What are three key performance needs for HVAC/harsh environment connectors?

By Jeff Shepard | May 15, 2023

Connectors are complex assemblies whose performance is based on interactions of materials with electrical, mechanical, and environmental factors. Extensive testing is required to ensure proper performance and numerous industry standards have been developed for that purpose.

This FAQ reviews 13 tests across three key performance indicators for connectors including electrical, mechanical, and environmental considerations for connectors used in heating ventilation, air conditioning (HVAC) systems, and other harsh environments. Some aspects of materials testing for connectors are discussed in the FAQ on "What's the Difference Between Needle Flame and Glow Wire Testing?"

Testing is the cornerstone for quantifying the performance of connectors in challenging environments. Three key areas are:

Electrical tests: the impact of high current and temperature rise poses challenges to components. Testing should evaluate wet insulation resistance, current carrying, impulse withstand, contact resistance, and other factors.

Mechanical tests: flexion testing, conductor pullout, cold impact, protection against contact, and low-level contact resistance are several of the basic mechanical considerations.

Environment tests: degree of protection, vibration, humidity, corrosion, and temperature shock, including highly accelerated life test (HALT) and highly accelerated stress screen (HASS) testing.

Whether testing is performed in-house or at an independent laboratory, the requirements of ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories, should be met. Accreditation is mandatory for testing performed as part of regulatory compliance. The need for accreditation applies to all types of tests including electrical, mechanical, environmental, and so on.

Electrical performanceElectrical performance testing is a broad area and can include a wide range of tests, a few of which are discussed in more detail below. Representative tests include:

Contact resistance is a basic connector performance specification. Bulk resistivity, or intrinsic resistance, describes the resistance of a material of a given size and area. Contact resistance, on the other hand, describes the resistance experienced when two conductors come into contact (Figure 1). IEC-60352-5 requires that contact resistance testing should be carried out using the millivolt level method specified by IEC 60512-2-1. Contact resistance is a complex phenomenon and is affected by constriction, film resistance, and stress relaxation. Constriction describes the narrowing of the true contact surface when two materials meet. Film resistance is the resistance created by unwanted resistive material (film) on the surface of the contact. Contact deformation can occur over time changing the shape of the contact as a result of stress relaxation.

Current carrying capacity should be measured in accordance with IEC 60512‑5‑2. Current carrying capacity is limited by the insulating parts of the connector housing and the thermal properties of the contact materials. It is dependent on the ambient temperature and self-heating of the contacts. The tests quantify carrying capacity versus ambient temperature.

The impulse withstand voltage test should be performed in accordance with IEC 60664‑1. This is an indirect test that the design has sufficient creepage and clearance distances for safe use. The magnitude of the test voltage is based on the rated surge voltage of the connector. The test uses three voltage surges (1.2/50 µs) per polarity. To pass, there must be zero spark-overs.

The wet insulation resistance test as described by UL 1703 and UL 27 is an important test for connectors used in outdoor applications like some HVAC and industrial systems. The standard requires that these connectors be able to withstand wind and weather for up to 25 years. Wet insulation resistance testing provides a quantitative measure of insulation performance. Performing the test in a still conductive aqueous solution enables the identification of changes in the air clearances and creepage distances that may be caused by hairline cracks and other difficult-to-identify sources.

Mechanical performanceMechanical performance testing is another broad area that includes a range of tests, a few of which are discussed in more detail below. Representative tests include:

The conductor pull-out test, in accordance with IEC 60998‑2‑1, applies a specified tensile load on the terminal point for one minute and the conductor must be held in position without damage. This test is performed after the flexion test (described below) and is a stricter requirement.

The cold impact test as described in UL 746C, 57/UL 1703, 30. Is performed after storing the connectors to be tested at -35°C. The test consists of dropping a test weight on the connector with a defined impact energy. This test can cause the creepage and clearance distances in the connector under test to change, as a result, a test of creepage and clearance distances should be performed following the cold impact test.

The flexion test, or rotation test, is detailed in IEC 60999‑1. A conductor is attached to a vertically fixed connector, and a test weight determined by the connector cross-section is attached to the opposite end of the conductor. The conductor is fed through a 37.5 mm circular hole in a rotating disc that is spun on its horizontal axis 135 times (Figure 2). Following the test, the contact must pass the IEC 60998-2-1 pullout test.

Touch-proof protection testing in accordance with IEC 60529 ensures that the connector design provides a high level of safety from live voltages to technicians when the equipment is being serviced or maintained. The test uses a test probe or ball that is placed on each opening of the connector with a specified force.

Environmental performanceEnvironmental performance testing is an important area of testing for systems like HVAC or industrial equipment that may be used outdoors or in harsh environments. Five of those tests are discussed in more detail below. Representative tests include:

The Corrosion test is defined in IEC 6988. Gas-tight connections are required to pass this test with corrosion-free and low contact resistance when exposed to salt spray and other aggressive materials. The Kesternich test involves exposure to a sulfur-oxidic atmosphere at an ambient temperature of +40°C. After a subsequent drying period, a visual inspection is performed, and the contact resistance is measured.

The degree of protection test in accordance with DIN 40050‑9/60529 measures the ingress protection (IP) level of the connector by testing its performance when exposed to water, including immersion and high-pressure sprays, and particles of various sizes including fine dust.

Humidity testing is conducted in combination with other qualification tests including mating and unmating, low-level circuit resistance (LLCR), and insulation resistance/dielectric working voltage (IR/DWV). Humidity testing is usually performed in a purpose build test chamber (Figure 3). It exposes the mated connector set to a varying range of temperatures, from 25 to 65 °C, with a humidity range of 90% to 98% RH over 10 days. To pass the humidity test, the connector must pass mating/unmating, LLCR, and IR/DWV testing after the humidity and temperature cycling.

Temperature shock testing defined in IEC 60512‑11‑4, test 11d is important for industrial applications and HVAC systems. To simulate rapid temperature changes, a two-chamber test is often used. The connector under test is moved between two chambers, one at the lower temperature limit and the other at the upper-temperature limit, within a few seconds, remaining in the second chamber for 45 minutes. To pass the test, the connector's overall functionality must be unimpaired.

Vibration testing includes harmonic and sinusoidal vibrations as defined in IEC 60068‑2‑6. Various combinations of vibrations are applied to the connector along all three axes to simulate oscillating, pulsating, or rotating forces. Typical vibration frequencies range from 10 to 2,000 Hz and are successively applied at a rate of one octave per minute. To pass the test, the functionality of the connector must be unimpaired.

SummaryTesting is important to ensure the reliable and safe operation of connectors in harsh environments and applications like HVAC systems. Electrical parameters, mechanical specifications, and environmental tolerance are key factors in connector performance. Accreditation to ISO/IEC 17025, which details the general requirements for the competence of testing and calibration laboratories, and is an important consideration when testing is performed as part of a regulatory compliance program.

ReferencesISO/IEC 17025 Testing and calibration laboratories, International Standards Organization.Mechanical tests for connectors and electronics housings, Phoenix Contact.Severe Environment Testing for Connectors, Samtec.Understanding Connector Contact Resistance, ATL Technology.