Correct choice of shielded and unshielded cables

A well-designed, properly installed FTP cabling network will undoubtedly improve signal integrity on unshielded systems. In order to work effectively, the shield must be properly grounded. In fact, this has not always been easy to achieve. Poorly grounded shielding systems will actually provide worse performance than unshielded systems at the beginning. In the worst case, the shield itself may become a source of radiation, generate a current ground loop, or receive ambient radiation, causing harmful interference to the associated data cable.

The type of shielding

There are several different types of screening. Two or more strands or four cables can be wrapped in a single shield. In addition, each pair of stranded cables can also be individually shielded. In addition, these individually shielded cables may or may not cover the second overall shield (referred to as a metal foil inner pair (PiMF)). Regardless of the design, the shielding must be electronically continuous from end to end and properly grounded to ensure effectiveness. When choosing a shielded cabling system, it is also important to choose an overall end-to-end solution. Mixing components with different systems may cause problems due to impedance mismatch. The cost of shielding the system, including the additional installation work required, is usually 20–30% higher than the equivalent UTP solution.

Standards and rules

EMI or Radio Frequency Interference (RFI) is any unwanted signal that adversely affects the operation of the equipment or system. In Europe, radiation and electromagnetic interference immunity requirements are specified in EMC Act 89/336/EEC and its Supplement 92/31/EEC. In the United States, the Federal Communications Commission (FCC) has made similar provisions for any equipment manufactured or sold in the United States.

Radiation can be determined by measuring the field strength of the generated radiation. It is in volts/meter and its usual frequency range is 30kHz to 1000MHz. EMC regulations provide a product family standard for information technology equipment, EN55022, that specifies the test procedures and acceptable limits for EMI emissions.

Passed the test, and confirmed that the device can continue to operate effectively when there is a corresponding level of EMI, and the anti-interference ability (or sensitivity) of EMI can be determined. There is currently no family of standards for anti-jamming capability testing of IT equipment, so it is currently included in the general standards EN50082-1 ​​(domestic, commercial, and lighting industries) and EN50082-2 (industrial environment). These standards and the IEC1000-4 series of basic standards are used to test the immunity of IT equipment.

It is not clear how the wiring structure fits into this situation. The recommendations provided by the DTI-appointed, well-known entity for the Molex Enterprise Wiring Network Division in the United Kingdom confirmed that structured cabling was excluded from the provisions of Article 13 of SI 2372: 1992, as it is acceptable that large installations in buildings cannot be evaluated by EMC. In addition, in the provisions of Article 17 of the SI, the wiring system is classified as a passive system and therefore it is not within the scope of the electromagnetic laws.

The EMC Electromagnetic Compatibility Testing Laboratory (EMCTLA) in the UK claims that wiring products “will not cause electromagnetic interference and their performance will not be affected by these interferences”. Therefore, the CE flag is not required to be displayed on the wiring component; in fact, this is illegal.

Prospects

Twisted-pair cables can cancel each other's signals by coupling the signal of each half of the line, thereby achieving electromagnetic compatibility. In order to work effectively, stranding must maintain a good balance. Longitudinal balance testing of cables is the focus of current controversy. The test method used is based on coupling attenuation, but many people believe that this method has many drawbacks, or even not enough. Other methods of vertical balance testing include line injection and antenna technology.

It is expected that this level of shielding must be used to meet the standard performance requirements for a specified transmission bandwidth of approximately 600 MHz. Alternatives must not require shielding and offer higher performance than copper solutions. This alternative is of course fiber optics.

At present, whether to choose whether to shield or not to shield and to choose which shield to use is still a complicated issue. The best advice end users can take is to consult senior network designers every time they install a new system.