Laying separate lives

Cables and cable separation

It is important to consider routing at the design stage. Normally the shortest distance allowed within an installation would be chosen. However, for emc requirements the shortest distance properly protected must be chosen.

IEC 61000-5-2 recommends the routing of cables along a pec. The purpose of this is to provide a continuous, well-conducting metallic structure that will divert disturbances from the cables. The pec should be connected at both ends to the earth of the apparatus. The effectiveness of the pec is related to its shape. If it is made of a number of elements care should be taken to ensure its continuity by correct bonding between the parts.

Separation distances

The use of screened or twisted pair cables allows the separation distances between signal and power cables to be reduced.

It was thought that incorrectly installed conduit and trunking would have little effect on the attenuation of fast transient disturbances, although it was clear that trunking and conduit made from ferrous metals could reduce the strength of 50 Hz magnetic fields. However, more recent research is showing that properly designed trunking and conduit systems are effective in the reduction of fast transient disturbances.

From recent work a table of minimum separation distances between power and signal cables has been devised (see table 1).

Separation distances for IT installations

There are many factors to consider when designing and installing IT systems, such as: voltage, power, the disturbing source frequency and the parallel length of run.

The draft standard prEN 50174-2 has tabulated separation distances for IT cables from power cables against typical installation scenarios. Experience gained first hand in the installation of IT systems will influence the separation distances that are decided upon for a given installation scenario.

Cables in parallel

A flat cable is often used for parallel data transmission between digital equipment. Each data line and return line should be at alternate positions. The flat cable can be shielded.

Some rules for parallel runs of cables should be observed in order to minimise any emc problem. Cables can run in parallel for 100 m if both the IT cable and power cable are installed in a completely enclosed, well-grounded aluminium or steel channel. These channels should be separated from each other by a common metal divider that is continuously bonded to the main channel.

Coincident runs of no more than 5 m are permissible if dividers or suitable retention hardware maintains a 25 mm separation distance. If the separation is less than 25 mm for a distance of up to l50 mm, it is necessary that no contact occurs between the IT and power cables.

Runs of no more than 9 m are permissible if dividers maintain a 50 mm separation distance. If the separation is less than 50 mm for a distance of up to 300 mm, it is important that no contact occurs between the IT and power cables.

In cases of multiple cables and limited separation, cables should be arranged in such a way that the same IT cable is not always nearest the power cable at every outlet or junction.

Electrical wiring closets and data wiring closets should ideally be in separate cabinets.

For parallel runs of under-carpet cables and power cables, maintain at least 130 mm of separation for the first 15 m of the run. Any cable in excess of this should be separated by at least 460 mm for the remainder of the run.

Cable screening and termination

The selection of the most appropriate cable screen and its correct termination will help to prevent radiated emissions from one type of cable being received by another.

Table 2 shows the effectiveness of various cable screens at 50 Hz and RF. Remember that the best screen without good bonding is ineffective in most situations.

The effectiveness of the cable screen will be significantly reduced if it is not terminated correctly.

Earthing and bonding

Without adequate earthing and bonding the effectiveness of other mitigation methods will be impaired.

It is important to note that the primary function of an earthing network is for safety. Safety must take precedence over any mitigation methods that are implemented to reduce the emi in the environment.

When making bonding connections between earthed metal conductors it is important to take account of a number of environmental factors to ensure a successful long-term connection – temperature, humidity, vibration and mechanical damage.

It is also essential that when making the connection the contact surfaces should be clean, ie free from grease, paint, corrosion, insulating materials and any other effect that may cause a high contact resistance. This is a practical point to note when bonding to cabinets, cable trays and trunking that have had protective painted coatings applied during manufacture. Any paint coating must be pierced.

When bonding connections are made it is important that compatible metals are used to minimise the effects of corrosion that would inevitably lead to a deterioration of the initial low contact impedance.

Two ways of achieving good bonding connections are by ensuring that earthing contact areas are of similar contact potentials, and by using a grounding strap or fastening of intermediate contact potential to separate the two.

For low-impedance bonding, the length of the strap between the apparatus and the earthing network should be kept to a minimum. In practice, this implies that apparatus should always be connected to the nearest earthing network conductor.

For bonding straps, suitable conductors include metal strips, metal mesh straps or round cables. Yet, round cables are not effective above 10 MHz, because they have higher impedance than flat conductors with the same material cross-section.

For cable terminations the most effective bonding is to have a 360o peripheral connection around the shield.