Selecting the right connector and cable assembly solution for an application is an important and challenging process, in particular when the operating environment requires sealed equipment. Stéphane Rohrbach reports.
The connector selection process becomes critical when the application involves sealing the connectors against various environmental conditions. Harsh situations include salt water for a remotely operated underwater vehicle (Fig. 1.) or vacuum in research instrumentation. Leakages occur due to gaps and other paths in the connector body, the insulating material, the contacts or the wires. They 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 navigational systems.
The key to achieving the optimum sealing solution is an accurate initial analysis of the specific conditions that the device will encounter in the operating environment. Parameters such as the type of substance blocked by sealing, the type of exposure (splash, jet or immersion) and duration, the pressure differential or the operating temperature range are crucial for sealed connectors. These restrictions can have a significant impact on connector performance. Critical applications require a thorough analysis and a risk evaluation. Fischer experts are trusted partners to help customers equip their application with the most suitable connector and cable assembly solution during the product development process.
Protection and sealing techniques
The careful analysis of operating conditions is essential to determine the appropriate sealing technique, along with the material to be used for the required degree of protection.
There are various levels of sealing protection available for connectors. They can be broadly classified into two groups: 1) External sealing which is achieved through a protective device such as a flexible boot. 2) Internal sealing which is using some combination of O-rings or potting material.
Some applications only require protection against dust or splashing liquid. They can use an unsealed connector with a flexible protective boot. When not in use, an unmated connector can be sealed with a protective cap. Using protective caps and boots (Fig. 2) is often a cost-effective solution to prevent mud, dirt and other foreign matter from deterioration, shorting or otherwise damaging contacts and connector locking mechanisms. In addition, mechanical damage caused by impact on hard surfaces can be minimized by using covers and boots.
Applications that require exposure to environmental factors like pressure, vacuum, liquids or steam demand a greater degree of sealing than provided by caps and boots. The connector needs to be designed intrinsically sealed (Fig. 3).
Elastomer O-rings are one of the most common mechanical gaskets used in connector technology (a, b and d in Fig. 3). They are 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.
The sealing technique generally applied to the contacts of a panel mounted connector utilizes various potting materials (c in Fig. 3) such as epoxy resin, rubber compounds, or for the highest levels of impermeability, glass. 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 different types of pressure such as under water at depths in excess of 100m or ultra vacuum with leakage rates below 10-8 mbar.l/s. There are greater levels of performance available by implementing specialised techniques and materials. Fischer Connectors design centre can provide assistance to customers with such special requirements. Customised product developments can be proposed, combining hermeticity with high strength mechanical design.
This initial assessment will allow selecting the most adequate connector using the adapted sealing technology. However, the design process does not come to an end here. The connector alone does not guarantee that the sealing specifications will be met. It is time to evaluate the interaction between the connector and the product where it will be integrated (panel and/or cable).
Installation a critical step
In addition to the design, the installation of the panel mounted connectors also plays an important role in sealing.
Efficient panel sealing can only be achieved if the contact surface is correctly prepared. No excessive torque should be involved to mount the connector, which generally implies the surface flatness to be <0.05mm. Following the O-ring shape of the seal, circular machining grooves will contribute to improve the contact between the two surfaces. And finally, the edges of the panel cut should be clean and free from burrs.
Cable assemblies for optimal sealing
Cable seal is one of the assembly solutions designed for optimal sealing to prevent ingress of liquids into the connector along the cable.
Cable sheath material and quality strongly affects long term behaviour and reliability of the plug seal. The grommet type seal acts through compression by tightening the connector back nut.
Soft cable materials may relax over time resulting in a potential loss of sealing efficiency and/or loosening of back nut. Trials are always recommended to determine the right grommet size.
Careful selection of cable diameter and shape is also essential. Some cable qualities show poor cross-sectional circularity. This can also have negative impact on short and long term sealing reliability. Sometimes selection of a smaller cable seal size can help to overcome this problem.
Through an adequate match of cable and seal, IP rating up to IP68 or even IP69K for a limited period of time, can be achieved.
Fischer Connectors recommends potting and overmoulding the plug back-end for the highest reliability of long term sealing in demanding applications. This offers a strong hermetic link between cable and connector even in applications subject to intense mechanical stress. Fischer Connectors is continuously developing customer specific solutions for potting and overmoulding.
The new Fischer UltiMate for instance (Fig. 5), with its highly performing epoxy potting and overmoulding designed for harsh environment, offers an excellent sealing IP68/69K even unmated.
After defining the complete interconnect solution (connector, panel interface and cable assembly) according to the sealing level required by the application, it is critical to verify that the selected solution fulfills these requirements.
Testing methods
The sealing test methods used by Fischer Connectors to evaluate the degree of sealing protection are categorised in three classes.
Environmentally sealed connectors designed to offer a sealing up to IP 68 are qualified through a 24-hour test at a water depth of 2m.
For hermetic connectors sealed IP69K and beyond which are designed to be used in high vacuum chambers, Fischer guarantees a leakage smaller than 10-8 mbar.l/s by testing each manufactured piece with a helium mass spectrometer.
For special applications exposed to liquid under high pressure, Fischer can test the parts up to 12 bar gradient in liquid environment (120 meter water column). For higher pressures, Fischer is working with different external labs depending on the customer's pressure requirements. Fischer Connectors' guarantees the customer that the solution meets their needs through the most adapted testing methods (internal, external labs or field testing).
The connector and its associated cabling solution must be considered integral parts of the complete system. They should consequently be integrated in the product development process at the earliest stage. When the operating environment demands faultless reliability and exposure to special conditions, engineers are indeed challenged beyond simply identifying and addressing basic connection requirements.
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Stéphane Rohrbach is Engineering Director at Fischer Connectors SA, Saint-Prex, Switzerland. www.fischerconnectors.com
