IEC 60335-1 Terminal Block Flammability Testing & Failure Analysis, UK
IEC 60335-1 Terminal Block Flammability Failures
This case study examines a flammability-related failure involving a terminal block used in a household appliance assessed against IEC 60335-1. The incident centred on overheating and ignition damage at the terminal block assembly during fault-representative testing (clause 19), revealing that the non-metallic insulating parts and connection design did not provide sufficient resistance to ignition or flame propagation under abnormal thermal stress.
In the context of IEC 60335-1, this type of failure is significant because the standard places particular emphasis on preventing fire initiation and limiting the spread of fire from overheated electrical connections and surrounding insulating materials.
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Overview
During product testing of a mains terminal block assembly in an unattended household appliance, we observed ignition damage and sustained charring around the terminal housing after abnormal heating at the connection point. The failure did not present as a simple electrical discontinuity; instead, it developed into a localised thermal event that affected the insulation around the conductor entry and clamping features.
The resulting damage pattern indicated that the terminal block assembly was vulnerable not only to overheating but also to secondary flame spread and material decomposition when subjected to connection-related fault heating.
Compliance Context
IEC 60335-1 addresses fire hazards in household and similar electrical appliances by requiring that components and insulating materials resist ignition and do not significantly contribute to flame propagation under reasonably foreseeable abnormal conditions. Industry guidance describing IEC 60335-1 highlights that glow-wire testing is used to simulate overheating arising from faulty connections, overloaded conductors, or overheated components, and that requirements for non-metallic insulating materials near live parts become more stringent in unattended appliances and for circuits above low current thresholds.
The associated glow-wire methodologies are defined in the IEC 60695-2 series and are commonly used to assess whether connector and terminal materials resist ignition, self-extinguish promptly, and do not ignite material below the specimen.
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Technical Background
Terminal block flammability failures in this context usually arise from a combination of an initiating thermal fault and insufficient fire resistance in the surrounding polymeric structure. The initiating fault may be caused by high contact resistance, conductor loosening, strand damage, plating degradation, contamination, undersized conductor accommodation, or poor clamping geometry, all of which are covered in the IE 60335-1 assessment.
If the material lacks adequate glow-wire performance, the terminal block can ignite, sustain flaming, carbonise, or propagate damage to nearby wiring and enclosure parts.
Requirements of Testing
The terminal block assembly was not robust enough to prevent ignition damage when exposed to abnormal connection heating, representative of the hazards addressed by IEC 60335-1. The torque of the screw terminal was not enough, and this caused the cable to become loose and cause heat.
Investigation determined that the terminal had adequate certification and was certified for glow wire at 850 °C, which is adequate for unattended appliances.
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