IEC 60112: CTI Tracking Index — The Invisible Safety Threshold for PCBs

CTI Below 175V? Your PCB Could Catch Fire in Humid Conditions

IEC 60112:2009 specifies the Comparative Tracking Index (CTI) test for solid insulating materials. This is the critical test for assessing PCB substrate and insulating plastic safety under humid, contaminated conditions.

Tracking mechanism: Under humidity and surface contamination (salt spray, industrial condensation), leakage current between electrodes on the insulation surface generates Joule heating that evaporates electrolytes, forming localized dry bands. Voltage concentrates across these bands, producing microscopic spark discharges. Repeated sparking carbonizes the surface, creating a conductive “track” — culminating in surface insulation failure and short-circuit.

CTI classification: PLC 0 (CTI≥600V): Superior, e.g., PTFE. PLC 1 (400–599V): Good. PLC 2 (250–399V): Standard FR-4. PLC 3 (175–249V): Caution. PLC 4 (100–174V): Tracking-prone. PLC 5 (<100V): Severe — unsuitable above Pollution Degree 2. For HV PCBs (power supplies), specify CTI ≥400V.

TN Lab — One neglected CTI value is one cause of PCB fires.