Signal Interference and Cable Shielding
A well-engineered cable is comprised of many crucial independent elements. Recently shielding has become just as critical as any other design element. The growing complexity of today’s communications and control systems, coupled with the increased distances signal and control communications are required to travel, have exponentially increased electrical interference (noise) related failures. Depending on the application, cables can be adversely affected by EMI/RFI/ESI (electromagnetic interference, radio frequency interference, electrostatic interference) also known as ‘signal interference.’ Insulation alone provides no protection from signal interference – so to combat the effects of signal interference, proper shielding is vital.
According to industry technical data there are essentially four sources of signal interference.
- Static Noise: Occurs when an electrical field distorts the signal and can be mitigated using continuous foil shields which offer 100% shielding efficiency and appropriate grounding techniques.
- Magnetic Noise: Comes from large AC motors, transformers, and knife switches, and can set up current flows in opposition to the instrument. The simplest and best means of eliminating magnetic noise is through the use of twisted pair signal wiring.
- Common Mode Noise: Results from current flowing between different potential grounds located at various points within a system. Solving this issue requires a carefully engineered and properly installed power and grounding system.
- Crosstalk: Refers to the super imposing of the pulsed DC or standard AC signals between two or more nearby wires or cables. The most effective means of mitigation is individually shielded twisted pairs.
If one finds that noise will pose a problem it is then necessary to determine if the noise is low, medium, or high level.
The table below displays generalized noise levels:
|Noise Level||Noise Sources||Typical Locations|
|High||Electrolic processes, large motors, generators, transformers, Induction heating, Relay controls, Power lines||Heavy processing plants such as steel mills and foundries|
|Medium||Medium-size motors generators, transformers Relay controls||Average manufacturing plants|
|Low||Small motors, generators, transformers||Storage areas, labs, offices and light assembly operations|
After determining the type/level of noise one can better choose the most appropriate type of shielding.
Shielding surrounds the power-carrying conductors of the cable and protects it by (1) reflecting signal interference as well as (2) picking up noise and conducting it to ground. Multi/Cable offers various shielding options and varying degrees of shielding effectiveness. When deciding on the type/amount of shielding required, consider the following factors:
- Type of signal interference – EMI, RFI, or ESI
- Noise level
- System configuration
- Cable cost – is more shielding necessary?
- Cable diameter, weight, and flexibility
Multi/Cable typically uses either foil or braid, or both foil and braid when shielding its cables.
Foil Shield: Good
- Protection at frequencies greater than 15 KHz
- 100% coverage over core conductors
- Low cost
Foil shielding uses an aluminum/polyester or an aluminum/Kapton foil shield (facing in) with 100% coverage and continuous contact to helically served tinned copper drain wire (one AWG size smaller than insulated conductors). The drain wire is used to create an electrical connection between the shield and the circuit ground. Foil shielding can be applied over individual conductors, twisted pairs or triads, or as an overall foil shield.
Braid Shield: Better
- Protection at low frequencies (up to 15 KHZ)
- EMI/RFI resistance in power, control and data applications
- High physical strength
Braid shielding is a woven mesh of bare, tinned, silver- or nickel-plated copper wires. Multi/Cable’s braid shield provides a minimum of 85% coverage. Braid shields provide a low-resistance path to ground, and they are much easier to terminate when attaching to a connector. Because copper has higher conductivity than aluminum and the weave provides more mass for conducting noise, a braid is more effective as a shield.
MultiShield (Foil & Braid): Best
- Protection across entire frequency range
- High physical strength
- Ease of termination
For extremely noisy environments and where physical strength is a factor, multiple shielding layers (foil/braid) are recommended. Multi/Cable MultiShield “foil & braid” shield uses triple laminate (aluminum / polyester / aluminum) foil with drain one AWG size smaller than insulated conductors, plus an overall tinned copper braid for increased physical strength and superior shielding from signal interference. In multiconductor cables, individual pairs are sometimes foil shielded to provide crosstalk protection between the pairs.
For more precise cable shielding information, contact your friendly Multi/Cable sales or engineering associate.