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Cable Insulation Resistance

S. K. SrivastavaProf / Inst.

IRIEEN

Nasik Road

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?

1.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

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charged. The current passing through the insulation gets stabilized to steady state ohmic current

and we get constant value of Insulation Resistance.

2.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.

3.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.

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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.

3.1.Insulation Resistance constant and Volume resistivity

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

L*R

Insulation Resistance Constant = -----------------------------

(M ohm km) 1000 * Log10 ( D / d )

2 T *L*R*108

Volume Resistivity = ------------------------------(ohm.cm) Loge ( D / d )

Where

R = Measured Resistance ( M ; )

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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

IR Constant for cable with 100 m length = 100 x 5 /1000x Log10 ( D / d )

= 0.5 / Log10 ( D / d )

or say = 0.5 C (C is a constant)

IR constant for cable with 10 m length = 10x5/1000x Log10 ( D / d )

say = 0.05 C (C is a constant)

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.

3.2Limiting values as per some IS's

Now we have reached to a point where we can discuss the limiting values of IR constant

and Volume Resistivity of some of the cables:

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XLPE cables

Type of Cable Governing IS Test method Volume Resistivity (ohm.cm)

XLPE insulated

PVC sheathed up

to 1.1 kV

IS 7098 Part -1

1997

As per IS 10810

Part 43 - 1991

At 270

C 1x1014

At 900

C Under

consideration

XLPE Insulated

PVC sheathed

3.3 kV to 33 kV

IS 7098 Part -2

1988

At 270

C 1x1014

At 900

C 1x1012

Elastomeric Cable (IS 6380-1984)

Test method - As per IS 10810 Part 43 - 1991

IE1 IE2 IE3 IE4 IE5

IR Constant

(Min) At 270

C

(M ohm km)

700 3670 3670 10 870

IR constant

(Min) at

maximum optg

temp.

(M ohm km)

--- 3.67 3.67 -- --

IE1 to 5 are the types of cables.

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PVC Insulated cables (IS 694-1990 and 5831-1984)

Test method - As per IS 10810 Part 43 - 1991

Type A Type B Type C

IR Constant At 270

C

(Min) in M ohm km

36.7 36.7 36.7

IR Constant At max.

optg. Temp.

(Min) in M ohm km

0.037 0.37 0.37

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 (Annexure A )

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 nominalthickness of insulation has been taken as the insulation thickness.

Conclusion

The main objective of this study is to emphasize upon the finer aspect of the Insulation

resistance, which is most commonly, used electrical parameter for checking the insulation

strength. The following conclusions can be derived:1) The cable insulation resistance is to be measured with 300 30 or 500 50 volt dc supply

2) The sample length should be full drum length or minimum 3 m piece.

3) Based on the IS by which the particular cable is governed IR constant or The volume

resistivity is to be calculated and then the strength of the cable is to be judged.

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ANNEXURE -A

These values are for 1 m length cable. To find out IR Min for any other length divide the IR

minvalue by the length of the cable.

All cable cores have been assumed circular in shape.

PVC insulated cable voltage upto 1100 volts governed by IS 694-1990 and IS 58311984

Size of Cablenominalthickness d D IR Min

(Sq. mm) of insulation (mm) (mm) (mm)(M

ohm)

16 1.0 4.51 6.51 5845

25 1.2 5.64 8.04 5648

50 1.4 7.98 10.78 4793

XLPE cables 7098 Part 2 1988 6.35/11 kV cable


(Sq. mm) of insulation (mm) (mm) (mm)(M

ohm)

70 3.6 9.44 16.64 90241

150 3.6 13.82 21.02 66764

185 3.6 15.35 22.55 61240

240 3.6 17.49 24.69 54912

400 3.6 22.57 29.77 44083

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Elastomeric Cable up to 1100 volt type IE4


(Sq. mm) of insulation (mm) (mm) (mm)

(M

ohm)

6 1.0 2.76 4.76 2364

10 1.2 3.57 5.97 2233

16 1.2 4.51 6.91 1851

25 1.4 5.64 8.44 1750

50 1.6 7.98 11.18 1464

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