How to select an industrial connector

An industrial connector has to carry power and signals reliably in a place that is rarely clean, still or gentle. Most field failures are not caused by a poor connector but by a sound connector in the wrong place: a series that performs well in a control cabinet can fail on a moving machine axis or in a washdown zone. This guide sets out the requirements that decide the choice — electrical, mechanical, environmental, termination, mating and supply — in the order in which it pays to settle them.

Start with the application, not the catalogue

Browsing a catalogue first and checking the application afterwards tends to produce mismatches that surface late, during qualification or in service. Working the other way round is faster: describe the application completely, then let each requirement eliminate connector families until a short list remains. Electrical requirements usually narrow the size and contact arrangement; the environment and the mechanical load then narrow the housing, coupling and sealing; termination, mating regime and supply decide between the remaining candidates.

Describe conditions at the connector, not for the machine as a whole. A connector mounted next to a motor, a hydraulic block or a cleaning nozzle sees a different temperature, vibration level and exposure than the enclosure specification suggests.

Connector family (series)

Connectors of one design — for example a circular screw-locking series — offered in several sizes, contact arrangements, codings and housing versions.

Free and fixed connector

A free connector terminates a cable; a fixed connector is mounted on a panel, housing or printed circuit board.

Mating

Engaging a connector with its counterpart so that the contacts make electrical contact and the coupling locks.

Termination

The joint between conductor and contact: crimped, soldered, screw-clamped, insulation-displaced or soldered/pressed into a printed circuit board.

Electrical requirements

The electrical load sets the minimum contact size and the spacing between contacts. Define it per contact rather than for the connector as a whole, and record how many contacts carry current at the same time.

• Current per contact and the number of simultaneously loaded contacts. A rating stated for one loaded contact is not valid when all contacts carry current.
• Operating voltage, together with the overvoltage category and pollution degree of the installation. These determine the clearance and creepage distances the insert must provide.
• Signal types: power, low-level analogue, digital data or RF. Combining them in one insert is possible, but it affects contact arrangement, crosstalk and shielding.
• Shield continuity: whether the connector has to carry a cable shield through to the housing or panel for electromagnetic compatibility.
• Data rate or bandwidth for data contacts, which determines whether a dedicated data coding or impedance-controlled design is needed.

For voltage, distinguish the rated voltage from the test voltage. The test (withstand) voltage is applied briefly during type testing and is always higher; it is not an operating limit. Insulation coordination principles for low-voltage equipment are set out in IEC 60664-1, and for connectors rated above 50 V up to 1000 V without a detail specification, IEC 61984 defines safety requirements and tests. Which standard applies to a given design depends on the end equipment and its product standard.

Contacts and plating

The contact has to keep a low, stable resistance over its life. Base material, spring design and plating are chosen for that. Gold plating is common on signal and data contacts because it does not form an insulating oxide film, which matters at low currents and voltages. Tin and silver are common on power contacts, where higher normal forces break through surface films. Mated pairs are specified with the same plating family; mating gold to tin is avoided because it promotes fretting corrosion and an unstable interface. The article on contact resistance and connection reliability covers the mechanisms in detail.

Mechanical load, mounting and coupling

Where the connector is mounted decides the housing style; how it is loaded decides the coupling and the strain relief. Establish the mounting arrangement (cable-to-cable, front or rear panel mounting, board mounting), the vibration and shock at the mounting point, the pull and bending applied to the cable, and the space available for a hand or tool during mating.

Under vibration, the coupling must stop relative movement at the contact interface, and the cable must be supported close to the connector so that its mass does not work against the contacts. Strain relief should take cable pull and bending into the housing rather than into the terminations. Connectors on moving machine parts also need a cable rated for the flexing regime; the connector and cable are best specified together. The guide on high-vibration environments goes further into locking and test methods.

Environment: sealing, temperature and exposure

The environment rules out more connector families than any other factor. Record the following at the connector location:

• Water and dust: is the connector splashed, jetted, washed down with hot high-pressure water or immersed? This leads to the IP code required under IEC 60529 — and to the question whether it applies mated, unmated or both.
• Temperature: the full range at the connector, including heat from nearby equipment and the connector's own temperature rise under load.
• Fluids and chemicals: oils, coolants, cleaning agents and disinfectants that housing, seals and cable jacket will contact.
• UV and weathering for outdoor installations, and salt mist in marine or coastal settings.
• Condensation and thermal cycling, which can pump moist air into a sealed connector that is not fully tight at every interface.

Sealing also depends on the cable. A cable gland or rear seal only seals within its specified cable outer diameter range, and a jacket material that swells in oil or hardens at low temperature can defeat a correctly rated seal. Specify cable and connector as one system. For applications with combined exposure — hot washdown plus vibration, for example — the harsh-environment connectivity overview describes the typical design responses.

Termination: how the cable is attached

The termination method affects reliability, assembly time and whether the connector can be fitted or repaired in the field. It should be specified explicitly rather than left to the assembler.

Field-attachable (rewirable) connectors give flexibility on site, while factory-terminated or overmoulded cable assemblies give tested, repeatable quality and usually better sealing at the cable entry. For volumes beyond prototypes, a cable assembly is often the more robust and economical route.

Mating, standards and long-term supply

Mating and maintenance

• Estimate how often the connector will be mated over the life of the equipment and compare that with the rated mating cycles in the datasheet, with margin.
• Where identical connectors sit side by side, use different codings or keyings so that each can only be mated to its intended counterpart.
• Check that service personnel can reach, see and operate the connector, with gloves if that is how they work.
• Decide whether damaged contacts or cables must be replaceable in the field and which tools that requires.
• Specify protective caps for connectors that are left unmated in service.

Standards and approvals

Establish which standards and approvals the end equipment and its markets require before comparing parts. Many industrial interfaces are defined in the IEC 61076 series, which makes parts from different manufacturers intermateable on interface dimensions. The M12 screw-locking connector, for instance, is specified in IEC 61076-2-101, with different codings for different uses. Test methods for connectors are collected in the IEC 60512 series. Approvals such as UL or VDE apply to specific part numbers and stated ratings; confirm them in the manufacturer's documentation for the exact part, not for the series in general.

Supply over the life of the equipment

A connector that is technically right but cannot be supplied for the production life of the equipment is not the right connector. Check lead times and lifecycle status, whether a second source or an intermateable alternative exists under a common standard, and whether loose parts or complete assemblies make more sense for the volumes involved.

Selection checklist and common mistakes

• Selecting on the headline rating and ignoring the conditions it was measured under.
• Assuming the IP code also applies to the open, unmated connector.
• Specifying cable and connector separately, so that the cable diameter falls outside the sealing range or strain relief.
• Leaving the termination method and tooling to the assembler.
• Designing in a part whose supply cannot be secured for the life of the product.

Where requirements conflict or the datasheets leave gaps, a connectivity specialist can help interpret the requirements and compare candidate series; the catalogue can be searched directly for interfaces such as M12.