High material requirements demanded by the glow wire standard

The glow wire test is one of the most important fire tests for electrical/electronic applications. This test simulates glowing wires or overheated resistors in component assemblies which occur as a result of overloads, for example, and can lead to extreme temperature peaks. A challenging requirement for thermoplastics has been set in the new household appliances standard IEC 60335-1 (4th edition, May 2001). This standard specifies a GWIT of at least 775°C with a maximum flaming time of 5 sec for carriers of live electrical parts in unattended appliances (e.g. washing machines, dishwashers, dryers, etc.) with a nominal current > 0.2 A.  

Topics:

• Test rig
• IEC 60335-1
• Testing Sequence
• Ticona grades

Test rig

In the glow wire test, a vertical sample of the material to be tested is held for 30 sec with a force of 1 N against a heated glow wire (fig. 1). A fundamental difference between the former standards IEC 60695-2-1 and IEC 60335-1 is that end users can choose between a material test on a sample plaque (GWIT – responsibility of the raw material supplier) and a finished-part test (GWT – responsibility of the end user). A basic requirement for electrical parts in unattended appliances with a nominal current > 0.2 A is to pass the GWFI test at 850 °C.

IEC 60335-1

The glow wire test comprises three practically relevant methods:

1.     Sample plaque test as  per IEC 60695-2-12:
        GWFI (Glow Wire Flammability Index)

        The GWFI is a general suitability test for plastics in contact
        with live electrical parts. The highest temperature is determined
        at which in 3 successive tests one of the following conditions is
        fulfilled:

• no ignition of the sample
• burning or glowing time after removal of the glow wire ≤ 30 sec and no ignition of the indicator

        Example of a 3 mm-thick specimen at 850°C:  GWFI: 850 / 3.0

2.     Finished-part test as per IEC 60695-2-11:

       GWT (Glow Wire Temperatur)The test specimens are exposed to the
       glow wire at a temperature of ≥ 650°C (nominal current < 0.2 A) or
        ≥ 750°C (≥ 0.2 A) for 30 sec.

       The test is passed if no ignition (max. 2 sec flaming) occurs.

3.     Sample plaque test as per IEC 60695-2-13:
         GWIT (Glow Wire Ignition Temperature)

         The test is carried out at 675°C (nominal current ≤ 0.2 A) or 775°C
          (>0.2 A). The test is passed if no ignition occurs. Ignition is
         deemed to have occurred if a flame burns for ≥ 5 sec.

        GWIT: 775 / 3.0 means,  that a 3 mm-thick specimen was not ignited
        at a test  temperature of 775°C.

Testing Sequence

The testing sequence for unattended appliances mainly depends on the nominal current > 0.2 A or ≤ 0,2 A), fig. 2. In view of the significantly shorter permitted flaming and glowing times (max. 2 sec (GWT) and 5 sec (GWIT) according to the new standard as compared with 30 sec after removal of the glow wire in the old standard), it may be assumed that many materials that are still permitted under the old standard will not pass the new standard.

Ticona grades

Ticona has developed special Celanex grades for household and appliances applications: These grades were tested along with Vectra and Fortron grades by an independent testing laboratory.