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Home » Power Brand » Solutions for testing Insulation resistance

Solutions for testing Insulation resistance

October 16, 2019 5:45 pm

Solutions for testing Insulation resistance
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Insulation testing is very critical type of test for checking the quality of insulation and prevent unwanted breakdowns. Insulation starts to age as soon as it’s made. As it ages, its insulating performance deteriorates. Any harsh installation environment, especially those with temperature extremes and/or chemical contamination, accelerates this process. This deterioration can result in dangerous conditions in power reliability and personnel safety. As such, it’s important to identify this deterioration quickly so that corrective steps can be taken.

What is Insulation?
In simple terms Insulation is nothing but a material which opposes the flow of current. Technically, it contains almost negligible free electrons for the flow of current. There are different types of Insulation material present on Earth which can be classified as Solid (Ceramic, Glass, plastic), Liquid (mineral oil, castor oil) and Gaseous (SF6, Air, etc).

The insulation material is classified according to the temperatures which they can sustain and not break down.

This evokes us with a thought that why do we need insulation? The first and basic need of insulation is to provide safety to the human beings and to the circuits in the instruments. In any electrical equipment, the current has to flow along designated conducting pathways (called conductors), Insulation prevents currents from taking other unintended path. Insulation has got the inherent high electrical resistance property which does not allow contact with live circuits and hence ensures safety also maintain the voltage at the terminals of the load, avoid arcing to a different voltage or earth, avoid electrocution. Hence all electrical equipment’s are manufactured by using insulation materials of proper grade in its construction.

So now it’s evident that how important insulation is, taking this ahead and as we wrote that every electrical/electronic equipment has some kind of insulation over it.

As Human beings our age increases we get affected with several diseases, our body starts degrading and similar are with insulation. As its continuously exposed to different conditions the degradation in insulation takes place.

There are five types of stresses which lead to degradation in quality of insulation such as, Electrical stress, Mechanical stress, Chemical attack, Thermal stress, Environmental conditions.

What will happen if this degradation of insulation goes undetected?
As a quality control measure at the time of manufacturing of the electrical equipment, a basic value of insulation resistance is available for reference

To ensure that the required specified insulation values are met for a new installation

  • Periodical preventive maintenance task
  • Used as a tool for trouble shootings in the power system /equipment’s
  • To avoid unexpected breakdown of the high voltage equipment
  • To monitor the rate of change of deterioration of health of insulation
  • Saving high cost of replacement of the material

Spot Insulation Test
The most basic test for insulation resistance testing is Spot Insulation test. It suggests the application of test voltage for 1 minute and then note the value of Insulation Resistance. Note when your devices have a very small capacitance then spot test will be all what you need. But mostly equipment’s are capacitive so spot insulation test is an indicator of how good or bad insulation is!

Whenever a Device under Test is subjected to insulation testing and if its highly capacitive in nature then, three types of current flows through the insulation which are:

Capacitive current
The initial burst of current that occurs when voltage is first applied to a conductor is called capacitive current. Like the first rush of water flowing through a hose, it typically starts out high and then drops quickly once the conductor is fully charged.

Absorption current
Like capacitive current, absorption current also starts out high and then drops. However, it falls at a much slower rate. As the voltage builds up, the absorption level in the insulation decreases. This gradual change reflects the storage of potential energy in and along the insulation. Incidentally, absorption current is an important part of the time resistance method of insulation testing.

Leakage current
Also commonly referred to as conduction current, the small, steady current present both through and over the insulation is called leakage current. Any increase in leakage current over time is usually an indication of deteriorating insulation. This would be noted on the insulation test meter as a decrease in resistance.

Most of the devices are capacitive so spot test is just an indicative of good or bad insulation. However, we have further tests which are also called as diagnostic test to have a detail analysis of quality of insulation.

The PI (Polarisation Index) measures the time required for molecules of insulation to polarise to resist the flow of current. For testing high capacitance systems such as large motors, generators, or apparatus with complex insulation systems. Steady readings are only possible after the time-dependent currents have almost died down.

PI Test measures the ratio between insulation resistance readings taken at one minute and ten minutes after the application of voltage to judge the rate of disappearance of capacitance and absorption currents. A low polarisation index usually indicates excessive moisture and contamination.

Dielectric absorption ratio is the ratio between insulation resistance readings taken at 30 seconds and 1 minute after the application of voltage to judge the rate of disappearance of capacitance and absorption currents.

Step Voltage Test / Ramp test is basically applying the different test voltages in steps for fixed duration of time to check the behavior of Insulation at different test voltages. As per standard generally the variation of insulation resistance between consecutive readings should not be greater than 25per cent if it is observed beyond 25per cent then there is some problem with the quality of insulation. Nowadays more popular test is the Ramp test where instead of in discrete steps, voltages are gradually increased and the leakage current is graphed continuously against voltage rise. Normally, the relationship is expected to be linear, but if current begins to rise more sharply, the material is breaking down and the test can be terminated prior to the occurrence of full breakdown. The Ramp test has been evaluated by standards agencies, and is described in IEEE 95-2002. The evaluation of test results first prescribes that insulation in good condition should produce a smooth, almost linear rising current curve against the applied voltage. Any deviation should be viewed as a warning that the test could be approaching a possible breakdown. A sudden increase in current usually indicates imminent breakdown, and it has been possible to approach as closely as 5per cent below breakdown voltage.

Thus we have seen different diagnostic test which can be performed to evaluate the quality of installed insulation on any electrical equipment and how these different test methods would be useful to assess the insulation health.

We have some recommendations published by international testing agencies which we will elaborate so that it would help us to know the different test voltages which would be suitable to test insulation resistance.

The different safety requirements to be followed while performing the insulation resistance test at different devices under test to ensure the safety of operator. Following are some of the safety requirements

  • All equipment under test must be disconnected and isolated
  • Equipment should be discharged (shunted or shorted out) for at least as long as the test voltage was applied in order to be absolutely safe for the person conducting the test.
  • Never use Insulation tester in an explosive atmosphere.
  • Make sure all switches are blocked out and cable ends marked properly for safety.
  • Cable ends to be isolated shall be disconnected from the supply and protected from contact to supply, or ground, or accidental contact.
  • Erection of safety barriers with warning signs, and an open communication channel between testing personnel.
  • Do not test when humidity is more than 70 per cent.
  • Good Insulation: IR reading increases first then remain constant.
  • Bad Insulation: IR reading increases first and then decreases.

Rishabh offers solutions for testing Insulation resistance through its various products like Rish Insu 10/20 1KV Digital Insulation Tester, Rish MIT 30- Multimeter cum Insulation Tester, Rish Insu 5000 5KV Analog Insulation Tester, Agam AIT 501- 5KV Digital Insulation Tester, Rish Insu 5Dx- 5KV Digital Insulation tester with diagnostic tests like PI, DAR and Ramp test suitable for testing in live switchyards.

Thus different important aspects on Insulation testing has been tried to cover in the article. But again lots of things yet remain to pen down however we need to curtail at some point.

For further details contact:
Vishwadeep Nanda
Product Manager- TMI,
Skype Id: vishwadeep.nanda
Landline : +91 253 2202071
Mobile +91 7757005930
Email: vishwadeep.nanda@rishabh.co.in
Web: www.rishabh.co.in

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