Conductor is material through which electrical current passes easily While insulator is the material which completely restrict the flow of current. Both conductors & insulators are opposite to each other. Copper, iron, aluminum are the types of conductors While Bakelite, porcelain are the types of insulators.
For a instant; It seems like conductors are more important than insulator. But Reality is, Both conductors & insulators are equally important. Both works Hand in Hand. Without insulator conductor is nothing, Conductor’s function only exist due to the insulator !
RESISTANCE
In Electrical, Resistance is the property of the material Which resist or oppose the flow Electrical current. Its been calculated with numerical value with a units ‘Ohm’.
” If we make a contact of an iron piece with potential/voltage, Iron piece will get the same potential. Further if the same piece get under the potential difference than current will flow through it. Similarly, If we do a same thing with a Bakelite material than no potential will exist on it & no current will flow under the potential difference.
An iron piece provides no opposition to current flow while a Bakelite provides the complete opposition to the current. We can say an iron piece has less resistance or no resistance while Bakelite is having a high resistance. “
Digital Insulation tester shows resistance value
INSULATION RESISTANCE
11KV Cable configuration | XLPE insulation around the conductor|
” Insulatedefinesseparatism & Insulation is a type of layer which prevents the energy leakages. Energies like heat, Electricity & light etc. In Electrical, Insulation is a type of layer around the conductor which prevents potential/current leakages. “
Insulator materials are used to make an insulation around the conductor, So that current only flows in defined path. The breakdown voltage of insulation around the conductive part; is multiple times higher than the system voltage. So that insulation works perfectly even in surge voltage scenario.
Motors, Transformer, Pumps- Maximum Electrical machines contains winding. Winding is a set of coils & each coil is prepared with multiple turns & Each turn is properly insulated so that current only flows in defined path.
Insulation been tested in term of resistance with the units of ‘ohm’. A routine test certificate of Induction motor is shown below which shows the insulation resistance value greater than 50 M-ohms.
Motor testing report by siemens shows the insulation resistance value of Induction motor
INSULATION RESISTANCE TESTING
Due to ageing of machines, environmental conditions, impurities, natural forces & continuous wear-tear, The insulation get deteriorated. Which result’s its insulation value get lowered. Lower insulation makes the chances of short circuit or system damage; higher. That’s why it is very important to record the insulation resistance of electrical machines; periodically.
“Maintained insulation record conveys about internal health of the system & it is the part of preventive maintenance program Which work as a savior for the critical systems. That’s why Insulation resistance known as a life predicator !”
For Insulation resistance measurement, Insulation testers are available. Insulation testers are available in both analog & digital forms. These testers are the types of voltage generating sources. Which generates the voltage comparatively at higher voltage than the working system’s voltage. That high generated voltage is get applied across the insulation of test specimen.
Through insulation tester, High potential difference get appear across the conductive part & metallic body of the machine. Due to high voltage, A resistance dependent current flows through the arrangement & finally; resistance value been calculated through the current flow at a fixed input voltage.
” A Digital insulation Tester by ESSMA shown in an image below. There are multiple voltage standards are provided like 750VAC, 500VDC, 1000VDC, 2500VDC, 5000VDC. “
Digital Insulation Tester by ESSMA
A GIF shows the working of Insulation Tester
A GIF above shows the working of insulation tester. One of the test lead of tester is connected to motor’s body. While another lead is connected to one of the terminal of motor. When test button is get pressed, Tester shows the resistance value of 52MΩ.
INSULATION RESISTANCE VS VOLTAGE
Both Resistance & voltage are the type of numerical values & numerical value has no end, Infinite !Insulation is proper insulator under the permissible volage limit. Beyond the voltage limit; insulation get break & starts conducting. That’s Why insulation is been chosen according the voltage level.
Voltage spikes & Voltage surges are also important to consider for the designing of insulation. The insulation resistance should high enough so that it withstand the voltage surges.
INSULATION RESISTANCE VS TEMPERATURE
“10°C temperature increase reduces the insulation resistance by half.” Temperature has drastic impact on insulation resistance That’s why a insulation level been chosen by considering the ambient temperature & System’s possible rise temperature.
BIL (BASIC INSULATION LEVEL)
Whenever a voltage surge appear across the system or lightning falls on the system, Lightning arrester diverts high potential surge towards the earth. Machines connected across the system, Should have the minimum insulation level such that it withstand the high voltage spike until surge arrester diverts the voltage surge.
That’s Why BIL– Basic Insulation level is defined for the Electrical machine according to the system’s rated voltage.
” A BILstandards for oil emersed power transformer by IEEE been shown below in a tabular form. In one of the example, We can see – Nominal system voltage is 25KV & Max system voltage is 26KV. The BIL standard is 150KV.”
IEEE standard for standard general requirements for liquid-immersed distribution, power & regulating transformers
SUMMARY
1. Resistance is the property of the material Which resist or oppose the flow Electrical current. Its been calculated with numerical value through the units ‘Ohm’. 2. Insulation is a type of layer which prevents the energy leakages. Energies like heat, Electricity & light etc. 3. In Electrical, Insulation is a type of layer around the conductor which prevents the potential & current leakages. 4. Due to ageing & Wear-tear, Insulation resistance get lowered which should be monitored at regular intervals.
5. To measure insulation resistance, There are multiple devices available in both digital & analog models.
Hope, You like the post. If you have any doubt, Suggestion or query please do comments !
