Shielded Cables: Why should I shield my cable?

 

Purpose of a shield:  To Prevent interference from entering or emanating from a cable.
The three types of interference are:

  • Radio Frequency Interference (RFI)
  • Electro-Magnetic Interference (EMI)
  • Electro-Magnetic Pulse (EMP)

RFI was the earliest interference engineers had to deal with.  Early radio signals easily found their way into devices.  Coaxial cables and shielded twisted pairs using copper can eliminate most of this type of interference.

EMI comes more into play today with high power transmission lines, higher magnetic fields, such as MRI machines in hospitals, and other high power applications.  Copper may not offer much resistance to higher magnetic fields; therefore the use of magnetic materials such as high permeable irons may be required.

EMP is produced by the detonation of nuclear devices.  When the Hydrogen Bombs were tested in the 1950s at Bimini Atoll in the south pacific, circuit breakers at power stations in Hawaii were tripped by the EMP wave from the detonation.  Critical military and civilian circuits have to be protected from an EMP condition.  These shields require the use of both high and low permeable materials to reduce the effect of an EMP.

Types of shields in use today:
The simplest is a plated plastic film (aluminized mylar) wrapped around a cable or twisted pair.  A drain wire contacts the foil along the cable to maintain a low resistance.

For increased shield effectiveness, a loose braid may be placed over the tape.  Many CATV cables use this technique.  This method works well as long as the cables are only flexed during installation and maintenance.

A served shield offers higher shield effectiveness than the film shield.  It can be used when the cable is subjected to moderate flexing.spiral cableWhen the cable is subjected to flexing, a braid shield becomes the best choice.  By choosing the right size of wire size, the braid offers the best shield method.

Shielded Cable

 

Caluculating the size of shield material
Calculating the size of the shield material used to be a chore.  The formulas for calculating the braid construction form an Eigen value problem for which there is no finite answer to the calculation.  Early engineers created tables of shield constructions and used them as a guide to designing the braid.  Today computers can quickly make the calculation.  The formulas can be found in cable design handbooks and military specifications.

The materials used for shields
Materials for shields include copper, tin plated copper, silver plated copper, nickel plated copper, high and low permeable irons, carbon fibers, tinsel wire and aluminum.

The choice of materials for the shield and the choices for insulation and conductor materials depend upon the environmental conditions to which the cable will be subjected.
Calculating the shield coverage and braid angle
The shield shall consist of a woven braid using strand material specified in the cable specification.  Coverage should not be less than 85% for most cables, but may be increased to 90%. The angle of the braid with the axis of the cable shall lie between 20º and 40º for diameters up to .600 inch (15.2 mm). For diameters larger than .600 inch (15.2 mm), the braid angle may be greater than 40º. Percent coverage, K, and angle of braid, a, shall be calculated as follows:
K = (2F – F2) x 100
F = NPd/Sin a
a = Tan -1 (2?(D +2d) P/C)

Where:
F = Fill or space factor
K = Percent coverage
N = Number of wires per carrier
P = Picks per inch of cable length
d = Diameter of individual braid wire in inches
a = Angle of braid with axis of cable
D = Diameter of cable under the shield in inches
C = Number of carriers

Shield effectiveness is expressed in decibels or DB.  For a single copper shield, the value is around 40 DB for 85% coverage and only climbs to 45 DB for 90% coverage.  By using two copper shields, the value rises to around 60 DB.  To go higher in effectiveness requires the use of high and low permeable irons.

For more information on shields refer to:  MIL-DTL-27500

For assistance with custom wire & cable design, contact a design expert at

Calmont Wire & Cable, Inc.

Website:         www.calmont.com

Email:             Contact Us

Phone:           800-905-7161 ext. 135

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