Installation Of Armoured Cable On Trench

Once the cable is installed a "sheath test" should be carried out before the trench is fully backfilled. Before the cable is terminated I test at 500V between the armourings and earth to show any damage to the serving that should be protecting the armourings.

It's easer to repair the sheath before problems appear years down the line.

In answering your question, 'What are the disadvantages when the underground armored cable is not installed in a well constructed trench?"

The answer is: Nothing! The trench has nothing to do with the performance of the cable in transmitting voltage.

It does however have a correction factor on current rating that the cable may carry if the cable is installed in a trench, or duct or direct buried.

And your proposal of a trench is useless, unless one is provided the ground conditions, (geotech), the trench depth, width, run length, surface conditions, (between houses, mud huts, shops, open ground, rocks, dense jungle, open plains, soft soil etc).

In my limited experience, the most important reason for a properly constructed and surface indicated trench is that one day someone needs to dig in the area. The trench gives added protection and an indicator to work carefully.

We have just started planning phase 2 of our parts warehouse and I am sure the effort we put in during phase 1 to do a proper tranch will pay off.

The immediate disadvantage is that one could not lay more. There is a risk of accidental damage. Buried cables should have markers and/or indicating tape. Due to the dissipation of heat, directly buried cables have a slightly higher current carrying capacity - see the tables. A slightly smaller size of cable might be used, with further saving.

Reducing cable size will not promote a cost saving if the required load has been determined. Going bigger allows for further expansion, slightly higher costs for a cable and less trouble in upgrading a cable for future loads and a 10% margin of safety. However, the trend has always been to load cables to the max to optimise the cable life span and initial cost.

A trench, duct or direct buried has the same costs to construct, Only the cable price varies and makes an impact on costs. The back fill materials may vary slightly if CBS, or river sand is used as the thermal medium, but that is negligible compared to cable price.

"...A trench, duct or direct buried has the same costs to construct,..." Really?

A 50HP DitchWitch can cut a 6" wide, 24" deep trench at about 10-12fpm in a single pass with a single operator (dependent on soil conditions of course). Compare that with creating a concrete encased trench or duct which you have to excavate, place forms, pour concrete, wait for it to harden, strip the forms, place the supports, etc.; time, materials, and labor requirements are at least 5 -10 times greater than direct burial.

That's one of the reasons why utilities would rather do overhead than underground where possible.

Indeed really. Read the entire statement. And I would assume, (which was wrong of me), that anyone knowing cable installations, that a BoM for a cable trench does not change anywhere in the world. Direct bury+X, ducted =X+1 and constructed=X+2. There is no cost saving to gain. But, on cable prices that fluctuates daily on a copper price this is the biggest cost of which very little control can be gained.

Given that the lad has provide zero detail if he has a road crossing, the type of medium to be excavated, if he has the reserve granted to install the cable below ground any other details I would not offer the in depth knowledge of costs of BoM for cable trenches, but would again assume that anyone doing excavations for cables would have an inkling on costs, again the costs of trenching all have the same universal costs and no cost savings.

OHL will be far cheaper if he was going a long distance, but as he has a cable in mind, OHL is of no value here. And OHL is a different ball game in costs.

You are correct, there are not enough details to assist the OP in going beyond his original question. But the argument that two men and a machine that can trench and fill 5-10ft per minute costs the same as a multi-person construction crew that takes a day to construct a 100 ft. lined trench is unfounded, unless of course the labor and materials have little or no cost.

The BOM for direct burial is simply the hourly rate for the laborer, operator, and the trencher divided by the length. The BOM for a three sided concrete trench includes a backhoe and its operator, two forms makers, the forms and concrete, labor for pouring and stripping, plus the cover/backfill material and labor. All that additional time, materials, and labor makes the cost per hundred feet at least an order of magnitude higher than direct burial, and this doesn't include the cable or its installation labor. btw- the cable is only 25-35% of the overall cost for anything other than direct burial.

Please use your copy of "RS Means Construction Cost Estimating Data" to prove me wrong.