re: Electricians - why are high-voltage power lines always exposed?

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They’re exposed to make it easier to tap into for free power.

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Power to the people!

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When they hit two of them and complete a circuit.

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i’m pretty sure if you hit two of them you are bucking a phase

Bridging two phases is called a line to line fault.

Found a image of insulated overhead distribution line, notice the gap in the insulation where the tap is connected.

Not true. The only thing in oil would be high voltage circuit breakers. The oil has a high dielectric value (High Resistance Value). It is used to dissipate the arc flash when opening or closing the high voltage breaker. In the industry those breakers are called OCB’s (Oil Circuit Breakers).

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Cost prohibitive?

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Bingo

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You can run them underground, but they are usually cooled with an oil circulating system and a pump on each end.

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Not bashing you, but worked for Entergy in Transmission Substation department (69kv up to 500kv) for over 20 years and I’ve never seen that type of setup. I’ve seen SF6 ( Sulphur hexaflouride) insulated buss work within substations, I’ve seen solid aluminum pipe used as a conductor, but never circulated oil, other than in a HV circuit breaker.

In the field, for distribution, underground utilities are extremely expensive, are difficult to troubleshoot faults and, as was said earlier, cannot dissipate heat as an open air setup does, therefore limiting current or necessitating larger conductors (re: higher cost). Same reason the spacing on poles is what it is - code and cost. However, depending on the subdivision/development, the aesthetics are worth the added cost.

Basically, all things (not mandated by safety etc) boil down to costs.

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Insulation on high voltage lines are not used because because it causes a voltage drop. Basically in simple terms it means it takes a higher voltage leaving the generating facility to get desired voltage at desired point in the grid system. Transformers are used to step down voltage, step up voltage or simply to boost the desired voltage. It is better to transfer voltages at higher values and then use a transformer to step the voltage down to desired voltage. When cables are run underground they are insulated but any place that a connection is made it has to be installed a certain way because of the way those cables are made.

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Insulated overhead high voltage primary distribution lines are in common use everywhere. Voltage drop is not caused by insulation, voltage drop in a conductor is a function of the loss resulting from the product of the square of the current flowing through the conductor, and the resistance of the conductor. This loss produces heat which must be dissipated to the air. Insulating a conductor reduces the rate of heat loss from the conductor to the surrounding air which requires a insulated conductor to be rated for less current than a equally sized non-insulated conductor, but for a given current and conductor resistance the voltage drop is the same.

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but worked for Entergy in Transmission Substation department (69kv up to 500kv) for over 20 years and I’ve never seen that type of setup.

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There was oil filled cables under the industrial canal for a while as well as under the MS river

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Insulated overhead high voltage primary distribution lines are in common use everywhere. Voltage drop is not caused by insulation, voltage drop in a conductor is a function of the loss resulting from the product of the square of the current flowing through the conductor, and the resistance of the conductor. This loss produces heat which must be dissipated to the air. Insulating a conductor reduces the rate of heat loss from the conductor to the surrounding air which requires a insulated conductor to be rated for less current than a equally sized non-insulated conductor, but for a given current and conductor resistance the voltage drop is the same.

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If the insulated conductor runs hotter due to the insulation, wouldn't the resistivity also be higher which would then cause more voltage drop along it?