Insulation resistance measurement | Insulation resistance testing

This page of test and measurement world describes Insulation resistance measurement and Insulation resistance testing. It mentions popular manufacturers of insulation resistance testers used for testing of insulation resistance.

What is Insulation resistance?

• Insulator is a material which do not allow conduction of heat or electricity. This is due to the fact that conductors do not have free electrons.
• Conductor cables are called insulated when they have insulating materials around such as PVC etc.
• This process of using insulator around conductor is known as insulation.
• Opposition offered by insulation material to the flow of current radially throughout the length of the conductor cable is known as insulation resistance.
• There are many benefits of insulation e.g. to prevent leakage of current, to provide resistance to cuts or tears, to provide resistance to oil and chemicals etc.

Single Core Conductor Cable

The figure depicts single core conductor cable with either copper or aluminium as conducting material. It shows XLPE material as insulation. PVC sheath is also used as protective layer.

Insulation layer functions opposite to conductor wire. It keeps current on the path along the conductor. It is similar to water hose protecting water from leak on its way from water pump house to residences.

Insulation Resistance measurement | Insulation Resistance Testing

Insulation Resistance test setup

Insulation resistance (IR) testing is used in quality control measurement. IR test is also known as megger. Megohmmeter or specially designed insulation tester is used for the measurement of insulation resistance. Megohmmeter has higher output compare to conventional multimeter. It uses megohms (MΩ) as units of measurement unlike Ohms used by multimeter. It has very high internal resistance to withstand against high voltage used during measurement. Make connections as shown in the figure-2 (test setup) for insulation resistance measurement of the conductor cable. One of the leads is connected to the conductor and the other lead is connected to insulation material during the testing.

Insulation resistance formula

DC voltage of about 250/500/1000/2500 or 5000 is used for the measurement of insulation resistance. Formula or equation for insulation resistance of single core cable is shown in figure-3. In the formula, r1 denotes conductor radius, r2 denotes internal sheath, l is length and ρ is resistivity of insulation material. The resistivity is measured in Ω-m. From the equation it is imperative that insulation resistance varies inverse to cable length.

Popular Insulation Resistance Testers

Following table mentions popular manufacturers of Insulation Resistance Testers.

Manufacturers Description
FLuke Model: 1507
Description: It measures insulation in the range from 0.01 MΩ to 10GΩ. It uses insulation test voltages 50/100/250/500/1000 Volts.
HIOKI E.E. CORPORATION Japan Company manufactures Insulation Testers (i.e. Megohmmeters) for low voltage and high voltage testing. They offer both analog and digital meters.
KYORITSU ELECTRICAL INSTRUMENTS WORKS, LTD. The company offers analog and digital insulation testers for various voltage ranges.
Powerpoint Engineering The company offers electrical test, monitoring and safety equipments. It offers insulation resistance testers for various applications.
AEMC Instruments The company offers various models of Megohmmeters for insulation resistance testing.

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