Glow-Wire Tests Go Wrong at the Temperature Setting

The glow-wire test is often described as simple: heat a loop of wire to a set temperature, press it against a sample for 30 seconds, and see whether the material ignites. Anyone who has sat through a disputed GWFI result knows the simplicity is misleading. The temperature you think you set is not always the temperature at the contact point.

The temperature is measured at the tip, not at the controller

IEC 60695-2-10 and -11 specify a nickel/chromium wire loop with a fine Type K thermocouple welded to the tip. The controller drives current through the loop, but the number that matters is the thermocouple reading at the hottest point. Thermocouples drift, welds degrade, and a loop that has been used for a few hundred tests changes resistance. A controller displaying 850 °C can easily be running the tip at 870 °C or 830 °C.

Check the thermocouple against a reference before a test series, and replace the loop when it has thinned or distorted. A wire loop that has sagged no longer applies the specified contact geometry, so the pressure and contact area change as well as the temperature.

Sample preparation decides more than people expect

Test specimens should be representative of the finished part: same material, same thickness, same colourant and same moulding direction. A 1.5 mm plaque moulded in the lab is not the same specimen as a 3 mm ribbed housing from the production tool. Thickness affects heat sinking, and colourants can change ignition behaviour noticeably. If a part is tested in its end-use configuration, keep any metal inserts and neighbouring components in place where the standard allows, because they act as heat sinks.

Condition the samples at the specified temperature and humidity before testing. Damp nylon and dry nylon do not behave identically, and a sample taken straight from a cold store carries moisture that will show up as a delayed ignition.

The 30 seconds, and what happens after

The wire is applied with a defined force for 30 seconds, then withdrawn. From that moment the observations are timed: whether flames appear, how long they last, whether burning droplets ignite the tissue paper or the pine board below, and whether the sample burns through. The usual acceptance for a GWFI pass is that no flame persists beyond the specified period and the underlying layer does not ignite.

• Use a fresh piece of wrapping tissue / pine board for each test; a scorched one gives false positives.
• Record ignition time and flame extinction time, not just pass or fail.
• Note whether the sample melted away from the wire — that is a valid observation, not a failed test.
• Vent the chamber between tests; residual hot gases affect the next ignition.

Reading a boundary result

When a material passes at 850 °C and fails at 960 °C, the GWFI is reported at the lower temperature. What should raise a flag is a batch that flips between pass and fail at the same temperature. That usually means the tip temperature, the contact force, or the specimen thickness is not under control, not that the material is marginal.

Before re-running a borderline series, verify the thermocouple, weigh or measure the specimen thickness, clean the loop, and check that the test chamber has returned to ambient. A set of three specimens tested under genuinely identical conditions is far more convincing than ten tested across an afternoon of drifting conditions.