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The high voltage wire and cable are used in different weathering conditions and situations
According to these factors, some standards regarding the installation of these wires and cables must be considered
Building underground tunnels, chambers, passages, and ditches can be dangerous
Knowing the types of wires and cables that are safe for underground construction can help keep electrical contractors safe
Wire or cable? Both wire and cable can be used in underground construction
Underground wiring can be made of copper or aluminum
The copper wire can be safely left exposed in the ground, or it can have a protective jacket or insulation
A cable is a set of two or more conductors that are sheathed together
Ground cables consist of one or more conductors with high electrical conductivity, such as copper or aluminum that can withstand high voltages
This will form the core of the cable
Surrounding the core is an insulating layer that helps prevent leaks and ensures a high degree of safety
Finally, there is a sheath or jacket that acts as a protective layer for the cable and seals the element
However, an overcoat is not always necessary if the insulation is thick enough and protective enough
Buried When working in underground works, use direct buried wires and cables
Direct buried wire and cable are specially designed for building trenches in the ground
This is because they can withstand harsh weather conditions in wet and dry environments
The wire’s insulation protects against the elements and any moisture in the ground, so it can be safely buried directly in the ground without a wire
The wire acts as a mechanical barrier between the wires and the ground
The most commonly mentioned types of direct buried wire are USE (stands for (Underground Service Entry
and Tracer Wire, both of which are typically rated at 600V or less
Stranded or rugged? Whether it’s USE cable or Tracer wire, electrical contractors can make different decisions when choosing the right wire for their project
It is primarily whether stranded or solid copper/aluminum is used as a conductor
A solid consists of a single conductor
Although it provides excellent strength, it is less flexible
Stranded wire, on the other hand, consists of a bundle of several wires and is easier to bend than solid wire, so it is a good choice when the installation requires flexibility
High voltage cable installation standards
As the different types of high voltage cable and wire are widely used around us, considering different standards for installation became more important
Utilities around the world are optimizing the equipment they use (reducing capital and operating costs)
Therefore, the Ministry of National Grid Standards and Regulations in Saudi Arabia formed a team of experts to study the possibility of standardizing/unifying the cable design in three sizes: 1200 sqmm, 2000 sqmm, and 2500 sqmm
The National Grid specification has a variety of high voltage cable conductor sizes based on IEC 60228 and different operating area requirements
However, National Grid more often chooses a conductor size of 1200 mm² to meet load requirements, based on the required capacity of each substation
The international standard IEC 60840 “Extruded insulated power cables of rated voltages above 30kV (Um = 36kV) to 150kV (Um = 170kV) and their accessories – Test methods and requirements” does not specify the nominal insulation thickness required for high-voltage cables
This is not the case with IEC 60502, which specifies the nominal insulation thickness required for medium and low voltage cables
However, the insulation thickness limit of high-voltage cables depends on the rated electric conductor voltage and should not exceed 8
0 kV/mm, and the insulation shield thickness limit depends on the rated electric insulation voltage and should not exceed 4
0 kV/mm
The required insulation thickness is 20
32 mm for 110/115kV cables and 21
6 mm for 132kV cables
These values meet the requirements of IEC 60840, but have a high safety margin for 132kV cables, as shown in Table 1
In the same way that IEC 60840 does not specify the nominal thickness required for the semiconducting layer of the cable, the team made changes to this layer based on the manufacturer’s experience operating the triple extruder and tried to maintain the eccentricity of the cable
National Grid has worked with thirteen (13) cable manufacturers worldwide to research and investigate the minimum required thickness of semiconducting layer capable of meeting IEC 60840-based eccentricity requirements, reducing conductor shielding and insulating shielding to 1
5mm
Various metal shields are available according to national grid specifications
The most common choice is shielded copper wire [Figure 2]
The choice is mainly based on project requirements, such as radial waterproof properties, electrical and mechanical properties
However, all types of metal shields have the basic ability to withstand the required short-circuit current
Therefore, an important part of designing a metal shield is to know how much of the required short circuit the area of the metal shield can withstand, IEC 60949 “Calculation of thermally permissible short circuit current taking into account non-adiabatic heating effects” is used to calculate high voltage cable metal can withstand the necessary short circuit
Necessary area for shielding
There are two methods of short-circuit calculations: using the adiabatic temperature formula and the non-adiabatic factor
In this study, the non-adiabatic factor was chosen using the thermal constant of the medium applied to both sides of the screen
Both sides are coated with a moisture barrier
This helps to reduce the area of the copper wire shield from 280 square meters “according to adiabatic calculations” to 261 square meters “according to non-adiabatic calculations” and still meet National Grid’s specifications to withstand 40kA for 1 second and with significant savings in cost
High voltage cables
The reduction in insulation thickness and metal shielding area also slightly increases the current capacity of the optimized cable compared to conventional cables used in the network
High voltage underground cable
Different types of underground cable, high and medium voltage, are used for the transmission and distribution of electricity, and the use of aerial structures becomes impractical
Such locations may be congested areas where right-of-way costs are prohibitive, or where overhead lines are prohibited by local ordinances for safety reasons, or at intersections around factories and substations, or wide bodies of water where overhead crossings are not permitted for various reasons
The type of cable used depends on the voltage and service requirements
Recent improvements in design and manufacturing have led to the development of cables suitable for high voltage
This makes it possible to transmit electricity over short or medium distances using ground cables
Underground cables consist of a central core or several (two, three or four) stranded tinned copper conductors (sometimes aluminum conductors are also used) connected by paper or painted jute or vulcanized asphalt or impregnated paper insulation
A metal sheath of lead or alloy or aluminum is provided around the insulating layer to prevent ingress of moisture
The initial high cost is the only thing holding back the use of underground cables for power transmission and distribution
Underground cable construction
The type of cable used in a particular location depends on mechanical factors and the voltage required for operation
These cables are divided according to the voltage into low voltage cables with working voltage up to 1kv, high voltage cables with working voltages up to 11KV, extra high voltage cables with working voltages up to 33KV, extra high voltage cables with working voltages up to
to 66KV and extra high voltage power cables with working voltages above 132KV cable
In addition to the above factors, the current rating of a cable depends on many other factors, such as the cable laying method used, the distance between cables, the number of cores and the thermal conductivity of the soil
Cable manufacturers address each of them with a variety of factors
Approximate indication of current carrying capacity under Indian conditions, based on a maximum conductor temperature of 80 °C for cables of 11KV and above
Use the following multipliers: For a soil temperature of 30 80 °C = 0
88
For 3,300 and 6,600 V cable = 0
95
For 11,000 V cable = 0
9
For 22,000 V cable = 0
8
15 cm 30 cm 61 cm
for 2 cables 0
84 0
88 0
92
for 3 cables 0
75 0
80 0
85
Choice of cables Factors to consider when selecting the correct size and type of cable for a specific application are: System voltage Operating system type such as DC (2-wire or 3-wire) or AC (single-phase, three-phase), grounded or ungrounded, operating voltage such as 415/240 V, 11, 33, 66, 132 KV, etc
Current carrying capacity The current rating is the most important factor
Permissible voltage drop The cable size is chosen so that the voltage drop in the cable is within the permitted range
Short circuit classification For the selection of low-voltage cables, this factor is not important, but for high-voltage cables, this factor is the most important
This is because in high voltage systems the fault currents (phase to phase or phase to phase) are very high
The cable should be selected so that it can withstand the load and the resulting temperature rise caused by a maximum short-circuit current for up to 1 second from a short circuit between the phases
Financial considerations The cable must also provide the lowest operating costs (power loss, capital cost interest and depreciation)
Cable degradation mechanism Cable breakdowns usually happen in two ways
One approach is progressive core and trace, which always starts from the core or sheath and ends up bridging the electrodes
Another way is thermal instability, which is due to the rapid increase in power factor with increasing temperature
A major difference between the degradation methods is that once nucleation occurs, it will continue until the cable breaks, another duration that can be quite long for a full action
However, with thermal instability, damage does not occur before failure, so if the load is reduced before failure, the cable will not suffer permanent changes
A very common situation is when core drilling starts and then thermal instability is introduced in the center of the core bore
Underground cable installation standards
The standards for the installation of different types of cable in the underground environment are different from the installation of the cable in an open area
When running buried cables – such as from the house to the garage or gazebo – it must be protected to prevent mechanical damage to the insulation
The National Electrical Code provides recommendations on minimum burial depth, and the Building Code specifies local requirements that you must follow
Check with your local inspector for special regulations and, if necessary, obtain a permit before doing any work
Buried depth of metal pipes Laying ground cables in metal pipes approved for buried cables is the most expensive method, but also the one that requires the least excavation
Generally, you should bury metal pipes at least 6 inches below the soil surface
You can also drive them to a depth of 4 inches under a 4-inch concrete slab
Under your driveway, pipes must be less than 18 inches deep, and under public roads or alleys, they must be buried less than 24 inches
In hard rock, sink them into the rock so that the top edge is 2 inches below the rock surface and cover them with concrete
Requirements for non-metallic pipelines Non-metallic conduits or conduits approved for buried cables provide less protection than metallic conduits and require greater burial depths
Generally, bury nonmetallic pipe 18 inches underground, or 12 inches if protected by 2 inches of concrete or equivalent
For all other applications, the burial depth for non-metallic wire is the same as for metallic wire
Direct burial of cables or conductors Underground feeder cables marked UF can be buried without additional protective pipes because they have a robust jacket designed for burial
Bury the UF cable 24 inches below ground level and 18 inches deep when protected by 2 inches of concrete
For all other applications, the burial depth is the same as specified for conduit, but UF cable requires conduit in 2″ thick solid rock
Special measures for low power cables If your cable runs on a 120 volt circuit, is protected by a circuit breaker rated for 20 amps or less, and has a ground fault circuit breaker to protect the underground portion of the cable, you can reduce the cable burial depth
A generally useful depth is 12 inches, if you protect the cable with 2 inches of concrete, this can be reduced to 6 inches
Under the driveway, you may only need 12 inches of burial
Pipe required for installation in 2″ thick rock material and concrete
These depth reductions apply regardless of the burial method chosen
Reduce the depth of control cables Carrying a control circuit below 30 volts is inherently less dangerous than carrying a higher voltage so it can be buried deeper
Typically a depth of 6 inches is required
For all other applications, use the same depth as buried UF cables, but control cables must be buried 18 inches below the driveway
