what is the Insulation Resistance (IR) Test for cables ?

What is the purpose of cable insulation?

Cable insulation is an important protective material for cable conductors. It is non-conductive, used to resist electrical leakage, prevents cable conductors from contact with other conductors, and protects the conductor from environmental threats such as heat, water, and chemicals. Poor or damaged insulation may result in short circuit, electric shock, or fire.
Because the insulation of a cable is so important in determining the cable's safety and electrical conductivity, at Keystone Cable, we ensure that all our cables are subject to passing the insulation resistance (IR) test (as part of our many tests) before product delivery to customers.

What is Insulation Resistance (IR) Test?

An insulation resistance (IR) test measures the resistance to current flow across it on a completed cable; it applies a test voltage to determine how effective the insulation is in preventing the flow of electric current out of the insulation. This is analogous to how you would pump pressurized water in a water pipe to identify leaks.

Since insulation starts to age after it is made, over time, the performance of a high quality insulation material versus one of lower quality will become more apparent. Hence it is important that after the cable is manufactured there is a good pass rate for the IR test to help ensure the longevity of your cable.


Insulation Resistance Test Process

IR test is conducted using an IR tester. The IR tester is a portable ohmmeter (MΩ.km) with a built-in generator that produces a high DC voltage. The DC voltage usually measures 500V and causes a current to flow around the surface of the insulation. This resistance reading measures leakage current; a high IR reading means very little current is escaping through the insulation and a low IR reading indicates stronger current leakage and may indicate a break in the insulation.

At Keystone's quality control laboratory, we adhere to International Standards IEC 60502-1 for our IR tests. To pass, the cables would need to obtain a minimum insulation resistance constant Ki (refer to the table below) while tested at its maximum operating temperature (e.g. 70 °C for PVC insulated cables and 90 °C for XLPE and rubber insulated cables).




For single core cables, the cables are tested in water while for multi core cables they are tested in air. Test results will also vary for cables across different types of insulation, length of run and ambient temperature. To be certain for your cable type, feel free to check in with our team on the IR tests we perform for your cables.

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INSULATION RESISTANCE OF A CABLE

Cable conductor is provided with an insulation of suitable thickness to avoid the leakage of current. The thickness of any cable depends on the purpose of its design. The path of current leakage in such cable is radial. The resistance or opposition offered by the insulation to the flow of current is also radial throughout its length.

For a single core cable conductor of radius r1, internal sheath radius r2, length l and insulation material resistivity ρ, the perimeter of the conductor is 2πrl. The thickness of the insulation will be given as dr.

Rins = ρdr/2πrl

When integrated, we will have:

Rins = ρ/2πl[loge r2 /r2 ]

Rins is inversely proportional to 1/l contrary to R = ρl. Where ρ (rho) is a constant known as resistivity.
There are some cables that have more than one insulating layers and more than one core. The main wire being at the center, serve as the main conductor. The other core serves the purpose of grounding and preventing the electromagnetic waves and radiations from escaping from the cabled. It serve as a shield. Cables under this category is the Coaxial cables.

Coaxial cable conducts electrical signal using an inner conductor (the inner or main conductor could be any good conductor but copper is mostly preferred because of it's low resistivity, the copper could also be plated) is contained in mostly PVC case. Before the outer PVC case, there are two or more other insulators with either aluminum foil or copper strand between them. The cables are protected from external environment by the outermost PVC case. While voltage is passed through the inner conductor, the shield or case has little or no voltage passing through it.

The advantage of coaxial design is that electric and magnetic fields are confined to the dielectric with little leakage outside the shield. Due to the level of insulation in the cables which prevents outside electromagnetic fields and radiations from penetrating into it, interference is avoided. Since conductors with large diameter have less resistance, less electromagnetic field will be leaked. The same goes for cables with more insulation. Knowing that weaker signals are easily interrupted by little interference, cables with more layers of insulation are always good choice for conveying such signals.

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R = KM log10(D / d )

R is insulation resistance (MΩ⋅km)
K is insulation resistance constant
M is the temperature correction factor to 15.6 °C
D is the diameter over the insulation
d is the diameter under the insulation

The insulation resistance of the sample under test is determined by the following formula:

R = 0.001* Rmeas

where
R is the insulation resistance (MΩ⋅km)
Rmeas is the measured insulation resistance (MΩ)
L is the length of the test sample (m)

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