Industrial environments are subject to electrical noise, which can cause electromagnetic interference (EMI) that can negatively affect the operation of equipment. Cables can act both as a generator of EMI, conducting or radiating noise to other equipment, or as a receiver of noise from other sources. Shielding, which surrounds the conductor within the cable to either reflect electromagnetic energy or pick up the noise and conduct it to ground, is used to prevent EMI in cables.
There are two main types of cable shield - foil and metal braid - which come in varying degrees of coverage and should be selected depending on the application, environment, required flexibility, etc. Foil shielding consists of a thin layer of aluminum attached to a backing material and requires a drain wire for terminating and grounding EMI. Foil can achieve 100% coverage, is lightweight, easy to terminate, and cheaper than braid, but it has limited flex life and offers little physical protection.
Braid shielding is a woven mesh of bare or tinned copper strands (though stainless steel or silver-plater copper is sometimes used) that surround the conductor to provide a low-resistance path to ground. Braid can provide anywhere from 40—96% coverage, depending on the tightness of the weave, though 75-85% is standard. Braid provides improved termination and lower DC resistance, making it ideal for low frequency interference. It has greater flex life and mechanical strength than foil and is generally more effective, but it is bulkier and costs more. In particularly noisy environments, braid and foil shields can be layered to improve performance.
Omni Instrumentation & Electrical Services, Inc. is a premier instrumentation and electrical contractor, providing superior total care solutions since 1986. Services include instrument installation, tubing installation, instrument calibration, control loop testing, startup and commissioning, power and lighting, process control wiring, BMS wiring, telecommunications and data wiring, fire alarm wiring, security wiring, process network wiring, and control panel fabrication. Omni Instrumentation & Electrical Services, Inc., is certified in Foundation Fieldbus, Profibus, DeviceNet and ASI-Bus installation.