Buried Electrical Power Lines

The main reason we in the UK bury our low voltage domestic lines, is that they present less of a danger to the public, and are less vulnerable to damage, and the weather, etc.

It costs a lot more in the first place witch is why the main power grid is via pylons.

The higher the voltage the higher the respective costs are. Insulation plus the cooling required all add up.

Some years back my father helped on a project to put a 400,000V link under the Wye river it cost £2,000,000 per 100m of cable. That was in the seventies.

It was a pressurised oil filled cable made exactly to the right length and with a specially designed monitoring system attached to it. Designed and installed by my father. It was quite a challenge when 400kv was wizzing through it.

There is the 2 million volt DC connector under the channel that connects the French and our selves. It has to be converted to allow for frequency and voltage fluctuations. Some very big semiconductors regulate and convert both the AC to Dc

and visa versa. The cost of copper has gone up so high that many new cables are now made from extruded aluminium, the only things being it is much more prone to failure due to fatigue and over heating. If you see a new cable being laid and it has a bright red outer coating it will by aluminium. Also the voltage drop is greater per 100m due to its higher resistance. The main over head lines have been copper steel and aluminium steel. The other reason they have gone from using copper is that it gets stolen and thieves won't take the aluminium it has next to not scrap value.

It is ultimately all about cost. They have to make huge profits so their chiefs can report back to their share holders and collect their bonuses.

We mere mortals don't come into their calculations we just have to grin and bear it.

It provides a clean look as compared to the lines laid outside in the open atmosphere. In my opinion when the power lines are laid on the poles in the open there is a loss of great deal of power due to friction and transmission of electromagnetic waves and in this way there is a lot of saving of electric power if the lines are buried. Further the buried power lines,though costlier at the initial stage, provide less disruptions due to natural calamities and also provide saving due to less maintenance and fare wear and tear in the long terms.

PAUL JK

Is the electromagnetic field shielded? In the event of an electrical leakage, what is the level of danger to barefoot children on wet ground, for example? In a residential area, what do industry codes specify as to depth of the power lines, above ground identification? Allowable exposure of electromagnetic field, if applicable. Sounds great for new construction, not so great for older neighborhoods and urban areas. I don't persist in these questions to reject underground power lines btw, I think the concept seems very logical. Ed

For human skin contact, 120vac-240vac is the most likely to end in death. Less is less probable. More is less probable to result in burns rather than death.

For human hazard, open air surface or pole mounts are the most hazardous. Conductive shields provide less hazard. Conductive burried is most safe. Point of use generation (solar, wind) provides less hazard than any open air, buried, or shielded.

A properly installed, insulated with breaker and grounded buried electrical is most likely to "safely trip" and present no hazard to a bare human skin contact. The buried cable is insulated and should pose very little hazard when inadvertant contact occurs. A digging machine that defeats the insulation should trip the electrical breaker but could be extremely hazardous for human contact between the power line and any electrical ground including surrounding soil, dry or wet.

An open air line is not insulated except for distance. Human contact is certainly hazardous if the distance is defeated from overhead open air uninsulated power lines. A fishing pole, kite string, sagging line, tree limb can close the insulative gap. A tree lime has been blamed for most of the extensive electrical blackouts ie California twice, entire NE for several days, east coast for three weeks, Katrina for several months.

Conclusion: For Electrical The most popular methods are the least expensive to install and likewise the most hazardous. 120-240 volt overhead uninsulated bare or barely insulated conductors. Adopted Local codes are written by profit concious utilities with little to no regard for hazard rating.

If the buried electrical power lines insulation is broken then the line is naturally grounded with the lowest potential. If breakage occurs in the insulation the high voltage lines are on a mechanical breaker and are fired a total of three times after going to ground in the initial break. Once that happens a light goes on at the regional board and they deal accordingly. the only way that barefoot children could be hurt is if they have a lower potential than the ground path and then complete a path from the high potential to the low. It sounds funny but I've learned a lot of grounding principles from cattle field studies of herds of cows being electrocuted from standing is a wet field with electric fences and somehow connecting a big enough difference of potential to fry them.

I would add that buried electric lines may cause galvanic corrosion to buried ductile iron pipe os steel water mains if they adjacent or cross the buried electric lines. Another main issue with buried electric lines is to communicate their existence. Make sure they accurately mapped. Also, bury indicator ribbon about 18 inches above the duct bank so an excavator seeing the ribbon knows he has to take special precautions. Make sure it is placed in the designated easement. Also, maintain consistent burial depths, usually 18 or 36 inches depending upon the duct material or if the wire was direct buried. In other words, follow the local electric code.

Buried electric lines are almost always buried in ducts here in the USA. Several ducts are usually grouped together to create a duct bank. Several ducts are left empty as a cooling mechanism. As the one writer indicated, the really high-powered stuff usually has to go overhead because it is quite impractical to cool it if it is buried underground.

In the UK buried electrical power cables are a direct result of the initial power transmission system when power was only available in cities and towns and it was generated in DC. Because DC could not be transformed it suffered drastically from volt drop to the end user. Towns and cities had many small local power stations and the DC cable ran along the gutter or pavement at the edge of the road. Obviously as we became more civilised and technically minded it was decided to cover these cables by burying them in the ground where they ran.

With the more condensed and populated towns and cities, overhead distribution was not an option. More so the size and weight of the equivalent overhead cable would be so large that the cable would sag and break. When AC generation became prevalent the small power stations could be replaced with substations transformers which would take take HV energy and transform it to the usable LV energy for underground distribution.

Electromagnetic waves and effects were never a consideration, but these are largely negated in underground cables which are predominantly multi-core cable with the Live and Neutral in the same cable. The opposing magnetic effects are in opposite directions and therefore cancel themselves out. Also underground cables are made in a concentric format with the outer cores either being earthed steel armour or latterly when armour was dispensed with, the outer concentric cores would be the earth or neutral conductor of the cable.

The idea being that the cable could not be punctured and contact made with the inner Live conductors without touching the earthed sheath which would automatically result in a short circuit and protective device operation

Overhead cables are obviously easier to install and the conductors can run at higher loads due to the free air cooling. The down side is security of supply, electromagnetic wave leakage is only cancelled out to a limited degree, and generally they are unacceptable in terms of their visual impact