Switching from Grid to RE mains

[alibro]

I am setting up a system for disconnecting some circuits from the grid mains in my house and onto RE mains. Some guys seem to think you can do this with switches or relays and others are saying this would be illegal and/or dangerous. As I don't want to get into trouble (or kill anyone) I will probably go with the solution TomW suggested ie Have two live sockets (one grid mains and the other RE mains) and a plug for the circuits. I can then move the plug as needed. In a perfect world I would make sure all TV's, computers, etc are turned off and disconnected before swapping from grid to RE or back. In the real world something will probably still be connected and my question is.

If I swap mains sources too quickly will I blow a power supply? If so how quick is too quick.

This may seem like a stupid question but I don't want to take anything for granted. At the moment I am using a APC 1400W UPS but when it dies I will get a more suitable inverter.

Cheers

Alibro

Resectioned. Not really about "Wind".

[AbyssUnderground]

I don't think you'll blow anything. If you really can change it quicker than 0.1 of a second then it will probably look like a brownout to most items otherwise they'll simply turn off. However as you stated, it is best to turn them off before changing over as the surge won't do the UPS inverter any good. It won't bother the mains too much though you might blow some fuses if the surge is great enough. Lights won't be fussed either way.

[zeusmorg]

 I think some clarification should be done in switching from mains or grid power over to an RE or any backup system. First unless you're using a grid tied system then ALL the grid connections must be dissconected. That means your hots your neutrals and your grounds.

Systems in different countries and areas have different connections and power schemes.

If you just disconnect the hot(by flipping off a breaker) you still have the neutral connected. Some systems intertie the neutral and the ground. Remember AC current flows bidirectionally, so if you disconnect the hot from the grid then power it from a different source, you've started using and have energized the neutral, and possibly ground. This may or may not, depending on what distribution scheme is used locally kill someone touching said neutral outside your home. Also, when the power DOES get switched back on by the power company, your inverter will smoke because the grid waveform and that of your inverter won't match. So if you do have some sort of "emergency bypass system" make sure the two do not share any common wiring when you switch over. So, in a nutshell if you disconnect your mains/grid make sure you disconnect ALL of it. If you're in the least doubt about what you're doing call a certified electrician and get his opinion,it doesn't take all that much electricity to stop a human heart. Play it safe if in doubt.

[icicle]

I may be nuts for saying this but.

I think it might be still legal or not, A person could use the old fashion knife switch.

you could wire the top half for your A.C., and The Bottom half for your inverter.

then wire the center to your fuse box.

[americanreman]

I live semi rural on grid but the power goes out frequently, for the last five years I have a circuit that feeds my computers and refrigerator through 2 double position relays that are nc by the grid, when the power goes out the relays switch position and connects a 2000 watt inverter that runs 24 hours a day waiting.

Hot, neutral and ground get switched when the grid fails all at once...nothing goes down, when the grid comes back it switches all 3 again back to grid.

Works perfect, high quality relays, continuous motor duty...not a problem.

It has been inspected, since the relays are housed in fire rated boxes and look professional with cutout switches and they PLUG into the inverter by 3 prong heavy duty plugs the inspector passed right by because (I think) they are not permanent, they can be unplugged from the inverter...I even showed him how it worked by tripping the breaker and walking into the office as he could see all my equipment was running without a glitch.

This by no means should be an example of what you should do, it works for me.

[domwild]

Alibro,

A running fridge works against a capillary tube and when the power goes off, then comes on again quickly, the compressor starts against a back pressure and the thermal overload trips. This overload protects the start windings. Those with fast automated switching via 3P2T relays may experience no problems.

If fridges/freezers are running and you disconnect manually, then I suggest waiting a while until the pressure in the fridge/freezer equalises; that is the hissing noise in the evaporator, when liquid is still being injected.

Good luck!

Regards,

[alibro]

Thanks for the responses guys, I knew to switch live and neutral both together as some other folks have mentioned that before. I don't think the earth should be a problem as it is a large spike in the ground for both UPS/inverter and grid mains.

I like the idea of the relays and have been mulling that over in my head but was concerned it might cause a problem with fast switching. If I routed the mains to the relays via a single throw, double pole switch, I can easily switch between sources. I won't have a fridge on the circuit I will be switching so that won't be a problem and the power cannot get from my RE system to the grid in the event of a power failure so the line men will be safe.

If anyone else has any thoughts on the matter please let me know.

Cheers all

Alibro

[terry5732]

Ground is Earth ground. You are suggesting someone can get electrocuted by touching two grounds. You are implying an RE system must be grounded to another planet. Ground is ground. In the US, by NEC, service entrance must be grounded as close as possible to the entrance to Earth ground. Most utilty poles are also grounded to the same planet. When you walk outside of your home you are walking on one conductor of the system - the neutral/ground. The only electrocution hazard can be with a hot and ground or two dissimilar hots. You cannot accidentally make the ground hot unless it is not grounded. Disconnecting it might make it not grounded. Your safety logic sounds flawed.

[alibro]

What Terry is saying makes sense (kind of). If you are using relays to break the earth bond to your system you are actually making it more dangerous and if something went wrong there is a potential for all the electrical goods attached to go live! Unless the earth is connected somewhere else I would take a look at rewireing for safety sake.

Alibro

[zeusmorg]

 Neutral in some systems (probably not yours) is not bonded to ground.If you're giving advice to someone in europe or asia, or even arizona, are you sure their system is like yours? Go read the wiki on different electrical systems worldwide, and even in some older wiring in some US systems that haven't been updated to current NEC standards, ground and neutral are not bonded at the service box. I'm sure you've been in a house that still had two socket plugs, not 2/w ground. Also if you don't dissconnect the neutral and the power comes back up and you're using a inverter that does not synchronize it's waveform to grid power, POOF! Do not assume that just because something works on your "planet" that there's not a floating neutral on a system somewhere else.

"Ground is Earth ground. You are suggesting someone can get electrocuted by touching two grounds. You are implying an RE system must be grounded to another planet"

 No, I'm suggesting that you have not disconnected the neutral, which will feed back. Would you feed live the hot on a system then grab hold of the neutral wire barefooted?

"You cannot accidentally make the ground hot unless it is not grounded."

 What does that even mean?

"Disconnecting it might make it not grounded. Your safety logic sounds flawed."

I did not say you shouldn't properly connect your RE system to ground did I?

This discussion is about properly disconnecting your system from grid power, and switching over to a different power source. Know and understand how your system is set up and disconnect it accordingly. If you were to just "flip a breaker" you have not disconnected from the system, you do not know where it will feed back! Let's say you just disconnected one of your 120 feeds, and you have a 240 service, are you sure that that power won't feed back through one of your 240 appliances?

[SparWeb]

Not all grounds are created equally.  If the ground from one system is not well connected, or is too small, then there can be a difference in potential between it and the other system.  Enough resistance in a ground wire can cause a voltage drop.  The same can be true of the earth itself, especially depending on the climate.

This isn't easy for me to explain, because I only picked up a few pointers in the National Electrical Code, but there are a lot of things that must be done to properly ground an electrical system.

I have two ground rods driven into the ground near my wind turbine.  One grounds the tower structure, the other grounds the electrical system beside the batteries.  When the inverter is on, I can measure a potential between them.  Not enough to be dangerous, but enough to reveal that I don't know everything about grounding electrical systems!

[americanreman]

In the US if you disconnect the ground and not the neutral or vice versa you are still connected to neutral and ground, plain and simple. Ground and neutral are one in the same and connected together.

I drop hot, neutral and ground at the same time making hot neutral and ground connection through the inverter as intended by the inverters design for hooking up grounded loads via 3 prong plug on the inverter itself.

You must completely disconnect your circuit from the grid and that means all 3, hot, neutral and ground when switching over to inverter power from mains.

This is the only way to totally isolate grid from inverter, protecting you and your equipment. It is the same as unplugging a plug from the wall and re-plugging it to another outlet so to speak, all 3 connections break.

Rv owners, boat owners have been doing this for years with relays, when you unhook the power cord from your campsite or dock it automatically switches your loads without shutting anything down.

Or you can spend a ton on transfer switches, your choice. Do buy quality relays though, don't save a buck here..I buy tyco brand 35 amp at about 30.00 a piece.

[americanreman]

What you are missing here, among some other things about ground and neutral is the fact that the inverter is not earth grounded at all. I sits on plywood mounted to the basement wall with it's hot, neutral and ground feeding the circuit that is now completely disconnected from the grid by relays.

[dpshort218]

Do not switch the ground if you go with a switch or relay!  I am assuming you have a small load to run. A small manual transfer switch could be used, but would be more costly. A simple double pole, double throw 15 or 20 amp switch would probably be ok. I am not up on the NEC about the exact requirements or power limits of this. You absolutely must insure that each source breaks before the other source makes. If a relay is used, use a force guided type where both poles are sure to either open, or stay made if either contacts happen to weld shut. You don't want a hot from one system and neutral from the other running to your load in the case one relay contact welds closed. I would leave the 2 system's grounds tied together. (even with the 2 socket - one plug method) This is NOT the same as tying the neutrals together at this point. Each neutral should be bonded only at their respected sources per NEC. The earth is NOT one of the current carrying conductors under normal conditions as I believe someone stated. The earth is used to basically dump fault currents, shorts, lightning, etc.  Any modern US home wiring system today above 48 volts will almost certainly have one of its conductors bonded to ground at the service entrance point to form this "neutral". The TomW 2 live plugs is about as simple, safe and cheap as you can get. I wouldn't think the power supplies in a computer or tv would be hurt by fast switching between the two. It would be a very short momentary frequency hicup like a brownout.

You also mentioned possibly upgrading to a more suitable inverter when the one you have dies. Is a grid-tied system worth looking into?  

Just my 2 cents worth.

[americanreman]

If you do not switch ground you're system is still tied to the grid, an inverter is grounded internally and this is the ground you should be using when running on inverter power, not earth ground.

In fact depending what you are running it is not a bad idea to cut ground connections completely from the inverter and just use hot and neutral from the inverter (lamps, lights, etc), your inverter may fault out and stop if your inverters neutral and ground wires are feeding to house ground. Even when switching grounds you may have to start the loads on inverter with the breaker off and then switch the breaker on or the inverter will fault (this is my experience with vector models).

Rv's, boats, cars, etc using inverter power are not connected to earth ground, obviously, neither should your house inverter unless it specifically built for the purpose, the inverters ground prong is there for that purpose.

[americanreman]

One more thing as zeusmorg pointed out, if you don't isolate your inverter completely from the grid, ground you may smoke it when the grid comes back online.

The grid and inverter power are 2 separate sources of power and should not be connected together by any means when one or the other is in use.

Disclaimer: The reason I got into this discussion is because I have personally hooked this type of system up on several occasions and run one at my home now, this doesn't mean you should, be smart and be careful. Isolate your 2 power sources from each other!

[alibro]

Thanks for your suggestions, I can see I have opened a can of worms here but I am a little confused now. From what you are saying

"What you are missing here, among some other things about ground and neutral is the fact that the inverter is not earth grounded at all. I sits on plywood mounted to the basement wall with it's hot, neutral and ground feeding the circuit that is now completely disconnected from the grid by relays."

Does your inverter not have a connection for earthing it?

It has been over 20 years since I did my electrical installation exams but one thing which has always been absolutely critical here in the UK was earthing any electrical item with metal parts. If you disconnect the earth as you suggest what is to prevent the chassis of a faulty electrical appliance going live? I know if I short out the earth and neutral at a socket I will trip the earth leakage breaker which feeds my house, so although they might be connected somewhere between the power lines and my house, they are not at the appliance side. Does this mean the potential for appliances going live would exist?

Maybe I am being silly here, If we follow what you are saying and a fault developed in an appliance and the casing went live then the inverter would sense a voltage on the earth pin and trip off. Am I correct? Has anybody ever tested this? Is this as foolproof as having a physically connected earth.

Answers to these questions and more in next weeks episode of SOAP!

I guess you needed to be there ;-)

Maybe I should do a little more research into how inverters are meant to be connected before going any further.

Cheers all

Alibro

[DamonHD]

"Answers to these questions and more in next weeks episode of SOAP!"

Ah, that brings back memories.  I'm no fan of TV at all, and that series does look very jaded in retrospect, but at the time it was fab!  Good old Jessica!

Rgds

Damon

[americanreman]

Personally I have never grounded the inverter case to anything because lets face it if you take a deep cycle battery, throw it and an inverter in the back of your pickup truck or van along with a tv and do a football tailgate party (blender included) nobody is gonna get hurt, and your tv won't fry, happens every sunday at sports stadiums around the country and when hurricanes, tornados and other disasters strike.

Now from a technical standpoint "they" say the inverter case should be connected to the negative battery terminal in most cases.

[terry5732]

If the inverter power is connected to ANY inside home wiring which has a neutral and/or ground (in the US), then it will be connected to Earth ground through a myriad of places. Assuming the home was up to code at some point with-in the last quarter century. If you can't get past this idea just use extension cords to what you want to power and don't use the home circuits.

You cannot send low voltage (120-240) AC through ONE wire except as it would be relative to a ground which is actually the second conductor. It is a two-way street only. You walk on one live conductor everyday without consequence. It's two that is problematic.

[dpshort218]

  If you have current running through a ground, then the system is already dangerous. It is true that a neutral is bonded to earth ground and the grounding electrode system of water pipes, buried bare copper wire, etc. on your house. Your equipment grounding is tied to this same point. The chief benefit of bonding the neutral is in case of a short from the hot to the metal parts of equipment. If the equipment ground was not tied to the neutral at the service point, then the breaker would not trip since there would be no short circuit. You would now have an energized piece of equipment. Suppose your stove shorted to its frame and then the neutral of the disposal shorted to the sink. With no bonding, there is no circuit short so no breakers will trip to clear the fault. You do have a lethal difference in potential between the stove and sink. If you touch both, you complete the circuit!

 I thought that RV electric was supposed to be supplied by ground fault protected circuits (GFCI).

   Truly isolated grounds of two "separately derived systems" can also develop a difference in potential as well. Depending on soil conditions, quality of connections and bonding, and faults of either system, this can be large enough for a shock. By tying the two system's grounds together, you remove o significantly reduce this possible danger. There should not be any current normally running through the RE systems equipment grounding either. The frame of the inverter should be bonded to the grounding electrode system of the house. The neutral is probably bonded to the case internally. Any wind tower frame or solar frames should have equipment grounding run back and grounded at the same point as well. Ground rods and water pipes of the grounding electrode system should be tied together. Again, it is when a ground fault occurs that the danger appears greatest. I would bond the neutral of a generator or inverter as a "separately derived system". The frames of generators are usually bonded to the neutral. The NEC allows this bonding to act as the grounding electrode system if it is a used as a portable unit like on the back of a truck at a ballgame. If they are to share a circuit in a building, they should be bonded together. This is no different in the way industrial transfer switched systems switch between the grid and an emergency standby generator. The generator frame and one point of the generator wiring is bonded to ground and the two systems are bonded together to the same grounding electrode system. NEC article 250 covers grounding and bonding. Here is a link to a short OSHA explanation. It does cover a generator and not an inverter.  

http://www.osha.gov/OshDoc/data_Hurricane_Facts/grounding_port_generator.pdf"

[americanreman]

"If the inverter power is connected to ANY inside home wiring which has a neutral and/or ground (in the US), then it will be connected to Earth ground through a myriad of places"

The word is "IF".

Taking lighting circuits and plug circuits, with relays breaking the connection at the breaker panel and making at the inverter you will be totally isolated from ground unless you have some screwed up grounding of an appliance or something directly to cold water pipe or other earth ground connection.

New appliances like stoves, washers and dryers no longer connect an additional green wire to cold water pipe..they seem to have done away with that.

I don't ground my inverters to anything making the backup power totally isolated from the grid.

[terry5732]

EVERY grounded outlet and permanently wired fixture or switch should be connected to the ground strip in the main box. This has been NEC for many years now. In the main box, the ground strip is to be connected to ground. The neutral is REQUIRED to be connected to ground from here also. Besides being grounded before entering, the neutral is to be bonded to incoming water supply (another ground). There is no seperation of ground and neutral. To do so is a violation of NEC. Ground/neutral is to remain in place at all times and should not be disrupted with any switch or relay. It is not even permissable to have any kind of switch in the ground/neutral system. If you had such in place, and there were an electrocution or fire, your insurance would have no problem denying a claim against you.

[zeusmorg]

 What you say is true, IF you're connected to the grid. What you say is WRONG if you have a backup/separate power system. If you do leave the neutral and ground connected when you have a power failure, AND you have a standard inverter powering a house system (not a separate system) AND the grid power comes back up then your inverter WILL GO POOF! no if's and's or buts. It will, simple fact. Since you seem to want to ignore our information, go ahead and wire yours that way, but I doubt you'll get your insurance company to pay for it, since it was improperly wired. Look up the NEC codes for RE anti-islanding. THAT MEANS WHEN THE  GRID POWER IS DOWN if you're using a separate power source YOU HAVE TO HAVE THE GRID TOTALLY DISCONNECTED. Legal NEC approved devices to provide backup power do this, look into the wiring of them. If you don't and your power company detects it, you WILL be disconnected until a TOTAL inspection is done on your house, and in some areas, you could be fined. Period, I'm tired of arguing this.

[Lumberjack]

First the ground. Your place should have a ground rod or two outside with a bare copper wire running into the service panel. This is earth ground and should never be cut or switched or interuppted in any way. If you dont have this, add it. Do not assume you have it since it was not required for a long time and some older places skipped it or simply tied into the plumbing. Any change to your system requires you to have a proper ground and it is not a terribly expensive job.

Assuming you have a typical system the power company dropped two 120 volt legs and a neutral wire. You should be able to see two insulated wires and one bare twisted wire between the pole and house. The bare twisted neutral from the drop should be tied to the ground preferably inside your main breaker panel. The power company neutral may not be tied to ground at the pole and should be considered a live wire untill it is tied to your ground.

There are a few ways you can handle the change over from grid to RE power but the overall procedure is about the same. You must disconnect both of the hot wires and the neutral from the grid before you connect your RE system. Disconnection of the ground is not needed or desired. Disconnection of the neutral should occur after it is tied to ground. Your RE system should be tied to the house ground permanently as well. Check the documentation for the inverter to get the correct ground connections.

To accomplish the switch-over you can use a manual switch or relays but the relays must be designed to handle the max current of the system. If you have a 200 amp service then each pole of the relay must be able to handle a full 200 amps. Since double throw relays of this size are not easy to find two interlocked relays can be used but must be wired so only one relay can energise at a time.

One way around some of this if you only want a small number of circuits powered up during outages would be to wire up the RE system like a UPS. The circuit itself is removed from the main breaker box and permanently routed to the Inverter which is always driven by the batteries. The batteries are set up to recharge from RE sources first and from grid power second. You do lose a bit of efficiency when recharging from the grid but It does eliminate the need to disconnect from the grid and also eliminates any change over glitches.

Much of this require work inside the main breaker panel and unless you are sure of what your doing it is best done by a licensed electrician.

   

[americanreman]

I don't think we are talking about switching the whole panel, ie, 200 amps...just a circuit or two, and in doing so you should switch all 3, hot neutal and ground..break grid before make inverter.

As I stated before from experience, if you do not switch grounds most good quality inverters will fault and shut down and in situations fry.

This really isn't brain surgery, you have 2 different power sources and grounds should not be shared because even though they are separate, when the grid comes back they are both outputting power at the same time. In the US neutral and ground are connected together.

[americanreman]

Thanks zeusmorg, you are right, I'm tired too...all they have to do is try it for themselves and see what happens.

The sole purpose of a transfer switch is to completely disconnect one source (grid) and connect another (inverter).

[TomW]

americanreman;

I think thats the bottom line. [complete isolation].

All this nit picking on what "should" be done with grounding is best left to the professionals.

One user posts the same misinformation every time this comes up which only serves to confuse the matter. I hope nobody follows wrong advice and gets into trouble. This "misinformer" will have no responsibility due to the improper information.

I can't get into correcting them they cry to Dan I am unfair.

When in doubt, follow manufacturers instructions and / or hire a professional. Saving a couple hundred bucks on quality advice and destroying equipment, etc will have little positive economic

impact.

Cheers.

TomW

[americanreman]

To clarify, when my grid is up, my backup circuits are powered and grounded through the grid as required by code.

When my grid fails, my backup circuits are powered and grounded by the inverters ground coming from the 3 prong plug on the inverter itself.

All 3 neutral, hot and ground connections break before they make from one power source to the other.

This is simply the correct way to wire a backup system, safe for me and my equipment and safe for the service personell attempting to bring the grid back online after a failure.

Your inverter has 3 prong grounded outlets on the front of it, use the ground from the inverter ONLY!!! when running on inverter power and use the grids ground ONLY!!! when you are powered by the grid. DO NOT!!! leave both grounds connected together at the same time and you will be grounded properly at all times, which will keep the magic smoke inside your inverter.

Some people choose to do this by mounting a plug on the wall next to the breaker box connected directly to a breaker, they pull the the romex out of the box that was connected to the breaker and put a 3 prong plug on the end. Then they plug this circuit into the inverter when the power fails, and plug it back to the plug connected to the breaker when the grid returns.

This breaks one circuit completely then makes the other completely by swapping outlets manually, this is exactly what I do with relays so there is no manual human intervention required and the loads stay powered up at all times.

This is the correct way to wire an AC backup system whether you are using an inverter or a gas genny. BREAK ALL on one source before you MAKE ALL on the other source.

As far as grounding the inverter case (larger inverters above 1200 watts will have a ground screw), I leave that alone and don't ground it at all...it is widely viewed as it should be connected to the negative battery terminal on your bank but I have never had a problem not doing this. This ground terminal is not to be connected to earth ground connected to your grid.

Hope you understand....

[Lumberjack]

In the US ground is physically tied to earth ground as required by the NEC. If the inverter objects in any way to being tied to ground then you have a wireing fault. If the ground rods are not making proper contact with the earth then they need to be fixed. You could establish a separate set of ground rods and so on and switch BUT I cant think of a single good reason to do so. The point of the US grounding system is to prevent voltage from existing between metal equipment chassis. Having everything tied to a single ground and earth is the only safe way to do it.

I wonder if your problems with inverters havent been because of floating grounds? The only way any voltage potential can exist is because of poor grounding. I realise in some older installations that ground was done by tieing to the water line and ground rods do fail eventually. Poor earth grounding is one of the major reasons why two or more ground rods are now rquired in many areas.

Neutral in the US is floating inspite of being tied to ground. It should be switched in all cases.

[Lumberjack]

Just to be clear...

Ground should always be 0 in the US. Any voltage present on ground indicates a problem that needs to be corrected.

Also you mentioned the panel. In order to switch only a few circuits you almost have to have a separate panel box for those cicuits. The circuits must have a breaker or fuse after the switch to protect the wiring and prevent fires.

To this end the correct arrangement is for the utitly power to run into the main panel. A circuit/breaker from the main panel should run to the switchgear. A line from the inverter should also run to the switch gear. The switch gear should output to a subpanel with breakers to the individual lines.

Everything including the switch gear must be in a proper enclosure. A single box can work but I have never seen a box that has the needed space outside of a custom industrial enviroment. It is simpler,easier and cheaper to use three enclosures. Finally even the switch gear must be in a proper enclosure. Someone mentioned the old sytle frankenstein type knife switches. These are illegal as the expose the user to live voltage on the blades and so on. It may look cute but sooner or later someone will get hurt on it.