Floating Neutral

[MikeM3]

I am a long time retired electrician, I have worked on many different
systems, I was High Voltage trained among other things.

I understand what floating means, but I don't understand a floating
neutral.

To amuse myself in my retirement, I decided to build a Solar System,
something that I knew nothing about, except that it is like black magic,
electricity from the sky!

I have 2 inverters, 12v X 3Kw and 12v X 4Kw, I have been advised that
I cannot parallel them, I have also found that they have floating neutrals,
the AC output of 230 volts, goes to a conventional UK socket outlet, but
without an earth, this causes the RCBOs on the lighting circuits to trip, I
tried earthing the unit, to the mains earth, but that didn't work either.

So, you know my next question, can I link this "Floating Neutral", to the
earth socket on the output?

Thank you for taking the time to read this, I hope that somebody with
more knowledge than I have, can advise me.

Mike.

 

[pc1966]

Strictly speaking you can't have a floating neutral as neutral is defined as the conductor that is earth-referenced. However, for many it is simply the conductor with the blue insulation!

It is not uncommon for small generators and inverters to be IT (isolated) supplies as it is simpler and largely avoids all of the issues of earth-referencing a supply where you don't have a direct means of earthing, you are relying on the CPC and it might not be guaranteed to be earth-referenced under fault conditions such as a loss of mains power.

 

[timhoward]

I suspect your current inverters are unfortunately going to be unsuitable for anything other than 'island' off grid installations.
It's crucial that inverters cannot back-feed a grid supply if the grid supply is lost, and grid-connected inverters either shut down or disconnect themselves from the grid in these circumstances to ensure things remain safe.
Other than safety there are bigger questions like how the waveform from your inverter would synchronise.

I think you need to start scouring ebay for inverters that were intended to be grid connected to advance your hobby.

 

[MikeM3]

Thank you both for your replies, why the neutral is called floating, I have no idea,
both the neutral and the positive go to the output sockets, I believe that I should
be able to connect the neutral to the earth without mishap, I only wanted some
advice from anybody who knows.

I think that the RCBOs are built to balance the neutral and earth, and since my
system doesn't have an earth, there is no balance, I could take a chance and just
try connecting to earth, but I don't want to ruin an inverter, they are quite dear to
me, as I am on a fixed income via my pension, I wouldn't want to buy a replacement.

I have no intention of connecting to the grid, the mains are off, before I connect
my version of solar power.

Thank you for taking time to read and reply.

Mike.

 

[newfutile]

Its AC after the inverter so there is no positive, (RCD) Residual Current Device and rcbo dont need an earth to work,these should never be connected to your consumer unit in any way.
As stated before they wont be in phase and there is a chance that in a power cut it will backfeed the grid ,this is a very dangerous situation . You would need a grid tie inverter which has protection to stop all the above problems.
4KW on 12 volts is a lot of power do you have many panels?

 

[timhoward]

Thank you both for your replies, why the neutral is called floating, I have no idea,

The best way I can put it, is that your inverter is ensuring there is a potential difference of 230v between the two lines. The potential difference between either of them and earth is not defined since the system is specifically designed that earth is not referenced for safety reasons. In theory neutral could be 600v to earth and live 830v to earth (silly example to illustrate the point)
Hence the term "floating". A bit confusing indeed.

 

[MikeM3]

Its AC after the inverter so there is no positive, (RCD) Residual Current Device and rcbo dont need an earth to work,these should never be connected to your consumer unit in any way.
As stated before they wont be in phase and there is a chance that in a power cut it will backfeed the grid ,this is a very dangerous situation . You would need a grid tie inverter which has protection to stop all the above problems.
4KW on 12 volts is a lot of power do you have many panels?

I just wrote a long reply, newfutile, but lost it because I was not logged in!

The panels that I have are supposed to be 1Kw each at 12 volts, they seem
to work fine, the problem that I have, is when I feed the flat, the lights trip,
because of the RCBO, in the circuit, I think that it needs a balance between
the Neutral and the Earth.

I tried connecting the circuit to the house earth and nothing happened, the
lights still tripped.

So I want to know, can I earth the floating neutral?

Thank you.

Mike.

 

[MikeM3]

The best way I can put it, is that your inverter is ensuring there is a potential difference of 230v between the two lines. The potential difference between either of them and earth is not defined since the system is specifically designed that earth is not referenced for safety reasons. In theory neutral could be 600v to earth and live 830v to earth (silly example to illustrate the point)
Hence the term "floating". A bit confusing indeed.

Another interesting suggestion Tim, if I cannot earth the floating neutral, I am
looking at Equalising/levelling transformers, that I believe is one way to get it
right, I also believe that transformers, will make paralleling possible.

Thank you for your time and reply.

Mike.

 

[littlespark]

I think what the question is, is can the cpc of the outgoing circuits be connected to the neutral being supplied from the inverter?

Not connected to grid in any way.

To “earth” the neutral suggests connecting to the grid supplied earth.

 

[brianmoooore]

(RCD) Residual Current Device and rcbo dont need an earth to work,

A RCD, or the RCD part of a RCBO cannot trip with a floating neutral on a single fault. Without a connection to 'earth' there's nowhere for the current that causes the imbalance detected by a RCD to go.

 

[MikeM3]

I worked as an electrician all of my working days, but I never came across this
issue before, I keep looking for some sense about it and get different ideas
from some posts on the net.

I know how to supply an earth to any circuit, I have done so many times, by
hammering a 3-foot copper plated steel rod into the ground, but this is a
different kind of neutral, it has been suggested that as it is an AC output, it
is not a neutral, but that it is a positive and is being switched rapidly to get
the AC effect, but it is not a modified sine wave machine, it is a pure sine
wave form, so as not to disrupt flat screen TVs and Monitors for computers,
so that could be reason for no earth.

A normal earth circuit has a resistance of around 9 ohms, I have thought to
add that kind of resistance to an outgoing earth connection, to prevent a
short circuit, or even put a lamp in the line, might work, I keep hearing that
a floating neutral cannot be earthed!

I might be able to do it, via a balancing transformer.

Thank you all for your contributions, they are gratefully received.

Even with the floating neutral, conundrum, I do have a working solar unit in
my one-bedroom flat on the ground floor, it is on wheels in order to take
to the sun shine, I think that I need to buy standard lamps if I can't get
the ceiling light to stay on.

Mike.

 

[Spanners]

I worked as an electrician all of my working days, but I never came across this
issue before, I keep looking for some sense about it and get different ideas
from some posts on the net.

I know how to supply an earth to any circuit, I have done so many times, by
hammering a 3-foot copper plated steel rod into the ground, but this is a
different kind of neutral, it has been suggested that as it is an AC output, it
is not a neutral, but that it is a positive and is being switched rapidly to get
the AC effect, but it is not a modified sine wave machine, it is a pure sine
wave form, so as not to disrupt flat screen TVs and Monitors for computers,
so that could be reason for no earth.

A normal earth circuit has a resistance of around 9 ohms, I have thought to
add that kind of resistance to an outgoing earth connection, to prevent a
short circuit, or even put a lamp in the line, might work, I keep hearing that
a floating neutral cannot be earthed!

I might be able to do it, via a balancing transformer.

Thank you all for your contributions, they are gratefully received.

Even with the floating neutral, conundrum, I do have a working solar unit in
my one-bedroom flat on the ground floor, it is on wheels in order to take
to the sun shine, I think that I need to buy standard lamps if I can't get
the ceiling light to stay on.

Mike.

Hi Mike, I am also a long time retired electrician but with extensive power generation experience.
You don't want to hear this, sorry, but you need to listen. What you are trying to do by connecting your invertor system to your consumer unit (or any interface with your flat wiring) is highly dangerous for you. Also, potentially your system could backfeed the grid under certain circumstances putting engineers at danger working on it.

Don't get tunnel visioned by the floating neutral problem. The whole subject is far deeper than you at first realise. In the UK the system you presently have (and I am guessing it is a basic invertor) is meant to be used as a self contained stand alone ( seperated from the flat wiring) system. No connection whatsoever to the consumer unit or incomming supply.

Solar systems designed to synchronise and connect with the grid require sophisticated control gear, regulation, software, compliance and final testing.

 

[MikeM3]

Thank you, Spanners, for your explanation, first let me clarify, that I have
no intention of connecting to the grid, I am well aware of the risks, the
few times that I did try, the consumer unit was isolated, twice, once out
in a cupboard beside the meter and also the unit itself, this exercise has
always been a R&D project.

I started with the wrong kind of solar cells, twice, I now have 12v X 1KW
panels, 6 of them, 4 X 12v 130ah batteries and a 4Kw inverter. it all works
very well and the neutral problem is my own research to see if it can be
overcome.

Now the only person at risk is myself and I hope that I am careful enough,
to be not at risk.

Mike.

 

[Spanners]

Thank you, Spanners, for your explanation, first let me clarify, that I have
no intention of connecting to the grid, I am well aware of the risks, the
few times that I did try, the consumer unit was isolated, twice, once out
in a cupboard beside the meter and also the unit itself, this exercise has
always been a R&D project.

I started with the wrong kind of solar cells, twice, I now have 12v X 1KW
panels, 6 of them, 4 X 12v 130ah batteries and a 4Kw inverter. it all works
very well and the neutral problem is my own research to see if it can be
overcome.

Now the only person at risk is myself and I hope that I am careful enough,
to be not at risk.

Mike.

Hi Mike,
Ok, I'm a bit confused. Lets take this in stages. Is your ultimate plan to manually turn off the grid every now and then and use the same Flat consumer unit, but fed then from the invertor? How / where are you physically terminating the invertor output at the Flat consumer unit for the test? When you say it all works very well. What works well? Do you mean the seperate mobile system you can move around works well? How do you presently utilise the 240VAC from your moveable system ? You have two systems right?

Regards Kim

 

[MikeM3]

Hi Kim, I back fed the system, via an unused cooker breaker
in the kitchen, I have found, but not yet purchased a special
change over breaker, where the Grid is on the top at one
side, the solar is diagonally across at the bottom and the
consumer unit is on one of the other empty places, I don't
remember which, I need more information, to see that it is
robust enough, to be safe, similar to the one below, which
is 125 amps.

Mike.