Why are shields used in electrical cable?
In addition to the benefit of protection against damage to cable components from cable motion and use, proper shielding is essential to many applications as it can keep out unwanted external interference. In many applications, electromagnetic interference (EMI) is a threat to signal integrity. Shield quality is of particular importance in small signal or high frequency applications where a slight variation can have significant impact. All electrical cable will radiate energy to, and pick up energy from, its surroundings. As such, shielding can also be used to contain the electromagnetic energy radiated by a cable, which can protect nearby sensitive components.
The types of shields used by New England Wire
The three most common shield types used by New England Wire for multi-conductor cable are braid, spiral, and foil. A braided shield is made of interwoven conductive wire. A braid can be manufactured using a wide variety of materials and wire sizes and can be applied over an extensive range of cable diameters. A spiral shield is made of conductive wire helically wrapped over the core. As with a braid, a variety of materials and wire sizes can be used, however, spiral shields are generally limited to small or mid-sized cables in order to achieve high coverage. A foil shield is a metallic foil helically wrapped around the cable with overlap. A variety of foil options can be utilized, however, most common is aluminum laminated with a polyester backing. Variations or combinations of the standard types are often used to provide a custom solution to a particular application.
In addition to wire and foil based shielding options, we can also manufacture with semi-conductive tapes, coatings or extruded layers. These are typically used in conjunction with wire based shields to provide additional shielding, but more notably, they are used for low noise or corona resistant high voltage cables.
So which shield type is best?
The ideal shielding method for any application depends on the specific application for which it is going to be used. For example, shielding effectiveness at low frequencies is largely influenced by resistance. Therefore, wire based shields such as spirals and braids are ideal. At high frequencies, coverage becomes the primary factor making 100% coverage foil shields the most effective option. At mid-range frequencies both resistance and coverage are influential so a high coverage braid is typically used.
While shielding effectiveness is often one of the primary concerns for most applications, other impacts of shielding should be considered. Design constraints such as flexibility, flex life, diameter, weight, and cost may influence the shield selection. Furthermore, in order to get the full benefit of shielding, the shield must be grounded properly, consequently, means of termination can play a role in shield selection.
Variations or combinations of the standard shield types are often used to maximize shielding effectiveness for a given application. For example, stacking a foil and braid will provide a shield with 100% coverage and low resistance making it effective from low to high frequencies.
| Braid | Spiral | Foil | |
|---|---|---|---|
| Core Size | 0.012″ and larger | 0.004″ – 0.450″ | 0.025″ and larger |
| Single End AWG Range | 30-46 AWG | 36-52 AWG | N/A |
| Flexibility | Good | Very good | Poor |
| Flex-life/Continuous Flexing | Good | Poor | Poor |
| Typical Coverage | 90% | 90-95% | 100% |
| Typical Frequency Range | Low to mid (up to 100 MHz) | Low (up to ~1 MHz) | High (greater than 100 MHz) |
| Ease of Termination | Medium Wire can be separated and terminated Drain wire can be beneficial | Easy Not interwoven so easy to twist together and terminate | Difficult Drain wire is beneficial |
| Advantages | Provides structural integrity Good flexibility and flex-life Lowest resistance Provides cut through/crush resistance | Most flexible option Less build to OD than braid Excellent coverage Lighter weight | Superior coverage Effective at high frequencies Aluminum offers low weight and cost |
| Considerations | Largest increase in OD Braid pattern limits max coverage Gaps in braid limit effectiveness at high frequencies | When flexed openings in spiral can limit effectiveness at higher frequencies Can create inductive effect | High resistance limits effectiveness at low frequencies Not recommended for continuous flexing applications Termination options are limited |