Insulation Resistance of Cables

Courtesy: study made by Prof. Shri S K Srivastava, IRIEEN


Insulation resistance is the most commonly measured parameter to check the health of Electrical Insulation. In case of cables the IR differs from cable to cable depending upon the length of cable, insulation thickness and type of cable. For example IR value of 1 meter cable and IR value of 100 meter of same cable are not comparable in absolute term as the later value will be 100th of the former. This paper tries to answer most commonly asked questions such as:

            1) What should be the megger voltage?

            2) What should be the sample length?

            3) What are the limiting values?

            4) What is the governing IS?

           

Insulation Resistance

            The Insulation resistance is defined as the Quotient of applied voltage divided by the current measured at a given time from the start of electrification.  We normally measure the insulation resistance by hand driven megger. Initially when we start rotating the instrument, the capacitance across the insulation gets charged due to which the initial current is high thus giving impression of low IR. When the megger is uniformly driven at constant speed for a definite time (1 Min), the voltage applied across the insulation becomes constant and capacitance gets fully charged. The current passing through the insulation gets stabilized to steady state ohmic current and we get constant value of Insulation Resistance.

Voltage and Sample length for IR measurement

           

            If we go through the IS specification for various types of cables i.e. XLPE cable, Elastomeric Cable and PVC insulated cable , we will find that the test method for Insulation resistance for these cables is as per IS 10810 part 43 - 1991 . This IS deals with the measurement of the insulation resistance of the cables. As per above IS the voltage for IR measurement should be 300 ± 30 or 500 ±50 Volt dc.

            At this point there is always one question whether HV cables are to be tested by 300 ± 30 or 500 ±50 volt dc IR meter then the answer is yes. If we go through the IS of cables even for HV cable the test method is as per IS 10810 Part 43. And this IS specifies the voltage for the measurement of IR as 300 ± 30 or 500 ±50 volt dc. Thus the voltage rating of cable has no relation to the voltage to be applied for the measurement of IR.

            As far as the sample length is concerned as per the relevant IS i.e. IS 10810 Part 43 it should be full drum length or not less than 3 meters.

Limiting Values

            Once we are measuring certain parameter to check the condition of insulating material or for that matter for any system then we must know the limiting values. Limiting values means what is the acceptable limit and what is the governing specification.  Since we are discussing here the Insulation resistance parameter for the cable then we must know the limiting values for this and governing specification.

            Before going to the parameters some fundamental observations regularly made in the measurement of IR of cable, needs to be discussed.

            Let us take a condition where we are carrying out the measurement of two cables of same type and size but with different length say 100 m and 10 m. The measured IR comes same say 5 M ohm then which cable insulation strength is better. The next question comes if one is better then how much better.

            Take another condition in which the length of cable is same say 100 m in both the case and type i.e. material of the cable is also same but size is different. In this case also if the measured IR value is same say 5 MOhm then which cable is better.

            If we go through the IS for cables we will find that the limiting values for Insulation resistance is not given directly. Instead in these IS specifications the limiting value for Insulation resistance constant or the volume resistivity is given. These parameters take into account the above factors i.e. the size of the cable and the length of cable.

Insulation Resistance constant and Volume resistivity

           

The insulation resistance constant and volume resistivity parameters are defined as under:

Where

            R         = Measured Resistance ( M W )

            L          = Length of Cable  (m)

            D         = Diameter over the insulation excluding screen if any ( mm )

            d          = Diameter over the conductor excluding screen if any ( mm )

Incase of shaped conductor D is to be replaced P and d is to be replaced by p where

            P          =  Perimeter over the shaped insulated core ( mm )

            p          =  Perimeter over the shaped conductor (mm )

            Let us take the same condition, which has been discussed in para 3. Two cables of same type and same size but of different length i.e. 100 m and 10 m are showing the same value of Insulation resistance ( 5 M ohm ). The IR constant for these two cases will be

 

We can see here a distinct difference in the health of two cable, which were showing apparently same value of Insulation resistance i.e. 5 M ohm. Similarly we can take the second condition in which the cable size is different but all other parameter like type of cable and length of cable is same. Even if it is showing same value of IR , the IR constant will be different for different size of cables, because of Log₁₀  ( D / d ) factor.  We will be able to differentiate between the health of two cables which apparently showing the same value of IR.

Limiting values as per some IS's 

 

 

Where Type A,B,C are the type of cable.

            From the above it can be seen that the minimum value of IR constant or Volume resistivity of the cable is specified in the specification of the cable. Also the test methods for these cable are as per the IS 10810 Part 43 1991.

            For better appreciation the IR min value of 1 m length has been calculated (see below) for certain typical cables from the limiting values of IR constant  and volume resistivity as per the corresponding IS. In all these cases circular cable has been assumed and the nominal thickness of insulation has been taken as the insulation thickness.

What is Dielectric withstand Test?

Dielectric withstanding test is used to evaluate the voltage at which insulation between two electrodes gets completely damaged allowing conduction. 

Dielectric testing is a simple, nondestructive method of verifying the adequacy of electrical insulation to withstand transient (surge) events. Transient voltage spikes on power lines are generally the result of nearby lightning strikes, but spikes can also occur for other reasons. In general, such transient spikes have a very short duration--the spike lasts for <20 microseconds. 

A dielectric test can verify the performance headroom of the insulation, ensuring that the insulation will not fail because of insulation degradation from aging, moisture, wear due to vibration, or other causes. 

The voltage level of the dielectric test is generally adjusted based on the environmental conditions to which the end product will be subjected. A higher dielectric test voltage is used for equipment located where environmental conditions are more severe. Passing this more-severe dielectric strength test when the end product is new indicates that the insulation being stressed has enough headroom to provide adequate protection after the end product has been subjected to environmental degradation.

Test Method: In dielectric testing, a high voltage (typically ≥1000 V) is applied between two conductors that are supposed to be electrically insulated from each other. If the two conductors (e.g., an insulated live wire and a metal enclosure) are completely isolated from one another, then the application of a large voltage difference between the two conductors would not allow current to flow between the conductors. In this case, the insulation is said to withstand the application of a large
voltage potential between the two conductors, hence the term dielectric withstand test.

In general, two dielectric test results indicate insulation failure. The first is excessive current flow during the test due to low insulation resistance of the insulating material separating the two conductors. The second is an abrupt dielectric breakdown due to electrical arcing or discharge,
either through the insulation material, over the surface of the insulation
material, or through the air. 

Test Voltage: If the test voltage is too low, the insulation material will not be adequately stressed during the test, allowing inadequate insulation to pass the test. On the other hand, if the test voltage is too high, the test could cause permanent damage to an insulation material that is otherwise adequate for the application. A general rule of thumb used for testing mains wiring that operates at voltages of 120–240 V ac is 1000 V plus two times the operating voltage. Using this rule, 120-V wiring
would be tested using a voltage of 1000 V + (2 x 120 V) = 1240 V ac. 

Test Duration: To adequately stress the insulation, the test voltage is  generally applied for 1 minute. However, many standards allow the test duration to be reduced to 1 second for production line testing to accommodate the large volume. For reduced-duration testing, standards often require the test voltage to be increased by 20% to ensure that 1 second is sufficient to test the insulation adequately.

AC-DC Dielectric Withstanding test- which is advantageous: For instance, An ac test voltage of 1000 V rms will have voltage peaks of 1414 V. Therefore, if a dc test voltage is used, the test voltage must be increased to 1414 V dc to produce the same level of stress to the insulation as would 1000 V ac rms.

The difference in test voltage for dc compared with ac is supported by national testing and standards writing organizations such as Underwriters Laboratories, Factory Mutual Corp., the Institute of
Electrical and Electronic Engineers, and the American National Standards Institute, as well as international organizations such as the International Electrotechnical Commission.

When compared with an ac dielectric test, dc testing offers many advantages. The maximum allowable test current can be set to a much lower level (1 mA is typical). The dc tester shuts down when more than 1 mA of current flows during the test. This highly sensitive test allows
the operator to identify marginal constructions that would have been overlooked by an ac tester.
The lower test-current levels are significantly safer for the operator. At 1mA, the current is enough to shock the operator, but the test current would be automatically shut off when the current flow exceeds 1 mA.

Dc testing offers significant advantages over ac testing. Dc and ac testing provide an equivalent level of breakdown detection due to total insulation failure. However, the heightened accuracy of dc leakagecurrent detection allows marginal insulation systems to be detected. Dc dielectric testing is superior for ensuring operator safety. Neglecting to consider dc testing as an alternative to ac testing potentially jeopardizes both the test operator (with shock hazards during testing) and the
consumer (with marginal insulation).

Courtesy: Compliance engineering; www.ce-mag.com

Does energy is associated with intrinsic inertia, the rest mass?

We could have the object-at rest-annihilate with an "anti-object", producing radiation possessing energy. so, yes there is energy associated with the rest mass.

While creating object, various particles like electrons, protons, neutrons are to be brought in together. Every bit of energy that goes into creating the object would be accompanied by an increment in inertia according to relation E = mC2

The attribute energy is always accompanied by attribute inertia.

In 1905, Einstein summarized his theoretical disclovery with the sentence,

" The mass i.e. inertia of a body is a measure of its energy content."

Energy in all forms has property of inertia, the reluctance to undergo a change in velocity. The more energy that went into forming a body, the more inertia the body has.

What is Equal Apriori Probability?

A fundamental postulate of statistical mechanics is that a macroscopic system in equilibrium is equally likely to be in any of its accessible microscopic states satisfying macroscopic conditions of system. It is called the postulate of equal apriori probability.

According to this postulate, the probability of finding the phase point for given system in any region of phase space is identical with that for any other region of equal extension or volume. This may also be stated as for a system in equilibrium, all accessible microstates corresponding to a given microstate are equally probable.

The basis for the postulate of Equal Apriori Probability is ergodic hypothesis. The constancy of volume element in phase space with time indicate validity of postulate of Equal Apriori Probability.