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FAQs Transmission Lines

Why doesn’t Transpower use underground cables instead of overhead lines?

We do use underground cables in some special circumstances, but not often, because:

  • Underground cables suitable for transmission voltages are very expensive - in the order of ten times the cost of overhead line.
  • Underground cables are difficult to maintain, and cannot be maintained live. In particular, it might take several weeks to identify and repair a fault on an underground circuit compared to about a day for an overhead line.
  • At long distances – i.e more than 30 kms, above ground reactive compensation stations would be required at regular intervals.
  • If long-distance power transmission used underground cables, the price of electricity would be several times what it is now.

Why do transmission lines buzz and crackle in damp weather? Is it dangerous?

Electricity flowing through a conductor at high voltage produces an energy field around the conductors. If a conductive path did exist, power would flow to earth. The insulators that hold conductors in position combined with the excellent insulating properties of dry air mean that electricity does not flow between ‘live’ components and the earth. Moist air and rain may cause unstable irregularities in the electrical field around the conductors and insulators, which can generate a crackling noise and under some conditions can produce a dull glow called corona. Under damp conditions, a small current may leak along the insulator string, between the conductor and the ‘earthed’ structure, as the live components attempt to go to earth potential. This current is very small and trickles harmlessly to earth through the tower. These conditions are normal and not dangerous.

What is a flashover?

A flashover is a fault, where the high-voltage electricity jumps from a conductor to earth or to another conductor. It is basically like a small bolt of lightning, and produces a similar noise and a bright blue light. Flashover can occur in very high winds when conductors clash together, or when the line insulation is compromised, or when trees grow too close to lines. Flashover can cause great disturbances in the power system, as well as damage to lines. Transpower goes to considerable trouble to prevent flashover by careful design and maintenance, and by controlling the growth of trees below lines.

How close can you safely get to a live transmission line or high-voltage conductor? Can electricity jump out at you?

Yes - electricity at high voltages used by Transpower can jump out or flash over to anyone, or anything that approaches too close to a live conductor. You don’t have to touch it to be seriously burnt or killed! The minimum distance that live equipment can be approached safely (or minimum approach distance) varies with the voltage. As far as members of the public are concerned, this distance is four metres. No person or object is allowed any closer than that to a live line. Check the Landowners and Occupiers booklet for more detailed safety information.

What is voltage?

Voltage represents the difference in charge between two conductors - such as the terminals of a generator, or between a transmission line and the earth. It can be described as the ‘pressure’ of electricity - it is also called ‘potential difference’. Conductors at high voltage need a lot of insulation to stop the electricity flowing from one to another when the potential difference between them is great. This is why transmission lines at many tens of thousands of volts need long strings of glass or porcelain insulators, but your house wiring at 230 volts only needs a simple plastic covering. (Note that the ‘volume’ of electricity flowing is called current, measured in amperes.)

Why are such high voltages used? Wouldn’t low voltage, such as used in our homes, be safer?

Long-distance transmission of electricity is only feasible at very high voltages. Long transmission lines have a relatively high linear resistance, which in turn regulates the maximum amount of current flow for a given voltage - Ohm’s law. This determines the amount of energy delivered - the product of voltage and current. The resistance can be lowered by making the conductor thicker, but the practical limit of this approach is reached very quickly. So to deliver a lot of power over cables of a practical diameter, a high voltage has to be used.

Is it true that electromagnetic radiation from transmission lines or substations is dangerous to your health?

No - there is no consistent evidence to confirm that there are any health hazards from high voltage lines and equipment - as long as you adhere to prescribed safe practices. See our safety section for more information.

Why are there three conductors on a transmission line?

Each high voltage circuit has three phases - labelled red, yellow and blue. The generators supplying the power system have their coils connected to the system through terminals at 120-degree intervals. As each generator rotates inside the coils through a full 360 degrees, the voltage and current rises and falls in each terminal sequentially. With three conductors, the whole power system will be kept in step, or synchronised, and the maximum amount of power can be transmitted.

Why don’t birds get electrocuted when they sit on power lines?

Animals are damaged by electricity only when it flows through them. A bird sitting on a power line is surrounded by air, which is a good insulator. No current flows through the bird, and there is no harm done. For the same reason, maintenance workers can sit on live transmission lines as long as they get there using insulated ladders and ropes, and keep a good air gap between themselves and the tower steel.