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Topic Title: PV connection to Fuse box
Topic Summary: Earth used as neutral?
Created On: 11 March 2017 02:51 PM
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 11 March 2017 02:51 PM
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pex

Posts: 40
Joined: 10 January 2008

The other day I decided to replace the earth block by my meter with a bigger one to accommodate another earth wire.

As I took one of the earths off there was a big flash and it turned out to be the earth to my PV system that has been working well for 5 years. It also turned out that my PV stopped working when the earth was disconnected.

It seems that the Earth has been used as a neutral return and indeed it has a bit of red tape on it so it looks like it was intentional.

I'm waiting for the installer to get back to me but in the meantime I've been trying to fathom out why they would do this - should there be a 4 core cable to the roof instead of the 3 core installed.

The inverter is a Sunny Boy (4kW) but their instructions leave a bit to be desired. Some say to connect wires to L1, L2 ,N and E. Others say connect L N and E.

I don't want to take the cover off the inverter to see what they've done because that would invalidate any warrantee presumably, so I'm asking here if anyone can shed light on why there would be an L1 and L2 in some inverters.

Also has anyone encountered such a thing as this.

Nothing on Google except the aforementioned useless instructions
 11 March 2017 03:32 PM
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broadgage

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No matter how the inverter is intended to be connected, I feel certain that no significant current is intended to flow to the earth bar.

I would expect the earth connection to be a safety feature and that under normal conditions it should carry only leakage currents of a few ma.

The references to l1 and l2 may be for overseas systems that have 208/220/230/240 volts between phases, with the inverter being connected between phases.
 12 March 2017 09:19 AM
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AJJewsbury

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As I took one of the earths off there was a big flash and it turned out to be the earth to my PV system

Well, let that be a lesson is safe isolation! It's unwise to work live on mains, even the protective conductors...

t seems that the Earth has been used as a neutral return and indeed it has a bit of red tape on it so it looks like it was intentional.

Presuming it's grid connected, running only in parallel with the grid supply, then there must never be a N-PE link and the inverter can only inject voltage between live conductors (otherwise you have a combined neutral/earth conductor in the consumer's installation and fall foul of the Law in the UK). The inverter should use the PE connection only for shock protection in the normal ADS manner, perhaps interferrence suppression and maybe for mains connection verification and other fault detection purposes (I think mine can detect faults to earth on the d.c. side too) - so as broadgage says they current flowing should be small, unless a fault was present at just the moment you disconnected it (back to point 1). Could tugging cables out of the earth block have disturbed any live conductors?

The installers of my PV system left tape on the cables too (brown I think) - but seemingly just to identify them as belonging to the PV system.

Is there no RCD on the PV a.c. side? Connecting to earth instead of N would surely have caused it to trip?

- Andy.
 12 March 2017 12:11 PM
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sparkingchip

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Remember to install one of these next to the earth block and then next time don't disconnect the earth without safe isolation of all the supplies to the installation.
At least there may be a next time as you got away with it this time.



Andy

B

Safety label
 12 March 2017 12:20 PM
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sparkingchip

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Actually the correct identification of a earth conductor that is carrying significant current is green / yellow conductor insulation over sleeved with blue either end.

Andy B
 12 March 2017 02:08 PM
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mapj1

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I do agree that it is a bad idea to unscrew stuff with the power on, but actually in a well designed system, the earth system should not be carrying significant currents or have an open circuit voltage capable of arcing - if it is then there is also a risk that apparently innocent activities on other earthed objects like plumbing repairs, as an example also become hazardous.

Either some filtering components that are wired phase to earth within the inverter have failed, or as you suggest, there is an NE confusion when the should be very clearly separated.
Let us know what the explanation is when the chaps come to fix it please.

-------------------------
regards Mike
 12 March 2017 08:57 PM
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sparkingchip

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The gas fitter should have a Temporary continuity bond those sparks and gas don't mix.

Why is it people assume earthing is just decorative and those do not remove labels can be ignored?

Andy B
 12 March 2017 09:52 PM
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mapj1

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well, at the risk of appearing inflammatory, perhaps because it often is - a lot of domestic stuff is class II, and in circuits supplying that, the CPC could safely be omitted, although of course this could not meet the regs. Then there is largly redundant 'boiler cross bonding'
The earth system is most useful to make the ADS work during faults - which are not that likely to occur in any one short period if the green and yellow wire is un-shipped for a few minutes.
In some ways taking the earth off is a bit like not wearing a seat belt or a crash hat - if something should go wrong what happens next is much worse than with it present.
However, in the same way, if nothing goes wrong, then there is also no benefit from it. A lot of countries regs do indeed permit an earth to be left off if the supply is RCD protected, and in much of the developing world you will see omission of both CPC and RCD, and they still mange to have accident figures similar to ours, and probably a lot of motorbike riders without hats as well..
And I reiterate - if it has enough energy to throw big sparks then there is a fault.
As a related thought, I am not sure what happens if the temporary bond is accidentally fitted straddling an insulating break
of the kind described here


ENA Engineering Recommendation G12 Issue 4 Amendment 1 December 2015 Requirements for the Application of Protective Multiple Earthing to Low Voltage Networks contains clause 5.2.1 which states:

Provision of earth terminal requires that where a metallic gas service is provided to a consumer's premises with a PME earth terminal, an insulated insert should be fitted in the gas service

bridging one of those could be worse than not bothering, and arguably depending where it is in the line it could make main bonding redundant as well.

-------------------------
regards Mike
 12 March 2017 10:29 PM
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AJJewsbury

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bridging one of those could be worse than not bothering

Unless of course you're about to grip the pipe to one side of the insert with one hand to steady it as you use a spanner to undo a joint on the other side of the insert...
- Andy.
 13 March 2017 11:24 AM
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sparkingchip

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Some years ago a guy working for the water board died whilst working on a water main in a Cotswold village due to the current running through the pipework. He cut a section out and then put a hand on the now separated pipe ends.

I'm sure there has been other such instances.

Andy B
 13 March 2017 07:17 PM
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sparkingchip

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Originally posted by: mapj1

I do agree that it is a bad idea to unscrew stuff with the power on, but actually in a well designed system, the earth system should not be carrying significant currents or have an open circuit voltage capable of arcing - if it is then there is also a risk that apparently innocent activities on other earthed objects like plumbing repairs, as an example also become hazardous.



Either some filtering components that are wired phase to earth within the inverter have failed, or as you suggest, there is an NE confusion when the should be very clearly separated.

Let us know what the explanation is when the chaps come to fix it please.


What If it is some DC leakage that reportedly produces a muscle spasm stopping the person holding the end of the disconnected earth conductor from letting go upon receiving an electric shock of a couple of hundred milliamps or more?

Having experienced fault finding on the mains side of PV installations I definitely close down the systems and mains, as there are some very dodgy installations out there, like the one I found last year where the main isolator only disconnected the neutral.



Andy B.
 13 March 2017 07:33 PM
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AJJewsbury

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What If it is some DC leakage that reportedly produces a muscle spasm stopping the person holding the end of the disconnected earth conductor from letting go upon receiving an electric shock of a couple of hundred milliamps or more?

I thought it was the other way around - a.c. "grabs" - d.c. "throws" (a.c. causes muscles to alternately contract and release a small amount on each cycle, while d.c. produces a continuous contraction).
- Andy.
 13 March 2017 08:29 PM
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sparkingchip

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AC drops to zero twice in a cycle, so you get one hundred chances per second to let go, once every 10 milliseconds, whereas the DC does not return to zero until you disconnect it.

So let's assume there is AC and DC circulating through that main earth giving both reactions, a double whammy, what then?

Andy B.
 13 March 2017 08:35 PM
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mapj1

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I thought it was the other way around - a.c. "grabs" - d.c. "throws" (a.c. causes muscles to alternately contract and release a small amount on each cycle, while d.c. produces a continuous contraction).

Not really, and please certainly do not rely on it. All current, AC or DC, makes the muscles move but depending on entry and exit and exact path, some muscles will more than others.
For example in most folk the muscles that close your hand (they are actually in your arm) are stronger than the muscles that open it, so as well as damaging both muscle groups, a in a shock situation the grip normally wins over the release - giving potentially fatal 'no-let-go' if you are holding a live metal part like a handle for example.
This is indeed more pronounced on DC, but I can say from painful personal experience, you can suffer no-let-go from mains as well.
Actually using the back of the hand with fingers straight is a good way to feel your way out of a problem if there is a real risk of accidental live contact, as then there is not that problem as you fingers wont bend the wrong way. Also the back of the hand is often harder drier skin than the touchy feely finger tips.

Oddly anecdotally at least, more folk seem survive DC HV shocks than might be expected from the official curves, at least if the peak current is limited - it may appear that being thrown clear is 'better' than staying stuck and cooking slowly.

But there have been some truly horrible incidents over the years, so the only sensible advice has to be don't deliberately get in the way of the electrons.
not that that is recommended either.

-------------------------
regards Mike
 13 March 2017 08:58 PM
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alancapon

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In addition to Mike's comments, I believe the human heart is particularly sensitive to 50 to 60 Hz. The two most dangerous connections are actually the neutral and the earth, as nobody ever believes there could be a voltage on them.

Regards,

Alan.
 13 March 2017 10:11 PM
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sparkingchip

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Originally posted by: mapj1


Actually using the back of the hand with fingers straight is a good way to feel your way out of a problem if there is a real risk of accidental live contact, as then there is not that problem as you fingers wont bend the wrong way. Also the back of the hand is often harder drier skin than the touchy feely finger tips.



Many years ago when I was in the Scouts I did the training for the firemens badge at the local fire station and we were taught to use the back of our hands to guide ourselves along a wall in a smoke filled and/ or dark room in case there is a damaged electric cable on it.

Andy B.
 13 March 2017 10:17 PM
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sparkingchip

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Originally posted by: alancapon

In addition to Mike's comments, I believe the human heart is particularly sensitive to 50 to 60 Hz. The two most dangerous connections are actually the neutral and the earth, as nobody ever believes there could be a voltage on them.



Regards,



Alan.



The fact that most people do not consider the neutral and earth to be dangerous is what I have been getting at, even in the posts above there has been a bit of a it will be okay to disconnect the earth whilst the installation is loaded.

Andy B
 16 March 2017 10:41 AM
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pex

Posts: 40
Joined: 10 January 2008

Thank you bondage, Andy and others for confirming things and the L1 and L2 explanation.

The installer is coming tomorrow (Friday) to have a look so all will be revealed then.

I'm hoping the clamp meter that I've ordered arrives before then so I can see current flowing through the earth.

I'll update in due course

Pete
 16 March 2017 10:47 AM
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pex

Posts: 40
Joined: 10 January 2008

Originally posted by: AJJewsbury

As I took one of the earths off there was a big flash and it turned out to be the earth to my PV system


Well, let that be a lesson is safe isolation! It's unwise to work live on mains, even the protective conductors...



t seems that the Earth has been used as a neutral return and indeed it has a bit of red tape on it so it looks like it was intentional.



Presuming it's grid connected, running only in parallel with the grid supply, then there must never be a N-PE link and the inverter can only inject voltage between live conductors (otherwise you have a combined neutral/earth conductor in the consumer's installation and fall foul of the Law in the UK). The inverter should use the PE connection only for shock protection in the normal ADS manner, perhaps interferrence suppression and maybe for mains connection verification and other fault detection purposes (I think mine can detect faults to earth on the d.c. side too) - so as broadgage says they current flowing should be small, unless a fault was present at just the moment you disconnected it (back to point 1). Could tugging cables out of the earth block have disturbed any live conductors?



The installers of my PV system left tape on the cables too (brown I think) - but seemingly just to identify them as belonging to the PV system.



Is there no RCD on the PV a.c. side? Connecting to earth instead of N would surely have caused it to trip?



- Andy.
 16 March 2017 10:58 AM
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pex

Posts: 40
Joined: 10 January 2008

Andy: No there is no RCD on the solar circuit, just a 16A MCB.

I think there is a good reason for not having one too.

Its interesting what you say about the brown tape on the Earth - that does suggest that it is 'live' rather than identification.
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