Glow wire testing for electrical plastics

What the test simulates

A flame test assumes something is already burning. The glow wire test assumes something is hot. In electrical equipment the common fault is a loose connection, an overloaded terminal or a failed contact that heats up without producing a flame. A plastic part in contact with that hot metal can soften, melt, smoke and eventually ignite.

The test reproduces that condition with an electrically heated loop of resistance wire, pressed against the specimen for a fixed time. The loop is at a set temperature, and the assessor records whether the material ignites, how long it burns after the loop is removed, and whether burning material drops onto a layer of tissue paper below.

That last detail matters. A material that self extinguishes but drops burning fragments onto tissue has still spread fire, and the test treats it as a failure at that temperature. This is the same logic as the drip criterion in a vertical flame test, applied to a different heat source.

Three tests, three different questions

The family is defined in IEC 60695-2. The glow wire test itself is IEC 60695-2-11 and is usually called GWT. It applies the loop to a specimen or a finished part at a specified temperature for 30 seconds and records a pass or fail. It is the test that appears in product standards, with a temperature chosen for the part.

The glow wire flammability index, GWFI, is IEC 60695-2-12. It is measured on a standard plaque and reports the highest temperature at which the material stops burning within 30 seconds of the loop being removed, with no ignition of the tissue layer, over three consecutive tests.

The glow wire ignition temperature, GWIT, is IEC 60695-2-13. It reports the temperature at which the material stops igniting during the application of the wire, again over three tests, and it is expressed in 25 K steps. Ignition here means a flame that persists beyond a short duration after the wire is applied.

The two indices answer different questions. GWFI asks how well the material stops burning once it has been heated. GWIT asks how hard it is to ignite in the first place. A material can have a good GWFI and a poor GWIT, and the product standard may require one, the other, or both.

The test parameters that matter

The glow wire is a loop of nickel chromium wire, about 4 mm in diameter, heated by a controlled current. It is pressed against the specimen with a defined force, and penetration is limited so the wire does not simply push through a thin wall. The application lasts 30 seconds in the standard test.

Temperature is measured and controlled at the wire, not at the specimen, and the calibration of the wire and the thermal couple is part of the method. Two laboratories running the same nominal temperature can produce different results if the calibration or the force is out, which is why the report should name the laboratory and the method revision.

The specimen matters too. A standard plaque is flat and uniform. A finished part has ribs, bosses, gate marks and thin sections, and the wire may be applied to the thinnest or most exposed point. Product standards usually specify where the test is applied, and a part that passes on a flat plaque can fail at a rib.

Reading a result correctly

A report should state the temperature, the duration, the specimen thickness, the colour, whether the tissue ignited, the afterflame time and whether the specimen was a plaque or a part. Without those, the number cannot be compared with another material or checked against a requirement.

Watch for the difference between an index and a test. A data sheet that says GWFI 960 does not mean the part passes a glow wire test at 960 degrees. It means a plaque of that grade, at the tested thickness, stopped burning within the criteria at that temperature in a flammability index test. The part test is a separate exercise.

Also watch the thickness. Indices are often reported at a specific thickness, and thin sections can behave worse. A grade with GWFI 960 at 1.5 mm may have no claim at 0.8 mm. If your wall is thinner than the tested plaque, the index does not apply as written.

Which materials do well

Flame retardant PBT and PA66 grades are common for parts that must meet a glow wire requirement, and both can reach high indices in well formulated versions. PBT tends to be dimensionally stable and consistent, while PA66 brings toughness and a wider processing window but absorbs moisture.

PPS and other high temperature grades perform well because they resist softening at the wire temperature and char rather than melt. They are chosen where the part is close to a hot element or must keep its shape during a fault.

Thermosets such as glass polyester and melamine are used where the glow wire requirement is severe and the part must not melt or drip. They char at the contact point and hold their form, which is why they appear in arc chutes and barriers.

The additive package matters more than the polymer name. Two PA66 grades with the same glass content can differ by hundreds of degrees in GWFI because of the flame retardant system. Select by the reported index at your thickness, not by the family.

What the product standard actually requires

The requirement comes from the end product standard, not from the material supplier. Appliance standards commonly require a glow wire test at 550, 650, 750, 850 or 960 degrees depending on the part, its position and whether the appliance is attended in use. Switchgear standards set their own requirements for insulation and barriers.

The standard also defines when the test is not needed. A part that cannot be contacted by a glowing element because of a barrier, a metal enclosure or a sufficient distance may be exempt. That exemption is worth checking before a more expensive grade is specified, because it can remove a requirement entirely.

Where the test is needed, the standard usually names the temperature and the point of application. The part should be tested as designed and as moulded, at the production thickness and colour, with the gate and weld lines in their real positions.

• Identify the part and its position relative to current-carrying parts.
• Check whether the standard exempts it through a barrier or a distance.
• Note the required temperature and the point of application.
• Test the production thickness, colour and geometry.

Writing the requirement on the drawing

State the method, the temperature and the specimen. A note such as glow wire test to IEC 60695-2-11 at 750 degrees for 30 seconds on the finished part is unambiguous. A note that says glow wire 960 without a method invites a plaque index to be offered in place of a part test.

Add the thickness and colour, and say whether the test is on a plaque or the part. If the part is large, say which area is tested. If there are several materials in the assembly, list the requirement for each.

Keep the reports with the first article and the approval file, and re test after any material, colour or thickness change. A glow wire result is evidence for a specific combination, and a substitution quietly breaks it.

1. 01

   Read the product standard

   Find the clause, the temperature and the exemption conditions for the part.

2. 02

   Fix the test basis

   State the method, the specimen, the thickness and the colour on the drawing.

3. 03

   Check the index data

   Compare the grade indices at your thickness as a first screen, not as proof.

4. 04

   Test the part

   Run the glow wire test on the production geometry and keep the report.