understanding star grounding

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#321
Actually, in a three phase power distribution system, the out of balance current load is carried by the neutral. This includes any harmonics present too.

This is why when you install a three-phase system, if it is properly balanced then you can safely touch any neutral-bars (in the distribution board) without getting a shock.

Earth is sort of a 'back-up' if the neutral fails, and also carries fault current back to the 'star-point' of the supply companies' generating set in the event of a fault appearing.

I have drawn a good diagram on paint but I have to re-size it as it is too big to go on here. It shows all of this quite well.

Cheers
Gareth
 
#324
gareth said:
Earth is sort of a 'back-up' if the neutral fails
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Appaling.

infinia said:
That's why Y is preferred over delta.
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Where Neutral is required as return path, because loads are not distributed equally across the phases, star connection is mandatory. Other than that there is no such preference.

infinia said:
I think ALL newer (<40yrs) systems use Y 3 phase.
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Not all of them.

infinia said:
That way earth is only needed to drain off excess charge.
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Excess charge?
 
#326
AndrewT said:
is there (always) a Neutral in a three phase distribution system?
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No is the answer to that one.
The "neutral" allows for connection of "low" voltage single phase loads. In the UK the "wire" in the road is normally 3 phase with an equal distribution of households on each phase. All the return currents are summed in the neutral. Things get interesting when that gets cut accidently. How the neutral is derived can vary country to country.
For higher voltage 3 phase supplies (industry ?) there is no need for a neutral as loads connect across phases. Motors etc are "3 phase devices".
It's all done in the name of efficiency both generation, and distribution.
 
#328
infinia said:
what would happen if the earth connections where cut (full floating)
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Depends. Usually you have two connections to Earth. One through the supply line, the other through earthing rods or tapes in or near the building. Usually there are jumpers between PE and N in the distribution boxes and in neighboring houses as well. Therefore it is quite improbable that a full floating situation ocurrs. But let us assume just for the fun of it, there was such a situation.

There are different types of supply lines. Such with a common PEN wire and such with separate wires for PE and N. In a system with separate wires, the N wire would take over the function of the PE. So as long as you don't cut that, everything keeps working, but you may get slight shocks from all appliances that are connected to PE due to the voltage drop across the N wire. How about a system with a PEN wire?

If all loads were tri-phasic or if all loads were distributed symmetrically across the phases, everything would keep on working.
If all loads were mono-phasic and all connected to the same phase, they would stop working.
In real life, where loads are distributed randomly across the phases, an unbalance would ocurr, because the return currents would have to flow through the other phases. The voltages from each phase to the star-point would depend on the load per phase and be different from their nominal value. Some devices would work on, others not, most would perform badly.

The star-point would be offset from 0 V and through the jumper in the distribution box the floating PE would be lifted off 0 V too. Someone, who touches a device that is connected to that floating PE, would not notice much, as opposed to the situation with separate PE and N wires. He is grounded to the floor, but the floor is not referenced to that floating PE. Very much like the supply line workers from the above post. Or there is a reference that goes through the floor from the building to the transformer station, which means a high earthing resistance and consequently the entire voltage drop across that and next to nothing across the person. As long as you don't touch two phases or one phase and the case at the same time, you are safe.
 
#329
Here is the diagram. It is very basic, but aims to clarify any problems.
 

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#330
If you called the center-tap ground instead of Earth, I would agree. Calling it Earth implies a connection to the Earth at the supplier's star-point, which should not exist in Class II equipment.

Although...
If you have a tuner or receiver and connect the antenna, satellite dish or network cable to it, you make a connection from the equipment's ground to the building's PE through the cable shield. You never really get the floating ground you seem to desire so much.
 
#331
pacificblue said:
If you called the center-tap ground instead of Earth, I would agree. Calling it Earth implies a connection to the Earth at the supplier's star-point, which should not exist in Class II equipment.
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Hi,
this has me confused.
The Class11 equipment is inside the consumers premises.

The three phase generator or transformer is outside.

We are not referring to the consumers distribution board (which is inside the premises) in that diagram.
 
#333
Thanks for all the useful info!

Thanks everyone for all the usefull info contained in this forum and in particular this thread. The info contained within enabled me to troubleshoot my latest project.

I added a Chipamp.com LM3886 dual mono kit to my buffered pre-amp (based on a Welbourne labs buffered linestage) and ended up with some serious hum. This was my first endeavor in a diy power amp so the discourse on star grounding the power source was key to eliminating the hum. As well, I very much appreciate the focus on safety. I work for an electric utility where our employees are exposed to electrical dangers daily, electrical safety is important to me, as it should be for all diy'ers.

Thanks again.
 
#334
Let's say I was building a headphone amp with 1/8" stereo TRS jacks. I can get isolated jacks, but since I need to ground the metal case anyway, which jack should I ground, if any - the input or output jack?

I may add 1/4" TRS jacks but they will be isolated and grounded to the board.
 
#340
454Casull said:
Let's say I was building a headphone amp with 1/8" stereo TRS jacks. I can get isolated jacks, but since I need to ground the metal case anyway, which jack should I ground, if any - the input or output jack?

I may add 1/4" TRS jacks but they will be isolated and grounded to the board.
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if there are only one ground point to the chassis.then i would like grounding the input,for perfect noise level.

for example,look at mine lm1036 tone preamp.the input RCA is non-isolated to the chassis.
http://www.diyaudio.com/forums/showpost.php?p=1400673&postcount=2

--
Zang
 
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