- The following technical article was current at the time it was published. However, due to changing technologies and standards updates, some of the information contained in this article may no longer be accurate or up to date.
Considerations in Structured Cabling Systems
Figure 1: Conductive coupling between a power line and a telecommunications channel
When cable is in close proximity to strong electromagnetic fields, unwanted current and voltage may be induced on it. If the power level is high enough, the electrical "noise" can interfere with voice and data applications running on the cabling. In data communication, excessive electromagnetic interference (EMI) hinders the ability of remote receivers to successfully detect data packets. The end result is increased errors, network traffic due to packet retransmissions, and network congestion. For analog voice communication, EMI can create psophometric noise, which degrades transmission quality.
The coupling between power lines and telecommunications cables may be due to one or more of the following mechanisms:
- conductive coupling
- capacitive coupling
- inductive coupling
Conductive coupling occurs when two circuits or channels have a common branch as depicted in Figure 1. If we consider Circuit 1 as being a power line and Circuit 2 as a telecommunications channel, the parasitic current introduced into Circuit 2 from Circuit 1 is detrimental because of its intensity. In commercial cabling installations, the occurrence of conductive coupling is fairly common when the bonding and grounding systems used for power and telecommunications are not sufficiently isolated.
Figure 2: Capacitive coupling between a power line and a telecommunications channel
Capacitive coupling is represented in Figure 2 as a simplified, discrete model of distributed coupling. Capacitive coupling occurs between power and tele-communications cables carried in parallel for some extent in a given installation.
The capacitance between the two lines, referred to in Figure 2 as CPT(Power line to Telecommunications line capacitance), is caused by coupling between these two circuits. The value of this capacitance will vary with the distance between circuits - higher for short distances and lower for large distances. To reduce the voltage noise level due to the capacitive coupling between channels, either the capacitance CPT can be decreased (decreasing the capacitive coupling) or the impedances ZPT and Z0 can be increased. In instances where it is not possible to change these parameters, screened cabling (ScTP) can be employed to shield the channel from the disturbing circuit, thereby reducing the value of CPT.
Other elements shown in Figure 2 include the Power line to Ground Capacitance (CPG ), Telecommunications Line to Ground Capacitance ( CTG), and the impedance between the Power Line and the Telecommunications Line (ZPT). All these elements can contribute to the interference in cable channels. Controlling their effect is important when designing and installing cabling infrastructure.
Figure 3: Inductive coupling between a power line and a telecommunications channel
Inductive coupling occurs via the mutual inductance (Lmutual) between two or more circuits or channels as represented by the simplified model shown in Figure 3.
When current flows in a circuit terminated with a load, it produces a magnetic flux proportional to the current. This magnetic flux may induce noise voltage (VN) into an adjacent channel, generating a loop current in the disturbed circuit. This type of coupling is one of the most common. The geometry of the conductors, as well as the geometric range between two lines in space, determines the value of Lmutual and, consequently, the intensity of the inductive coupling. Another important factor is the environment that contains the lines. For example, metallic raceway or cable tray can help attenuate or propagate unwanted signals beyond the initial source of interference.
In order to reduce the effect of inductive coupling between circuits, it is important to maintain cable geometry along the entire channel length and to keep adequate separation between circuits. The intensity of the magnetic field is directly proportionate to the current present in the disturbing channel (i.e., power line in Figure 3) and inversely proportionate to the distance between the lines (i.e., power lines and telecommunications lines).
Combined Coupling Effects
Figure 4: Combined coupling effects
When considering interference between circuits, we have to consider all of these effects together. The combined effects are represented in Figure 4, where ZGG is the impedance ground-to-ground between the telecommunications line and the power line as a result of conductive coupling between these two lines.
Protecting Cabling Systems against EMI
While electromagnetic interference can affect the performance quality of structured cabling systems, there are two effective methods to help avoid this: shielding and physical separation.
SHIELDING. Shielding is one of the techniques employed to protect telecommunications cabling systems from EMI. In shielding, noise voltage is induced into a foil or braid surrounding the twisted pairs, instead of onto the conductors. Two types of shielded cables are available: ScTP (Screened Twisted Pair) cables which are balanced and surrounded by an overall foil screen; and SSTP (Shielded Twisted Pair) cables in which each pair is surrounded by a foil and also surrounded by an overall shield. A properly installed shielded cabling system performs about 20dB better than UTP cabling system performance in terms of interference immunity (coupling attenuation).
When designing and installing shielded cabling systems, the grounding/earthing and bonding have to be very carefully considered. Proper grounding and bonding are mandatory to assure the effectiveness of shielded systems. For example, to maintain high frequency shield effectiveness while avoiding ground loop difficulties associated with shield grounding at multiple locations, horizontal ScTP cabling should have only one DC ground connection at the telecommunications room. This connection should be independent of network equipment. In addition, all applicable codes and regulations must be met for circuit protection, safety, and electromagnetic compatibility. Another important note for shield effectiveness is that shielding must be complete - that is, shield coverage must exist throughout the Permanent Links or Channels, including both cable and connecting hardware. High frequency shield performance for cabling components is characterized by transfer impedance. Also, shield effectiveness for equipment cables should be maintained up to and including the equipment interface (unless specifically prohibited by the equipment manufacturer).
PHYSICAL SEPARATION. The other way in which a telecommunications cabling system can be protected from EMI is to ensure some degree of physical separation between the telecommunications cabling lines, cross-connects, electrical power lines, distribution panels, secondary branch circuits, and electrical office equipment. Tables 1 and 2, developed by The Siemon Company, specify the minimum distance recommended for both 100 Ohm UTP and ScTP cabling, as well as the pathways and spaces used to carry or house the telecommunications cabling. These distances have been provided by The Siemon Company to assist in reducing the effect of EMI on cabling systems and are not intended to be used in place of local codes and regulations.
|Table 1: Minimum power separation for UTP cabling systems|
|Power Level||< 3 kVA||> 3 < 6 kVA||> 6 kVA|
|Pathways||50mm (2 in)||1.5m (5 ft)||3m (10 ft)|
|Spaces||50mm (2 in)||3m (10 ft)||6m (20 ft)|
|Table 2: Minimum power separation for screened and shielded cabling systems|
|Power Level||< 3 kVA||> 3 < 6 kVA||> 6 kVA|
|Pathways||0m (0in)||0.6m (2 ft)||1m (3 ft)|
|Spaces||0m (0in)||0.6m (2 ft)||1m (3 ft)|
By applying the proper physical separation distances, UTP cabling can be used in a cabling system, while still avoiding EMI. In fact, in situations where proper physical separation can be maintained,The Siemon Company recommends UTP cabling as the preferred cabling media. However, in situations where minimum separation distances cannot be met for UTP cabling, ScTP or SSTP cable can be used.
In conclusion, installing cabling without regard to sources of electromagnetic interference can be detrimental to network performance and transmission quality. However, this can be prevented by either maintaining proper physical separation between power and UTP telecommunications circuits or using screened or shielded cabling in installations where sufficient separation is not possible.