Bars on top are arc catch plates, you will have a big arc on breaking under current, and closing on current. Contacts are large to handle the inrush current, which can be 2kA, and the flexible copper braid has to be rated to handle this inrush current. Common parts in all AC contactors and all AC breakers, to quench the arc and connect the moving contact to the terminal. Ideally you want a separate delay relay in the feed to the incoming supply, so that power has to be applied for 30 seconds before it is reconnected, so that all the transients on the line can die down, and also the massive inrush load can subside from all the other loads being turned on cold.

When you get into larger transfer switches they of course get a lot more sophisticated. Automatic delays to ensure the grid is actually stable before switching over, voltage/frequency checks, and feedback to modulate the generator speed to match phases and frequency before switching.

Let's take a moment to appreciate how strong those solenoids are. Incredible!

Hi Dave, Imagine this scenario: Your freezers are powered by battery in the night and the battery is empty. Then the ATS switches to mains. However, since the battery is not under load anymore, the voltage increases back again. Then the ATS switches back to battery. Then because of not having enough power, ATS switches back to mains...then loop.... I am sure that your BMS is smart enough to cut entire battery output off (high Z) after the battery depleted and not enable it again until it is charged. But anyway keep this problem in mind ;) Cheers.

My guess is that the switching time varies depending on where in the sine wave the switch happens. If it's near a zero crossing it will take longer because the solenoids may not pull the rod until closer to peak power. So you would get some ~10ms (half cycle) variation in the timing.

congratz on 1500 videos(+ or - a few not in the main series) dave! i've found a few pieces of industrial kit like this before, some beefy switches and motor contactors from some pumps, amazingly simple and yet surprisingly complex for what they do

Cute little baby transfer switch.😁 The data center at the bank I used to work for was more the size of a refrigerator, and had a huge lever, not a tiny knob. Those microswitches are effectively the negative feedback side of the circuit. Typical sort of a limit switch setup: apply power until you're in the right position and then stop.

LOL as I sit and watch this video in South Africa 🇿🇦, we currently have whats called "load shedding" stage 4. Power off for 2 hours at a time, 3 times a day minimum. In stark contrast to experiencing only a couple of hours in a lifetime in Sydney.🙈 Great video as always! Thanks Dave!

This was a really great tear down and explanation. I really like the simplicity of the design for this device. Thank you for making this video.

I have a similar setup at home to use battery power when electricity is expensive. I have found that switching while a fridge is running and the voltages are too far out of phase between inverter and grid the fridge kompressors will stop abruptly. I am aware double positive (or negative) ac pulse essentially is DC and acts as a brake on induction motors when using vfd's, I suspect my fridge compressor have a very low rotating mass as well and out of phase power upsets it. However measuring voltages with a multimeter and transfering between sorces when both are near 0 volts betwheen line 1's keeps the fridge happy and it continoues to run. Inverter fridges dont care, but I recommend testing this in your setup.

Heads up with refrigeration ! Watch out for locked rotors on switchover . A off delay of ten minutes is advised .

Great content. I have learned so much, Bob is now my uncle.

18:30 Those metal fins are there to help quench any arc that's formed as the contacts separate, I'd imagine -- same as for the fins you find in protective circuit breakers.

The quality of the first unit you tore down is much better than that of the second with the green selector switch. The one with the green switch is a "stacked" design used to add more switch sets. I have some of these I used for my generator transfer switch with 4 contacts (I only needed 3) and the outer most switch fails spectacularly when there is a surge. I already lost 2 during power outages. Always the outer most switch assembly. It seems since these are stacked and each one "plugs" into the adjacent switch plastic lever pivot, the strength holding the contacts closed gets weaker the more poles are added due to poor tolerances! The original one you tore down is not stacked and should not suffer from this defect.

Happy 1500!

Brilliant tear down and explanation. Thanks

I wonder if the plastic in those transfer switches meets the necessary flammability requirements? I have a feeling it doesn't. But I at least like how the slightly crooked "QC PASS 08" stamp in the manual is actually just an image in the pdf :) The metal sheets at 18:38 look like arc splitters to cool and extinguish any arcs from breaking loads (particularly inductive ones), not sure if I'd trust them at 400 VAC 63 A but probably good enough at 240 V 10 A. However one thing I don't understand is that both units have speed holes on the front plastic cover presumably either to ensure the impending explosion is vented forwards towards the user to cause maximum damage or maybe they had issues with the low quality contacts overheating and their solution was just to add ventilation slots, but seems like it's just begging for contamination issues. I also assume this change over switch and battery inverter will not be considered a 'fixed installation'?

Good informative video. I bought similar from same manufacturer and now I learned how to wire it up thanks. $25 here in Thailand off Lazada. Same project, running guest house off solar and a Bluetti AC200MAX with mains grid as backup.

Simplicity is the ultimate sophistication! Even with a 5,000-cycle rating, that thing will probably last forever.

I like this switch, it's so simple, not over-engineered and should be reliable. Automatic transfer switches make sense 1) when you have two separate utility connections (primary and backup), or 2) you've got a generator that can be remotely started. I wonder if the relay uses its auxiliary contacts for controlling the generator startup; it would make sense since as the mains dies and the relay turns off, the aux contact gets toggled, starts the generator, and when the generator is up and running and supplying power, the solenoid in the ATS gets energized and the changeover happens. Fridges won't care, but for any servers, computers etc. get yourself a UPS that can keep them powered through the changeover process. You got a lot of distortion in your mains. All that SMPS rubbish - forget about clear radio transmissions... It's sad.

I've seen these used within the water industry along with other sine wave normalizers, delays, soft starts - soft stops, VFD switches and current shunts for very high voltage electric motors.

I'm actually doing the same thing but my continual load is a rack of servers so the changeover needs to be smooth and phase synced. Instead of a timer though and a commercial battery/inverter, I picked up an old 3000VA UPS and modified it so it can't charge the batteries attached to it. I gutted the batteries from it and connected a 14.4KWh LiFePO4 array to it's "extended runtime" input and built a little circuit with a STM32F103C8T6 at it's heart that measures current consumption, battery charge state, among other things. When the charge state is high and solar input is high it disconnects the UPS from grid power making use of it's internal switchover function. The batteries are just connected as per normal to a MPPT via a BMS to charge from solar. Because I am using a high grade UPS that is designed for 100% duty, it has the added benefit of locking it's AC output phase with the grid phase, and when it takes over the phase is in sync with the grid, and when it goes back to mains power again it takes about 3 seconds while it re-syncs it's output to match the grid phase before switching back over.

Transfer switches are an extremely important part of backup generators

I have Tested with Timer, and it works fantastic. The Timer just have to be connected to the green poles AR and AN or BR and N. And it work with the Time you want. :)

“It is rated for 5000 cycles” *click *click *click *click* Maybe 4000 by the end of the video. Thanks for another informative and useful video!

These ATS are not for rotating devices as one cannot ensure that both power supplies are (were) in-phase. The worse case scenario is opposed phase which would surely stall an rotating device.

These are very simple, very crude, but also very effective for small single phase loads. As pointed out by many others, the strengthening bars are intended to break/split/dissipate the arc when switching loads. For small single phase it obviously works, I doubt they would come anywhere near the rating claimed (50kA) or the full load for the no. of operations claimed. As also pointed out by others, professional units will have multiple time delays built in to prevent hunting in the event of voltage fluctuations, and also run-on timers for generators. But they also cost accordingly :-) Great video, thanks.

Thats a pretty nifty setup, my grid here in the rural US sucks. During the winter we routinely loose power for week's/month's. Ive been wanting to build a system to automatically switch my homelab server rack (with UPS) to a generator.

Just a thought Dave... Do you really want to switch your fridges (essentially single phase motors under load) from one source to another almost instantaneously??? Will the solar inverter power source be synchronised to mains frequency? If so, no wukkas But I'd think you might get into difficult territory if the two sources are not in synch When I was responsible for a data centre, we were always in synch (courtesy of a UPS on the output side of the transfer switch) for most things, but we still enforced a minute or two gap between power sources (so we didn't damage the HVAC)

I have one of these, they work great. Dave, whats involved with replacing the contactor side of it with solid state switching? Is it worth it?

Very good useful video I just ordered one of these (Jotta Brand same thing off Ebay) Thank you! cant beat the price of these $35.26. Of course I am sure they are not UL rated, but reviews are all good.

brilliantly simple ..

Hi Dave, very interesting. I wanted to use this but was concerned about safety, so built a servo to a manual transfer switch (video on channel!). But this looks great - and now I can see inside I'm happy to spend the $50. Cheers!

Thanks Dave! Just about to put one in my caravan to switch between mains (aka “shore”) power, and my inverter.. allows me to use my microwave and coffee machine (and any other 240vac) when travelling

How’s there people commented 8 hours ago while the video just been uploaded 20 seconds ago? 😅 Btw, this subject is literally important for any beginner electrical engineer, thanks in advance ❤️

Thanks for sharing. 😉👌🏻

Does that circuit have sufficient isolation between the grid and backup sides? The distance between the relay's pads is awfully small. With the right fault condition it seems like there would be a possibility of back-feeding power into the grid through the control circuitry (creating a hazard for linemen etc.).

Yep I have a couple, different style and brand, but the same! thanks for the reversEE :). I do wish they x'fered faster though. But considerring the likely mismatch in phase at the time, then maybe you do need some 5ms+ delay. Any thoughts on that?

I found that very interesting, to see how it works. Hope to see the whole system working soon. Will you have an under voltage output alarm, from your battery inverter?

I'm guessing the solenoids are so powerful that it can't run them continuously, so the microswitch turns off the solenoid as soon as it has done its job.

18:34, those "bars" are not for strenghtening, they are spark arrestors.

Used to work on megawatt backup gen's - when the xfer switch on those kick over, you better be paying attention and plug your ears! Some sounded like a car crash!

Big Clive took a similar unit to bits a while back, but it used a motor, much smother but the transfer time is longer

The “strengthening bars” are probably arc extinguishers

is there a posibility that one of those switches gets stuck and you have 2 supply connected at the same time to the load... or is there something that prevents that...

Nice Video!!! Some risk could be when your battery inverter (connected to your A input) goes into a "brownout. ---> (i) Battery "empty" - turns off the power - (ii) Relay swaps to B (grid in your case) - Battery has no load at that time (iii) battery and inveter recovers - since there is no load - (iv) relay swaps back to source A ---> we are back at the start.

Did I miss them or does that unit lack the required compliance markings for use in Australia?

Was very impressed with the portrait DaveCAD. Indeed rare and it looks like you have plenty of portrait cycles left based on the thickness of the DaveCAD Pad. DCP

The first box internally it looks real cheap-arsed. The way the solenoids are positioned seem a tad wrong. The later switch box looks a bit better... 10/10 for the vid

Has anyone looked into what a typical blackout looks like over ~1 second? I'm wondering if there's normally behaviour that would make a 240v relay across it "chatter". Do UPS's normally have a minimum time to run off battery before re-connecting to the mains once it's back up?

Can i use this to my PC and not restart when main power is off, it is about switching speed from main power to backup power?

Our CBD office has independent connections to two separate grids because it's in a bank building, which is great because it basically meant we didn't have to have a UPS for our data centre. It's practically impossible for both grids to be down at the same time, because that would mean the head offices for most of the major banks and financial institutions in the country would go down. Well, one day there was a major power outage in the CBD, it took out one of our grid feeds, but when our automatic transfer switch switched over, it decided half way through that it was too much work and exploded, taking out the second power feed and our entire office and data centre. Everyone was basically left standing around twiddling their thumbs for a couple of hours until they could get an electrician to bypass the transfer switch and reconnect the one good power feed. Unfortunately the disruption probably cost the business about half a million Aussie dollary doos. But that's not the end of the story. Do you think this convinced them to install a UPS? Nope! Instead they decided that in the event of losing one grid feed, they'd have an enormous generator on the back of a truck, which someone would drive into the CBD and park on the street outside the loading dock. Then they'd roll out a huge cable up the ramp, through the loading dock, and into a panel to feed another transfer switch that they could use to power up the missing feed. Yep, a flawless plan. Thankfully we haven't yet needed to implement it, since the grids have been stable since then. Unfortunately the UPS in our head office wasn't as reliable ... but that's a story for another time.

Can't you switch off the power to the batteries, based on the current provided by the solar panels?

Hi Dave, Great video but i was wondering how this arrangement is grounded? Thanks

One of the first bigclive videos i watched was about a transfer switch like this.

I'd be curious if you could just run the fridges when the sun is shining and turn them off at night. It would depend on now well insulated they are and thermal mass. Not sure this would work for a fridge, but I think it would with a deep freezer.

The vents on the stationary contacts are for arc flash suppression.

Bridge rectifiers and associated circuitry are possibly only protected by the 100A main fuse. The control circuits need their own low amp protection device.

Doest it turn off the light during switching?

Your going to need to think about RCD protection when running off the battery.

Nice video. Didn't quite follow the trigger setup. Curious if a rigol 1054Z is capable,

Would love to see Dave explaine the PEN and MEN connections, when using a transfer switch. There's a good article from WorkSafe QLD on the dangers of switching Neutral

Hi Dave ! futher to the SeanBZA comment below, there is the problem also fixed by the timer, the compressors having a power cut and reapplication with the compressor still under head pressure usually that time is around five or ten minutes otherwise the compressor just keeps cycling till that pressure reduces hard on the overload and the motor, Cheers!!! Rick Armstrong from Canada

Hopefully you can tell your battery inverter to not re-energize until it reaches a certain SoC, otherwise this might chatter as the inverter energizes, then quickly browns out. Nicer ones have time delays to prevent that (wait 30+ seconds to make sure grid is actually back on), and also zero crossing switching. Funny timing on this video. My campus is doing ATS testing today and switching to “island mode” (local power generation) off the grid and making sure everything works.

Just make sure you're installing to AS3000 and AS3010. You might also want to use a PE cell instead of your manual switch.

I wonder if you could use a UPS to do this exact same setup, depending if the UPS had certain features that you could utilise? I'm thinking something like a self test feature that you can set on a time schedule, and that will allow the battery to drain down almost completely, then switch back to AC power afterwards. So you could make it at a certain time every day, it disconnects AC power and uses it's batteries until their depleted, or until a certain timeout is reached perhaps.

Could you replace the timer with a external adjustable photocell?

SCRs and IGBTs are awesome

You might consider using one of the easun/powland cheap Chinese inverters with automatic bypass and built in charger when you have direct 24/48v battery connection. 5kW are around $400 and unlike typical ATS when battery is low it switches like ups without circuit disruption to grid power. Your ats is here really very fast! You may also configure safe buffer, to reserve a bit capacity should grid fail. those inverters are really good and configurable as for that price.

So how do you monitor and direct the solar power into your EV or Battery bank? My Solar has an inverter that shuts down if the mains is shutdown, but If I run a backup battery off the grid, it cannot detect if the solar is providing the power or the grid. Do you have a method for detecting when the solar is providing the power so that all the power generated is used by the battery charging. It would help to charge local batteries during the day when I am at work and then allow me to high voltage charge an EV during the night.

Would it be possible to have the unit control an EV charger to change over to the cheep Tarif.

Here in europe where power is expensive i do measure actual power into/from the grid and switch extra consumers according to solar surplus. Measuring is done with 3-phase shelly-3em over WiFi, switching load is done with sonoff-basic over WiFi.

Seems like a design flaw to me. Shouldn't the tamper switches be arranged so that the solenoid circuit is broken at the end of the throw, not the beginning? Breaking the circuit at the beginning of the throw could result in oscillations if the initial force of the solenoid is not sufficient - perhaps due to increased friction after years of use and accumulation of dirt.

Daiyve, Daiyve, Daiyve, this lash up may be frowned upon by Aussie government. Your refrigerators must never disconnect from the grid, even for 20 milliseconds. It's all on page 278 of the International Internet of Things Treaty, IIoTT.

Nice video Dave. Those U shaped metal lets call them bars are actually for ark reduction or reduction of the arc that will start ot end in that place..so they are there only for that.. btw. I created similar circuit.. you need two main relays and then one smaller relay... and also two extra contacts..exactly how it is in that package.. I will add one small thing..they should add small capacitors parallel with coils to reduce voltage spike and it would good design

Your optimal cost timer should be set to a random time offset from exact hours, to avoid grid impact from other people running similar clocks. This smoothes out the load discontinuity for the grid backbone system, giving them time to startup power sources.

Why are coils DC ?

Barring aside the manual switching, would it be feasible to use a DPDT contactor with the coil wired to your priority source to do a job like this?

In my experience anyway, those contacts on the Source B/Generator are used to trigger the generator's start sequence.

Clive, I think you know that all this can be done much easier with some available home automation thingies. They measure the consumption and control without steps the output of your batteries as well as the charging. But well, your oldschool solution might not be too bad.

Is there some hysteresis to the micro switches that control the main switch solenoids, or are they relying on momentum to complete the switch change?

For your 3 fridges, I have seen someone make a 3 relay box, to prevent all 3 loads starting at the same time. It's a very low chance of this happening, but with thermostats opening & closing, it can happen.

it doesn't seem to be very concerned about run with no working earth connect at any of the contacts?

Do your refrigerator/freezers have protection against short cycling? I don't know if the 20-50ms glitch is enough to get the compressor motors to lock up.

dont know if I got lost in the sauce, but I assume the battery its only being charged when solar is working? from the diagram it seems that battery is charged from mains (which it may be) but I assume there is some configuration to make work only on sunlight time, right?

Have you considered all fridges starting at the same time?

Be worth using an ON DELAY circuit on the output, domestic refrigeration equipment usually needs the pressure generated by the compressor to have equalised off cycle for the compressor to start equalised and therefore unloaded, short breaks in supply don’t allow this to happen and the compressor will generally trip on over current and it’s not a good thing to do frequently. What you described as strengthening bars on the first unit are most likely arc chutes to break the arc current during switching

What happens if the control wires aren't connected. Can it still be used manually as a mechanical switch? It's too late to get my head around the circuit.

Forgot the most important rule of the EEVBlog! Don't turn it on take it APAAAT!

I know you are running freezers through this but I would think the delay of it switching over wouldn't even cause your digital clocks to go to "12:00" and blink. (If you were using it as a backup for grid failure). Hey! Try that, Dave! "Plug" a digital clock into it and see if the toggle action is fast enough to keep it going. I would try that before seeing if it crashes a desktop computer (or not)! 😱

18:27 - these are for absorbing and quenching the arc produced when making and breaking the current.

Heat pumps generally can't immediately restart. If there's a power failure, it should stay off for a minute or two before applying power again. They have a soft-start accumulator tank which fills when first turned on. Applying power while that is full may stall the compressor. I suspect inverter-driven units take care of this electronically, but I'm not certain.

Is the degeneration of the 3000$+ battery and the inverter running every night, or 24/7 actually, (caps?) actually worth the lower energy intake?

Looks like Dave is going to build overcomplicated nuclear power station from it :D What I didn't understand are these points: 1. How can help these mechanical contacts help with glitches caused by another mechanical contacts of built-in relays ofthe battery? 2. Why the fridge must be powered permanently? We have two days with eight hours power outages here per year due to maintenance and it is not absolutely problem - the freezer goes from -18 degC to -12 degC at this time. I don't understand why can 5 to 10 minutes cause a probelm, it would be below scale of the thermosat hysteresis. If the internal temperature rises so fast it is not problem for electrician but for the fire department. 3. The fridge is a compressor. The compressor is a motor with hard start. It means 10 to 15 times normal current. It is stupid idea to connect three of them in paralell and conect at the same moment, even if it runs form a convertor. This will activate the overcurrent protection. And my recommendations: 1. If there is no extra reason, NEVER switch the neutral line in mains circuits. Really, NEVER. And if you need it be sure the neutral is connected the first and disconnected the last. I know you are not familiar wit it but imagine a board powered by +24V and +5V with cutted off the ground. This is similar. 2. This switch is nice to be a bistable relay on the output. The sensing inputs A and B are good to realize separate control circuits with delay etc. in that. 3. If it is possible to connect N without switching the N contact can switch on and off charging of the battery (the charge time will be shotrer by few minutes but you won't need the extra relay for it). 4. If the invertor has power indication contact it should be used to control this crazy invention. 5. You can also use current sensing relays and switch the output when there is no power consummed from the battery ouput. In this case you have no inrush current...

"I'm not going to take it apart further "

Wouldnt the solenoids have opposite polarity?

Hi regarding the fridge circuit, will you pull a new circuit through your house just for this setup? As I could imagine the complexity of putting this in line to an existing circuit may cause confusion down the track. How will you identify this in the mains board and on your GPO outlets. There could be other devices plugged into that circuit in the future or more GPO added? (PS I am not a qualified sparky so take my comment with a pinch of salt)

At about 18:45, the two perforated bars may be arc extinguishing means? You see such on higher voltage and power switches or any breakers capable for over 10 kA breaking. The idea is to let the arc extend by its own magnetic effekt and push it to multiple gaps that also cool the arc.

I have a question, my ATS switches nicely but sometimes my house fuses flip during the transfer switching from battery to main. What could be the reason?

The problem with AC is if you want zero switching time, you need to phase lock the supply's. Or convert both to DC then invert back to AC the same way a lot of high power UPS do.