making your own ATS (Automatic Transfer Switch)

[relihanl]

I am trying to figure out how to make my own 240V Automatic Transfer Switch using off the shelf parts. In the states you can buy these things pretty cheaply but here in Ireland (and the UK) they are expensive and difficult to get hold of.

I have been poring over the electrical supply catalogs and I reckon that two SPDT contactors are required. I reckon that one should be normally open (N/O) while the other should be normally closed (N/C) and the coil of one should be energized by the other.

The problem is that the catalogs are godawful confusing and I don't have a clue what to order. Can anyone tell me whether my basic strategy of using contactors is correct. If it is, can anyone point me to the exact models of contactors (Siemens, Telemecanique, ABB, etc)

If I can get this thing assembled I promise to take some pictures, write up an article and post it here.

[dinges]

That's not so hard to build? Don't know what a contactor is, I was thinking about using a relay.

If you use a BIG relay, you should be fine, I think. I have some 3phase 380VAC magnetic switches (as big as a fist). They contain NC and NO contacts. Power the coil by mains AC (the grid), so the center contact is connected to the NO contact. When grid goes off, the relay de-energizes, falling back to the NC contact (where your inverter etc. are connected to).

(BTW, the coils of my switches are 220V; if they are only 12V, 24V or 48V in your case, use a transformer; primary connected to the grid, secondary to the coil of your relay)

Sounds straightforward enough, unless I'm overlooking something here?

Peter.

[relihanl]

Contactors are an alternate name for relays.

what you are saying makes sense but I am having difficulty visualising it. Do you have the model number of your relay so I can look it up. By the sounds of it your relay has the following contacts:

2. for the coil
   
2. in for the NC
   
2. out for the NC
   
2. in for the NO
   
2. out for the NO
   
   

giving a total of 10 contacts. Is this correct

[Ungrounded Lightning Rod]

Don't know what a contactor is, I was thinking about using a relay.

A "contactor" is a BIG relay, suitable for switching large loads with significant inrush on turnon or inductive kick on turnoff.  They get a special name for historical reasons.

They may have extras, such as auxiliary contacts for position monitoring, feedback in relay logic circuits, or multiple coils - one for pull-in, one for hold, or one for pullin and another for dropout, with the relay latching mechanically.  They may have special contacts with some arrangement for extinugishing an arc when they open.  And so on.

Though they may be a somewhat fancy machine, they're still basically a big relay for seitching a massive load.

[dinges]

Understood. Didn't know they had a different name in English; in Dutch those heavy relays I was thinking about are called 'magneetschakelaars' instead of 'relais'. The ones I have come from an old crane, heavy duty 3phase stuff, 380V, etc.etc.

Made a quick schematic of what I had in mind. However, beware. The fact that the original poster asked this question indicates to me that he lacks the knowledge/experience to safely deal with these matters. The schematic is just one thing. Selecting the right relay/contactor is a whole different matter. Wiring it up another one. So, continue at your own risk.

Here's the quick schematic of what I had in mind. But continue at your own risk.\

In the resting position (no grid power present) the relay is in the NC positions (only 2 of my 3 switches are used. I've drawn my 3phase contactors). So, users are connected to inverter. When grid voltage becomes present, the coil (the rectangle at the right) energizes and switches to NO; users are connected to grid.

Like I said, it's pretty straightforward, but why/how you ask the question indicates to me you may have to think twice before continueing.

Good luck,

Peter.

[dinges]

Perhaps other posters can indicate whether ground should be switched over too. Personally I'd think I would wire all grounds (grid & inverter & user) together, but there may be other issues that I'm not aware of. I'm not an electrical engineer!

Before building such a thing you should check your local electric codes whether and how such an ATS should be used.

Peter.

[kurt]

dinges. most inverters use what is called a hot neutral which means that if you hook the neutral up to a neutral that is grounded like a house wiring system the magic smoke comes out of the inverter. see the post over in diaries were the guy fried his inverter by hooking it to his house wiring. if you want the inverter to live you had better switch the neutral. that is also why most transfer switches sold in the U.S. will not work with inverters (unless the inverter is specifically designed to work with a grounded neutral system) because they do not switch the neutral.

[dinges]

Yes, good point!

I was assuming that the battery was electrically completely floating. I.e., battery ground (- pole) NOT connected to AC ground/neutral/earth. But, whether AC ground of the inverter is connected to AC earth of the grid should NOT be a problem. I think.

As far as connecting battery minus-pole to AC ground (output of the inverter), I understand it's only the cheap/simple inverters that can't deal with input-ground and output-ground connected.

It's these little details that can really ruin your day...

See e.g. this current thread:

http://www.fieldlines.com/story/2006/7/4/193146/8689

[nanotech]

Actually, I think he was talking about the neutral wire being electrically the same thing as ground.

In US household wiring that is up to code, the white wire (neutral) is connected to the grounding bus bar in your breaker panel.  And since the circuit breakers on household wiring only go on the hot lead (black wire), your inverter will see an unprotected dead short across two of its wires (neutral and ground) thereby causing the magic smoke to appear.

[windstuffnow]

  You can use 2 breaker boxes side by side mounting one upside down and bolt the two handles together.   One is off and the other is on.   Wire one with the genny the other to the grid.   When you pull the handle One goes off and the other goes on.   The basic breaker boxes are pretty cheap.

.

[dinges]

You serious? In USA, neutral is connected to earth in the breaker box?

This was (is?) very illegal in NL. For examply, in my house there's a real earth connection. A copper pipe of a few meters length that's in the ground (I mean, soil). That is the earth point.

The neutral is provided by the electrical company, just like the live wire (or, in my case, 3 phases).

I know it was very forbidden to have ground and earth connected together anywhere in the system. My dad is an electrician and he told me often enough about it. The reasons behind it are not very clear to me though.

Absolutely amazing that what is very forbidden in one country is obligation in another one.

[TomW]

Dinges;



You serious? In USA, neutral is connected to earth in the breaker box?

Indeed it is and required by NEC our electrical code.

Neutral and earth ground tied together in breaker panel only tho.

Cheers.

TomW

[nanotech]

In your case the ground and earth WOULD cause problems tied together as you're on a 3 phase system.  We are essentially on a two phase.  Actually we are on twin 110V circuits, each 180 degrees out of phase with the other to make 220V, with the common wire being the earth/ground.  So we actually have a "positive" hot and a "negative" hot and a ground.  

In the breaker box the two phases are split, one down each side for the 110V and if you need 220V (range, clothes dryer, etc) you tap off both rails.

It's actually quite the cool system, but I never understood the need for the ground lead when the common was already grounded :confused:

[Ungrounded Lightning Rod]

Hot (black) and neutral (white) should be switched when you're using a "hot neutral" inverter.

(If you're using a generator you need to tie the neutral to the bond.  You can do this by just tying the neutrals of the load, line, and generator together rather than switching them.  If you plug your hot-neutral alternator and floating-neutral generator into the same plug depending on situation, you can use a disconnect (a high-current switch that LOOKS like a circuit breaker with no current rating label) to tie the genny-plug neutral to the line neutral.  Label the "on" position "generator" and the "off" position "inverter".)

Ground (green) should NEVER be switched.

You can use the third contact to transfer the other side (red) of the 240v service if you have a 120/240 genny or 120/240 inverter.  (If you only have a 120 backup supply, ignore it.  Hook the protected loads to the transferred black side and leave the non-critical and 440 stuff hooked to the red side and the non-transferred black side.  Include some lights on an always-line circuit to let you know when the grid is back.)

[Ungrounded Lightning Rod]

I never understood the need for the ground lead when the common was already grounded

The US code requires the neutral, the protective ground house wiring, the grounding rod or network, the water main, and the gas main, to be "bonded" at exactly one point.  (Typically this is the box containing the main breaker.)  In particular, they MUST be separate in subpanels.

Apparently some European standards go farther and have the one bonding point at the transformer.

The point of having only one bonding point is multifold:

 - It keeps the voltage drop from return currents in the neutral wire - or faults in the neutral wire - from producing dangerous voltages in the ground network.  (The ground network is connected to things like swiming pool water and metal-cased electrical equipment.  A nonzero voltage there might electrocute someone leaving a hottub, holding a tool while touching a pipe or stepping in a puddle, etc.

 - It prevents diversion of neutral return currents from the circuit's neutral wiring to another circuit's ground wiring - which might not be adequate and thus might start a fire.

 - It helps keep ground currents from nearby lightning strikes out of the house neutral and ground wiring, and tends to confine lightning current to a circuit's own ground path in a direct hit.

The "one bonding point" standard in the US is apparently a compromise allowing lightning ground current to flow in the heavy neutral drop wire in return for providing SOME protection for the house in a situation where the neutral wire to the pole opens or the earth ground is lost at the pole.  (The latter situation, with no grouning bond at the house and a Y configuration for the pole-pig, could end up with the house wiring floating at 8KV - as happened to my brother's house a few years ago.  B-( )

[Opera House]

There are three phase motor reversing contactors that would do this.  These are basically two motor contactors that are mechanically linked so that only one can close at a time.

[Nando]

LET ME CLARIFY something:

The power transformer that feeds a certain number of houses ( it is a 3 phase transformer).

Each phase may feed a row of houses in a street, in this case it is 230 volts with center tap ( Neutral) grounded at the transformer.

The wiring is sent to the house, 230 Volts, 3 wires to the BOX where the Neutral is grounded to the BOX and the box GROUNDED to the ground next to the house.

The rest of the house wiring has the "GREEN" wire from each outlet power box to the entrance BOX in addition of the neutral (White) -- good use for the GFI protection circuits.

GFI = Ground Fault Interrupt

So USA has the way to have 115 Volts and 230 Volts with a common incoming set up

Nando

[Drives]

"It prevents diversion of neutral return currents from the circuit's neutral wiring to another circuit's ground wiring - which might not be adequate and thus might start a fire."

Additionally, if a neutral and ground were tied together at an outlet for example, and the neutral was disconnected before the bond, the circuit would continue to work properly by using the normally denergized ground wire as the return path. (Provided the ground wire was sized adequate.)  Yet, when the poor electrician decided to disconnect the ground wire to that circuit one day in the future, he would be killed by the return current flowing through him vs. the neutral wire.  

Electricians are taught that the ground wire is designed to carry current only in abnormal conditions, and should never be "live".  However, I have pulled ground wires off grounding bars, and drew arcs before....the best lesson electricians are taught is "Never assume anything".

[WXYZCIENCE]

Has anyone address the phase relation of the grid power and the inverter. My computer backup inverter uses a phase locking circuit to zero cross the grid when it switches to backup and again when it goes back on grid. Joe

[Ungrounded Lightning Rod]

Reminds me of an electrician's story about trying to debug some problem and hanging his clamp-on ammeter on a handy water pipe to get it out of the way while doing something else - only to discover it reading somethig like 50 amps.  Building had a multiple bond to the water pipe, putting return current in the pipe.

[Ungrounded Lightning Rod]

Was intending to mention that but got sidetracked.

Using just a cutover will produce a nasty phase hit when switching.  Machinery that's operating will be in and/or cause trouble.

Motors will pull an ENORMOUS current when the switchover occurs.  Will probably pull down your inverter and may blow the breaker ahead of the motor when going back to line power.

Older electronics with transformer-based power supplies may see a big spike from the phase hit.  (Stuff from the early era of computing often used a regulating transformer, which could produce a major overvoltage from such a phase hit.)  Modern stuff with switching power supplies normally wouldn't be bothered - though the capacitors might not be able to handle the dead time before the relay falls out in a power outage.

Phase matching during transfer requires electronics or a human watching a phase indicator and throwing a switch at the right moment.  (I'm not aware of any electromechanical devices designed to do it though it wouldn't surprise me if there were but they were uncommon.)

This is why phase-up and changeover is usually a function of the inverter - where it is just a modest add-on to the other circuitry.

[relihanl]

Peter, I understand perfectly what you are talking about :-) My lack of official qualifications has not kept me from working inside consumer units and other dangerous metal boxes.

I have only ever seen contactors that are capable of making and breaking contacts. However, the relay in your diagram essentially acts like three interconnected selector switches (essentially a coil activated triple pole double throw switch).

Ideally I want to mount this solution on a din rail (along with an MCB) and I have only ever seen din rail contactors that can do make/break such as this (i.e. Single Throw):

What I was thinking was to use one of those contactors that have an auxillary contact that is closed when the other three contacts are open (and vice versa). I would use the auxillary contact to energize the coil of a second contactor. This would have the effect that there would be a guaranteed full disconnect of the primary power supply (across all poles) before the secondary power supply cuts in.

[WXYZCIENCE]

Lightning Rod, when the UPS kicks on(power fail)it happens very fast. But when the grid comes back I can hear the frequency come up to the line frequency it takes a lot longer. A simple straight transfer switch needs a turn on delay connected to the power on switch of the inverter so it will use the inverters soft start. I think we have to clarify uninterrupted switch over. Most cheap inverters are not intended for this UPS. Your thoughts? Joe.

[stephent]

Making a homebuilt ATS is usually frowned on by most grid suppliers in the world these days. Most countries have an electrical  code or at least the electricity suppliers do. And most if not all require a "mechanically interlocked" ATS type disconnecting device. i.e if one is engaged, the other will be mechanically (physically) locked out. Although it can be electrically locked out (relays or switches) in addition as well, the mechanical interlock is still required.

And at least the US has the little "UL" approved device label requirement on the whole assembly as well. (Canada--CSA??)

But the simplest ATS I ever worked on was just 2 breakers side by side with one upside down and a common "tie" handle between them--if one is turned on--the other was off, mechanical and very simple, but very manually operated.