Lux Power Hybrid - EPS signal

[Swwils]

Hello,

Does anyone know on a lux power hybrid inverter if there is a signal output indicating EPS status so a N-E bond relay can be triggered? Or what is the usual arrangement for achieving this?

Thanks

[Oldgreybeard]

No need for an E to N connection in EPS mode, as the earthing is provided via the earth electrode, and I believe there may be an internal N to E path within the inverter, albeit not at a very low impedance (enough to stop N from floating, perhaps?). The E to N connection is needed on some generators as they may not have any sort of earth leakage detection, I believe. It's not normal to connect N to E in a TT wiring installation, AFAIK (I've never seen it done, and there is no N to E link shown in the regs for a TT installation.

[Swwils]

It was my understanding that when operating in an island mode the distributor’s neutral-earth link also cannot and must not be relied upon, as this is switched out when the live conductors are disconnected.

And the island mode is treated as TN-S. If N and E were linked in the inverter the RCD would trip, hence need for N-E relay.

So to comply you need a switching mechanism to provide a neutral for the island mode and The IET Code of Practice for Electrical Energy Storage Systems calls this an "N-E bond relay".

I am aware lux offer a "mode b" configuration without this, but it would not be allowed in the UK.

I guess I will need to get the N-E relay mechanically linked with the island mode isolator.

[Oldgreybeard]

Why not just make life easy and make the system TT all the time? That removes any requirement for any sort of N to E link and also removes all switching devices from the earth path. Although having switches in the earth conductor is allowable, as I understand it, I'm of the view that it's probably slightly better to not have them if there is an easy way out.

[sharpener]

It was my understanding that when operating in an island mode the distributor’s neutral-earth link also cannot and must not be relied upon, as this is switched out when the live conductors are disconnected.

And the island mode is treated as TN-S. If N and E were linked in the inverter the RCD would trip, hence need for N-E relay.

So to comply you need a switching mechanism to provide a neutral for the island mode and The IET Code of Practice for Electrical Energy Storage Systems calls this an "N-E bond relay".

Yes, that is correct.

I am aware lux offer a "mode b" configuration without this, but it would not be allowed in the UK.

I guess I will need to get the N-E relay mechanically linked with the island mode isolator.

If as you say there is a "mode b" then by implication there is also a "mode a" where there is an internal N-E bond relay. Without this the inverter would not have got UK type approval - if it has.

If it hasn't then you will have to fit an external one. Has it also got the internal anti-islanding relay which disconnects the grid? I think you can get both incorporated in a separate unit which is itself type-approved, the downside is that AFAIR they cost several hundred quid.

The rationalisation of all this with OGB's comments is that a public TT supply has the neutral grounded at the substation, this is the first "T" in TT [https://en.wikipedia.org/wiki/Earthing_system]. So the RCD will trip on the first earth fault to occur as there is a complete circuit for the leakage current.

In island mode this connection needs to be replicated by the N-E bond or it will need two different faults to trip the RCD. So even if you rewire your installation as TT - which I agree with - you still need the bond relay one way or the other. Fortunately in my case Mr Victron has taken care of it all.

[Oldgreybeard]

It was my understanding that when operating in an island mode the distributor’s neutral-earth link also cannot and must not be relied upon, as this is switched out when the live conductors are disconnected.

And the island mode is treated as TN-S. If N and E were linked in the inverter the RCD would trip, hence need for N-E relay.

So to comply you need a switching mechanism to provide a neutral for the island mode and The IET Code of Practice for Electrical Energy Storage Systems calls this an "N-E bond relay".

Yes, that is correct.

I am aware lux offer a "mode b" configuration without this, but it would not be allowed in the UK.

I guess I will need to get the N-E relay mechanically linked with the island mode isolator.

If as you say there is a "mode b" then by implication there is also a "mode a" where there is an internal N-E bond relay. Without this the inverter would not have got UK type approval - if it has.

If it hasn't then you will have to fit an external one. Has it also got the internal anti-islanding relay which disconnects the grid? I think you can get both incorporated in a separate unit which is itself type-approved, the downside is that AFAIR they cost several hundred quid.

The rationalisation of all this with OGB's comments is that a public TT supply has the neutral grounded at the substation, this is the first "T" in TT [https://en.wikipedia.org/wiki/Earthing_system]. So the RCD will trip on the first earth fault to occur as there is a complete circuit for the leakage current.

In island mode this connection needs to be replicated by the N-E bond or it will need two different faults to trip the RCD. So even if you rewire your installation as TT - which I agree with - you still need the bond relay one way or the other. Fortunately in my case Mr Victron has taken care of it all.

There's absolutely nothing wrong with having a TT system with an isolated supply emergency back up (although I thought that all UK approved inverters had an integral N to E link anyway). All that happens when the power fails with an isolated backup supply is that the installation goes from being TT to IT, and there's nothing at all wrong with having an IT supply (in large areas of Norway IT is the normal domestic supply). Just because IT is uncommon in UK domestic supplies (but is common in some commercial installations) doesn't mean that IT is in anyway unsafe. Wikipedia has a ranking of different earthing systems, in this graphic, and this shows that IT isn't inherently unsafe, safer than TN-C, less safe than TT:

Earthing systems.jpg (50.14 KiB) Viewed 1539 times

I'm a great believer in keeping things simple, if at all possible, something that some in the IET regularly seem to lose sight of, in their seeming desire to make things ever more complex. In general, simple stuff tends to be more reliable.

[Swwils]

The only thing in the manual I have to go on this this:



As far as I am aware this unit does have UK rating - I didn't know that there was a requirement for internal N-E bond relay. That's great if it has. I've seen various installs online and it's a mixed bag, so I can't tell if it also has anti islanding.

The discussion about TT is interesting as island mode requires you to install a consumer earth electrode even if you have a TN earthing arrangement from distributor and PME conditions continue to apply in island mode. So that will be going in.

[Oldgreybeard]

The only thing in the manual I have to go on this this:



As far as I am aware this unit does have UK rating - I didn't know that there was a requirement for internal N-E bond relay. That's great if it has. I've seen various installs online and it's a mixed bag, so I can't tell if it also has anti islanding.

The discussion about TT is interesting as island mode requires you to install a consumer earth electrode even if you have a TN earthing arrangement from distributor and PME conditions continue to apply in island mode. So that will be going in.

My personal view is that it is potentially risky to have any connection to the incoming PME when the system is running in emergency back up mode, just because there is a fair chance that the incoming PME could well be sat at a high voltage relative to the actual earth potential, and because an earth electrode probably won't have a low enough resistance to stop the earth voltage getting to the point where it could cause a shock hazard.

Say, for example, that the power fails because the neutral conductor is cut somewhere nearby. That would cause the cut off bit of the neutral to rise to a high voltage from all the loads connected to the three phases probably being out of balance and so dragging the cut off bit of neutral to a high voltage. An example might be a street of twelve houses, all affected by a break in the neutral out in the street. A couple of the houses are empty and drawing no power and a couple of others have high loads turned on. The cut off bit of neutral could easily get to a couple of hundred volts or so, and with a local earth electrode being maybe 50 ohms the circulating earth current via the earth electrode from all the other houses in the street would only be a few amps. No RCDs would trip as there wouldn't be an imbalance, this circulating earth current would seem normal.

My view is that it's better to just do away with the PME installation and make the whole house TT. That way there are no risks in the event of a fault and the backup system is safe with no need for any earth/neutral switching.

[nowty]

Latest (I think) regulations are linked below for EPS.

https://electrical.theiet.org/wiring-ma ... e-systems/

The summary is, (bold is my annotations)

"An installation that operates in island mode therefore requires:

• a switching mechanism to disconnect live conductors of the installation that are to be powered in island mode from the grid. The IET Code of Practice for Electrical Energy Storage Systems calls this an island mode isolator

• a switching mechanism to provide a neutral for the island mode The IET Code of Practice for Electrical Energy Storage Systems calls this an N-E bond relay, and

• a consumer earth electrode. In TT systems, this may be the TT system consumer electrode, if it meets specific technical requirements."

And for the absence of doubt about the live conductors which must be disconnected from the grid,

"All live conductors, that is line(s) and neutral, that are to be powered in island mode must be disconnected from the grid. The installation may remain connected to the distributor’s means of earthing (where this is provided)."

But the distributor’s means of earthing cannot be relied upon, hence the need for the earth rod and for the N to E reconnection at or near the EPS.

[Swwils]

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.