs and one output going to the load, and meets all the code requirements. It is two pole and has a neutral bar and ground bar. The neutral is not switched to meet code - only the "hots". If you are switching 120 volt power then you'll only use one pole of the switch. If you are switching 240 split phase, then you'll use both poles.

It probably wouldn't hurt to look at your system grounding issues and install a lightening arrestor if you have had multiple problems with blowing inverters.
--
Chris

[Jim Mac]

Ralph- Yes, I think I do need some ground improvements. I have 2 8' ground rods. One goes to the grid supplied power. The 2nd ground-rod goes right to the inverter chassis. But I think this can cause an impedance mismatch between the 2 grounds. So I am thinking maybe I should put both the grid ground rod and the inverter rod together to make 1 large ground rod for everything.. Would you suggest?

Chris- Right On.... The term I needed was "manual transfer switch". I definitely want to get this for the exact reasons you mention.. As well as avoiding storm damage. SO I ordered this cheap one. http://www.northerntool.com/shop/tools/product_200196574_200196574

Thanks dudes

[Chris Olson]

Ralph- Yes, I think I do need some ground improvements. I have 2 8' ground rods. One goes to the grid supplied power. The 2nd ground-rod goes right to the inverter chassis. But I think this can cause an impedance mismatch between the 2 grounds. So I am thinking maybe I should put both the grid ground rod and the inverter rod together to make 1 large ground rod for everything.. Would you suggest?


I'm not Ralph, but you should only have one ground point unless certain conditions are met (distance apart, only one neutral ground bond, etc..).

Like you surmise, if you have potential between grounds, you'll have all sorts of stray voltage problems. The only way to really check it is to have an electrician come and ohm out your grounds with a DLRO.

Typically you're going to have just one ground at your service entrance and the service neutral is bonded to the ground at that point. To prevent potential between grounds your inverter chassis ground should be connected to that same ground as well as the battery bank ground. It is important to not have any difference in potential between the AC and DC grounds of the system to prevent electric shock and stray voltages that will fry electronics.

Also make sure you don't have an extra neutral/ground bond at the inverter with your second ground rod. That's another problem of a different color, and will not even meet code if its inspected.

When you put that transfer switch in, DO NOT switch the neutrals. Only the "hots". Tie the grid and inverter neutrals together by connecting them to the same neutral bar in the box. There will also be a ground lug or bar in that transfer switch box and your grounds go to that from both the grid and the inverter. If there is a provision in the box to install a green "bonding screw" do not put the screw into the neutral bar. That is only to be done at the service entrance.

If you're off-grid then things are different and the neutral/ground bond is made at the inverter because it is the service entrance and will feed subpanel(s) (assuming you have an AC conduit box on the inverter with mains in it). However, if you feed an outbuilding with the inverter, then the service box for outbuilding still has to have a main disconnect in it, and again, the neutrals and grounds are kept separate on the feeder to the outbuilding.

Power flows in the neutral during normal operation. Power should never flow in the ground unless there's a fault. If you have currently have more than one ground and more than one bond, you're going to have all sorts of issues.

If you have any doubts consult an electrician that is familiar with generator and inverter backup or prime power systems, and the proper way to configure the neutrals and grounds. The dollars he or she costs you will save you another blown inverter.
--
Chris

[Rob Beckers]

You can't have too many ground rods, and a potential difference between them is not important. What is important is that they are all connected together with sufficiently sized wire (6 AWG minimum in most jurisdictions).

Unless you have current flowing through your ground wiring there should be very little potential difference between unconnected ground rods (there may be some galvanic action, but that's about it). With lightning hitting anywhere near that situation will be entirely different though; at that point there will easily be 10,000+ Volt difference between ground rods! That makes no difference to their job though, they're there to conduct energy to ground as efficiently as possible.

I suppose what Chris meant is that there should be only one point where your house-ground connection hooks up to those ground rods, ie. a single-point ground. You do not want to have electronics that are interconnected, hooking up to different grounds (and their potential difference when lightning strikes). That will set up very large currents through those electronics and let the magic smoke out in a hurry.

I've written a rather long Web article (actually a set of articles) on lightning, wiring for lightning, grounding, and surge arrestors. You can read it here (http://www.solacity.com/Lightning.htm). That should help explain how lightning does its dirty work, and what you can do to mitigate the damage.

-RoB-

[Chris Olson]

I suppose what Chris meant is that there should be only one point where your house-ground connection hooks up to those ground rods, ie. a single-point ground.

Hi Rob/Jim,

This is correct - you should only have one ground, and one bond to the neutral of the system at the service entrance. Depending on whether you are on or off-grid, the service entrance will vary as to what it is. But in this case, it is the grid service panel.

The code in the US (Section 250.70) deals with grounds and how they are to be connected for a residence. Like I said, if you have any doubts consult an electrician because done improperly you will have problems (and probably be out of compliance with code requirements).
--
Chris

[Rob Beckers]

Chris, with respect, the US and Canadian electrical codes are only interested in public safety with respect to electrocution and fires caused by the grid. There is nothing in the electrical code that helps mitigate lightning damage. From a lightning perspective, the request for a single ground plate or rod for a residence is pitiful. Now, you can make decent lightning protection, and comply with the electrical code at the same time. But I would certainly not look to the electrical code when it comes to guidance for lightning-proofing.

-RoB-

[Chris Olson]

It should be noted the NEC and CEC doesn't limit an installation to only one ground rod. Just one grounding point. Multiple ground rods can be used as long as certain conditions are met for distance apart, size wire used (6 AWG) and method used to interconnect multiple rods.

However, driving one separate ground rod for the service and another separate rod for an inverter or generator is not allowed. It sounds like Jim has done this, and chances are there is also a second neutral/ground bond at the inverter. It is not hard to see why he would have problems with lightening EMF surges with such a configuration.

There is virtually nothing that can protect equipment from a direct lightening strike. You can try to prevent direct strikes with proper grounding schemes for the tower, etc.. But if Mother Nature decides to unleash the incredible power of a direct strike, all you can do is run and hide.
--
Chris

[Rob Beckers]

There is virtually nothing that can protect equipment from a direct lightening strike. You can try to prevent direct strikes with proper grounding schemes for the tower, etc.. But if Mother Nature decides to unleash the incredible power of a direct strike, all you can do is run and hide.
Chris

Not exactly...
There are lots of communications towers that get hit on a regular basis (some of them weekly during the season!), and it is very, very rare for there to be any damage. Neither to tower, antennas, nor electronics. Lightning protection can be made very nearly nature-proof, and it is well-known how this can be done. No voodoo or black-magic involved (nor prayer ;) ).

The downside is that such a level of protection is expensive for the home owner, and therefore normally not done. Besides telling you how the pros do this, my Web article tries to point out what you can do on a modest budget.

-RoB-

[Chris Olson]

Yes, while this is true and lightning protection systems do work, it is important to remember that just because you install a lightning protection system that you are never safe if Mother Nature decides to strike. According to Sandia National Labs, which has done a lot of lightning research, Mother Nature can deliver up to 300 million volts and 200,000 amps at a temperature of 54,000 degrees F. There is nothing mankind has ever built that can control it. You can only attempt to direct this power to ground.

In Jim's case I don't believe he is seeing the results of direct strikes. I believe he is seeing the results of nearby strikes to other objects. I remember as a kid living on our home farm when lightning stuck the phone lines. This was back when they were strung all over the place on telephone poles. The power traveled thru the tiny wiring in our house without melting the wires or burning the walls and blew the handset right off the wall. I was only about nine years old at the time and have been scared crapless of lightning ever since.
--
Chris

[Ralph Day]

Hi Jim
I don't know if i had a paranoid installer, paranoid electrical inspector or just got lucky. On my house system there are ground plates at the array sites, generator site, quite a few at the wind turbine site...but the micro\FIT only has ground plate where the meter bases are, not the array site. Maybe code requirements changed, I don't know.

We were doing things according to the book (only one ground for the entire system) and when the ESA inspector saw the generator he said drop a ground plate/rod within 10 feet of that please and thank you. No problems so far.

The wind turbine site has a ground plate at each of 4 guy points and mast with #6 wire joining all the plates together. Lots of copper out there, and no problems so far. I personally feel more comfortable with multiple grounds.

Good luck

Ralph