Tracking is a surface phenomena on an insulating material. When you have two conducting terminals or tracks at a high voltage (higher than 100 VAC) separated by an insulator, a combination of environmental factors such as dust, moisture and thermal cycling could cause minute leakage currents to flow on the surface between the conductors. Over time, the deposits carbonize and the surface current increases. Eventually, a carbon track forms over the surface of the insulator making it conductive at a particular “tracking” voltage. Finally, a short circuit is created between the two conductors which may also lead to fire. Worse, it’s possible that the tracking current could be lower than the rating of the protective fuse in the appliance, which will prevent the electrical supply from being cut off, creating a fire hazard. Tracking can be avoided by using the right kind of insulating materials and adequate creepage and clearance distances. One of the reasons for adding a slot between adjacent high voltage terminations or tracks on a PCB is to take care of tracking.
It’s impossible to conduct such tests according to real world conditions, so a standardized procedure is needed which can produce results that allow different materials to be compared. Considering the importance of this standard and its wide implications, this work is now handled by Evaluation and qualification of electrical insulating materials and systems.
IEC 60112 defines a “standardized method for the determination of the proof and the comparative tracking indices of solid insulating materials” for voltages up to 600 VAC, and provides information on how to design a suitable test equipment. The ASTM has an equivalent document — ASTM D3638 as does the UL — UL 746A-24. A more severe test is covered under IEC 60587 — “Electrical insulating materials used under severe ambient conditions – Test methods for evaluating resistance to tracking and erosion”. This test is often referred as the inclined plane tracking and erosion test and specifies test voltages up to 6 kV. But for now, let’s just look at the low voltage test as per IEC 60112.
It’s always fascinating looking at a sample undergoing the Tracking Index Test — check out the video below. When you look at data sheets for plastic materials, the Tracking Index value will always be reported under it’s electrical properties. Paper Phenolic, which was the PCB substrate used before the advent of fibreglass, usually has a very low tracking index value (depending on its composition), ranging between 100 V to 175 V. On the other hand, depending on composition and filler materials, fibreglass substrates such as FR4 can have CTI values ranging from 175 V up to about 300 V or higher.