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The actual continuous maximum working voltage, wave shape (AC, frequency, DC, pulse etc.), duty cycle (short-time/long-term on-off time), operating conditions and severity of application must be first ascertained.  SIZE TABLE gives a choice of various conductor sizes/insulation wall thicknesses for the specified working voltage as well as the optimum smallest size (O.D.) cable.  For example, for 10 KV AC or 22 KV DC, the cable would be designated as AWG 18/19/30 X 5 CR, with 3.8 mm OD.  The first numbers give conductor AWG and stranding; X 5 represents minimum 1.2 5mm (50 mil) radial insulation of CR (Corona Resistant) PTFE.  For the same working voltage, above the stepped solid line, a wide choice is available for combination of conductor sizes, strandings, and radial insulation.  A careful study of this literature will help the user to properly specify his chooice, or answer our questions, or at least ask the right questions.

The cables must be suitably de-rated for use at higher temperatures, higher altitudes, and higher frequencies, typically to about 40% under combined effect of operation at 20 km altitude at 100°C and at 400 Hz.  It should be carefully noted that DC ratings are about 2.25 times the AC (50/60 Hz) ratings.  While describing the applications, the voltages must at all places be clearly stated as DC or AC.  Distinction must always be made between the actualworking voltage (AC/DC, worst-case, transients, etc.), proof/test voltage (AC or DC), and the desired minimum Break Down Voltage (BDV).  Each of these must be accurately and separately specified.  Also, the test procedure and the desired tests on end-cut samples and on 100% lengths should be specific.

The life of insulation under corona attack is influenced by:

1. Insulation Type: CR PTFE is by far the best amongst PVC, silicone, PE, plain PTFE etc.
2. Frequency of Corona Bombardment: Life is sharply higher for DC voltages but inversely proportional to voltage, from power frequencies to about 100 MHz.
3. Intensity of Corona increases with increase in applied voltage, altitude, and decrease in conductor size.
4. Conductor size and Insulation Thickness:  The smallest size cable for a given rating has a ration of about 3 between Overall Dia (OD) to Conductor Dia (CD).  For higher voltages, the CD, and consequently the OD, has to be increased.  Power handling capacity and cable weight considerations may dictate OD to CD ration lower or higher than 3.

SIZE TABLE gives nominal dia of HVCR cables for conductor size ranging from AWG 28 to AWG 10 (7, 19, 37 or larger numbe of strands) with CR insulation thickness ranging from 0.5 mm (20 mils - designated as X 2 CR) to 2.5 mm (100 mils - designated as X 10 CR).  The actual continuous working voltage (for 10,000 hour life) ranges from 4 KV to 22 KV AC (or 9 KV to 50 KV DC).  Top-right portion of the charte above each step line is the safe selection zone for the voltage specified on each line.

For use upto 200°C, Silver Plated Copper (SPC) conductors are standard; high-strength copper alloy is recommended for sizes finer than AWG 22/19/34 when insulation thickness beyond X 5 is selected.  For up tp 260°C, Nickel Plated Copper (NPC), or alloy, or stainless steel conductors can be ordered.  Alloy conductors are recommended for high vibration/flex-life applications.

Many types of shielding are available to reduce the corona RFI pick-up on nearby circuits.  The shield also equalises high electric fields caused by grounded sharp-pointed objects.

Overall jackets of several types including PTFE are available.  All solid or striped (one or two) colours, and formations of HVCR cables into flat bonded or woven, or multicore cables are available.  For larger sizes, varnished fibre glass (VFG) jackets are advised because PTFE jackets are difficult to process.

In some recent tests on long samples of AWG 18/19/30 X 10 CR, rated at 16 KV AC/36 KV DC, repeated application of HV DC of 140 KV did not cause any breakdown,.  Similarly, for AWG 20/19/32 X 4 CR rated for working 8 KV AC/18 KV DC, no breakdown was found at 115 KV DC applied repeatedly.  Work is in progress on 100 KV DC HVCR cables (within 8 mm size), and lighter weight and more flexible HVCR cables.

SELECT GRADED (SG) HVCR cables for up to 30% higher ratings or longer life (than our normal cables of the same size), for critical applications, are available on demand.
 

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