Cable shielding is used to prevent electromagnetic interference;

• Keeping the energy conducted through the cable from escaping
• Preventing outside energy from being conducted by the cable conductors to effect sensitive circuits

There are 3 conditions where cable shielding may be needed:

• Low-frequency electric fields,
• Low-frequency magnetic fields, and
• High-frequency electromagnetic fields

With low frequency fields, copper braid or aluminum foil with drain wire with any termination will be adequate. In some medical applications, for low frequency electrostatic shielding, extruded or tape wrapped semi-conductive coatings applied over a drain wire have been used.

However, as frequency increases to 10 MHz and above, shielding quality becomes critically important. Cable designs must include:

• shielding capacity for the desired frequency range, and
• proper shield termination.

For frequencies above 10MHz, an expensive shield is not always needed. It depends on how much shielding is needed. Options include:

• Loose-weave copper braid will provide moderate shielding (20-40 dB) up to 1 GHz.
• Tight woven or double braid will provide better shielding up to 1 GHz
• Aluminized foil shield with drain wire will provide even better shield effectiveness.
• Aluminized foil shield with drain wire and braid provides the best shielding performance combination.

As circuit speeds increase, sensitivity levels and immunity requirements increase, requiring more effective shielding and termination strategies.

Shielding Methods

• Braid

Braiding is the diagonal crossing of metal strands, usually tinned copper, so that one or more strands pass alternately over and under one or more strands laid in the opposite direction over an insulated conductor or cable core.

• Spiral Serve

To save weight and increase flexibility, shield wires are applied helically in one direction over an insulated conductor or cable core. However, the shield effectiveness is significantly reduced.

• Foil

Aluminum laminated to a polyester/polyethylene composite film gives this shield mechanical strength. The tape is applied spirally around an insulated conductor or cable core. A drain wire in contact with the aluminum side of the tape throughout the length of the cable is used for easy shield termination. The tape may be folded to improve shield effectiveness at higher frequencies.

See Wire & Cable Designer's Handbook, p. 24-25, Insert table 1-8 w/o served flat wire.

Northwire Design Assistance.

We strongly recommend that you consult our technical staff when designing your shielded cable. Many factors influence shielded cable designs. Let our over 150 years cumulative design experience help you design your cable just the way you want it.