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Electrical

Electrical Properties



Dielectric Constant  ASTM D150

  • Measure of the capacitance of the insulation or the ability of the material to store electrical energy
  • Low dielectric constant is desirable to minimize electrical losses and minimize interference with the passage of high frequency energy
  • Capacitance of the insulation material determines the maximum length of line over which the signal may be transmitted without excessive distortion
  • The dielectric constant of air, a perfect dielectric is 1.0
  • Non polar polymers like PE, PP have lower dielectric constant (2.2- 2.6) than polar polymers such as PVC, Nylons and PBT 


Dielectric Constant (Relative Permittivity)

                                                                        ε = Cs / Cv 

                                                where Cs is the capacitance with the specimen as the dielectric, 
                                                and Cv is the capacitance with a vacuum as the dielectric.

The ratio of capacitance of a capacitor with test material as the dielectric to the capacitance of a capacitor with a vacuum as the dielectric. For materials that are to be used to insulate electrical components the dielectric constant should be low. For materials that are to be used as a dielectric in a capacitor, the dielectric constant should be high so that the capacitor dimensions can be minimized. The dielectric constant can be calculated using: 


Power factor/Dissipation factor ASTM D150


  • Measures the power loss dissipated as heat due to build-up and collapse of the electric field within the insulation
  • Power factor and dissipation factor are numerically equal to each other when the dissipation factor does not exceed 0.10
  • A perfect insulation would have a power factor of 0.0
  • To ensure maximum efficiency and predictability of cable performance the power factor of the insulation needs to be low and uniform throughout the length of the cable
  • Non polar polymers like PE, PP have lower power factors (0.001 - 0.1) than polar polymers such as PVC, Nylons and PBT (0.1 - 1.0)Dissipation Factor


The ratio of the power dissipated in the test material to the power applied. Equal to the tangent of the loss angle, or the cotangent of the phase angle. The dissipation factor can be calculated using:

                                            D = tan δ = cot θ = 1 / (2π f RpCp) 

                                            where δ is the loss angle, 
                                            θ is the phase angle, 
                                            f is the frequency, 
                                            Rp is the equivalent parallel resistance, 
                                               and Cp is the equivalent parallel capacitance.

Dielectric Strength ASTM D149

  • Measure of the voltage at which the insulation breaks down electrically and abruptly becomes a conductor
  • Varies with the thickness of the material, the rate and time of application of voltage and temperature.
  • Expressed in volts/mil or volts/mm
  • The dielectric strength of a material decreases with an increase in temperature
  • Non polar polymers like PE, PP have higher dielectric strengths (600- 800 v/mil) than polar polymers such as PVC, Nylons and PBT (400-600 v/mil)
Scope:
Dielectric Strength is a measure of the electrical strength of a material as an insulator. Dielectric strength is defined as the maximum voltage required to produce a dielectric breakdown through the material and is expressed as Volts per unit thickness. The higher the dielectric strength of a material the better its quality as an insulator. 

Test Procedure:
There are three basic procedures that can be used to determine the dielectric strength of an insulator. These procedures are the short-time method, the slow rate-of-rise method and the step-by-step method. Each of these three methods has the same basic set-up, which consists of the test specimen placed between two electrodes in air or oil. 

For the most common test, the short-time method, voltage is applied across the two electrodes and raised from zero to dielectric breakdown at a uniform rate. Breakdown is when an electrical burn-through punctures the sample, or decomposition occurs in the specimen. The rate of voltage rise is determined by the time it takes the sample to reach dielectric breakdown. 

The slow-rate-of rise method starts at 50% of the breakdown voltage as determined by the short-time-method and is increased at a uniform rate. 

The step-by-step method starts at 50% of the short-time-test then voltage is increased at equal increments for a specified time period until breakdown. The test is sometimes performed in oil to prevent arcing from the electrode to the ground. 

Specimen size:
The recommended specimen type for this test is a 4 inch plaque or larger. Any specimen thickness can be used, however the most common thickness is between 0.8 to 3.2 mm (0.032 to 0.125 inch). Specimens over 2 mm thick are typically tested in oil to decrease the chance of flashover before breakdown. 

Data:
Dielectric strength is calculated by dividing the breakdown voltage by the thickness of the sample. The data is expressed in Volts/mil. The location of the failure is also recorded. A higher dielectric strength represents a better quality of insulator.

 
SOP:

   1.   Rotate the voltage control knob (1) to its maximun ccw position. Lock the cutoff arm in the 10 o'clock position.
   2.   Place the High voltage on-off switch (2) in its off position.
   3.   Rotate the rate of rise (3) control to its maximun ccw position.
   4.   Meter function switch(4) to .000V.
   5   Current set lock(5) control maximun ccw. Current set control to its maximun cw position.
   6.  Throw the power switch(5a) to on. Allow unit to warm up two minutes.
   7.   Digital panel meter should read .000  +- .003.  Rotate Meter Function(4) switch to 1.900V. Meter should read 1.900  + - .003. If the above values were not  
         obtained,  calibrate the digital panel meter per paragraph 5.1.                    
   8.   Secondary current limit adjustment.
         (a) Set the meter function switch (4) to the current set .
         (b) Turn the rate-of-rise control(3) control clockwise until the desired current ( 12.0 ) is displayed on the digital panel meter. Insure that the limit range 
                specified in paragraph 1.3 is not exceeded. If the desired current is less than 10.0 milliamps, depress the EXT CALIB switch and keep it depressed   
                while performing steps c and d.
         (c) Turn the current set(5) control ccw until the limit lamp glows. Readjust the current set control to a point where a slight adjustment of the control will cause
                the limit lamp to change state.
         (d) Lock the setting by turning the current set lock(5) clockwise.
   9.   Insert the test specimeninto the the fixture.
 10.   Varify the black ground lead to the low terminal on the test fixture is in place.
 11.   Set the Meter Function (4) to kilovolts. Zero # 5 to reset Secondary light.

 12.   SHORT-TIME TEST
 13.   Set the rate-of-rise to the desired rate-of-rise using the chart at the end of the manual. ( 500 volts second)
 14.   Depress the zero(5) push button switch. Switch the High Voltage on-off (2) switch to on.
 15.   Place the Forward-Reverse switch(8) in the forward position.
 16.   Press the Drive button.(9)
 17.   The instrument will now automatically increase the voltage across the test specimen, at the desired rate, until breakdown occurs. When breakdown 
           occurs, the reset circuit breaker will automatically remove the voltage.The digital panel meter will retain the breakdown voltage until the switch is 
           activated.
 18.   The voltage control should be rotated to its maximun ccw position before the circuit breaker's reset switch is activated.
 19.   The voltage control may be reset by hand or can be driven back at full speed by throwing the forward-reverse switch to the reverse position.
 20.   With the voltage control in its maximun ccw position, press the zero(5) switch and then reset (7) switch. This will cause the digital panel meter reading to 
          decrease towards zero. A small residual voltage will be noticed when the zero(7) switch is released. This is normal and will not affect the calibration of the
          instrument.    
 21.   To check volts per second, use stopwatch to check amount of time verses breakdown during a test. # of seconds from start to breakdown.
          Breakdown  volts / seconds . You shooting for 500 volts / second. 



Volume Resistivity / Surface Resistivity (ASTM D257)

  • Generally two leakage paths, one through the insulation and the other along its surface
  • Volume resistivity is the resistance in ohms 1 cm in length and 1 cm2 in cross-sectional area, expressed as ohm-cm.
  • Surface resistivity is the surface component of resistance between opposite sides of a square sample of any size and expressed as ohms/cm2
  • Volume and surface resistivities are both adversely affected (decrease) by the presence of absorbed moisture
  • Non polar polymers like PE, PP have higher volume resistivity values (1014 – 1017 ohm-cm) than polar polymers such as PVC, Nylons and PBT (1010 – 1014 ohm-cm)

Scope

1.1 These test methods cover direct-current procedures for the measurement of dc insulation resistance, volume resistance, and surface resistance. From such measurements and the geometric dimensions of specimen and electrodes, both volume and surface resistivity of electrical insulating materials can be calculated, as well as the corresponding conductances and conductivities.

1.2 These test methods are not suitable for use in measuring the electrical resistance/conductance of moderately conductive materials. Use Test Method D 4496 to evaluate such materials.

1.3 This standard describes several general alternative methodologies for measuring resistance (or conductance). Specific materials can be tested most appropriately by using standard ASTM test methods applicable to the specific material that define both voltage stress limits and finite electrification times as well as specimen configuration and electrode geometry. These individual specific test methodologies would be better able to define the precision and bias for the determination.

1.4 The procedures appear in the following sections:

This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.


SOP

 Using Keithley Model 6517A

Turn power on ~ allow ~ 15 minutes to warm up.
To select volume on surface resistivity – press config. Then press R on front panel.
Arrow > right through menu until you come to MEAS – Type
Press enter – resistivity will be blinking – Press enter
Config. Resistivity screen will appear – select vol. Or surface.
The one you choose should match the one you select on the 8009 Test Fixture box – change setting by depressing button in lower left of 8009 Test Fixture box to match selection in 6517A.
If volume is selected – the next screen will be thickness.
Thickness will be blinking – press enter – use micrometer from the intron to measure thickness of specimen in MM – one measurement will do.
Enter the measurement by using  (◄►) and range (▲▼) buttons.
Press enter after done.
Press exit four times to get to the test screen.
Press Seq. (sequence = ALTPOL Rs) – press enter.
ALTP Seq. - waiting for trigger screen – press auto on right side
Center the specimen above the rubber contacts in the 8009 Test Fixture then close lid.
Press trig (trigger) to begin the test.
Auto should be lit on screen.
At completion of test – record data (1.21 x 1015)
Remove specimen.
Press config. Then R and start measurement sequence again.

 Surface Resistivity – Keithley Model 6517A
                 (NO MEASUREMENT NECESSARY)

Press Resistivity button on 8009 Test Fixture – Surface.
On 6517A Box, press Config., then press R.
Arrow ► right to meas – type – pres Enter – Resistivity – Enter.  ◄ Arrow Left to select Surface – Enter.
Model – 8009 user screen.
Press Exit four times to get to test screen.
Verify Unit is set to Auto mode.
Press Seq. ( sequence=ALTPOL Rs) - press enter
ALTP Seq.- waiting for trigger screen-- then press auto on right side
Test is now ready to run.
Center specimen over rubber contacts in the 8009 Test Fixture.
Close lid.
Press Trig (trigger) on 6517A.
Test will begin.
After completion – record data on Surface Resistivity Test Sheet.
Remove Specimen.
Put next specimen in fixture.
Press trig to begin next test.



Arc Resistance/ Track Resistance (ASTM D495)

  • Ability of the material to withstand the discharge of a specified voltage across its surface
  • Test sample is placed between tungsten electrodes in a test assembly with 12,500 volts AC placed across the electrodes.The arc resistance of the material is determined by the total elapsed time of arcing exposure until tracking occurs.
  • Non polar polymers such as PE, PP are highly track resistant as compared to polar polymers. The addition of polar additives such as FRs or fillers can significantly lower the track resistance of the product.


Arc Resistance

The number of seconds that a material resists the formation of a surface conducting path when subjected to an intermittently occurring arc of high voltage, low current characteristics. The results at 3 mm thickness are considered representative of the material's performance in any thickness.

Performance Level Categories (PLC) were introduced by UL to avoid excessive implied precision and bias.

Mean Time of Arc Resistance (sec)PLC
420 and longer0
360 through 4191
300 through 3592
240 through 2993
180 through 2394
120 through 1795
60 through 1196
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