عکس العمل عایقهای مختلف در برابر ولتاژ تست اعمالی، بستگی به وقوع پدیده های مختلف که عمدتاً تابع زمان و ثابت زمانی خاص خود می باشند، دارد. در ثانیه های اول شارژ خازنهای معادل دسته ای از رفتارها را موجب می شود. در فاصله یک دقیقه اول دسته ای دیگر از رفتارهای پلاریزاسیون که منجر به جریانهای absorption می شود خود نمایی می نماید و در فاصله حدود 10 دقیقه جریانهای نشدی و کانداکتیو خود را کاملاً نشان می دهند. بنابراین تست عایقی تجهیزات الکتریکی نمی تواند مستقل از زمان باشد. در استانداردهای مختلف از جمله IEEE منحنی جریانهای مذکور بر حسب زمان آمده است.
بر همین اساس رفتار عایقی عایق های مختلف در شرایط AC و DC متفاوت خواهد بود.
The dielectrics have the property of both temporary and permanent absorption of electrical charges and property of conduction. When a voltage is applied to a dielectric, forces on the positive and negative charges inherent in the particles which make up the dielectric tend to orient the particles in line with the applied field. Some dielectric materials have molecules that have uneven number of atoms, that is, having asymmetrical arrangement of charges. When such a molecule is placed in an electrical field, it will migrate in an electric field, thus become polarized with the electric field. Such a molecule is called a dipole. Dipoles play an important role in the electrical characteristics of the insulation. A dipole may be represented by a particle having small positive charge at one end and a small negative charge at the other end. When these dipoles are subjected to DC voltage, they are polarized and become aligned with respect to positive and negative polarity of the DC voltage. This phenomenon is known as dipole polarization. Polarization phenomenon is influenced strongly by the material properties, structure, and condition of the insulation.
On the other hand, charged particles, that is, particles with positive and negative charges, which are not interrupted by interfacial barriers, and can travel through the dielectric from one electrode to the other, constitute the leakage current, and are not part of the polarization phenomenon.
After a time when the applied voltage is removed from the dielectric, the polarized molecules will eventually revert to their initial random arrangement so that the polarization approaches zero. The time it takes for the polarization to drop to zero when the dielectric is short-circuited is known as relaxation time. It should be noted that the large dielectrics have a much longer relaxation time, and appropriate measures should be taken to discharge the released energy (voltage and current) to ground, which is given by the polarized molecules when they revert to their original state.
This test simply measures the insulation resistance value for a short duration of time, such as 30 or 60 s, through a spot reading that lies on the curve of increasing insulation resistance values. The reading only allows a rough check of the insulation condition. However, comparison of this value with previous values is of importance. A continued downward trend is indicative of insulation deterioration ahead. For interpreting the results, the values used for comparison should all be normalized to 20°C with humidity effects considered.
A good insulation system shows a continued increase in its resistance value over the period of time in which voltage is applied. On the other hand, an insulation system that is contaminated with moisture, dirt, and the like will show a low resistance value. In good insulation, the effects of absorption current decreases as time increases. In bad insulation, the absorption effect is perpetuated by high leakage current. The time-resistance method is independent of temperature and equipment size. It can provide conclusive results as to the condition of the insulation. The ratio of time-resistance readings can be used to indicate the condition of the insulation system. The ratio of a 60 s reading to a 30 s reading is called the DAR:
DAR = (Resistance reading at 60 s)/ (Resistance reading at 30 s)
A DAR ratio below 1.25 is cause for investigation and possible repair of the electrical apparatus. Usually, the DAR readings are confined to the handdriven megohmmeter.