# High voltage three phase power lines and ground danger

In Europe, most power lines have three cables for each phase and no neutral (usual UK line pictured):

The three-phase AC generator at the power plant probably use a Δ-phase winding (simplified):

(image extracted from this website)

What I don't understand is — why is the current flowing through those power lines trying to rush to ground if there isn't a neutral wire connected to the ground for the zero potential reference?

There is often a sign on electric poles warning not to attempt climbing at the risk of dying — you'd end up electrocuted if you happen to touch one wire. But, according to the schematics above, touching one wire shouldn't be a dangerous attempt as the ground isn't the zero potential. Touching two wires at the same time, however, would be deadly.

Apparently, there are two possibilities:

• corner-grounded Δ-phase system where one phase becomes the ground;

(source: netdna-ssl.com)

• wild-leg Δ-phase system by adding a center-tap fourth wire between two phase that becomes the ground-neutral wire.

(source: netdna-ssl.com)

(images extracted from this website)

Also, the step-down transformers reducing HVAC to 120/240V AC must be connected so as to provide a neutral wire:

(image extracted from this website)

Homes are then connected between one LV phase and the neutral wire. The neutral wire is earthed and becomes the ground wire in each individual home.

• @DanSheppard Don't try to answer OPs question in the comment section, especially so when the answer isn't trivial.
– pipe
Jun 28, 2022 at 13:24
• @DanSheppard interesting, so even if a system totally isolated from the ground, capacitance could be induced in such a system? Some HV power lines have corona discharge when the electric field surrounding the cable reach a certain threshold, I wonder if this is related. Jun 28, 2022 at 13:25
• @DanSheppard You may wish to make that an answer - either a partial one or expand it. Jun 28, 2022 at 13:33
• @explogx Corner grounded phase page here Jun 28, 2022 at 13:36
• Sadly, I don't have the time for a proper answer which requires more deal of rigour than I have time for, for example calculating the capacitance of a typical transmission line and impedance to earth at line frequencies, detailing how switching and momentary faults to earth can induce artifacts which elevate frequencies, and so on. My area is MV: in the case of the transmission network I don't even know if this is the dominant factor (eg over High-Z instrumentation pulling stuff to earth or corona discharge). I'd be more than happy to upvote an answer along those lines of course.
– Dan
Jun 28, 2022 at 13:44

## 4 Answers

There may not be a neutral conductor in the overhead distribution system but there are many points throughout the system where it is earthed or earth leakage occurs. For example the centre of a star winding is often earthed and used as a local neutral. Hence there is a conductive path from the overhead wire back to the generator via the ground.
I agree that, if you had a totally isolated generator connected to a similar isolated load and no point in the system was earthed or provided earth leakage then it would be safe (if ill advised) to touch one of the conductors.
It's similar to the demonstration of someone touching a Van de Graaff generator whilst stood on an insulating platform (Google "Van de Graaff" and switch to images) Their hair stands on end but they are otherwise unaffected. This is the way "hot working" is carried out by highly skilled maintenance engineers.

• I edited my answer as I found some schematics that can explain the possible wiring system used in power lines. They must use a corner-grounded phase which then becomes the zero potential reference point, but I don't see why. Jun 28, 2022 at 13:17
• Re, "if you had a totally isolated [system] and no point in the system was earthed...then it would be safe...to touch one of the conductors." If it's an A.C. system, operating at high enough voltage, then it would be unsafe even to come close to one of the conductors. Search YouTube for videos of crews who approach, and climb onto high-voltage transmission lines using helicopters. They wear full Faraday cage suits. Without the suit, the intense, A.C. electric field surrounding the conductor would induce dangerous A.C. currents in their bodies even before they actually touched it. Jun 28, 2022 at 15:03
• @SolomonSlow "A.C. electric field surrounding the conductor would induce dangerous A.C. currents in their bodies even before they actually touched it." do you have any reference for that? Jun 28, 2022 at 15:15
• @explogx, You mean, like in an academic journal? No. But have a look at this video. It shows a lineman climbing onto a high voltage line from a hovering helicopter. At 1:05 in the video, He reaches out with a probe to electrically connect the helicopter to the line. It draws an arc, half a meter long. That's current flowing between the completely isolated, hovering helo and the wire. Can you imagine him doing that with his hand instead of the probe? Also, see physics.stackexchange.com/q/191503/74763 Jun 28, 2022 at 15:29
• @SolomonSlow That's what I meant by "hot working" . Only for the experienced linesmen who, I believe, wear conductive clothing effectively putting their bodies in a Faraday cage. Jun 28, 2022 at 19:44

In my area (Vancouver BC), the local HV distribution system (13.2 KV) must originate with star-connected transformers with the center of the star groundeds, as the step-down transformers reducing that HV to the 120/240 V for homes are connected between one HV phase and Ground/Neutral.

• It's the same in the UK. Area distribution HV (typically around 10KV 3 phase delta) is transformed locally to 440V phase to phase star. The common point is grounded and taken as neutral along with one phase to a domestic premise or street, Thus supplying 240V single phase. The other two phases are run to adjacent houses / streets thereby balancing the load. 440V 3-phase is available for any industrial premise in the area or at extra cost to the domestic premise, although the latter is very rare. I believe that in some European countries 3-phase is supplied as standard to all customers. Jun 28, 2022 at 20:24

The line voltage for distribution networks is not 400 VAC, but 100 to 400 thousand VAC.

And even if there is no neutral being distributed in the transmission lines, that does not mean that the three 400kV phase wires are not referenced to earth potential.

The trasformer used to step up the voltage might have a center/neutral tap which is connected to earth potential, so that the three phases of distribution network are not floating with regards to ground/earth potential.

• Three phase power uses three 400 kV phase wires, not four. The fourth wire if there is one is for lightning protection and is grounded.
– Uwe
Jun 28, 2022 at 19:17
• Thanks. It was a typo. Jun 28, 2022 at 19:22

Based on the length of the insulators, this power line is transmission voltage level, not distribution levels. (See here for a reference of number of bells in porcelain insulators for different voltage levels.)

Many transmission lines include shield wires to protect the phase conductors from direct lightning strikes. These shield wires are usually made of steel unless they contain fiber optic communication cables, in which case aluminum is also used. The shield wires do not carry load current and are connected to earth ground at every structure.

In areas with low lightning rates, such as the west coast of the U.S. and much of Europe (ref), overhead shield wires may not be installed, or may be installed for a limited distance adjacent to a substation to reduce the risk of a direct strike damaging substation equipment.

Although the shield wire is not used to carry load current, transmission systems are generally "effectively grounded" such that a ground fault on one phase will draw significant fault current and the voltage rise on the unfaulted phases will not be "too severe". The reason for this is that ungrounded systems require full phase-to-phase voltage insulation from phase-to-ground on all phases. Locating ground faults on ungrounded systems can also be problematic compared to effectively grounded systems with their relatively high fault currents.

To be clear, the transmission system is generally NOT as shown in the schematics that you included with your post.

• Would you die if you touched one of those HV cables? I can't see the shield wire on the picture, hence no effective grounding here. Jul 4, 2022 at 9:10
• Use of a shield wire or not has nothing to do with whether the system is effectively grounded. Jul 6, 2022 at 8:15
• And effective grounding has nothing to do with whether the conductor can be safely touched while in contact with ground. Even ungrounded systems will be coupled to ground through stray capacitance, and de-energized conductors can pick up voltage from electric fields from nearby lines. It doesn't take much current to harm or kill a human. The safety procedures used by transmission line technicians to work on even de-energized lines are significant. Jul 6, 2022 at 8:21