"Hybrid Inverters" in the TT-Earthing System. How to N-G Bond in different modes of operation

[@inothome]

Finally a decent video showing and explaining why you do not bond the earth and neutral in a TT system. I live in Vietnam and it's a TT system here, but no grounds at the buildings either. Outlerts are two prong and not polarized either. If you do see a three prong outlet, half the time the ground isn't connected. But when I do install grounds for people some ask for me to bond it and I then have to explain why you don't bond on a TT system. Another potential (pun not intended) problem with bonding is if the neutral is broken between the transformer and the bonded location, whatever is downstream from the bonded location will use that ground to get back to the transformer. Which of course depending on how much load that is, can easily overload the neutral at the bonded location. RCDs are not widely used here either for some reason. I always recommend them to customers but not many want to spend the extra $10, literally $10 USD extra for an RCBO vs just a normal breaker that here costs less than $10 to begin with. I also refuse to install a ground unless they also install an RCD. As like you explained, there may not be enough fault current to trip a breaker and can energize everything that is grounded to line potential. But, that is one of the benefits to a TT system, reduced ground fault current. But that is more applicable to industrial situations where you have a higher chance for ground faults with higher currents since there may be 200A service vs a 60A household service, the TT limits your ground fault current over a TN-C system. But the flip side is it may not trip a normal breaker, as discussed.

[@RolandW_DIYEnergyandMore]

Yeah. This sort of half way solutions like we do have in many countries where people have to think twice about every Cent spent, is unfortunately dangerous as the grounding of any metallic structures and enclosures shall protect humans to come in touch with the live potential. That is the duty of the mandatory RCD to trip when such a fault current to the grounded object is detected. Many people think that when they do not ground at the consumer side, fault current cannot flow so all is OK, right. The problem is that Neutral is still bonded to Ground at the transformer. So if anyone comes into contact with a live wire will still be shocked as leakage current will go over the persons body to ground and via ground back to the transformer. So yeah. Mandatory Class 2 equipment grounding, mandatory RCD/GFCI, no bonding at the consumer... this will save a life for an investment of maybe just a few dollars!

[@inothome]

@@RolandW_DIYEnergyandMore Exactly, except I do work for westerners, not locals. The price of electrical equipment is a fifth of what it costs in the US. Except wire, that's only a little cheaper. But I had a 50A three phase panel built, with about 24 circuits in it for what one three pole, DIN rail RCD would cost in the US. Yet, they didn't want any RCDs and this was a commercial application. I didn't do the install, just consulted on it.

[@RolandW_DIYEnergyandMore]

@inothome yeah. It's a shame. Some must feel before they believe. Here in Thailand you will hear from buildings and factories burning down from electric failures every few days. One breaker for a whole building, no grounding, etc. Keep on fighting :)

[@codertao]

Thanks for bringing up "what happens during a neutral break (in a TT system)"- I'm not an electrician, so I was asking during the video "what is the problem with having a gnd/neutral bond in this system"- at least one of the issues would be during a neutral break, you've converted the system into an unintended single-wire-earth-return.

[@RolandW_DIYEnergyandMore]

@@codertao Neutral breaks are bad in any TN-C or TN-C-S earthing arrangement which comes with a grounded transformer Neutral and equipment grounded to the separated grounding connected to the Neutral. A break here would set all grounded equipment live down streams of the break! In the TT-system breaks initially wouldn't be to serous. The whole areas electricity supply would stop, so the grid supply would be notified quite quickly and could shut down and repair the affected transformer branch. All grounded equipment in this installation without a bonded N-G would just stay without potential and safe as there shouldn't be a return path if there is no fault. RCDs should stay operational as they wouldn't see flow on its N leg while current could go from L to PE in case a further leakage in the installation would happen.

[@cyber5515]

I just googled "battery backup system wiring diagram" and it appears the changeover switch only disconnects the active line. So the off-grid system's neutral wire remains connected to the grid neutral wire which is grounded at the power pole supply transformer with a TT earthing system. We have the TN-C-S earthing system where I live, so the neutral wire is also grounded at the meter box with a ground rod, which I expect removes a lot of noise from the neutral wire. We still have a separate earth wire to the power points, which is also connected to the ground rod. The TN-C-S earthing system is the best system for solar and battery backup. With the TT system, if a falling tree takes down the neutral wire near the supply transformer, everyone's battery backup system will be activated and everyone will have a common unearthed (live) neutral wire except for the guy who connected his neutral wire to earth - who will become the whole town's earth connection.

[@RolandW_DIYEnergyandMore]

Hi, yes. The guy with the Backup would just create that unwanted extra earthing point if his inverter isn't isolating N from the grid. But I thought that the inverters internal ATS should switch over L and N.

[@hasger1941]

Hello Roland, wonder is there a solution for the all in one inverters when it comes to the mains input side which we normally plug into a wall socket ( to charge batteries), however we need to remember to switch it off before switching the transfer switch to off grid mode as failing to do so it tries to charge the batteries from the batteries i.e from the wall socket feedback ?

[@RolandW_DIYEnergyandMore]

Is there a sort of schedule when you do the grid charge or is it just random? You could use timer relays to set for example a 3 hours charge and then the relay automatically disconnects. And next time you just unplug and plug in again for a new cycle...

[@hasger1941]

@@RolandW_DIYEnergyandMore Hello Roland, it is random, the inverter will use the grid to charge batteries when insufficient solar (sol or sbu in settings). We thought about connecting the inverter ac input to the grid input side of the transfer switch which would work fine unless we needed to bond earth and neutral this would not be ok.. It seems there would be no option other than disconnect grid input to the inverter when switching to off grid..

[@RolandW_DIYEnergyandMore]

@hasger1941 yes. That's the problem with the All-in-One Off-grid type inverters. But i think that only the Grid Bypass is the problem. The AC charging might actually be isolated and not creating a bond between output and input...???

[@hasger1941]

@@RolandW_DIYEnergyandMore Ahh this is an interesting point, we will need to check this... Thank you Roland, Also some of these inverters support parallel function and automatically do the ground and neutral bond internally when in off grid mode.. The question is when using say 2 or more in parallel is it now a problem since the bonding is happening in more than one place ? This is since some say only to bond in one place i.e not at inverter and also the distribution board ??

[@hasger1941]

ok update: the inverters have communication ports so when using in a parallel system we will assume the dynamic bonding will hopefully only bond in one inverter ?? maybe maybe not nothing seems to be documented.. Regarding the charge mode the earths will still be linked inside the inverter i.e. input side and output side.. So in order to have earth ground bonding on the system the input side will need to be disconnected from the ring main in off grid mode.. it still all seems a little hit and miss🙂