how determine load wire on AC line

danieru · May 19, 2012

Hi,

Just bought a house (in Sweden, so 240V) and am trying to figure out what's going on with the wiring, as it seems a little strange.

I have typically have two black wires coming out of my wall, and one green(earth). I would assume that one black wire is load, the other black is neutral, and green is of course earth, so using my multimeter, I've tested the three possible circuits from those three wires with the following results:

black 1 + black 2 = 411V ??
black 1 + green = 237V
black 2 + green = 236V

...I am puzzled by the reading between black1 and black2...I'm not an electrician but assuming everything was wired properly, I would expect to see (assuming black 2 was neutral):

black 1 + black 2 ~= 237V
black 1 + green ~= 237V
black 2 + green ~= 0V

Someone please correct me if I'm wrong! My only guess is that the load and neutral wires are mixed up thus I'm getting load on both? Still don't know what to make of the 411V though... Any comments/insight is welcome!

telectrix · May 19, 2012

if this was in UK i would assume that you have 2 phases of a 3 phase supply.

danieru · May 19, 2012

ah, I think you're right! The wires coming out of the wall, used to be attached to an electric radiator I think. On another part of the same wall is a radiator, so I'm assuming it's on the same circuit as the wires I've tested. I looked up the installation manual for the radiator, and it can be wired for either 240V (L1, N and earth) or 400V (L1, L2 and earth). On the radiator, it has black 1 to L1, and black 2 to L2.

For 400V, the radiator instructions show connecting L1, L2 and earth, so when using 2 of 3 phase, a neutral return wire isn't needed?

Knobhead · May 19, 2012

Yes you have 2 phases out of 3.
411 / √3 = 237V
The neutral will be connected to your earth wire (green) at the transformer which is why you get a reading to it. This wire is for protection only.
For your heater as you say no neutral is needed.

telectrix · May 19, 2012

but how is he going to get 240V supply without a neutral?

Knobhead · May 19, 2012

He has heaters that are dual voltage 400/230V so on the 400V connection no neutral is required.
For appliances then obviously one phase and the neutral is required at the outlet.
Earth might help as well :yes:

danieru · May 19, 2012

Thanks both of you for the replies, it's been helpful for me!

@Tony: So if I understand you correctly, you think that the green earth wire is connected to neutral at the transformer, effectively making it the neutral, and not really earth at all? Is this standard practice/ good or bad idea when wiring for 2 or 3 phase?

Would it be better to still have a separate ground wire when running 2 or 3 phase?

Is it safe then to create an outlet for normal 1 phase appliances by taking one of the load lines for load, then the green earth wire for neutral? (I suppose the outlet would have no ground in that case...there are already several of those in the house)

EDIT: One more quick question. If I wanted to install another radiator of the same type, is there any difference between L1 and L2 when wiring? In other words, I've got two black wires, each with load, does it matter in what order I connect the two loads to the radiator?

Engineer54 · May 19, 2012

NO .... It will be a normal safety earth conductor, and should never be used as a return conductor!! If your never going to use this point again as a heater connection point, then get a local electrician in to convert the present wiring to 230V, so that it can be utilised as a standard socket outlet!! Please don't try and do this yourself, be safe rather than sorry!! lol!!

danieru · May 19, 2012

Sure, I wouldn't go ahead with trying to wire it myself unless I'm completely sure I know what I'm doing, but I'd still like to understand how it's wired.

Tony had said that the green earth wire was probably connected to neutral back at the transformer, thus I'm reading 237V between one of the phase lines and the green, but he said that it's just for protection (path for the current in case of a short, right?) But now Engineer54 is saying that the green earth wire is a normal safety earth conductor...so is the wire connected to both?

Richard Burns · May 19, 2012

Within your house the neutral and earth must be separate wires, outside your house (back at the suppliers transformer) they are joined.

To supply the radiator you need Line1, Line2 and earth.

To supply sockets and other single phase appliances you need a single Line, Neutral and earth (though the earth may be omitted in some unsafe cases)

This diagram shows a depiction of how the wiring is arranged, this is just to show the principle not an actual layout.

[ElectriciansForums.net] how determine load wire on AC line

danieru · May 19, 2012

Thanks to everybody for your comments, and especially to Richard Burns, your explanation along with the layout is very clear, I have no questions now

Big thanks!

Knobhead · May 20, 2012

Tony said:
Yes you have 2 phases out of 3.
411 / √3 = 237V
The neutral will be connected to your earth wire (green) at the transformer which is why you get a reading to it. This wire is for protection only.
For your heater as you say no neutral is needed.

To emphasis what I said the earth is a protective conductor. NOT to be used for carrying load, that is the job of the neutral.

danieru · May 21, 2012

@Tony: You were clear in your earlier comment, I just needed to look more closely at it. Thanks for taking a moment to emphasis this to make sure I got it

What I'm still curious about is why, as you say, the neutral is connected to the earth wire at the transformer? For protection's sake, wouldn't the earth wire serve it's purpose if it's simply connected to earth as usual and kept out of the load circuit? Is this standard practice when wiring for 2 or 3 phase power?

Knobhead · May 21, 2012

This may help Earthing system - Wikipedia, the free encyclopedia

Inteificio · May 23, 2012

danieru said:
@Tony: You were clear in your earlier comment, I just needed to look more closely at it. Thanks for taking a moment to emphasis this to make sure I got it

What I'm still curious about is why, as you say, the neutral is connected to the earth wire at the transformer? For protection's sake, wouldn't the earth wire serve it's purpose if it's simply connected to earth as usual and kept out of the load circuit? Is this standard practice when wiring for 2 or 3 phase power?

Your question was a bit unclear, so sorry if I answer what you have not asked =-)

On a 3 phase system (all normal trickery is generated 3 phase) if there is even load on the 3 phases the net neutral current will be zero.
So at the transformer, if the system is designed in balance there is no need for an expensive copper run back to the power station.
However as the balance is slightly off in practice they need to 'dump' the extra electricity, so the send it to ground.

So it is more that the N is earthed at the transformer, not the other way around.

So in your house (when everything is working well) you will have the same amount of trickery coming out of the N as comes in the L, and nothing going down the earth.
At the transformer there will be lots coming in and out of the lives, and a small amount in the N which gets dumped to ground.
Although there is a small amount of trickery in the N it will not go back up the E as the path of least resistance is too ground.

If you have an RCD in your house that 'measures' the amount of trickery coming in the L and subtracts the stuff going out of the N, if there is a net then it cuts the supply (as it means some it going out of earth).
If you had no earth, there would not be an imbalance and RCD would not work until an earth was introduced (eg you touching the faulty item and acting as an earth).

This is embarrassingly simplified, but I hope it gives a better understanding.

how determine load wire on AC line

danieru

telectrix

-

danieru

Knobhead

telectrix

-

Knobhead

danieru

Engineer54

danieru

Richard Burns

-

danieru

Knobhead

danieru

Knobhead

Inteificio

News and Offers from Sponsors

Similar threads

OFFICIAL SPONSORS

YOUR Unread Posts