Volts Between N And Earth

[luggsey]

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

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

Current without voltage, well if the neutral is at earth potential it can't show any potential to any other part of Terra Firma, a bit like touching a live wire with an insulator?

The star point is fixed at earth potential so it can't drift, except as our friend described there are some transformers that can, I know nothing about these but can compare them to a generator, a normal portable generator will present 110 volts to it's own earth and 240v across phases (live and, er, neutral which is really a phase!) try it on a genny and see, this is why plugging in a RCD to a generator supply won't work as it has no earth refrence to terra ferma to allow the imbalance detector to work.

I don't get the last bit about fuses and star point though? If a phase fuse disconnected then the imbalance current would rise, got that bit, and the current would flow through the star point to earth, that's ok isn't it? Why would it be so huge, do the formula predict that I wonder? The current may be high but I suspect there is detection equipment at the transformer to allow for this as it happens on a regular basis...

A neutral is a neutral only if it presents no PD to earth, if the neutral is not earthed it is no longer neutral but carries voltage and current, the yanks would call it hot! We call it a phase or a live.

I find this stuff really interesting as the more you enquire the more you learn!

[james.wilson]

I find this stuff really interesting as the more you enquire the more you learn!

Agreed, but you also need to accept that you need to learn. Some dont.

try it on a genny and see, this is why plugging in a RCD to a generator supply won't work as it has no earth refrence to terra ferma to allow the imbalance detector to work.

Im gonna go all theory again now.

I cant see why not. An rcd works by detecting an imbalance between what goes out and whats returned, so even with a floating isolated neutral an rcd would know if any current had leaked as it would not be on the retuen and would disconnect.

I don't get the last bit about fuses and star point though? If a phase fuse disconnected then the imbalance current would rise, got that bit, and the current would flow through the star point to earth, that's ok isn't it? Why would it be so huge, do the formula predict that I wonder? The current may be high but I suspect there is detection equipment at the transformer to allow for this as it happens on a regular basis...

I dont know. But my assumption that if the 3 phases at the boards tx are supply say 1000A. and then Ph 2 disconnects then 1000A needs to flow to earth. I dunno is this normal?

A neutral is a neutral only if it presents no PD to earth, if the neutral is not earthed it is no longer neutral but carries voltage and current, the yanks would call it hot! We call it a phase or a live.

Ok i hear you but a disagree. I may be wrong but i cant see how you can say a neutral with a volage on it is a phase. Hot i can go with, live also. But a phase?

James

[luggsey]

Agreed, but you also need to accept that you need to learn. Some dont.

There are a few round here you know!

Im gonna go all theory again now.

I cant see why not. An rcd works by detecting an imbalance between what goes out and whats returned, so even with a floating isolated neutral an rcd would know if any current had leaked as it would not be on the retuen and would disconnect.

Well the test button won't work for sure! Also seeing as there are two phases then any drain to earth will mean the other phase balances the load, floating load, hence no imbalance seen, I think.

I dont know. But my assumption that if the 3 phases at the boards tx are supply say 1000A. and then Ph 2 disconnects then 1000A needs to flow to earth. I dunno is this normal?

You got me there, dunno.

Ok i hear you but a disagree. I may be wrong but i cant see how you can say a neutral with a volage on it is a phase. Hot i can go with, live also. But a phase?

The two wires on a generator are both phase because they both supply electrical energy, I was just comparing what I know to our question, if it's no longer neutral ie. at or about the potential of ground and it's not a phase then what is it!

James

[james.wilson]

luggs

Well the test button won't work for sure! Also seeing as there are two phases then any drain to earth will mean the other phase balances the load, floating load, hence no imbalance seen, I think.

I dont think so. Floating voltages in reference to earth yes, but if the rcd was connected to the gen and say 50A go out, it would expect 50A back. 30mA difference and it would trip due to the way it detects.

However the test button may fail if there is no earth for the test button (well it will fail)

The two wires on a generator are both phase because they both supply electrical energy, I was just comparing what I know to our question, if it's no longer neutral ie. at or about the potential of ground and it's not a phase then what is it!

I sort of agree but cant agree thats its a phase.

In theory even in single phase the neutral (maybe the old name of return was better) should still be at zero pd to earth (except for volt drop issues) But in 3 phase you have said yourself the star point (neutral) is tied to earth so under what circumstances would a tied neutral have a voltage on it?

[luggsey]

luggs

I dont think so. Floating voltages in reference to earth yes, but if the rcd was connected to the gen and say 50A go out, it would expect 50A back. 30mA difference and it would trip due to the way it detects.

However the test button may fail if there is no earth for the test button (well it will fail)

If you connect a RCD to a floating supply generator and plug in a RCD tester it won't work [RCD], there is no imbalance in the RCD coil as it has a phase on both sides of it which are always balanced as there is no earth reference. This happens on installations where a standby generator is connected to a normal household supply without being modified with an earth tap, the shock protection [RCD] will stop working.

I sort of agree but cant agree thats its a phase.

In theory even in single phase the neutral (maybe the old name of return was better) should still be at zero pd to earth (except for volt drop issues) But in 3 phase you have said yourself the star point (neutral) is tied to earth so under what circumstances would a tied neutral have a voltage on it?

I was making refrence to FS... comments about 'some' transformers that he said were not earth tapped, they could well show voltage on the neutral.

[DirectFS]

Guys.

Lots of talk about when a neutral isn't a neutral, some RCD chatter, and imbalances.......

Let's try to clear up some things.....

Easy first. RCD's do NOT detect anything on earth - there is no physical connection from an RCD to Earth, and therefore it DOES NOT detect, or rather, measure, any current, voltage, mushrooms, or even stars on the earth.

What an RCD does, simply, is detects a difference in the potential between phase and neutral - and by reason, this "missing current" would be present on the earth wire - hence, earth leakage - it has "leaked" from the circuit, and logically that current can only run to earth.

Ok, if voltage, and by extension, current, exists on a neutral line, above a few volts, it is generally termed as being line voltage, rather than neutral - line, is specific, and distinct from phase.

Now, as for imbalances, earthing theory, and transformers in general.

UK power comprises three stages, generally, generation, transmission, and distribution. In the first two stages, there is no concept of neutrals, and no concept of earthing anything. Purely electricity at very high voltages, and nothing but. Indeed, imported electricity from the continent is brought in on undersea cables, which are actually DC rather than AC.

When we get to the distribution stage, many things happen before the power is available on the grid, for consumption. Key among these things are:

Step down to distribution voltage.

Rectification, filtering and harmonisation of incoming power.

Connection onto grid at distribution voltage.

Distribution voltage is normally approximately twice what you'll find present in a sub station output - so around 1.1kV or so.

This voltage is further stepped down at the substation, and is then connected multiply to the local distribution network - hence, grid.

Of this voltage, we're interested in the bit that comes down the street to us, either overhead, or underground.

This bit, for our purposes is the output of a transformer in the substation -

This transformer can be one of two types, depending on the local distribution network - if it is supplying TT, or TN-S systems, then the transformer will, in all probability be a DELTA type transformer. If it is supplying TN-C, (PME), then typically, the transformer will be a STAR type transformer.

The differences between the two essentially are in the way they are wound, and the resulting outputs -

For a Delta type, the three phases, for our purposes, are wound on in series with one and other, at 120 degree intervals, making up the three phases - the end winding of the third phase simply connecting back to the initial end of the first winding.

In this type of transformer, the reference to ground, or earth, is floating - that is, you will rely on a seperate connection to a copper or aluminium pole driven into the ground, whether at the transofrmer site, or at the supply destination. This opens the way for potential difference between Phase and earth, as the ground reference will likely NOT be the same in two different locations, even if only a matter of tens of feet apart.

Star type transformers are wound on differently. In this type of transformer, the phases are wound on again, at 120 degree angles, but this time, in parallel, thus creating three "end of winding" phases, and also three "start of winding" points, which are bonded together, and connected to that fabled great copper pole driven into the ground.

Now, star type transformers are usually bigger, because they need more turns per winding, and there are more windings, to create the required phases.

Either way, the resultant output is a Sine Wave - Phase one being the peak of the sine, phase two being the flat or middle of the sine, and phase three being the trough of the sine - in total, 360 degrees.

So, this understood, floating earths or potential imbalances, can only generally occur on Delta type transformer outputs, and NOT on star type transformers, unless under very specific fault conditions - normally only when the transformer windings fail, and connection to earth is lost on at least one phase - this would generate a fault on the transmission side of the network too, and so is always dealt with very quickly - from our perspective on the grid, it would most likely result in a power cut and load difficulties on the other two phases.

In a star type transformer, the earthed star point is always referenced as zero potential - as it is understood that the reference will be the same anywhere on the connected network as it is only ever taken from one point on the earth. As such, the phases will always be 120 degrees aligned, and hence, remain at the correct voltage, referenced to earth, again, anywhere on the connected network (ignoring volt drop issues). If a phase winding fails, the result is NOT a mass rush of current onto other phases, simply the failure to generate power on one phase output - power cut, without issues on other phases, which remain connected, wound at the correct rate, and correctly referenced to zero potential (earth).

Bill.

[steven sneddon]

wow, fantastic topic guy's. Following it or trying to , the only bit of this topic I actually had a clue about was the RCD as I must have been trying to pay attention in college that day and remember the tutor talking about this.

[luggsey]

Good information thanks Bill, a couple of points/questions.

Easy first. RCD's do NOT detect anything on earth - there is no physical connection from an RCD to Earth, and therefore it DOES NOT detect, or rather, measure, any current, voltage, mushrooms, or even stars on the earth.

The only point being made was that without a reference to earth on the neutral side of RCD's they don't detect an imbalance, this is what I was describing when trying to make an RCD work on a generator output with no earth tapped neutral connection, they don't work as the output of the generator is two phases. I have proved this when installing standby generators and testing it myself, the generator needs to have a small mod to the winding output making in similar to a earth taped transformer, then RCD's work as normal. I really don't know how that compares to a delta wound transformer with a floating neutral?

Ok, if voltage, and by extension, current, exists on a neutral line, above a few volts, it is generally termed as being line voltage, rather than neutral - line, is specific, and distinct from phase.

There we go James, it's a line!

[DirectFS]

Good information thanks Bill, a couple of points/questions.

The only point being made was that without a reference to earth on the neutral side of RCD's they don't detect an imbalance, this is what I was describing when trying to make an RCD work on a generator output with no earth tapped neutral connection, they don't work as the output of the generator is two phases. I have proved this when installing standby generators and testing it myself, the generator needs to have a small mod to the winding output making in similar to a earth taped transformer, then RCD's work as normal. I really don't know how that compares to a delta wound transformer with a floating neutral?

Ah - yes, indeed. On an isolated system, such as TI, or even on a TT with the earth disconnected, an RCD will fail to function.

The reason for this, simply is that all earth leakage, stays in circuit - it has nowhere to go, and so would exhibit, and dissipate, as heat equally from both conductors, until it is earthed through something - even you.

RCDs will work on a system supplied by a Delta transformer, because you still provide a path to earth, and the neutral is still referenced to phase.

There we go James, it's a line!

Bill.

[esp-protocol]

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.

Hello Bill... lots of good stuff but I wasn't after an explanation of EFLI... just trying to get across that if the OP understands the paths that current can take to get to earth then he might better be able to work out the solution to the potential problem.

And the fact that in order for the overcurrent device to operate (not an RCD) then in some way shape or form their must be a fault path involving the particular phase conductor entering the property and eventually passing through the "fuse" protecting the sub-circuit.

As has been said... a lot depends on the method of distribution/earthing arrangements of the particular property.