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Difference between HT and LT lines

by i.mehrzad
Tags: power distribution
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i.mehrzad
#1
Jun8-09, 01:22 AM
P: 83
I am doing a power distribution project, and i have come across power distribution panels, some of which are High Tension and some of which are Low Tension.

I have assumed that the power lines that carry a lot of current are the ones that contribute to high tension and the compliment being low tension. Is this a safe assumption, cause i don't find a definition for the two.
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vk6kro
#2
Jun8-09, 02:01 AM
Sci Advisor
P: 4,032
No. High tension and high voltage mean the same thing.
So it is the voltage that is high, not necessarily the current. Probably not the current.

For the same power, high voltage means less current than for a lower voltage.
i.mehrzad
#3
Jun8-09, 04:18 AM
P: 83
Thank you,

I got this impression because all the distribution panels which were towards the supply end read HT on it and those that were read towards the ground end read LT on it.

I also have another doubt,
Wires that come from the supplier are normally four in number(3 phase + 1 neutral). But the output plugs that we have in our household are three point plugs, so what is the relation between the 4 core wires that we have and the three point plugs??

vk6kro
#4
Jun8-09, 04:50 AM
Sci Advisor
P: 4,032
Difference between HT and LT lines

Sounds like you are outside the USA.

Australia uses a system much like Europe's. Maybe it is the same.

There are high voltage (330 KV and lower ) lines that eventually feed street transformers.

The output of those is 3 phases that are all 230 volts relative to a neutral point but 120 degrees out of phase with each other. Many houses have just one phase but, by paying extra, you can have all 3 phases brought into the house.

This would be done if you had an electric oven or hot water system that needed 2 or 3 phases.

So, we have 3 phases and some of the wall power outlets are on different phases to others.

The voltage between phases is about 400 volts. This is available at the power meter for running 400 volt equipment or specialist motors like lathe motors that need 3 phase.

The outlets on the wall have one phase, which is 230 volts 50 Hz, one neutral and one earth. So 3 pins.
i.mehrzad
#5
Jun8-09, 05:05 AM
P: 83
Yes i stay in India.
So it is the live wire the one with 230 volts which is essential for the working of your device.
Then i am slightly confused about the neutral and the earth wire. Which of the two acts as the opposite end of the live wire.

Meaning to say that, current that is generated from the power plant comes in the live wire, and the neutral wire also comes from the power plant (the center point of the three phase star connection). So these two wires are essential for the entire circuit. Because it completes the entire circuit. So all the electricity that enters the device should end up into the neutral wire. Now what about the earth wire?? Where does that come into play???

And moreso i have read about the power distribution grid and there they mention about the power plant producing electricity at a voltage and the earth acting as the other end of the line, therefore there is a potential difference which is maintained causing current to flow. Why can't the center point of the star connection be used as the so called Ground.

I am sorry but my concepts are all out of place, and i wish to straighten them. I would appreciate any help.
vk6kro
#6
Jun8-09, 06:43 AM
Sci Advisor
P: 4,032
OK. It sounds like India has the same system we do.

The essential lines to make power flow are the active and neutral line in the house.
The "earth" wire is a safety connection which is connected to any metal parts of appliances, so that touching them should always be safe, even if the appliance develops a short circuit. It is always connected to some metal buried in the ground.

When power is distributed from the street transformer, each phase can return power through the neutral, but since these are out of phase with each other, if the loads are equal, the currents cancel each other out.
The loads will not normally be exactly equal so the neutral can have a small voltage on it relative to earth.

The neutral is normally earthed at each house in Australia. We have sandy soil and this helps keep the earth voltage low.

I don't know if the neutral in the house goes all the way back to the power station. That is part of a high voltage system which is stepped down to safer voltages for household distribution.
i.mehrzad
#7
Jun8-09, 12:02 PM
P: 83
If the neutral wire that we use in electrical circuits goes directly to the ground, then what is the need of pulling the fourth wire millions of miles, when we anyway have wires which are going to the ground which is also neutral??

One more clarification, is it that both the neutral and the ground wires end up in the ground??
vk6kro
#8
Jun8-09, 07:06 PM
Sci Advisor
P: 4,032
The ground wire does end up in the ground, but the neutral wire can optionally be connected to ground.

The neutral wire would not be necessary if we could make a perfect connection to ground everywhere.
There used to be systems where there was only an active wire and the return was through the ground, but it was very difficult to make a good enough connection to ground.
i.mehrzad
#9
Jun9-09, 12:44 AM
P: 83
Well then is it safe for me to assume that the fourth neutral line that is drawn is just so that there is a safe ground, which i mean as an actual zero volt, (this being the center of the wye connection).
vk6kro
#10
Jun9-09, 02:42 AM
Sci Advisor
P: 4,032
No, the neutral line is the return path for the power coming from each phase.
That is its main purpose.
Some houses might only have single phase and the neutral is their return path.

Grounding it is another issue.
It is done to make a more effective earth connection for each house by putting the earth systems of all houses in parallel.
i.mehrzad
#11
Jun9-09, 03:45 AM
P: 83
Thank you,
This is what i have understood. There is a star connection in the secondary terminal of the transformer. This results in four lines, three for the phases and one for the neutral. The neutral line is grounded at the transformer end, and whatever current that returns in the circuit goes to the neutral wire and finally into the earth.

Simultaneously there is a earth point which is separate and nowhere related to the four lines aforesaid. This earth connection is just a safety measure for leakage current, which may land up in the metallic body of the equipment.

So most of the three point plugs in our household are single phase. Therefore the three point plug should have the following connections.The live wire which is one of the phases of three phase transformer. The neutral is basically the fourth wire which is from center point of the star connection of the transformer, and the third which is the earth is what is separately connected, and which lands in the earth pit somewhere near our household. Is the brief description of the power grid connection correct.

In the case of a two phase connection the neutral wire is replaced by another phase wire which results in a potential difference
vk6kro
#12
Jun9-09, 04:21 AM
Sci Advisor
P: 4,032
Mostly OK

whatever current that returns in the circuit goes to the neutral wire and finally into the earth.

This is not right. Any current flowing in the earth is not normal. Earth current is purely for getting rid of any dangerous voltages due to fault conditions.

The normal supply of power to the house does not depend on the earth at all, except to hold the towers up.
i.mehrzad
#13
Jun9-09, 05:28 AM
P: 83
Well i think i will need to correct you here. If all the branches in the wye carry the same load then the neutral wire should not have any current in it. But suppose if the load distribution in all the three branches is not the same then the load difference will result in earth flowing current.

Please correct me if i am wrong, i am still slightly confused.
vk6kro
#14
Jun9-09, 07:26 AM
Sci Advisor
P: 4,032
No.

If the load current is not the same for all 3 phases, there will be current flowing in the neutral.
The Earth connection has nothing to do with it.
i.mehrzad
#15
Jun9-09, 10:58 AM
P: 83
Thanks i got it finally. I appreciate your effort to have single handedly explained to me something that i have been trying to figure out for some time now.

Thank you vk6kro once more.


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