What happens if "hot" wire touches Earth ground?

[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?

 

[William White]

He was safe in the tractor because it has great big 6 feet rubber tyres.

The tractor was at the same voltage as the overhead line.
The ground was at zero Earth potential.

As he stepped out of the tractor and put one foot down on the ground there was a massive voltage across him and he fried (HV, because of the power involved, tends to cook you alive rather than shock you into death)The auto-reclose is normally a relay on the supply CB.
Overhead lines are prone to nuisance tripping. Wildlife can short out the conductors, so can flying debris in the wind (branches etc). High winds can clatter the lines together.

All these sorts of faults clear themselves (the dead animal falls to the ground, the branch falls down etc). So after a few seconds (generally 3-5 seconds) the relay sends a signal to the CB to close. If the CB trips again (the fault is still standing) the relay is either programmed to lock the breaker open or try again.A load spike at 11 kV or LV would generally not even be a blip at 33kV. Thats one of the reasons for discrimination.

A dead short across a 33kV line can be several hundred MVA. There is no load spike that could require such power - the maximum load of the 33 transformer is typically 15/30 MVA. (15 continuous/30 short time, or with coolant pumps and air fans running(

(an oil filled CB contails several hundred litres of mineral oil - a large fault will cook the oil black)

Earth faults generate lower fault levels, but last longer. So the trick is to set different protection schemes to operate at different rates.

Designing protection curves is not a trivial task. The protection closest to the fault should trip first and it should diffentiate between load and faults, and different types of faults (ie do you want to take out the LV breaker and just disconnect load, or the HV breaker and take out the transformer)In the UK 33 is a now distribution voltage, but many still call it subtransmission (distibution was below 22kv; subtransmission was historically up to 132 kv; transmission 275 kv and the "supergrid" 400 kv. The distinction was generally due to the different departments that looked after the work)

 

[rollingstein]

He was safe in the tractor because it has great big 6 feet rubber tyres.

The tractor was at the same voltage as the overhead line.
The ground was at zero Earth potential.

As he stepped out of the tractor and put one foot down on the ground there was a massive voltage across him and he fried (HV, because of the power involved, tends to cook you alive rather than shock you into death)

Understood. Stepping out is a sure way to get yourself killed.

I was imagining a guy who didn't step out but jumped and then while standing on the ground the recloser closed and the step potential between his feet got him killed. That would sure be bad luck.

As I understand, it's very tricky to estimate the effect of a step potential while standing on the ground close to a downed conductor. e.g. the gait / stride varies, ground resistivity varies etc.

Have you heard of fatalities due to the step potential in real life? i.e. People who didn't actually touch a conductor nor a piece of live equipment nor were in the water but got killed merely by the current flowing through the ground from one leg and out through the other?

I've seen calculations of this but never heard of any anecdotal electrocutions of this nature.

 

[William White]

Understood. Stepping out is a sure way to get yourself killed.

yes, if he jumped clear, he would (could) have been okay

As I understand, it's very tricky to estimate the effect of a step potential while standing on the ground close to a downed conductor. e.g. the gait / stride varies, ground resistivity varies etc.

They teach people who operate HV switches to keep their feet planted firmly together when operating the switch in case the earth-mat fails (or is missing - farmers love to plough up Earth mats near switches in fields)

Have you heard of fatalities due to the step potential in real life? i.e. People who didn't actually touch a conductor nor a piece of live equipment nor were in the water but got killed merely by the current flowing through the ground from one leg and out through the other?

I've heard of plenty of people getting to close to the safety clearance and getting fried - idiot metal theives.

And this
http://news.bbc.co.uk/sport1/hi/other_sports/horse_racing/9400599.stm

horses being much more likely to die than people

 

[rollingstein]

I've heard of plenty of people getting to close to the safety clearance and getting fried - idiot metal theives.

Indeed! But that's more like an arc from the conductor to the idiot's body.

 

[rollingstein]

horses being much more likely to die than people

Reminds me of another anecdotal story I've heard that stray potentials can cause cows to reduce milk production. Sometimes a tingle below the perception threshold of a human will be enough to severely impact the milk production of a dairy barn.

 

[jim hardy]

horses being much more likely to die than people

"Step" potential is called that because it's the voltage that would appear between a man's feet when he takes a step
as opposed to a step change in time
it'd be volts per meter or yard or foot ; volts/lengthclearly a horse has more distance between his front and rear feet than the span of a man's step
and the path from horse's front to rear legs goes through horse's chest, unlike a man on two legs,

Lightning strikes produce a lot of Earth current and it goes a surprisingly long way from the strike.
Once while i was swimming in a freshwater lake lightning struck about a mile away and the shock was surprising but not very painful. "Wow , " i thought, "glad that wasn't any closer !"Once upon another time when returning from a fishing trip in Everglades, with the afternoon thunderstorms menacing us from behind, we passed a flock of flamingos standing on one leg.

That's when it dawned on me, that one legged stance protects them from a shock should lightning strike anywhere nearby.
i wonder...

 

[rollingstein]

That's when it dawned on me, that one legged stance protects them from a shock should lightning strike anywhere nearby.

If a one legged stance would protect a flamingo from feeling a shock then how come a "zero legged" swimmer you still felt a shock?

 

[William White]

make up some numbers (just for example - it doesn't matter what the numbers are for illustration)

There is a lightning strike in the water which results in a voltage gradient. Say it is 1000 volts per metre in the region of interest.

The body of the swimmer is 1 m long. There is a voltage across the swimmer of 1000 V AND the flow of current is across his organs!

The flamingo has one leg in the water. Its tiny little foot is 2cm across. There is a voltage gradient of just 20V across its foot BUT the current will not flow into its organs because there is no circuit.

Its exactly why birds can sit on a live wire. The voltage gradient along the wire is (for all intents) zero. They sit on the wire and their entire body is at the same voltage as the line and there is no voltage gradient.

If it was a big bird (like a swan) and it opened its wings and touched the adjacent line then there is the full voltage across the bird body. Its fried.

 

[rollingstein]

Its exactly why birds can sit on a live wire. The voltage gradient along the wire is (for all intents) zero. They sit in the wire and their entire body is at the same voltage as the line and that voltage is fixed. No gradient.

But humans too can sit on live wires right? In spite of having much larger bodies. e.g. Those guys doing hot work on high voltage transmission lines? Or is their protective suit a Faraday cage?

 

[William White]

But humans too can sit on live wires right? In spite of having much larger bodies. e.g. Those guys doing hot work on high voltage transmission lines? Or is their protective suit a Faraday cage?

well they wear a protective suit but the principle is that same with the bird

at any two points on the same line the voltage difference is (for all practical purposes) zero (significant volt drops occur over very long distances - hundreds of metres).

What the linemen cannot do is touch two lines that are at different voltages.What is interesting, is that you will not see birds sitting on EHV lines (above 132kV). The air is conductive around the line and there is a voltage gradient in the air. When the birds get close enough it is very uncomforable for them and they fly away.slightly off topic - but if you have a flourescent tube and live near a supergrid line (400kV); walk under it with the tube and it will light up. You can plant the tubes in the ground and they will light up. An artist made a electric forest of flourescent tubes

http://www.richardbox.com/