Volts Between N And Earth

[james.wilson]

lol

rulland i dont think anyone is on an ego trip here i really dont.

Regarding my posts i am talking theory as i have never worked on anything above 1kv.

I knew someone would know, and i knew i didnt.

Direct

So then if in theory the star point drifts (assuming it isnt tied to earth) with an imbalanced load, you would end up with a pd on the star point (neutral that i accept is now floating as it inst tied)

WHat happens if the star point is tied to earth and there is a phase imbalace, does the current flow to earth to from the phase with less load, thus balancing the phases? Or am i way off?

[DirectFS]

So whatever happened to the earth fault loop impedance path... ?

I'm not saying a fuse blows only because of earth faults... this was just the example....

Ouch - you want me to explain what earth loop impedance is?

No, I appreciate that you might not have been saying a fuse blows *only* because of earth faults - just trying to clarify that an earth fault isn't really the best way of desciribing why a fuse might melt.

It is, essentially, an overcurrent protection device, designed to protect the supplier, rather than the installation, or the dummy operating the installation...... whatever the reason for the overcurrent. No?

Bill.

[luggsey]

lol

rulland i dont think anyone is on an ego trip here i really dont.

Regarding my posts i am talking theory as i have never worked on anything above 1kv.

I knew someone would know, and i knew i didnt.

Direct

So then if in theory the star point drifts (assuming it isnt tied to earth) with an imbalanced load, you would end up with a pd on the star point (neutral that i accept is now floating as it inst tied)

WHat happens if the star point is tied to earth and there is a phase imbalace, does the current flow to earth to from the phase with less load, thus balancing the phases? Or am i way off?

Hey everyone's joining in now, good, we may get to the bottom of this interesting discussion and actually learn something from each other!

For the record, my ego is about as big as my wage packet.....nuff said! (I was going to compare it to a part of my anatomy but decided not to!) I don't think there has been any argument here, just honest discussion which is lacking a lot of the time on other threads so don't knock it!

I think the reason transformers star points are earthed is to ensure that no current on PME systems should return through customers earthed metalwork, otherwise all the bonding in a house would do was act as a neutral if it's potential was lower then the transformers star point right?

So the star point is earthed for safety?

I'm still thinking of the 'drifting' starpoint in James example that is not earthed, I think the phase imbalance/out of phase current and calculations are way to deep for any of us to comment on here [maybe?] but it's not a 'real world' example for us really as it's not used here as far as I know? However, it could well be that the op's supplier has a faulty earth to their transformer locally to him which may be the cause of the problem to start with so maybe James your example could be relevant after all!

I still suggest, as I did to start with the op measures neutral to earth with all other circuits isolated 'and' with all circuits isolated at the incomer. The external EFLI (Ze) can also be used to see what's going on can't it?

I hope this thread continues with some more input from the op.

[Cubit]

Whats the possible fault when you get a voltage between neutral and earth trying to sort a job where im getting about 15 volts.

Unbalanced load?

Dodgy connections on Neutral, undersized core on neutral run (somewhere in system)

[DirectFS]

I think the reason transformers star points are earthed is to ensure that no current on PME systems should return through customers earthed metalwork, otherwise all the bonding in a house would do was act as a neutral if it's potential was lower then the transformers star point right?

So the star point is earthed for safety?

Essentially, the star tap is put in place to ensure that no phase can exceed significantly the voltage or current of any other. Don't forget each phase is connected to the star tap too. It has a fringe benefit that it holds all three phases to zero potential when you plug it into the ground. If not grounded, it will still maintain a reference potential in respect of phase, which is calculated by the equations James pointed out - for most purposes, that would be taken as zero potential, though not zero volts.

Someone figured out that the only way to ensure a zero potential that wasn't actually up around 50 or 60 Volts, was to nail the star tap to the floor. It then means that each phase will maintain a constant frequency relative to mother earth. The trick then, is to make sure it is the right frequency - and that's the other part of where James' equations come in.

PME is a relatively new concept in electrical distribution - and is essentially a TN-C supply system, where the neutral and earth are supplied along one cable (or core, if undergorund as most incoming supply cables in the UK are concentric). The idea of PME is that neutral is bonded to earth at several points along the supply cable - normally at the star tap, then at every OH pylon, or at every substation. Finally, the neutral and earth are split again at the incoming point to the premises.

The idea is that there will be no variance at all between the potential on neutral and that on earth. It works pretty well.

I'm still thinking of the 'drifting' starpoint in James example that is not earthed, I think the phase imbalance/out of phase current and calculations are way to deep for any of us to comment on here [maybe?] but it's not a 'real world' example for us really as it's not used here as far as I know? However, it could well be that the op's supplier has a faulty earth to their transformer locally to him which may be the cause of the problem to start with so maybe James your example could be relevant after all!

Oh, floating star taps are real enough luggsy - you'll find them (not) attached (to earth) on any TN-S system in the UK today (and there are still more than a few).

The only circumstances the supplier would be to blame for an erath leakage fault would be on a TN-S system - on TT systems, the sub is responsible for earthing, and on TN-C it is pretty well taken care of. The OP's problems are more than likely related either to white goods on the same phase, or to a dodgy earth rod if a TT system.

This time of year is one when earth faults can become apparent in droves. As the ground dries out, resistance increases, and those earth rods not properly installed will start to exhibit higher resistance - which in turn leads to the earth fault loop impedance mentioned earlier...... This is why you used to see electricians watering copper rods in the ground - literally to drop resistance (rather impedance) prior to testing.

I still suggest, as I did to start with the op measures neutral to earth with all other circuits isolated 'and' with all circuits isolated at the incomer. The external EFLI (Ze) can also be used to see what's going on can't it?

Well, it is a fairly simple matter to identify whether or not the fault is on the installation side or on the supplier side. It does involve removing all Neutrals from the dis board, and taking appropriate readings.

Most likely by far, that the OP has a leaky device plugged in somewhere, such as a washing machine, or tumble dryer, or some dodgy IT kit.

I hope this thread continues with some more input from the op.

Would be nice to see what he's made of his request, yes....

Bill.

[luggsey]

Essentially, the star tap is put in place to ensure that no phase can exceed significantly the voltage or current of any other. Don't forget each phase is connected to the star tap too. It has a fringe benefit that it holds all three phases to zero potential when you plug it into the ground. If not grounded, it will still maintain a reference potential in respect of phase, which is calculated by the equations James pointed out - for most purposes, that would be taken as zero potential, though not zero volts.

Someone figured out that the only way to ensure a zero potential that wasn't actually up around 50 or 60 Volts, was to nail the star tap to the floor. It then means that each phase will maintain a constant frequency relative to mother earth. The trick then, is to make sure it is the right frequency - and that's the other part of where James' equations come in.

PME is a relatively new concept in electrical distribution - and is essentially a TN-C supply system, where the neutral and earth are supplied along one cable (or core, if undergorund as most incoming supply cables in the UK are concentric). The idea of PME is that neutral is bonded to earth at several points along the supply cable - normally at the star tap, then at every OH pylon, or at every substation. Finally, the neutral and earth are split again at the incoming point to the premises.

The idea is that there will be no variance at all between the potential on neutral and that on earth. It works pretty well.

Oh, floating star taps are real enough luggsy - you'll find them (not) attached (to earth) on any TN-S system in the UK today (and there are still more than a few).

The only circumstances the supplier would be to blame for an erath leakage fault would be on a TN-S system - on TT systems, the sub is responsible for earthing, and on TN-C it is pretty well taken care of. The OP's problems are more than likely related either to white goods on the same phase, or to a dodgy earth rod if a TT system.

This time of year is one when earth faults can become apparent in droves. As the ground dries out, resistance increases, and those earth rods not properly installed will start to exhibit higher resistance - which in turn leads to the earth fault loop impedance mentioned earlier...... This is why you used to see electricians watering copper rods in the ground - literally to drop resistance (rather impedance) prior to testing.

Well, it is a fairly simple matter to identify whether or not the fault is on the installation side or on the supplier side. It does involve removing all Neutrals from the dis board, and taking appropriate readings.

Most likely by far, that the OP has a leaky device plugged in somewhere, such as a washing machine, or tumble dryer, or some dodgy IT kit.

Would be nice to see what he's made of his request, yes....

Bill.

Some good points to chew on there Bill, I wasn't aware there were any floating star point transformers in the UK as I have never seen one in my area. I have seen standby generators with floating star points but even these are normally bonded to earth on the star point when installed. If the star point is floating then the 'neutral' is no longer neutral, it becomes a phase.

This also means there is no reference to earth from the supply and hence the reason we used to use voltage trips in days gone past....

The op needs to tell us what the earthing is as I said in my first post as if there is PME earthing there he should never see a potential between neutral and earth! (If there is TN-S or TT earthing I wouldn't be so surprised to see a small voltage but even so 15 volts is getting worrying.) That could only mean a break or high z in the neutral, as I already said neutral should be as near as dammit 0v on PME systems and as James pointed out if balanced there should be 'little' current.

PME is also cheaper for the supply company, they can use a cheaper cable with neutral wires doubling as earthed armour and direct bury concentric cables. How c##p is that!

I found this on the web:

Let's first differentiate between a floating Neutral and a loose Neutral. A loose Neutral is where there is no solid connection between the star-point of the supply and the star-point of the load. Please note we said 'not solid', and not "disconnected". 'Not solid' or 'loose' means any Neutral that is not bonded with at least 2x phase and through this can lead to imbalance on the load side.

A floating Neutral (star-point not bonded to Earth) is actually a floating supply i.e. the whole supply can, as an entity, float from Earth. But we need to cover one very important fact first.

Neutral can no longer be called Neutral! The term is given to this current carrying conductor through it being 'neutralised' with Earth (and finally Ground). As it can no longer hold on to Earth, it can no longer hold on to the name usually given it. We therefore have to revert to the place where this conductor is connected, this being the star-point of the transformer - and we call it such.

There are two inherent 'dangers' with floating systems, although neither 'danger' damages equipment operating on the floating supply. As is, ships have used this form of supply system very successfully for quite some time now! But first,...

On 3-phase systems there is a tendency for the star-point and Phases to want to 'balance out' based on the ratio of leakage on each Phase to Earth. The star-point will remain close to 0V depending on the distribution of the load and subsequent leakage (higher load on a phase usually means higher leakage).

If the supply is single phase then it is not uncommon to find the two current carrying conductors balancing out at about half-supply voltage each (on 230V this would mean 125V on both). Investigating leakage currents will show that, if anything, a floating supply significantly reduces leakage currents and therefore minimises damage to equipment through leakage via signal cables.

On to the 'dangers'.

The first 'danger' is; Should a Live-Earth fault occur then it is highly likely that NO protection will trip as there is no return for the fault currents. The star-point will no longer be 0V and will now rise to 230V. The second 'danger' is now present as only on a second fault will any protection operate. This danger increases with 3-phase in that a Phase-Earth fault will cause not only the star-point to rise to 230V, but the other phases will now reach 400V. Should the second fault occur then this will occur at the said 400V rather than the 'designed for' 230V.

Equipment designed for 230V can usually withstand the 400V from Phase to Earth without the danger of flash-over (note, not 'Phase-Neutral' which still remains 230V). It may stress the insulation a little, but nothing is likely to come of it (modern design specifications ensure that desigers are aware of what's required).

Based on the above, it is safe to say that a floating 'Neutral' is not the common cause of the equipment failure. Such damage is more attributable to the fact that gen-sets are not clean sources of energy, and it was probably one of a range of factors from high peak voltages to just simple overvoltage or something similar. Or, simply, that someone forgot to connect the Neutral......

[james.wilson]

phew

So if the star point is tied to earth, what happens if there is a phase imbalance. Does the current from the less loaded phase or phases flow to earth? ie how does it balance.

[luggsey]

phew

So if the star point is tied to earth, what happens if there is a phase imbalance. Does the current from the less loaded phase or phases flow to earth? ie how does it balance.

I don't think it has to balance James?

There is just more current in the neutral if there is imbalance, which in reality there will always be.

[amateurandy]

I've been reading this with interest, but have just realised that Installer44, who asked the initial question 3 days ago, hasn't come back yet to clarify the initial points that were queried.

[james.wilson]

andy were on a roll now, the original question is pretty irrelevant lol

I don't think it has to balance James?

There is just more current in the neutral if there is imbalance, which in reality there will always be.

luggs

Yes i can see that but if it doesnt balance then the star point will not be central if it doesnt. But you say that if it isnt there will be a current in the neutral. IF there is a current there will be a pd.

But thats what i thought. But then we have the fact that in most cased the star point/neutral point (which i have since learned that if it isnt tied isnt a neutral but a 4th phase but im lost on that as i cant see how we can have a 4th phase but thats by the by lol) is tied to earth.

If its tied to earth then there should in theory be no way for a pd between neutral and earth. I can see that this is possible from the site end now (ie volt drop and current due to phase imbalance) but this is corrected at the local tx.

What i still cant get my head around is if there is a phase load imbalance then there will be an additinal load on the neutral to correct this imbalance. (assuming the star point doesnt drift)

But to my mind the only way the star point cant when tied is when the under loaded phase would flow current through the star point to earth. THis would then balance all phases again.

But the potential current through that star point tie could be huge, ie if a phase fuse blew or disconnected then in theory the whole of the load now missing would be flowing through the star point? If not then it has to drift.

Thats why im lost now.

James