What happens if "hot" wire touches Earth ground?

[dmtyper]

Greetings !New to the forums.
Here expanding my concept of what AC Electricity is
My question (correct me if wrong)..
If AC power alternates between +,- at a rate of 60Hertz/second and negative is used as -earth- at the AC generator

If the hot wire touches the ground would happen? (I think Nothing)

Cause by the principle of my understanding if I touch a live "Hot" ac wire and I am standing still to the ground current from the "hot" wire will travel through me to reach the potencial difference cause there is no voltage in ground .

But since the current is actually passing through my body I will get electrocuted.

Question is if this happens would the voltage go back to neutral?

 

[jim hardy]

I think you need to get straight in your mind the basic concepts of

energy
work
power

charge
potential
potential differencecurrent

and learn the units of each

before you become totally confused.

 

[jim hardy]

Cause by the principle of my understanding if I touch a live "Hot" ac wire and I am standing still to the ground current from the "hot" wire will travel through me to reach the potencial difference cause there is no voltage in ground .

But since the current is actually passing through my body I will get electrocuted.

Current will return to the source from which it came.
If enough of it it travels through you to get there, you will indeed be electrocuted.
If Earth also provides a path back to that source, current might go through Earth too.

 

[Nugatory]

My question (correct me if wrong)..
If AC power alternates between +,- at a rate of 60Hertz/second and negative is used as -earth- at the AC generator

If the hot wire touches the ground would happen? (I think Nothing)

That is not right. If the hot wire touches the ground, there will be a somewhat exciting spark (I have some melted screwdrivers to prove it) and a rush of current from the hot wire to the ground. If a circuit breaker or other protection device does not open, things will explode, melt, catch on fire. If you happen to be part of that path to ground, you will receive a dangerous, perhaps lethal, electrical shock.

What you may be missing is that the Earth at the AC generator is not negative, it is zero. In a North American 60Hz 120 system, the hot wire is alternating between negative -170 and +170 (120 volts is the RMS average - google for "root mean square electrical power" for more), so there is a voltage difference from ground, and hence current flow in one direction or the other throughout the cycle.

 

[jim hardy]

a compilation of electrical explosions

 

[dmtyper]

Thanks very much for the replies !

 

[berkeman]

a compilation of electrical explosions

Yikes! Did you see the Fire Engine that pulled up to the burning substation around 2:44 in the video? They thought better of it and drove right on by. Not a thing in the world they could do until the power was turned off...

 

[jim hardy]

They thought better of it and drove right on by. Not a thing in the world they could do until the power was turned off...

Indeed you don't put water on an electrical fire . The stream will conduct current right back to the fire hose and whoever's holding it.

Country wisdom : never pee on an electric fence.

 

[dlgoff]

They thought better of it and drove right on by.

Indeed you don't put water on an electrical fire . The stream will conduct current right back to the fire hose and whoever's holding it.

That and ...

http://www.esgroundingsolutions.com/about-electrical-grounding/what-is-step-and-touch-potential.php

 

[DrClaude]

Country wisdom : never pee on an electric fence.

City wisdom: never pee on the third rail

 

[jim hardy]

Don - you taught me that nuance of term "Step Voltage".
DrClaude - I married a NYC girl. She confirms your advice .

Thanks, guys !

old jim

 

[zoki85]

City wisdom: never pee on the third rail

Hehe.

 

[jim hardy]

We tried an insulator wash system that used the principle of breaking the stream into droplets separated by enough distance to prevent conduction..
It worked okay the first couple of times... wish i had a video of that 240KV flashover

Thanks for the confirmation !

 

[zoki85]

We tried an insulator wash system that used the principle of breaking the stream into droplets separated by enough distance to prevent conduction..
It worked okay the first couple of times... wish i had a video of that 240KV flashover

Thanks for the confirmation !

I've heard of power line insulators washing systems where they use water spray under high pressures. Water is of low conductivity or even tap water.
Sounds insane but...

 

[jim hardy]

Very pure water is a good insulator. Some generator windings are water cooled.

I've heard of success washing insulators on transmission lines. Our trial on a transformer didn't work out well.

 

[zoki85]

I've heard of success washing insulators on transmission lines. Our trial on a transformer didn't work out well.

System was the same used for washing insulators on transmission lines? Maybe it was used in a wrong way...

 

[jim hardy]

System was the same used for washing insulators on transmission lines? Maybe it was used in a wrong way...

Maybe. It was one of the earliest offerings. I see it's still done a lot of places.

On a high line like that there's nothing immediately below the insulator, so runoff falls and separates.

I'd be doggone nervous around this:

Our failure was on a transformer with vertical insulators like these.

 

[rollingstein]

Not a thing in the world they could do until the power was turned off...

Why is it not possible to design the analog of an automated circuit breaker? A short like that, doesn't it have some signature? I suppose you cannot just detect the high current because there might be legitimate reasons for a high current too?

Some of those shorts seem to last for ever. It'd be nice to have an automated system trip the feeders. Too many false alarms?

 

[dbc]

k... now I am confused...

taking what Nugatory said into account... A/C transformers primary coil are a direct path from hot ( >4000V ) to ground/neutral (unless you are considering the reactance of the inductor coil)... so why don't A/C transformers in substations and on poles blow up when they are hooked up since there is no resistance? Is the coil's inductor's reactance actually acting as the resistance of the circuit thus preventing current from flowing at a substantial rate? And what happens when the secondary side is disconnected? Does the primary current continue flowing?

 

[rollingstein]

@dbc

How so? On a delta-star AC transformer (assume you mean step down) the neutral grounding is on the secondary not primary side.

 

[EverGreen1231]

k... now I am confused...

taking what Nugatory said into account... A/C transformers primary coil are a direct path from hot ( >4000V ) to ground/neutral (unless you are considering the reactance of the inductor coil)... so why don't A/C transformers in substations and on poles blow up when they are hooked up since there is no resistance? Is the coil's inductor's reactance actually acting as the resistance of the circuit thus preventing current from flowing at a substantial rate? And what happens when the secondary side is disconnected? Does the primary current continue flowing?

(We'll talk about single phase for ease of communication) If the secondary side is disconnected so there is an open, current will cease to flow in the secondary and current in the primary will depend on the equivalent values of the primary circuit - coil resistance and impedance, along with magnetization current and hysteresis losses of the core material. When the secondary is connected to a load, the load will draw some amount of current (whatever is needed to drive whatever device is connected or there're multiple references to ground). This current draw will have an effect on the voltage and current phasors of the primary side.

In three phase transformers this is complicated slightly depending upon how the respective sides are connected. For instance: if you have a Delta-Wye transformer and you have a fault on the Wye side that goes to ground, it will manifest itself as a 2 phase fault on the Delta high side. There are also zero sequence networks thrown into the mix, it's these currents that make it important to ground a system - or have some form of stability insurance. If you had a large power transformer that didn't have some type of grounding, you could have massive circulating currents in the core and other parts where current was not designed to flow (this could also happen if it is improperly grounded): The ground is used to stabilize the system (though you can have ungrounded systems so long as the relays monitoring the presence of ground faults are set to operate on a very slight disturbances, or so I'm told). There are some large transformers that have three and four windings to eliminate as much as possible these zero sequence currents (along with other reasons).

Hope this helps...I'm still not sure I completely understand your question.

 

[EverGreen1231]

@dbc

How so? On a delta-star AC transformer (assume you mean step down) the neutral grounding is on the secondary not primary side.

On large Power Transformers, there is also a grounding (Wye) tertiary winding usually included on the Delta side of the transformer that has a 1:1 ratio and is connected to ground.

 

[EverGreen1231]

Why is it not possible to design the analog of an automated circuit breaker? A short like that, doesn't it have some signature? I suppose you cannot just detect the high current because there might be legitimate reasons for a high current too?

Some of those shorts seem to last for ever. It'd be nice to have an automated system trip the feeders. Too many false alarms?

There is, and it is, itself, and entire profession.

 

[psparky]

k... now I am confused...

taking what Nugatory said into account... A/C transformers primary coil are a direct path from hot ( >4000V ) to ground/neutral (unless you are considering the reactance of the inductor coil)... so why don't A/C transformers in substations and on poles blow up when they are hooked up since there is no resistance? Is the coil's inductor's reactance actually acting as the resistance of the circuit thus preventing current from flowing at a substantial rate? And what happens when the secondary side is disconnected? Does the primary current continue flowing?

The neutral of the secondary is grounded just like the neutral is grounded in a house panel. The only thing going thru here is perhaps a small trickle current, a perhaps a ground fault or things of that nature.

When the secondary of transformer gets disconnected, the primary sees an open circuit and NO power flows.
Why, because the neutral of the transformer has a potential of zero volts...and the Earth ground has the potential of zero volts. Zero minus zero is...Zero. Therefore, according to V=IR, Zero current flows.

Hypothetically speaking, read the voltage in your house receptacles with a meter. Hot to neutral or hot to ground you get 120 volts. Now measure the voltage from neutral to ground. You will get zero volts. Same thing for 3 phase panel, neutral to ground is zero volts...same thing for said transformer above.

To answer the question about what happens when hot wire touches ground...again, we'll use our friendly V=IR. The current will simply be the supply voltage divided by the resistance of the ground back to the neutral of the panel, or the neutral of the secondary of its transformer. Or a combination of both would be even more realistic. Both of these neutrals are obviously tied to ground.

 

[dbc]

K. I guess I must be misunderstanding something. This is how I've always visualized how high voltage (power line) transformers (7 kV, 36 kV, 500 kV, etc...) are hooked up. Am I wrong?

 

[davenn]

K. I guess I must be misunderstanding something. This is how I've always visualized how high voltage (power line) transformers (7 kV, 36 kV, 500 kV, etc...) are hooked up. Am I wrong?

you missed the connection from the other side of the generator ... as shown, the circuit is incomplete

 

[dbc]

you missed the connection from the other side of the generator ... as shown, the circuit is incomplete

transmission circuits don't have a return (neutral) wire. so it must go to ground.

also... neutral wires on distribution circuits are connected to ground at poles so it never makes it back to the substation.

in either case, the circuit never makes it back to the source.

nevertheless, is this acknowlegement of the fact that there is a constant flow of electricity since the circuit is complete regardless of whether the secondary is hooked up?

 

[davenn]

transmission circuits don't have a return (neutral) wire. so it must go to ground.

also... neutral wires on distribution circuits are connected to ground at poles so it never makes it back to the substation.

dbc, I know how its supposed to be connected !, just making sure you did/do

...in either case, the circuit never makes it back to the source.

that's incorrect, of course it does. The Earth return IS part of the circuit

it pays to show everything, else misunderstandings ariseDave

 

[davenn]

nevertheless, is this acknowlegement of the fact that there is a constant flow of electricity since the circuit is complete regardless of whether the secondary is hooked up?

a constant flow would be indicative of DC. You pointed out in your diag. text that it's AC and is oscillating, correct.
keep in mind that the electrons in the wire in the coil of the generator never get to your house and likewise the ones
in the cable in your house, never get to the generator at the power station. They are just osc. back and foreward in situ

Yes, there is still current flowing in the primary, even when the secondary is open circuit ( no load connected)

some years ago in this thread ...
https://www.physicsforums.com/threads/transformer-secondary-coil-question.187441/

Claude made a couple of excellent posts ...

There are 4 laws at work simultaneously. All of them must apply, namely, Ohm's law, Ampere's law, Faraday's law, and the conservation of energy law.

A transformer cannot output more power than inputted, due to CEL (conservation of energy law). Right off the bat, it should be obvious that the secondary power, which is Isec*Vsec must equal the primary power, Ipri*Vpri, minus losses.

Faraday's law, FL, relates the magnetic flux density, B, to the voltage, V, and the frequency, f. Ampere's law, AL, relates the magnetic field intensity, H, to the current, I. Also, B and H are interrelated through the permeability mu, which is analogous to Ohm's law (for magnetics). Ohm's law, OL, relate the current and voltage at the secondary with the load resistance.

Most transformers are driven at their primary by an independent power source which is generally a "constant voltage" source. Current transformers can be discussed later. The voltage source magnitude, V, frequency, f, and primary turns number, Np, and the core area, A, determine the magnetic flux density, B, per FL. This magnetic flux almost completely links, or couples into the secondary winding. Since the core area is the same, as well as the frequency, only the number of turns differs, Ns. Just as FL describes the relation between Vp, f, A, and Np, it holds equally for Vs, f, A, and Ns. Again, f and A don't change, so that the ratio of volts to turns cannot change. Hence Vp/Np = Vs/Ns, since B, f, & A remain constant.

When current is drawn by loading the secondary, a magnetomotive force, mmf, occurs, which tends to counter the existing core flux, which tends to reduce the voltage. But, by definition, the primary power source is a constant voltage type, which will supply whatever current needed to maintain a fixed voltage value. The primary current increases to a value needed to maintain the core flux. AL describes the relation. For a given secondary current, Is, and Ns, a magnetic field intensity, H, is given by AL. In the primary an equal and opposite H, or mmf if you will, must exist. Since H is almost equal in the primary and secondary, Np*Ip = Ns*Is.

Since the volt per turn RATIO must be the same on each side, the winding with higher turns has a higher voltage, or emf. Since the amp-turns PRODUCT must be the same on both sides, the winding with the higher turns has the lower current, or mmf.

It can't be any other way, as the CEL would be breached. Does this explanation make it clear?

and ...

Because the primary has a constant voltage source, CVS, connected across its terminals. Remember the definition of a CVS. It will output any current neded to sustain a fixed voltage value. When the secondary is open, and a CVS is connected to the primary, a small current flows, called exciting current. The B-H curve of the core material, and the CVS value of voltage determine the magnetizing portion, Imag, of the exciting current. Hystersis and eddy current losses determine the loss portion of the exciting current. FL describes the relation between V, f, N, A, and B. For a transformer whose core has area A, and primary turns Np, the flux density B will be determined by the voltage and frequency of the CVS. AL describes the relation between magnetizing current Imag, primary turns Np, magnetic path length including gap(s) lc, and magnetic field intensity H.

This B value links the secondary almost entirely. For a given B, f, A, and Ns, Vs is determined. Just as a fixed voltage determines the flux density value, a fixed flux density determines the induced voltage value. FL is bi-directional. Likewise with AL.

When a load resistance is connected across the secondary, Is is induced in accordance with OL, Is=Vs/Rs. This current Is is accompanied by an mmf which counters the original emf, known as Lenz' law, or LL. This mmf would tend to reduce the flux density B, and the field intensity H. But remember that the primary flux (& secondary since the coupling is very good) is determined by the value of the CVS. A CVS forces a specific voltage value, and consequently a specific B & H value. When the primary voltage Vp is countered by the mmf of the secondary load, the CVS outputs a larger current whose mmf opposes the secondary mmf. In other words, just as Is and its mmf tend to reduce the flux and emf, Ip does just the opposite and restores them. Otherwise power would diminish greatly as soon as a load is connected.

Does this make things clear? BR.

Claude

I wonder if those help ?Dave

 

[dbc]

this confirms my suspicions. thank you.

most people never realize this until the see both the abstract and physical versions of electrical construction, and it is never actually mentioned point blank in physics texts, but AC primary sources are actually connected directly to ground and is constantly flowing.

coming from a background of an electrician to lineman to potential engineer... this at first seemed ludicrous. all my life I've been dealing with power on a source > breaker > ground relationship. the fact that something operates on a source > ground basis and doesn't cause an explosion somewhere was hard for me to get my hands on.

taking all this into consideration now... I'm curious now how many amps are flowing in the transmission lines when there is no load connected at the substation (secondary is open circuit). what's actually going on at the power station? do the generates go into some idle state?

 

[davenn]

coming from a background of an electrician to lineman to potential engineer... this at first seemed ludicrous. all my life I've been dealing with power on a source > breaker > ground relationship. the fact that something operates on a source > ground basis and doesn't cause an explosion somewhere was hard for me to get my hands on.

if you directly ground the live ( phase) of your mains outlet, yes of course large current flows and there will be sparks and a bang.
I saw this in action a few months ago on a wet /raining nite when the dropper line ( 2 cored) from the power pole to my neighbour's house developed a fault. The insulation around each of the live and neutral wires has failed, as had the insulation covering them both.

Rain water was running down the cable and causing arcing between the live and neutral wires. after about an hour and several phone calls to emergency services, the cable finally completely failed and fell to the ground. NOW, the live ( phase) wire was shorting out to ground via the wet concrete footpath.
The emergency services finally took me seriously and turned up to make the cable safe so that passing pedestrians didn't get zapped if that walked into the wire in the dark

in the case of your transmission line from power station to substation or your local street transformer. Yes, again you would be getting lots of sparks IF that phase wire was directly grounded. but it's not, its grounded via the primary transformer winding. And it's the load impedance of that primary winding @ 50/60 Hz AC means that the source voltage isn't looking at a short circuit.

taking all this into consideration now... I'm curious now how many amps are flowing in the transmission lines when there is no load connected at the substation (secondary is open circuit). what's actually going on at the power station? do the generates go into some idle state?

I personally cannot answer that completely
hopefully someone else will chime in with some answers

note that the transformer primary ( in the substation) or the one in your neighbourhood is still presenting a load to the power source ( the generating station)
so there will be some current flowing.

As Claude said above ...

When current is drawn by loading the secondary, a magnetomotive force, mmf, occurs, which tends to counter the existing core flux, which tends to reduce the voltage. But, by definition, the primary power source is a constant voltage type, which will supply whatever current needed to maintain a fixed voltage value. The primary current increases to a value needed to maintain the core flux. AL describes the relation. For a given secondary current, Is, and Ns, a magnetic field intensity, H, is given by AL. In the primary an equal and opposite H, or mmf if you will, must exist. Since H is almost equal in the primary and secondary, Np*Ip = Ns*Is.

cheers
Dave

 

[William White]

transmission circuits don't have a return (neutral) wire. so it must go to ground.

This is only true in VERY rare constructions (SWER).

SWER is illegal in many countries and where it is allowed, the power distributor has to apply for special dispensation. Because of the considerable Earth currents, it is only used where the danger arising is limted - the limitation usually being remoteness (i.e. the Australian desert!)AC transmission circuits are usually three-phase; and any two of these phases can be used for a spur.

Current flowing through the Earth (unless it is SWER) is, by defintion, fault current.

Transmission and distribution circuits employ sophisticated protection systems to detect Earth currents and disconnect the circuit when they arise.

also... neutral wires on distribution circuits are connected to ground at poles so it never makes it back to the substation.

Where the neutral-earth is physically earthed in multiple places (be it a link box or a pole) this is known as protective multiple earthing.

The fault circuit certainly DOES include the Earth point (star-point of transformer or a earthing resistor) at the substation.

 

[William White]

taking all this into consideration now... I'm curious now how many amps are flowing in the transmission lines when there is no load connected at the substation (secondary is open circuit). what's actually going on at the power station? do the generates go into some idle state?

If you have a circuit that goes something like:

...132k transmission >>>> 132 circuit breaker >>>> 132/33 transformer >>>> 33 circuit breaker >>>>> 33 substransmission...

If the 33 breaker was open, the current flowing beyond that - in the 33 subtransmission line - is zero (obviously).The current flowing in the circuit which includes the 132/33 transformer's HV winding is proportional to the voltage and impedance of the circuit.
Power stations regulate their output due to demand by varying the reactive power that is generated.

 

[rollingstein]

@William White

In the distribution transformer I've seen the primary seems a delta. So there is no point to attach a neutral right?

Even in utility pylons I see wires in groups of 3 (plus the protective ground on top for lightning strikes). So those are three phases with no neutral correct?

What you seem to be describing with the SWER use-case is the final distribution leg. But high voltage transmission seems mostly without a separate neutral conductor, correct?

 

[William White]

Its probably best to ignore SWER. It is so uncommon as to not be worth considering - and it just causes confusion!YES, When you see a three-wire, three phase system, the neutral is not distributed.

However, (at least this is the case in the UK, (mandated by law) and I can't see it being different anywhere else) there must be an earthed reference point at every voltage level. So if your transformer primary (say 11kV) is delta wound, then this will be connected to a transformer that is either star (or sometimes zigzag) wound to provide an Earth point so the 11KV circuits are earthed.

Say you were looking at an 11kV/400V TX where the HV side of the TX is delta wound. The HV windings would be connected to power lines that go off somewhere to a 33/11kv TX where the 11kV winding is star. So your distribution/transmission circuit can - and usually will - contain a mixture of star/delta; delta/star; and star/star transformers (and possibly some zigzags) at each voltage level.

ie. you might have a sequence of transformers in your system:

400V (star) / 11KV (delta)
connected to
11 KV (star) / 33 KV (star)
connected to
33 KV (delta) / 132 KV (star)
connected to
132 KV (delta) / 400 KV (star)
so at each voltage level, 400V, 11KV, 33KV, 132KV, 400KV there is a star point for earthing. Here the 11/33 is a star-star TX which is quite common.All of this is three wire except the 400V distribtion network which is four-wire so customers are provided with 230V single phase. (the voltage levels obviously vary by country).

 

[rollingstein]

@William White

Very interesting! I was under the mistaken assumption that only the final distribution transformer's secondary is star wound and all upstream transformers are delta wound.

I never realized that the 11kV primary on my TX (which is delta wound) would be connected to the 22 kV feeder's secondary that would be star wound.

Can you elaborate about why a Earth reference is needed at every stage? What would be the problem in having a voltage level that has only deltas i.e. no Earth reference?

Also, in your description some stages are star-star. What's the choice that goes into deciding if to have a delta-star or a star-star etc.

 

[William White]

What would be the problem in having a voltage level that has only deltas i.e. no Earth reference?

a) A reference provides the quoted voltage with a physical meaning. This reference is the mass of earth, which is taken to be 0V.

b) Say you had a (33kV) circuit with no earth, how would you achieve safety in Earth fault conditions?

Safety is the most important reason for earthing an electrical system. A lot of money is spent on protection to detect Earth faults in EHV networks.

this might be of interest
https://en.wikipedia.org/wiki/Zigzag_transformer
What's the choice that goes into deciding if to have a delta-star or a star-star etc

Its to ensure that you have a Earth point at that voltage. If you look at the list above, if the 11/33 TX was delta-star there would be no star point on the 11KV network. If that was the case, there would need to be another transformer (connected somewhere) at 11KV that did have a star point.
I've attached (a small part of a) schematic of one of the primary s/s in my town. It shows two 33/11 TXs connected in parallel. The HV and LV windings are both star.

If you look carefully the star points of both TXs LV windings are connected to the same Earth point, which is a liquid-water resistor (just a great big tank of water which limits Earth fault current - its rated at 1,200A for 30sec).

There is the ability to close the neutral of either TX down to "solid" Earth (instead of through the resistor). You might see those switches in the middle of the diagram. You can also see the CTs on the TX neutral - these are there to detect earth-currents. They are connected relays which will trip the transformers CB if Earth current is flowing (for too long)

This is the law in the UK regarding earthing
http://www.legislation.gov.uk/uksi/2002/2665/regulation/8/made
I guess many countries would have the same meaning (but obviously worded differently)

 

[jim hardy]

Also, in your description some stages are star-star. What's the choice that goes into deciding if to have a delta-star or a star-star etc.

it's almost off-topic and trivial to mention this , given the overwhelming importance of earthing

...but for the record

when you go through a transformer ΔΔ or star star there's no phase shift
when you swap between Δ and star there's 30 degree phase shift
since the system designer must arrive at his destination with all his feeders coming from different directions still in phase
sometimes that'll play a part in his choice of transformer connection.

 

[rollingstein]

since the system designer must arrive at his destination with all his feeders coming from different directions still in phase
sometimes that'll play a part in his choice of transformer connection

Ah! That makes sense. I had never thought of that part!

Out of morbid curiosity, say a mistake was made and an out of phase feeder was connected. Would the results be spectacular?

PS. The linemen, for example, typically carry no easy tool to detect relative phase in the field do they?

 

[William White]

yes, one will see TXs described as DY11 and such like

D - delta
Y - wye(star)
11 the o'clock phase shift (one -hour being 30°)

 

[rollingstein]

yes, one will see TXs described as DY11 and such like

D - delta
Y - wye(star)
11 the o'clock phase shift (one -hour being 30°)

So that's why they have all those esoteric vector groups (Dyn5, Dyn11 etc.) they sell transformers in, eh? To give the designer the freedom to get his phases all lined up before he ties them in?

 

[William White]

Ah! That makes sense. I had never thought of that part!

Out of morbid curiosity, say a mistake was made and an out of phase feeder was connected. Would the results be spectacular?

HV switchgear is normally rated for a short circuit right up on the TX:

The rating is, say, 250MVA at 11kV; 750MVA at 33kV for three seconds.

So if you closed a breaker in onto such a situation it would (should!) trip instantaneously. Depending upon how good the breaker was, it might grumble for a split second or so (and that can be quite alarming!)

The speed of protection is determined by how much current is flowing (except in instantaneous trip relays which can be set to trip instantly for any fault current or for fault current above a certain threshold)However, you would not want to be using a manually operated switch (rather than a breaker) to close onto such a fault. That is madness and dangerous, and against every rule in the book!

The rule is, that if connections have been unmade, and then remade, or new connections are made, the circuit is made live remotely (after phasing checks have been confirmed - see below) whilst everybody is clear of the point of work.

PS. The linemen, for example, typically carry no easy tool to detect relative phase in the field do they?

yes, you use a voltage stick to detect HV

Its an insulated rod, with a voltage detector that beeps when close to HV.

So, say you wanted to Earth a HV line for maintenance; you would make it dead, isolate it and then test dead using the stick before you put your Earth's on.

You approach the line with the stick and as it gets close it beeps if the line is live.If you wanted to check phasing; then you use phase sticks (which are glorified test lamps). You check l1-l1 (should be in phase) then l1 to l2 (should not be) and so on...

 

[William White]

So that's why they have all those esoteric vector groups (Dyn5, Dyn11 etc.) they sell transformers in, eh? To give the designer the freedom to get his phases all lined up before he ties them in?

that's it,
the n is saying the Y has a neutral connection

 

[rollingstein]

Its an insulated rod, with a voltage detector that beeps when close to HV.

I've seen the voltage sticks in use. But never knew they carried phase sticks too.

The closest I can recall is those set of lamps and the synchroscope / spinning needle when bringing a generator on-line the old school way.

 

[jim hardy]

Out of morbid curiosity, say a mistake was made and an out of phase feeder was connected. Would the results be spectacular?

Spectacular is in the eye of the beholder.
Some protective device should gracefully subdue the overcurrent.
The distribution fuses in your neighborhood sound like a shotgun when they go.This is extremely unlikely but possible

http://www.newson6.com/story/195654...sion-damages-ponca-city-electrical-substation

expanding vapor , be it dynamite or copper, follows the same gas laws...

 

[William White]

rolling:
Its very important to confirm phasing before remaking conductors.

If there is an LV backfeed available from another HV source; the phasing can be checked on the LV.

I think this is safer - can be done with a test lamp, and no special safety procedures and documents required - but it is not always possible.

 

[William White]

Spectacular is in the eye of the beholder.
Some protective device should gracefully subdue the overcurrent.
The distribution fuses in your neighborhood sound like a shotgun when they go.

When I worked on networks, I would often get called out to replace LV fuses after they popped on a fault.

You had to keep banging the fuses in until the fault cleared (ie the cable blew in half, or blew back together).

Once, the fault was right under my feet, I slapped the fuse in and the ground lifted about 3 inches. I was out of the sub like a rocket!

 

[dlgoff]

Once, the fault was right under my feet, I slapped the fuse in and the ground lifted about 3 inches. I was out of the sub like a rocket!

Taking LONG steps might not be the best thing to do.

A couple of my favorite images. Compliments of http://ecmweb.com.

 

[William White]

no worries with that
a) substations are equipotentially bonded
b) one wears heavy duty insulated boots when working live

c) if there is danger of an imminent explosion, you get out! (before working live, you ensure you have a clear, quick escape route: doors are left open, no tools or rubbish lying around, nobody else in the substation etc)

sad story about step potential

About 10 years ago a colleague of mine had to walk a 33 kV line to look for damage (it had tripped earlier)

He found a dead man on the ground next to a tractor. The dead man's young son was in the tractor.

The farmer had hit a 33kV line with his tractor (the machinery on his tractor was raised and he drove under the line).

There was a bang as the conductors shorted and then the circuit tripped. He jumped out of the tractor onto the ground to see what happened just as the circuit reclosed. Dead.

His young son had either the sense, or fear, to stay in the tractor. (It was about two hours before the linesman turned up and called the police)

 

[rollingstein]

There was a bang as the conductors shorted and then the circuit tripped. He jumped out of the tractor onto the ground to see what happened just as the circuit reclosed. Dead.

You mean he was killed by the "step potential"? He was not in contact with the tractor when he was killed?

As an aside, are reclosers triggered by the fault current? In such a case like when a tractor contacts the line is the fault current much higher than any normal load current through the 33 kV line? i.e. How easy or difficult is it to select a threshold that can cleanly separate a fault like this from just regular load spikes.

Also, is 33 kV a transmission voltage or a distribution voltage? How far apart are these reclosers on a transmission line?