How about a DIY three-input Automatic Transfer Switch?

[Crossy]

It's been a while since we had a thread from Crossy Labs. So here we go with something to make the little grey-cells work.

 

I put this unit together to meet a specific need but the basic design could be adjusted for different scenarios.

 

This design uses relay-logic and commercial timers and under/over units so there's no software to worry about.

 

When developing any kind of transfer switch it is imperative that :-

• The mains and either of the other inputs cannot be connected to the load at the same time (not good for the inverter or genset).
• It is vital that it is impossible for the inverter or genset to back-feed into a dead grid (to protect the chaps trying to get your power back on).

The contactors used are good for 40A continuous current, this should be adequate for a 15/45 supply if you move your big loads (water heaters) to the non-controlled side of the switch and only run your domestic loads via the switch. Other than cost there's no reason that bigger contactors with the same contact arrangements cannot be used.

 

The contactors are electrically interlocked so that, if the mains is on (and within the required voltage range) neither of the other two contactors can be energised thus isolating the inverter and genset from the load. As a "just in case" backup the contactors are also mechanically interlocked with the same requirement. The mechanical interlock is there as a final safety function (should a relay get stuck or similar), in normal operation it never comes into play.

 

This transfer switch is intended to work with the EPS (Emergency Power Supply) output of our Sofar grid-tie hybrid inverter.

 

The logic works like this:-

1. Mains on = load powered by the mains (the grid-tie inverter is also operating).
2. Mains off = load powered by the inverter EPS output (some loads are shed)
3. Mains off + Inverter off (batteries flat or overload tripped) = load powered by the genset (shed loads are restored as the genset has a bit more oomph)

To whet the appetite, here is a photo of the completed unit.

 

 

And the circuit diagram.

 

 

Note that there are a couple of differences between the diagram and the unit as implemented due to the topography of our distribution boards.

 

The original TinyCAD file should you want to fiddle with it. 

3-way transfer switch for AN a.dsn

 

If that's not scared you off then more details are coming in the following posts.

 

 

[Crossy]

Contactors are not inexpensive and the accessories (like mechanical interlocks) can be difficult to come by locally.

 

In fact, I wasn't going to build this unit until I came across this item on Lazada https://www.lazada.co.th/products/i3007531853-s11043074833.html

 

 

That's a 3-phase "reversing" contactor, but it actually consists of two normal contactors plus a mechanical interlock.

 

 

At 450 Baht it's an absolute steal, so I bought two, giving me the required 2 mechanical interlocks plus a spare contactor for under a grand! Result!

 

At time of writing there is still stock, so if you're thinking of doing this get in there quick ???? 

 

It all just clips together so re-configuring to a group of 3 contactors plus 2 mechanical interlocks was a piece of cake. Clipping it all onto a DIN rail holds it together, although a couple of extra screws into the backplate is a wise addition.

  

IMPORTANT NOTE: As designed, the mains contactor is the one in the centre, ensuring that if it is energised the other two cannot operate. If you re-configure the logic I would ensure that the mains contactor is still the centre one for maximum safety.

[bluejets]

Have to take a closer look at the complete arrangement ( couple of minutes spare )

However my first concern  is the switched neutrals, depending on the rest of the earthing system not shown naturally.

[Crossy]

 

5 hours ago, bluejets said:

Have to take a closer look at the complete arrangement ( couple of minutes spare )

However my first concern  is the switched neutrals, depending on the rest of the earthing system not shown naturally.

 

Always a point of concern.

 

All commercial single-phase ATSs are 2-pole and switch the neutral. Three-phase is another animal altogether, an open neutral on a live system is very bad news.

 

EDIT This is from the manual of a commercial ATS system.

 

 

 

The incoming supply is TNC-S with MEN, standard Thai installation, the N-E bond is at the mains incoming breaker. So, when on mains there's no issue.

 

The genset has no N-E link, it's a baby (6kVA) beast. So, on genset we have an IT (floating) system, it actually wouldn't be a major job to add a N-E link at the genset and make it TT. Adding that to my list, although it's been operating for 10 years wired as IT.

 

The inverter is possibly more, er, "interesting". The mains inlet comes directly from the incoming mains supply (after our under/over voltage device) so in normal operation the neutral is bonded. When in EPS mode it's unclear what it does with the outgoing neutral so I'm assuming it to makes an IT supply. The inverter is a transformerless, high-frequency type so I'm loathe to start adding earths anywhere that's not specifically intended to be grounded.

 

From the inverter manual, I see an RCD on the EPS output so there should be a neutral ground somewhere.

 

 

I just did a test with my patented RCD tester (2W lamp on test probes).

• Running on mains. RCD trips when lamp connected L-E = PASS.  
• Running on inverter (incoming mains isolator open meaning incoming N-E bond is still in place). RCD trips when lamp connected L-E = PASS. 

So, whilst I'm not totally sure how the inverter handles the N when in EPS mode, I do know that any RCD protection connected to that output functions as expected.

 

 

 

[bluejets]

https://www.worksafe.qld.gov.au/news-and-events/newsletters/esafe-newsletters/esafe-editions/esafe-electrical/2021-bulletins/september-2021/beware-of-switched-main-neutral-conductors

[Crossy]

This is the relevant illustration from AS3000 (2018), note that the mains neutral is NOT switched by the transfer device (but the aux supply neutral IS switched).

 

Since Thailand has a very similar TNC-S (MEN) system to Oz I may have to modify the design. Thanks for the heads-up @bluejets.

 

 

I shall have to locate the relevant UK document.