Lux Power Hybrid - EPS signal

[Oldgreybeard]

The key reason for both L and N needing to be disconnected from the grid (apart from the obvious one of potentially back-feeding the grid and creating a safety issue) is that the N is really the E for the majority of UK installations, so to comply with the letter of that regulation requires the PME earth to be disconnected too, as that's hard wired to the incoming neutral and would otherwise bypass one pole of the changeover/disconnect switch.

I still don't see why a simple TT installation, that has double pole switching of both grid live conductors, needs anything else in order to be safe. As long as the supply has RCD protection on every circuit and the earth electrode has a low enough value of Ra, then it doesn't really matter much if there is the required N to E link in the inverter or not.

If there isn't a N to E link (which shouldn't be the case for any UK approved inverter with an EPS) then the system operates with TT earthing when the grid is up, and reverts to IT when the grid goes down. Both are safe and approved earthing systems, even if IT isn't that common in the UK. It's a moot point, anyway, as a UK approved inverter with a backup power output will have the N to E link anyway, so the system would stay as TT whether running in grid mode or backup mode.

PME creates so many potential issues that it's best to just forget about using it as long as it's practical to install an earth electrode, in my view. I was fortunate, in that we had to use TT for our TBS (the DNOs will not allow a TBS to use PME) and then when it came time to convert our TBS into a permanent supply Ze was a bit too high for us to use PME, so I opted to just keep the installation TT. That's been a benefit twice, now. It made installing the charger simpler, as there was no need to bother about PEN fault protection, and then it's been a boon when getting the battery backup installed, as it's meant there was no need to be bothered about earthing in backup mode.

[sharpener]

Yes hence my question if I can somehow get the EPS signal out of this particular inverter.

If not I will just get a mechanical interlink to a N-E relay from my NC switchover isolator.

I think to be sure we need to know more about this inverter e.g.

(i) Does it actually have UK type approval
(ii) if so which mode should it be used in to be compliant
(iii) does it have an internal anti-islanding relay in the incoming AC connections
(iv) is the neutral link in Mode A hardwired or is it only there in EPS mode
(iii) what exactly is the external neutral switch for?

I still don't see why a simple TT installation, that has double pole switching of both grid live conductors, needs anything else in order to be safe. As long as the supply has RCD protection on every circuit and the earth electrode has a low enough value of Ra, then it doesn't really matter much if there is the required N to E link in the inverter or not.

If there isn't a N to E link (which shouldn't be the case for any UK approved inverter with an EPS) then the system operates with TT earthing when the grid is up, and reverts to IT when the grid goes down. Both are safe and approved earthing systems, even if IT isn't that common in the UK. It's a moot point, anyway, as a UK approved inverter with a backup power output will have the N to E link anyway, so the system would stay as TT whether running in grid mode or backup mode.

I disagree.

IT earthing is deprecated for domestic installations as it is less safe than TN-C-S for the reasons given in the wikipedia article. It will not trip on a first earth fault as there is no grounded supply conductor. So you may not even know that the fault has occured

In IT systems, a single insulation fault is unlikely to cause dangerous currents to flow through a human body in contact with earth, because no low-impedance circuit exists for such a current to flow. However, a first insulation fault can effectively turn an IT system into a TN system, and then a second insulation fault can lead to dangerous body currents. Worse, in a multi-phase system, if one of the line conductors made contact with earth, it would cause the other phase cores to rise to the phase-phase voltage relative to earth rather than the phase-neutral voltage. IT systems also experience larger transient overvoltages than other systems.

So that is why it is not allowed under the IET CoP for domestic ESS installed in the UK.

I do however agree that if you are going to add an earth electrode then converting the premises to a TT supply is a good deal better for a variety of reasons. But for a compliant system you do still need the N-E bond relay either inside or outside the inverter.

[nowty]

The key reason for both L and N needing to be disconnected from the grid (apart from the obvious one of potentially back-feeding the grid and creating a safety issue) is that the N is really the E for the majority of UK installations, so to comply with the letter of that regulation requires the PME earth to be disconnected too, as that's hard wired to the incoming neutral and would otherwise bypass one pole of the changeover/disconnect switch.

Except that the new regulations specifically allow you to keep your existing PME connected. I'm sure this is a change and begs the question, what is the point now of disconnecting the N from the grid ?, as a L only disconnection is much simpler to implement for an EPS with regards to earthing.

I still don't see why a simple TT installation, that has double pole switching of both grid live conductors, needs anything else in order to be safe. As long as the supply has RCD protection on every circuit and the earth electrode has a low enough value of Ra, then it doesn't really matter much if there is the required N to E link in the inverter or not.

That changes your earthing from a TT to an IT system, all well and good if your happy running off floating voltages.


TT Grid System



EPS disconnecting the L and N from grid and not reconnecting the N to the local E changes installation to IT system.
There is no longer a reference connection from N to E.

If there isn't a N to E link (which shouldn't be the case for any UK approved inverter with an EPS) then the system operates with TT earthing when the grid is up, and reverts to IT when the grid goes down. Both are safe and approved earthing systems, even if IT isn't that common in the UK. It's a moot point, anyway, as a UK approved inverter with a backup power output will have the N to E link anyway, so the system would stay as TT whether running in grid mode or backup mode.

No, the regulations require you to have a N to E link so why not have it in the EPS. Your correct that you revert to an IT system in EPS mode but the regulations want you to have a TNS system for your EPS. If the inverter EPS has a N to E link then you have changed from a TT to a TNS system in EPS mode, you do not stay as TT. From Table 1 of the regs, https://electrical.theiet.org/wiring-ma ... e-systems/




TT Grid system, now running off EPS with E to N connected, turns into TNS system. RCDs now work better than the grid connected TT system.

PME creates so many potential issues that it's best to just forget about using it as long as it's practical to install an earth electrode, in my view. I was fortunate, in that we had to use TT for our TBS (the DNOs will not allow a TBS to use PME) and then when it came time to convert our TBS into a permanent supply Ze was a bit too high for us to use PME, so I opted to just keep the installation TT. That's been a benefit twice, now. It made installing the charger simpler, as there was no need to bother about PEN fault protection, and then it's been a boon when getting the battery backup installed, as it's meant there was no need to be bothered about earthing in backup mode.

For running an EPS the regs now say you have to install an earth rod unless you already have a TT earth rod which you have.

[Oldgreybeard]

A fair part of Europe (including most of Norway) has been using IT for years without killing their population. Same goes for many commercial installations in the UK (mostly in the comms sector) where IT is normal.

I do think that we (more specifically the IET) tend to get so far up their own backsides when dreaming up new regulations that they lose sight of the simple and common sense solutions. There is also an obsession with trying to achieve absolute safety, when the reality is that it's both unachievable and that it is human nature to bypass safety provisions as soon as they become too complex.

Quite why there is such a UK obsession with both dreaming up complex regulations and then what seems to be a national trait to behave like policemen to anyone that suggests a simpler, but safe, solution that doesn't fit with the letter of some obscure clause in some guidance note somewhere I just don't know. Perhaps the sticklers for making life overly complicated need to go and live somewhere else for a while and realise that millions don't get killed just because some arcane regulation hasn't been followed to the letter.

A dead simple TT installation only has one disadvantage, and that is that it needs to be checked to ensure that the earth electrode, its associated wiring and the RCD, is all still functional. However, given that every UK home is supposed to be inspected at least every 10 years, and in some cases (like PV installations) that reduces to 5 years, and given that 99% of the UK population completely ignore the recommendation for regular periodic inspections, in addition to ignoring many other factors associated with the safe operation and use of electrical installations I do wonder quite why there are so many very detailed and very critical observations made of the slightest theoretical non-compliance.

[Caesium]

However, given that every UK home is supposed to be inspected at least every 10 years, and in some cases (like PV installations) that reduces to 5 years, and given that 99% of the UK population completely ignore the recommendation for regular periodic inspections

I consider myself of above average clue of electrics when it comes to the general population and honestly I had no idea that was even a thing I've never had one done.

Is this publicised anywhere? Where should I have seen this?

[Oldgreybeard]

However, given that every UK home is supposed to be inspected at least every 10 years, and in some cases (like PV installations) that reduces to 5 years, and given that 99% of the UK population completely ignore the recommendation for regular periodic inspections

I consider myself of above average clue of electrics when it comes to the general population and honestly I had no idea that was even a thing I've never had one done.

Is this publicised anywhere? Where should I have seen this?

Should be a label on, or near, the consumer unit, giving the date of the last inspection and the date when the next inspection is due. The form of the label is in the wiring regulations. For rented properties the requirement for periodic inspections is the law, they must be done at the set interval or whenever there is a change of tenant. For non-rented homes it's up to the householder to ensure inspections are done to ensure their own and their families safety - they aren't breaking the law by not getting inspections done, although there have been suggestions that insurers may be starting to insist that the guidance in the regs (been much the same for decades) is complied with. There is guidance here: http://www.energylegislation.co.uk/eicr/

The label should look like this (they vary a bit, the ones I use have no logo):



The other labels that get ignored all the time are the ones stating that RCDs and RCBOs must be periodically tested, to make sure they still work. Again this is in the wiring regs and printed on the front of some RCDs, yet the majority of people just ignore them.

[Swwils]

I don't have an earth rod yet. But will get one once I've sorted what to actually do for circuit switchover. The inverter does have UK ratings. But I can't find any info about internal relays. It does click clack when it loses grid power and click clack a few minutes after grid AC is returned.

[Oldgreybeard]

I don't have an earth rod yet. But will get one once I've sorted what to actually do for circuit switchover. The inverter does have UK ratings. But I can't find any info about internal relays. It does click clack when it loses grid power and click clack a few minutes after grid AC is returned.

Easy way to check would be to measure the earth loop impedance from one of the emergency power outlets, when the unit is in EPS mode. If that's sky high then there is no N to E link, if it's around the same impedance as the earth electrode resistance then there' s a link.

[sharpener]

I don't have an earth rod yet. But will get one once I've sorted what to actually do for circuit switchover. The inverter does have UK ratings. But I can't find any info about internal relays. It does click clack when it loses grid power and click clack a few minutes after grid AC is returned.

Sounds promising, that will almost certainly be the anti-islanding relay(s), the delay is while the inverter is matching its voltage, frequency and phase to the grid.

Might be worth checking if you get the same result to OGB's test whether it is in Mode A or Mode B.

[Oldgreybeard]

I don't have an earth rod yet. But will get one once I've sorted what to actually do for circuit switchover. The inverter does have UK ratings. But I can't find any info about internal relays. It does click clack when it loses grid power and click clack a few minutes after grid AC is returned.

Sounds promising, that will almost certainly be the anti-islanding relay(s), the delay is while the inverter is matching its voltage, frequency and phase to the grid.

Might be worth checking if you get the same result to OGB's test whether it is in Mode A or Mode B.

Would be interesting to check and add to the pool of collective knowledge, especially as it seems that details like this are a bit thin on the ground for some makes and models of inverter. When our batteries are charged later today I may try throwing the main switch and doing a quick measurement of Zs at one of the spare EPS powered outlets. If that comes out close to the around 20 ohm electrode resistance then that confirms that the Sofar does connect E and N when in EPS mode (I'm pretty sure it does, although I only checked Zs on the circuits in grid mode).