The right cable selection is critical not only for power supply reliability and the safety of equipment and people present around them, but it also eliminates asset loss and saves valuable business hours.
Any electrical network's nerves are electrical cables and Fire Cable. Cables account for a significant portion of any electrification project's capital investment. They are also the most susceptible to failures. The majority of cable problems can be traced back to poor selection. The purpose of this post is to discuss how to choose the right Flexible Power Cable.
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Parameters for Selection
i. Voltage Rating: This is the system's rated voltage, which the cable will be installed and utilised in. It's also crucial to understand how to earth your system. The cable's rated voltage is usually given as a dual rating (e.g., 6.6kV (UE)/11kV) (E).
'UE' indicates that the cable can be used in an uncovered or ineffectively earthed system for the stated voltage. The letter 'E' indicates that the cable can be used in a solidly earthed system for the specified voltage. As a result, a cable with a rated voltage of 6.6kV (UE)/11kV (E) can be used in a 6.6kV unearthed or 6.6kV non-effectively earthed system, as well as an 11kV solidly earthed system.
ii. Conductor Type: Copper or Aluminum are the most often utilised conductors in cables. As is well known, the continuous current rating, short time current rating, and per unit length cost of a Copper cable are significantly higher than those of an Aluminium cable for the same voltage rating, type, insulation, cross sectional area, and method of installation.
iii. Insulation: The majority of today's cables are insulated with PVC or XLPE. The continuous current rating, the short time current rating, and the per unit length cost of an XLPE insulated cable are obviously higher than those of a PVC insulated cable for the same conductor material, voltage rating, type, insulation, cross sectional area, and mode of installation.
iv. Unarmoured cables are used in indoor installations and on above-ground installations, such as in cable trays, in pre-built concrete cable trenches, and so on. Armoured cables are required for any subterranean cable installation.
The armour might be formed of galvanised iron or aluminium wire or strips. This armour is frequently connected to the plant's earthing system, preferably only at one end, usually the sending end.
v. Continuous Current Rating: The continuous current rating of cables with an aluminium or copper conductor can be found in the catalogues of various cable manufacturers. However, it should be remembered that these catalogues only provide continuous current ratings under particular conventional laying conditions.
In practise, obtaining or maintaining these standard conditions is impossible. To get at the practical continuous current rating, various rating parameters are employed.
vi. Rating Factors: The following are some of the most important rating factors to consider:
Group Rating Factor – Vertical Spacing 
Group Rating Factor – Horizontal Spacing 
Rating factor for variation in ground temperature or duct temperature 
Rating factor for fluctuation in ambient temperature 
Rating factor for variation in thermal resistivity of soil
The cable manufacturers' catalogues provide all of these rating variables for various circumstances.
vii. Voltage Drop: Cables have resistance and reactance in them. As a result of the current passing through such an impedance, a voltage drop occurs. The loads connected by the cable should not be affected by this cable.
Actual voltage dips in cables are listed in V/km/A in the catalogues of cable manufacturers for various types of cables. IS 1255, an Indian standard, also mentions it (Code of practise for installation and maintenance of Flexible Power Cable up to and including 33 kV rating).
Not only should the steady-state voltage drop be calculated, but also the acceleration state voltage drop when significant loads are started.
Furthermore, the steady state voltage drop at the load terminals must be less than 10%, and the acceleration state voltage drop at the load terminals must be less than 15%.
viii) Short Circuit Current Withstand: The short-term current withstand capacity of any cable can be estimated using the formula below:
[(It) / K] = S
Where I is the Short Circuit Current in Amperes, t is the Short Circuit Duration in Seconds, and K is the Adiabatic Constant (= 115 for PVC/Copper, = 143 for XLPE/Copper, = 76 for PVC/Aluminum, and = 92 for XLPE/Aluminum).
Note: The duration (t) should be carefully chosen based on the fault clearing time of the isolating devices installed upstream of the cable. It must be assured that the highest isolating device's coordinated fault clearing time is taken into account.
How to Select Fire Cable
When multiple cables are densely arranged, their flame retardancy must meet the requirements of the appropriate category in accordance with "the experiment method of bundled wire and cable burning," cable configuration, catastrophic accident prevention, and the principles of economy and reasonableness.
(1) It is not advisable to run non-flammable and flammable cables in parallel in the same channel.
(2) The exposed cables in the following locations or circuits that must be powered within a particular time due to an external fire should be fire-resistant or flame retardant cables. Fire protection, alarm, emergency illumination, interrupter operation DC power supply, and emergency shutdown power supply of generator sets are all important circuits. When the same channel is not isolated from each other, computer monitoring, dual relay protection, security power supply, and other dual circuits share one of the circuits.
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