The process of selecting a connector becomes critical when the application involves sealing the connectors against various environmental conditions, such as salt mist in an oceanographic project, or vacuum in physical instrumentation.
Leakages due to gaps and other paths in the connector body, the insulating material, the contacts or the wires impact insulation characteristics as well as the functions of the equipment and eventually cause complete failure.
In some cases, the efficiency of an application can become critical; imagine a faulty diver’s communication equipment or defective weather and sailing navigational systems (Fig. 1).
The key to achieving the optimum solution is an accurate, up-front analysis of the specific demands that the device will encounter in the operating environment.
Degree of protection
There are various degrees of sealing protection available for connectors, these can be broadly classified into two groups: external sealing, achieved through a protective device such as a flexible boot; and internal sealing, utilising some combination of o-rings or potting material.
Most applications requiring protection against only dust or splashing liquid can use an unsealed connector with a flexible protective boot (Fig. 2). When not in use, an unmated connector can be sealed with a protective cap. Using protective caps and boots is often a cost-effective solution to prevent mud, dirt and other foreign matter from fouling, shorting or otherwise damaging contacts and connector locking mechanisms. In addition, mechanical damage caused by impact on hard surfaces can be minimised by using covers and boots.
Applications requiring exposure to environmental factors like pressure, vacuum, liquids or steam demand a greater degree of sealing than that provided by covers and boots: the connector needs to be intrinsically engineered sealed.
Elastomer o-rings are one of the most common mechanical gaskets used in connector technology. Designed to be seated in a groove and compressed between two parts – for example between two mating connectors, between a connector and its mounting surface (typically a chassis-panel), or between a cable and its attached connector – o-rings create a seal at the interface (Fig. 3).
For the contacts of a panel mounted connector, the sealing technique generally applied exploits potting material, such as epoxy resin, rubber compounds, or for the highest levels of impermeability, glass (Fig. 4). Sealing this area of the connector guarantees that no fluid or other contaminant will enter an enclosure through the connector, even when the connector is unmated.
These sealing methods can achieve reliable and economical sealing performance for both pressure, like under water at depths in excess of 100m or ultra vacuum with leakage rates below 10-8mbar·l/s. Even greater levels of performance are available from some manufacturers by utilising specialised techniques and materials.
The nature and number of seals used in a connector directly relate to the level of sealing it provides. Distinction must be made between liquidtight connectors and gastight connectors.
For the first ones, the International Electrotechnical Commission (IEC) established Ingress Protection (IP) standards, classifying the degrees of sealing protection against the entry of solid objects and liquids. This system provides a reliable method of comparing relative levels of sealing between various connector products. While the first number describes the level of protection from solid object, the second one relates to liquids protection – from dripping water to continuous immersion. For example, a connector rated as being sealed to IP55 will provide some degree of protection from penetration by dust and a jet spray of water, but it is not expected to be completely sealed from all dust or water immersion (see Table 1). To achieve an IP68 rating, or even IP69K, high performance sealing resin, as well as specialised o-rings, are integrated into the connector.
Gastight connectors are hermetically sealed and therefore exceed the IP68 rating, which only takes liquids into account. Usually gas-tightness is a characteristic associated to fixed connectors (feed-through and receptacle) and their mounting areas. As they sometimes play an interface role between two different environments, a pressure differential for instance, they need to prevent any contaminations from one environment to the other. To achieve hermetic sealing, a combination of an epoxy composite material and o-rings seals needs to be integrated into the connector.
The connector and its associated cabling must be considered integral parts of the complete system. Starting to work on the design of the application’s interconnection at the initial stage of a project contributes to the development of a complete solution that supports targeted performance and cost objectives.
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Daniel Spycher is Sales & Marketing Director with Fischer Connectors, Apples, Switzerland. www.fischerconnectors.com
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