Choosing shielded conduit

Radio frequency interference (RFI) and electromagnetic interference (EMI) – stray voltage, current, and high frequency noise – can damage circuits, interrupt performance, and initiate potentially dangerous actions. Shielding reduces these potential problems and shielded conduit, especially where flexibility is required, provides a significant solution for the engineer.

Conduit types for high-performance shielding applications are: braided shield; interlocked bronze strip inner core; and interlocked bronze strip with braided shield. The good, better, and best solutions provide shielding from RFI and EMI interference.

Markets, applications

Variable speed drives – also called variable frequency drives – are extremely susceptible to RFI. Shielded flexible conduit from the drive controller to the motor eliminates the potential for hazardous interference in operating the drive. Flexible shielded conduit also is used in blower motors, electronic controllers, and data center wiring to keep power wiring from interfering with signal and data lines, and, for the same reasons, in test and measurement applications.

In applications where space is at a premium and flexibility is an advantage, such as in aircraft, flexible shielded conduit protects the jacket and inner conductors of the cable from damage.

Good

Braided shield conduit, such as Liquatite Shield-Flex Type SLA (see sidebar, below), consists of an inner core of interlocked steel with a tinned, braided shield over it, and covered by a polyvinyl chloride (PVC) jacket. UL listed, it offers a wide temperature range, is chemically resistant, and is suitable for use in some hazardous areas. The conduit can be used as an equipment grounding conductor in most cases and accepts standard metallic liquid-tight connectors. Type SLA flexible conduit has a working temperature range of -30°C to 80°C dry or 60°C wet/70°C oil.

Braided shield conduit has a high reduction level of RFI and EMI in the lower frequency ranges, from 150kHz to 10MHz, with a sharp roll-off at higher frequencies. These characteristics should be considered when choosing the shielded conduit type.

Better

Shield-Flex Type EMS does not have a braided shield; instead, an interlocking bronze strip inner core provides RFI and EMI shielding. This flexible shielded conduit can be used across a wide temperature range, its PVC jacket material has strong chemical resistance, and this type accepts standard metallic liquid-tight connectors.

Interlocked bronze-strip inner core flexible shielded conduit has a wider shielding effectiveness than the Type SLA braided flexible shielded steel conduit. Its effectiveness has a more gradual roll-off until the higher frequencies are reached, approximately from 150kHz to 6MHz. Type EMS conduit has a working temperature range of -55°C to 105°C.

Best

Shield-Flex Type EMCS has the same type of interlocked bronze strip inner core as the Type EMS and the same type of tinned, braided shielding as the Type SLA conduit. It has a wide operating temperature range and excellent chemical compatibility because of the PVC jacket. It is compatible with standard liquid-tight conduit connections and has a working temperature range of -55°C to 105°C. Interlocked bronze-strip inner core flexible conduit with a braided shield has the best shielding effectiveness of the three types of conduit, from 10kHz to 18GHz.

Halogen-free jackets

Applications needing low smoke and low flame-spread construction materials require zero-halogen conduit jacketing instead of the PVC jacket material used in the three conduit types discussed. These applications include confined spaces such as airplanes where the public could be exposed to hazardous smoke and off-gassing from PVC in the event of fire.

The three types of flexible shielded conduit can also be obtained in the halogen-free jacketing material polyurethane (PU). Electri-Flex adds HF to the conduit type designation – HFSLA, HFEMS, HFEMCS – to denote this variation. Both PVC and PU offer wide chemical resistance.

Conclusion

Using flexible shielded conduit provides a solution to the problem of noise and interference from stray voltages and current. When high-performance EMI/RFI shielding is needed, these three conduit types offer quality across a wide frequency range.