Earth Neutral bond for hybrid inverter (again)

[jimseng]

At the risk or reopening the can of worms and revisiting an old thread I am still wondering about the earth neutral bond relay in the event of a power cut. I have now ordered a Growatt 10k hybrid inverter and according to this diagram (from the Growatt manual) the earth neutral bond is permanent and provided by the DNO at the meter, or wherever it is. (Or at least it seems to be relying on the DNO E/N bond). In the event of a power cut it looks to me like the earth neutral bond still remains in place as the grid and home loads neutrals are connected and therefore the E/N bond is still intact.

I suppose here the additional E/N bond  relay would be necessary if there was a cable fault between the house and the DNO E/N bond, but doesn't that take place in the meter box?

I'll have to have a ground spike anyway given that my grid connection is a way off but once it is in place...what then?

[Mattg4321]

You won't have an earth-neutral bond once the grid is disconnected from your installation by the inverter in the event of a power cut. DNO earth-neutral bond isn't necessarily at your intake position either. It certainly won't be in a TN-S earthing system (although yours is likely to be TN-C-S (PME) if a new connection).

[jimseng]

In the image the load and grid neutrals are permanently tied so even in a power cut there is still continuity to the grid neutral. Surely, unless there is a cable fault, the bond would still remain?

[Dillsue]

8 hours ago, jimseng said:

In the image the load and grid neutrals are permanently tied so even in a power cut there is still continuity to the grid neutral. Surely, unless there is a cable fault, the bond would still remain?

As both the house loads and the inverter are connected to both the earthbar and neutral bars in the schematic, I take it that Growatt expect that E-N link to be within your house. It's a poor schematic if that's depicting the link to be out in the grid somewhere.

 

Probably worth asking Growatt for clarification??

[jimseng]

57 minutes ago, Dillsue said:

As both the house loads and the inverter are connected to both the earthbar and neutral bars in the schematic, I take it that Growatt expect that E-N link to be within your house. It's a poor schematic if that's depicting the link to be out in the grid somewhere.

 

Even if the link was in the grid somewhere, isn't it electrically the same? The trouble is it probably covers different countries and is a bit generic. I am wondering what UK people think. The Sunsynk manual has the same schematic.

[Dillsue]

3 minutes ago, jimseng said:

Even if the link was in the grid somewhere, isn't it electrically the same? The trouble is it probably covers different countries and is a bit generic. I am wondering what UK people think. The Sunsynk manual has the same schematic.

Its not electrically the same if the DNOs cable gets disconnected/severed which seems to be your concern?? Outside of your house isn't under your control but inside your house is, so you can ensure that link remains.

 

If your inverter meets UK grid regs then there will be people at Growatt that understand the UK grid so should be able to clarify things for you??

[SteamyTea]

Just a thought, possibly a bad scenario.

If there are a number of grid connected system, and one of them is an islanding system that is incorrectly wired in, so it does not disconnect from the grid, would the other systems still be connected and running?

I know that most inverters check that grid impedance as part of the running/disconnect protocol, but that may be within tolerances in a rural setting.

An unlikely scenario I know, but with millions of systems fitted, unlikely becomes inevitable.

[JohnMo]

18 hours ago, jimseng said:

Growatt 10k hybrid inverter

Are you trying to use a hybrid inverter in wrong situation - it's designed for grid connection - you don't have one.

 

Isn't the correct inverter an off-grid inverter?

[jimseng]

3 hours ago, JohnMo said:

Are you trying to use a hybrid inverter in wrong situation - it's designed for grid connection - you don't have one.

 

Isn't the correct inverter an off-grid inverter?

I don't have a grid connection yet, but I will have. Maybe not until later in the year.
I think the schematic is not really correct. I think the permanent neutral link is for other countries. I was under the impression that the neutrals are common until the grid goes down, at which point the grid is disconnected internally with a double pole relay, hence the need for an earth neutral bond relay, but it is hard getting the information. I have reached out to Growatt, I'll see if they come back to me.

[Mattg4321]

That schematic is wrong and potentially dangerous in the wrong hands. All 'live' conductors (this includes neutral, don't confuse live with line) must be disconnected from the grid when in island mode. This is a safety issue to protect people working on the DNO side.

 

The schematic shows the neutral from the load connectors connected to a common neutral bar with the grid neutral. This is wrong. Most, if not all, of these inverters are made for foreign markets and the instructions are either nonsense or dangerous when applied here in the UK imo.

 

I would like to imagine that the L&N are both disconnected from the grid entirely when in island mode, but I've not fitted a Growatt before. My Sunsynk does do this.

[Dillsue]

11 hours ago, Mattg4321 said:

That schematic is wrong and potentially dangerous in the wrong hands. All 'live' conductors (this includes neutral, don't confuse live with line) must be disconnected from the grid when in island mode. This is a safety issue to protect people working on the DNO side.

 

The schematic shows the neutral from the load connectors connected to a common neutral bar with the grid neutral. This is wrong. Most, if not all, of these inverters are made for foreign markets and the instructions are either nonsense or dangerous when applied here in the UK imo.

 

I would like to imagine that the L&N are both disconnected from the grid entirely when in island mode, but I've not fitted a Growatt before. My Sunsynk does do this.

Wanting to understand more about this, what is the risk to the DNOs linesmen if the neutral is bonded to earth but the DNOs neutral isn't disconnected from the customers inverter, seemingly as in the schematic?? 

[SteamyTea]

1 hour ago, Dillsue said:

Wanting to understand more about this

This is not for normal low voltage, or high voltage, but shows how transformers can be wired differently.

 

 

You may find a bit more in this, long times since I have read it.

 

Students_Guide_to_the_IET_Wiring_Regulations_-_The_institution_of_engineering_and_technology.pdf

[Mattg4321]

11 hours ago, Dillsue said:

Wanting to understand more about this, what is the risk to the DNOs linesmen if the neutral is bonded to earth but the DNOs neutral isn't disconnected from the customers inverter, seemingly as in the schematic?? 

 

If live conductors are left connected to the grid the inverter could backfeed the grid causing unexpected voltages for DNO linesmen. In practice it may well not be a problem if only the neutral was left connected to DNO grid, but that would be against the regulations and have the potential to be dangerous to DNO under certain fault conditions/if the inverter was incorrectly installed.

[Dillsue]

But if there's a neutral to earth bond, as in the schematic, anything the inverter might apply to the neutral line will run to earth?? I appreciate UK regulation/best practice may want the inverter neutral disconnecting from the grid as well as line, but if the neutral is earthed surely there's no hazard??

[Mattg4321]

In theory perhaps not, however this is all academic, because that's what the regulations say. You can't leave live conductors connected to the grid when operating a generating set/inverter and you can't make your own neutral - earth link when connected to the grid.

 

In any case, it won't work when connected like this as you'll have parallel neutral paths and any RCD protecting the grid supply to the inverter will trip out as some current will go via the neutral at the RCD and some via the neutral bar. That schematic is a load of %@!$. I'd be interested to hear what Growatt come back to you with!!

[jimseng]

14 hours ago, Mattg4321 said:

I'd be interested to hear what Growatt come back to you with!!

I'll certainly report back if I hear anything. It is perhaps only fair to post the other schematic from the manual which depict double pole MCBs and no link. I thought I was asking an easy question. 

[Beelbeebub]

I'm pretty sure uk regs say double pole breaker. 

 

I think the issue is if the cable is severed then the neutral can become line and someone outside working could get a shock between the neutral and local earth. 

 

Your electrican should know all this and sort it out

 

The single pole seems to the Australian regs. 

 

[Nickfromwales]

1 hour ago, jimseng said:

I thought I was asking an easy question. 

But you’re getting all the answers, however difficult that becomes, lol. 

[Nickfromwales]

4 minutes ago, Beelbeebub said:

I'm pretty sure uk regs say double pole breaker. 

 

I think the issue is if the cable is severed then the neutral can become line and someone outside working could get a shock between the neutral and local earth. 

 

Your electrican should know all this and sort it out

 

The single pole seems to the Australian regs. 

 

Changeover, as it’s referred to, should be exactly that. A switch that breaks all connection from the grid supply and swaps you to the ‘generator’ (whatever source that is), and any electrician should know the fundamentals here; if they don’t, there’s a book in their van that they can go and consult. 
 

If they still, then, don’t know, show them the door and give the same money to someone who does.

 

If you don’t know, and you (any member of the public) attempt to DIY, then that’s about as dangerous as it can get.

 

Foreign manufacturers installation guides need proper interpretation by the sparky / installer in the relevant country, so there shouldn’t be any ambiguity if you’ve chosen the installer well.

 

17 hours ago, Mattg4321 said:

 

If live conductors are left connected to the grid the inverter could backfeed the grid causing unexpected voltages for DNO linesmen. In practice it may well not be a problem if only the neutral was left connected to DNO grid, but that would be against the regulations and have the potential to be dangerous to DNO under certain fault conditions/if the inverter was incorrectly installed.

 

It would need to be the ‘perfect storm’ though, as one dwellings output would be entirely consumed by the neighbouring properties sharing the line, plus the line / local transformer losses etc, so even if you had a huge inverter the chances of zapping a linesman, in the real world, would be rows of noughts after the decimal place.

 

Imagine your microgenerator being asked, in a blackout, to reverse feed all of your neighbours houses. It wouldn’t even get the lights to flicker.

 

A person would have to be working on the severed DNO cable feeding your house to go meet Jesus. 

[Nickfromwales]

1 hour ago, jimseng said:

I'll certainly report back if I hear anything. It is perhaps only fair to post the other schematic from the manual which depict double pole MCBs and no link. I thought I was asking an easy question. 

Those AC isolators are physically connected in a changeover switch, so it’s one or the other in a flip/flop arrangement.

 

 

[jimseng]

5 minutes ago, Nickfromwales said:

Those AC isolators are physically connected in a changeover switch, so it’s one or the other in a flip/flop arrangement.

 

 

I'm not sure what you mean here. I don't see a changeover. There is an internal disconnect in the inverter so the on grid load goes down in the event of a power cut. Have I misunderstood what you are saying? 

[Nickfromwales]

4 minutes ago, jimseng said:

I'm not sure what you mean here. I don't see a changeover. There is an internal disconnect in the inverter so the on grid load goes down in the event of a power cut. Have I misunderstood what you are saying? 

I’m saying what the usual arrangement is for swapping between grid and generator. If the inverter has approval for use in the UK then happy days. 
 

The changeover is done electronically in an approved device. The last pic you shared is helpful, and more relevant btw!!

[jimseng]

10 minutes ago, Nickfromwales said:

The last pic you shared is helpful, and more relevant btw!!

Yes. In fact it pretty much makes the whole thread moot.
But why double pole breakers in the second schematic? They aren't going to trip in a power cut. Are they also for manual disconnect?

[Nickfromwales]

4 minutes ago, jimseng said:

Yes. In fact it pretty much makes the whole thread moot.
But why double pole breakers in the second schematic? They aren't going to trip in a power cut. Are they also for manual disconnect?

Over reverse current is usually dealt with by a double pole MCB type device.
 

It’s quite a shit diagram tbh, with “breaker” used where I suspect they should be saying (perhaps) isolator or over current device.

 

As there’s no current rating stated I’m assuming they’re just dumb DP isolators. Does the manual state otherwise? 

[jimseng]

1 minute ago, Nickfromwales said:

As there’s no current rating stated I’m assuming they’re just dumb DP isolators. Does the manual state otherwise? 

Yes. The manual states current so they are all breakers. (100 amp grid, 80, home loads).

I guess it is a serving suggestion and to consult local regs. I tend to take things too literally.