Comparative Tracking Index of Electrical Insulating Materials

What the test measures

Moisture and ionic contamination can create a conductive path across an insulating surface. Repeated current flow may carbonize the material and develop a permanent track between conductors. CTI testing accelerates that process under defined laboratory conditions so materials can be compared consistently.

A higher CTI indicates better resistance to tracking under the selected test conditions. CTI is used for material screening and, where applicable, as one input to insulation-coordination decisions.

How CTI is determined

Two electrodes are positioned on a clean test surface and an AC voltage is applied. A specified electrolyte solution is deposited between the electrodes at controlled intervals while the test system monitors current and other failure criteria.

1. A fresh specimen or independent test area is prepared for the selected voltage.
2. Electrolyte drops are applied between the electrodes at the interval required by the governing standard.
3. The specimen is monitored for sustained overcurrent, tracking, or other specified failure criteria.
4. Testing is repeated at additional voltages and fresh locations until the comparative tracking index can be reported.

The CTI is based on the voltage associated with the standard's 50-drop performance criterion. IEC 60112 can also be used to determine a proof tracking index (PTI), which evaluates whether the material passes at a specified voltage.

Sample guidance

Flat specimens with enough surface area for independent trials provide the most straightforward setup. Lectromec's typical submission guidance is:

• At least 8 in² of usable material for testing.
• A 4-inch-diameter disk or an equivalent flat plaque is preferred.
• Enough material for several independent test areas.
• A clean, representative surface free of handling residue.

Required thickness and specimen count depend on the governing method and material. Thin films, curved parts, multilayer constructions, coatings, adhesives, and finished components may require stacking, support, or a different preparation plan. Contact Lectromec before fabricating or shipping those samples.