Key takeaways
- UL 94 is a small-flame material test; glow wire simulates a hot element and is often the more relevant test for an appliance.
- CTI describes how a surface resists tracking, and it sets the creepage you need, not the other way round.
- The temperature index is a long-term property, not a short excursion limit.
- Glass fill and flame retardants change flow, toughness, CTI and colour, so the grade matters as much as the family.
Three jobs, one housing
A connector housing holds the contacts in place, keeps the current paths apart and stops a fault inside the connector from becoming a fire outside it. It also has to survive assembly, the environment and the life of the equipment. Those jobs are tested in different ways.
The structural job is set by contact retention, mating force and panel fixing. The electrical job is set by the creepage and clearance between contacts, the tracking resistance of the surface and the dielectric strength of the wall. The fire job is set by the material's response to a flame or a hot element.
A data sheet lists a UL 94 rating, a glow wire temperature, a CTI value and a temperature index. Those four numbers answer different questions, on different specimens, with different failure modes. They are not interchangeable.
UL 94 and what the ratings test
UL 94 is a laboratory test on a standard plastic specimen. In the horizontal test, a small flame is applied to a bar held horizontally, and HB is given if the burn rate stays below a limit or the flame stops before a marked line. In the vertical test, the flame is applied to the bottom of a bar and then removed.
The vertical ratings are graded by afterflame time and by dripping. V-0 allows the shortest afterflame and no flaming drips. V-1 allows a longer afterflame with no flaming drips. V-2 allows the same afterflame as V-1 but permits drips that ignite a cotton pad below the specimen.
The 5V ratings use a larger flame and a plaque. 5VA requires no hole through the plaque; 5VB allows a hole but limits the afterflame. All ratings are thickness dependent, so a material that is V-0 at 1.5 mm may be HB at 0.8 mm. A callout without a thickness is incomplete.
UL 94 is a coupon test
A V-0 specimen does not prove that a moulded housing with a thin wall, a weld line and a metal insert will behave the same way. The end product test covers the geometry.
Glow wire: heat source, not flame
The glow wire test presses a heated wire loop against a specimen for a set time. It simulates a hot element, such as a glowing connection or an overloaded resistor, rather than an open flame. The specimen is watched for ignition, flame duration and droplets that ignite tissue paper below it.
Three variants are used. The test for end products applies the wire to the finished part. The flammability index and the ignition temperature are material tests that rank the grade at temperatures from 550 °C to 960 °C.
For appliances and many power products, the glow wire result is more relevant than UL 94, because the likely ignition source is a hot part rather than a match. A material with a good UL 94 rating can still ignite at a temperature that occurs inside the product.
CTI and tracking across a wet surface
Comparative tracking index measures how a surface resists a conductive carbon path when voltage is applied across a wet, contaminated surface. A standard test drops electrolyte between two electrodes and raises the voltage until tracking occurs.
The result is grouped into performance levels. PLC 0 is 600 V or more, PLC 1 is 400 V to 599 V, PLC 2 is 250 V to 399 V, PLC 3 is 175 V to 249 V, PLC 4 is 100 V to 174 V and PLC 5 is below 100 V. A higher number is better.
The CTI feeds the creepage calculation. A low-CTI material needs a longer creepage path for the same working voltage and pollution degree. A high-CTI grade can shrink a connector, but only if it also meets the mechanical and flame requirements.
Temperature: RTI and thermal class
The relative thermal index is a long-term property. It comes from oven ageing tests and represents the temperature at which the material retains a chosen percentage of a key property for a defined time. It is not a short excursion limit or a melting point.
The IEC thermal class uses letter designations, such as class B at 130 °C, class F at 155 °C and class H at 180 °C, for the insulation system rather than the plastic alone. The system can be the lower limit.
The practical question is the temperature at the housing, not the room. A connector next to a current-carrying contact, inside a closed enclosure, can run far above room temperature. Measure or calculate the local temperature, then choose a material with margin.
Material families and what they trade
Polyamide 66 is common because it is tough, moulds well and takes glass fill. It absorbs moisture, which changes dimensions and mechanical properties, and it needs a hydrolysis-stabilised grade for warm, humid service. Polyamide 6 is tougher but absorbs more water.
PBT has lower moisture absorption, good dimensional stability and good electrical properties, which suits precision connectors. It is less tough and is sensitive to hot water and some chemicals. PPS handles higher temperatures and aggressive chemicals, but it is expensive and brittle.
Polycarbonate gives impact strength for larger housings but is sensitive to solvents and stress cracking. PPO and PPE blends give good dielectric behaviour at moderate cost. In every family, the flame retardant package and the glass content change the data sheet values.
Wall thickness, creepage and the moulded part
Wall thickness affects flammability, strength and flow. A thin wall can be V-0 in the data sheet and fail in a real part because the flame sees a different geometry. A thick wall can sink, warp or trap voids. Choose the wall for the moulding process, then check the flame requirement at that thickness.
Metal inserts, weld lines and gate positions create local weaknesses. A weld line across a thin wall is a path for a crack and a place where the flame retardant is less evenly distributed. A brass insert can conduct heat into the plastic and change the glow wire result near the insert.
Creepage and clearance belong to the finished geometry. Ribs, slots and barriers add surface path, but a rib that is too thin may not mould cleanly. The CTI sets the creepage requirement and the moulding sets what can be produced.
What to ask for
Ask for the material grade, not just the family. PA66 covers a wide range of glass content, flame retardant packages and heat stabilisers. The grade determines the UL 94 rating at a stated thickness, the glow wire temperature, the CTI and the RTI.
Ask for the test report, the thickness and the conditioning. A UL 94 rating is tied to a thickness and a colour, and some flame retardants perform differently in dark colours. A glow wire result is tied to the temperature and the variant, and a CTI value to the test solution.
Ask how the part will be tested as an assembly. The end product glow wire test and the dielectric test across the contacts cover geometry that the coupon tests cannot. The data sheet is the start of the evidence, not the end.
| Requirement | Test or standard | What to state |
|---|---|---|
| Flame, small source | UL 94, horizontal or vertical | Rating, thickness and colour |
| Flame, hot element | IEC 60695-2-11, end product | Temperature, duration and result |
| Material glow wire | IEC 60695-2-12 and 2-13 | GWFI and GWIT at thickness |
| Tracking | IEC 60112 | CTI value and performance level |
| Long-term heat | UL 746B and IEC 60085 | RTI or thermal class, with margin |
| Creepage and clearance | IEC 60664-1 | Working voltage, pollution degree |
| Dielectric strength | IEC 60664-1 or product standard | Test voltage and duration |
A data sheet value without its thickness, conditioning and test variant cannot be checked against the drawing.
Frequently asked questions
Is UL 94 V-0 the same as glow wire?
No. UL 94 applies a small flame to a coupon in a controlled laboratory test. Glow wire applies a heated element and is closer to a hot connection inside a product. A V-0 material can still ignite at a glow wire temperature that occurs in service.
What CTI do I need for a connector housing?
It depends on the working voltage, the pollution degree and the creepage the design can afford. A higher CTI allows a shorter path. Choose the grade from the insulation coordination calculation, then confirm the mechanical and flame properties at the same thickness.
Does a higher RTI mean a better material?
Not necessarily. RTI is a long-term temperature index, not a quality score. A material with a high RTI can be brittle or hard to mould. Match the index to the local temperature with margin, and check the other properties separately.
Why does the same plastic behave differently in a moulded part?
Wall thickness, weld lines, flow orientation, inserts and colour all change the local behaviour. The coupon tests use a standard specimen. The end product test is the one that covers the real geometry.
Sources and further reading
- UL 94, Tests for Flammability of Plastic Materials for Parts in Devices and Appliances
- IEC 60112, Method for the determination of the proof and the comparative tracking indices of solid insulating materials
- IEC 60695-2-11, Fire hazard testing, Glow-wire flammability test method for end-products
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