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Author Topic: Joining mains neutral to inverter/charger output neutral  (Read 3770 times)
Clive F
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« on: May 01, 2016, 10:30:59 AM »

If I have 4 core wire (including Earth), and I want both mains and the inverter output at the other end of it, can I combine the mains neutral with the inverter's neutral, such that I end up with:

1) Earth
2) Mains neutral+Inverter neutral
3) Mains live
4) Inverter live

Or, is this going to upset a typical inverter/charger?

Presumably, earth is supposedly connected to neutral in any case (at the building entry for mains, and at the inverter for that)?

Or perhaps the inverter output is supposed to remain floating/unearthed and for use with double-insulated items only?

Perhaps I've opened a can of worms...
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jonesy
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« Reply #1 on: May 03, 2016, 10:46:03 PM »

If you want to use both supplies at the same time, you cannot share a neutral.  It's a bit like running 2 radial circuits from your dis board with 2 lives and 1 neutral. The neutral would be over loaded at some point. Not normal installation practice.
I can't think of any domestic use where the inverter neutral was not solidly earthed. RCDs will not work correctly either if the earthing is absent.
SMA have a few documents on the subject of safe inverter use in stand alone grid, including MCB selection, which is really quite difficult if you don't want to lose the entire board on a single fault.
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Nickel2
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« Reply #2 on: May 03, 2016, 10:56:02 PM »

Not a good idea.
 If there was ever a fault on one of the circuits, you may end up with the neutral of the other circuit floating at mains potential.  sh*tfan Shared return lines are definitely high on the list of 'things you should never do'. I suspect you are thinking of having a switch at the 13A wall outlet between grid/inverter... facepalm
Run another cable, keep your installation/family safe!
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1.140kW mono south-facing at 49*
EpEver 4210A at 24v
24V 400 Ah battery. (4x200Ah FLA)
EpEver STI1000-24-230 pure sine inverter
Of course it'll work. (It hasn't caught fire yet).
Clive F
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« Reply #3 on: May 05, 2016, 08:50:39 AM »

Thanks very much for the advice.

So, yup, I accept that I need 5 core (even if only one supply is used at a time).
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knighty
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« Reply #4 on: May 05, 2016, 12:04:15 PM »

what size cores is the 4 core and how much power do you want to run down it ?

I don't see the problem at all as long as you fuse both the live cores at half the amps the neutral is rated for
(so you can't overload the negative - or just put a fuse on the negative too?)


plenty of power suppliers/inverters/ups join their negative  and the mains negative together

pretty much every transformer joins them, and plenty of them run multiple voltages down different cores of the same sable
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paul149
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« Reply #5 on: May 05, 2016, 12:30:50 PM »

Is it a true 4 core or a 3 core + Earth?, I say this because if it was a SWA cable then all cores will be the same csa, but if it is a pvc/pvc cable then the (bare) Earth conductor is not normally the same size csa as the phase conductors.
So if it is a 4 core SWA and if you could run seperate Earth conductors then you could potentially have two circuits within the SWA. But if it is a pvc/pvc then the bare subsized Earth can't be used as a conductor.

Always a good idea when doing any refurbishment to install cables for "future use" a tip I follow when 's permit but with the price of cable it is then it has to be considered if it is better to install ducting of some kind to enable the exact cable to be installed when required with minimal works at that later date.

Paul m.
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3.5 kWp (14 x Sanyo H250E01 + SB3000TL) 225' (SW) at 35' Pitch Lat 51.30' (Bristol) Installed cost 2.63/Wp (Nov 2011) now with solarimmersion and 250L unvented dhw (Almost Zero gas input for DHW from Apr to mid Sep)
Clive F
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« Reply #6 on: May 05, 2016, 02:31:48 PM »

The 3 phase cable (fairly old) has four insulated wires of the same size. Looks slightly thicker than 13A single phase cores. I wouldn't want to run more than 13A from it anyway.

The cable feeds sockets once used for 3 phase radiators, and appears a professional install. Trouble is, later sockets were wired up with 5 core, but earlier sockets with 4 core. So, I'm wondering if I can still use the 4 core ones.

NB It is intended to select between two supplies at point of use - never for simultaneous use of both supplies.

Interested to hear knighty's point about a UPS or inverter/charger's neutral being joined with the mains neutral. This is what I was wondering, but then I wouldn't be surprised if in some cases it may cause problems.
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Clive F
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« Reply #7 on: May 05, 2016, 03:06:14 PM »

Thinking further about it, although it could work, I conclude that it's not really safe enough.

The inverter neutral needs to be connected to earth, and if that neutral is then connected to mains neutral, then the mains neutral is consequently reconnected to earth, which is not good practice.

There is also the possibility of the inverter/charger getting confused (detecting a fault) if, upon mains failure, it finds some proportion of its output appearing at its mains input.

I think I'll just have to limit myself to the 5 core cable, i.e.
1) Earth (mains earth + new earth at inverter bonded to inverter neutral)
2) Mains neutral
3) Mains live
4) Inverter neutral
5) Inverter live

But then given the mains neutral is ultimately connected to mains earth, this still effectively means there's a connection between the inverter's neutral output and the inverter's mains neutral input.

Which then raises the question as to whether an inverter's neutral should be earthed at all, and if it should be, whether it can be joined with the mains earth? If it can't be joined, then you have a weird situation of having to have a switched earth...   facepalm
« Last Edit: May 05, 2016, 03:09:39 PM by Clive F » Logged
Nickel2
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« Reply #8 on: May 05, 2016, 03:52:51 PM »

If I was spending your money  Grin  I'd put a single solar socket in each room, run back to the solar dist board/CU; a roll of 2.5 T&E  is not desperately expensive.
If your house is old enough to warrant a rewire, the chances are that sharing neutral conductors between supplies would cause a current imbalance and you'd spend a lot of time resetting trips. (RCD/RCBO/ELCB etc).
(If you were ever ill, someone else would have to deal with the electrickery. If they weren't aware of dual-sourcing, they could get seriously smoked)
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1.140kW mono south-facing at 49*
EpEver 4210A at 24v
24V 400 Ah battery. (4x200Ah FLA)
EpEver STI1000-24-230 pure sine inverter
Of course it'll work. (It hasn't caught fire yet).
Clive F
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« Reply #9 on: May 05, 2016, 04:05:21 PM »

Nickel2, I think I'll turn the 4 core sockets into inverter dedicated sockets, and the 5 core into dual/switchable sockets. I suspect they'd still be safer than 3 phase sockets.

I now find I need to check the manual of my inverter/charger, as it seems in some cases they don't like either output connected to earth (with the mains input earth connected to chassis and battery -ve).
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Clive F
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« Reply #10 on: May 05, 2016, 05:04:19 PM »

I've checked the manual of my 6kW inverter/charger and:

1) it has a chassis connector for bonding to earth - with a diagram indicating this is connected to battery -ve (though not whether this is performed internally, or recommended externally).
2) It has 3 mains input connectors (including an earth terminal)
3) It has 3 inverter output connectors (including an earth terminal)
4) It says (including poor spelling & punctuation):
Quote
When the PS LW is in the inverter mode
the "through the box" earth line
is automatically disconnected from the
input earth and joined to the output
neutral, complying with the
natural earth inverter requirements
if you what to maintain a trough earth simply
connect the input earth to the output earth

This is not really very clear.

However, I deduce that upon inverter mode, the output earth is simply connected to output neutral (and is disconnected from the chassis earth), so the output earth is not earthed at all.

However, as indicated in 4, it seems it is permissible to connect the output earth (and thus output neutral) to mains earth (input earth).

And I deduce that it is permissible because there is probably a high resistance between mains earth and local/chassis earth (high enough not to upset the inverter). In which case, it may well cause a problem were the chassis earth connected to mains earth.

But then that seems to imply that there is a voltage between the locally earthed chassis and the earth of any mains operated appliance in the vicinity, perhaps around 120V AC?  Lips Sealed
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Nickel2
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« Reply #11 on: May 05, 2016, 06:53:11 PM »

If you know what you are doing and are comfortable with what you are doing...

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1.140kW mono south-facing at 49*
EpEver 4210A at 24v
24V 400 Ah battery. (4x200Ah FLA)
EpEver STI1000-24-230 pure sine inverter
Of course it'll work. (It hasn't caught fire yet).
Clive F
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« Reply #12 on: May 05, 2016, 08:21:53 PM »

If you know what you are doing and are comfortable with what you are doing...

I know enough to know that a little knowledge can be a dangerous thing...

but I think I'm still within my comfort zone.
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jonesy
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« Reply #13 on: May 05, 2016, 09:43:41 PM »

Without exception
1. You must earth the inverter neutral
2. You must not switch the earth. You have no way of knowing if the switch fails.
3. Follow the wiring regs, not the inverter manual. This following statement is intended to be factual, not emotive. Whilst the regs are not law, it's what the judge will use if you are unfortunate enough to injure someone. Chartered electrical engineers will only very rarely and very reluctantly stray from the regs. And you can bet they will have had any digression peer reviewed, and will have a fat folder of calcs.
Even if your installation is PME, you must not add further connections from grid neutral to earth. Grid neutral moves around during fault conations,  and you don't want those fault currents coming your way.
It would be unusual, but not impossible, to find an inverter that connects the battery to earth. The chassis will be where the mains filters are wired to. A floating chassis ie one where you haven't bonded it to earth, will often sit at 120VAC, at a lowish current. It feels feathery.
Wise words from Nickel.
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billi
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« Reply #14 on: May 06, 2016, 07:29:36 AM »

Hi ,

What s the make of the 6 kw inverter/ charger ?  And why not connect the mains power on the AC-in side of the inverter ?


Billi
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1.6 kw and 2.4 kw   PV array  , Outback MX 60 and FM80 charge controller  ,24 volt 1600 AH Battery ,6 Kw Victron inverter charger, 1.1 kw high head hydro turbine as a back up generator , 5 kw woodburner, 36 solar tubes with 360 l water tank, 1.6 kw  windturbine
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