Using Bluetti AC500/B300S for home backup (UK)

The Combi inverter / charger units with ATS used on some narrowboats and motorhomes have built into them an extra relay which when the mains input to the device is turned off automatically connects Earth to Neutral on the output terminals.

You’re alive then

Ooh, mixing brands in your CU? The internet electrical police will be coming for you!

FWIW in building-scale UPS systems there's generally an additional E-N bond on the output side of the UPS so I imagine the scenario where you bond them on your backup inlet socket should be pretty much equivalent.

Worth investing in a copy of BS7671 18th Edition which covers this scenario in detail. IIRC the advice around earthing of islanded systems is to have an extra pole on the changeover for earth. When off-grid the incoming earth is disconnected and the backup neutral is connected to the building earth. You also need an earth rod or even an earth loop round the whole property.

For the workshop backup location you might consider a captive key system instead of having to run an extra wire for the interlock. I've been contemplating a solar setup that would normally operate in grid-tie but I want the option to isolate it in case of a power outage. Unfortunately my layout means I can't really use transfer switch without a lot of expensive digging and wire. My plan is to add an interlock to the main breaker which is secured with a captive key. Only once the main breaker is locked off can you remove the key and walk it over to the solar system and put it in stand-alone mode. If you're not going to switch on the backup very often, this might work.

Great stuff Mike. I suspect this will become an increasingly necessary part of our house-wiring as the world continues to fall apart 🙂

Just been through a complete rewire - type B rcd for my victrons then all my critical loads via a changeover switch for my critical loads all with type A rcbos - changeover switch allows me to bypass my victrons for maintenance. added an earth rod to the existing pme setup. The victrons have an internal relay that connects earth to neutral together when they go into island mode. I didn’t want to go full tt or switch the earth. will be making a video shortly showing how it all works

Here in the US, we also do the "bonded neutral" which is done only at the service entrance, and combined with a ground electrode driven into the actual earth. In the event of backup, we usually don't switch the neutral, so the bond remains in place. You could use a 3-phase transfer switch, and then use the 3rd leg to affect a bond when it's in the "backup" position. Maybe you should just replace the transfer breaker with a standard 2 pole disconnect, but get one with a supervisory switch, so that the backup is connected by a contactor (one at either location?) and that is interlocked with the supervisory switch in the main breaker. I've looked at the commercial solutions, and they use a special 3-way latching contactor that's wired up so that when grid power reappears it will pulse the contactor to drop the backup input and restore the grid, likewise when the grid goes down, it will fire the contactor in the other direction when it sees backup voltage present. This way you aren't paying to keep a coil energized all the time.

In the enterprise server market there’s a device called STS (source transfer switch) that’s used for alternating two power feeds like a UPS. Most houses here are capped at 3KW so one of those units would do the trick just fine

Thx Mike, well explained I think ... only slightly distracted by watching you waving a screwdriver inside a consumer unit 😀👍 Hopefully that excellent chap Mr John Ward @jwflame will be able to make a video and give us his (and the regs) take.

BS7671 contains very little on this situation, Chapter 82 is all there is, with what would be 81 and 83 missing, as those parts of 60364 haven't been included. Chapter 82 has only existed for a short time, being introduced in the 2022 version of BS7671. For earthing, an earth electrode is required. Connection to the grid supplied earth can be maintained - and with TN-C-S there really won't be much option anyway, due to other connections via gas/water etc. For practical purposes, this means a local earth electrode permanently connected to the TN-C-S earth terminal. If either conductor of the backup supply is referenced to Earth (which it should be to allow RCDs/RCBOs to operate), then that connection needs to be switched, so that it's not connected when the grid supply is in use. For this example, that could either be in the Bluetti device itself (if the earth connection at the output is already referenced to one or both of the L/N outputs), or if not, then a connection could be added in the cable to the Bluetti, so that it's not there when the changeover switch is in the grid position, or when it's unplugged from the inlet connector. For more permanent installations where the backup supply is not removable, an interlocked switch is required so it can only be disconnected with the other conductors. Other considerations are protection against overcurrent, as the fault current from the backup supply will be extremely limited. In this instance, the Bluetti device already contains overcurrent protection, so any short circuit faults should be covered by that, and if overloading did occur on things like lighting circuits (with current over that of the particular circuit but still within the output current of the Bluetti), the existing circuit breakers/RCBOs would cover that. This is an example of practical installations already being far ahead of what's in BS7671, and it's inevitable that there will be significant changes to all of it in the future. The situation with mixed manufactures devices in a consumer unit is a requirement of type testing as covered in BS EN 61439, and 412.2.1.1 in BS7671. Consumer units are only type tested using devices from the original manufacturer, and therefore any other devices installed put the whole assembly outside of that scope, and not compliant with 412.2.1.1 In many cases there are no ill effects, but there can be issues with thermal and electromagnetic compatibility if mixed circuit breakers, RCDs and so on are used together in ways that were not intended.

I’ve been looking at a changeover switch for my house in order to accommodate my petrol generator in the event of a power cut, but also had to design in a contactor to isolate my solar inverter when in generator mode as I don’t think the two would cooperate together……LOL…….but haven’t pulled the trigger yet after thinking about the RCD issues.

You can split the board, high loads on grid side and low loads on the UPS side, grid side feeds the UPS , as for earthing, use an earth rod and bond it all together, bridge the neutral to earth on the UPS, its legal as that setup is for backup generators, a Caution two feeds label is needed, I live in England and my house is fed with a PN supply, earthing comes from a ground rod

Your video is very useful and informative. Thank you brother

During a power cut you are not supposed to rely on the suppliers earth. As you highlight, a PEN fault on a TN-C-S supply could leave the earth connection at the head end of your installation floating and your RCDs (mostly) inoperative. There is another implication that you may need to take into account. Many central heating boilers will use flame rectification to detect successful ignition. So if they don't have a proper earth connection they may detect a fault during their ignition proving cycle and lock out. In these cases you will need your own neutral to earth bond on the output of your backup power generator, and also a local earth spike. This basically runs the installation as a system with TT earthing. Older style consumer units without RCD protection for all circuits would be inappropriate for TT operation since the earth loop impedance on the local earth may be two high to reliably trip a normal MCB under fault conditions. (the local earth spike can be left connected on TN-C-S installs - it just becomes yet another earth connection point in the Protective Multiple Earthing (PME) configuration of a TN-C-S supply).

The manufacturer thing is probably more about them than safety For instance, if you put one wylex breaker in a crabtree box, and one of the crabtree breakers failed and caused a fire, crabtree would deny responsibiloty

Alternative supplies should have their own ground reference, but connecting the output of most inverters to ground can be bad, as they often have half bridges driving both sides, and thus float from ground. The alternative supply should have it's own ground connection and protection built in to be safe. For your long cable connection you would need some version of the automatic transfer switch, which does have microswitch contacts in it to allow sensing of both switch position, and as well also includes a set of latching relays that change over the supplies. Big Clive did them a while ago.

Here in Germany I would use something like the Hager SFT440 for the switchover. It does the exact same thing like the switch Mike used, but for 3 phase power. ABB E214-16-202 might be worth a look for single phase. Usually buildings here have there own grounding (a ring of thick wire burried around the building or inside the concrete). This building grounding is then usually connected at a central point with the grounding/neutral from the grid (TN-C-S) and everything other condictive like water pipes. This combined ground is then used to form the N and the PE for the electrical installation. When you switch the N to the N of the generator, you have to make sure, that the N of the generater is grounded in the generator. Gas generator usually have a ground connection, that often nobody bothers to connect. I don't know if there is such a proper grounding point on the Bluetti unit. Doing the grounding of the N in the breakerpanel is a bit problematic, because another generater you might connect to the power input later may have it's own grounding and an RCD and this won't work then. Probably most people can live with this limitation...

Bonding the backup neutral to PE: If the backup Live would be connected to the physical earth (during a fault scenario of course) you could leak power back onto the grid putting people working on fixing the gris in harm's way, depending on how much and where the incoming PE-N is bonded to earth. Your incoming PE-N would become neutral via the PE connection to the backup and the earth itself would become Live. It is all "but sometimes" arguments.