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21 June, 2021

Cabling best practice Part 3: Routing

05 April, 2021

Have you ever experienced instruments giving readings from variable-speed drives that are clearly inaccurate or sporadic? The problem may lie, not with your drives, but with the way their cabling is installed, as ABB's Martin Richardson explains.

Because of the way that variable-speed drives (VSDs) work and use power electronics, it is essential that the routing of their cables is considered carefully, otherwise other electronic equipment in their vicinity could suffer from interference.

Firstly, ensure that the motor cable is separated adequately from the signal or control cables (by at least 500mm) and from other power cables (by at least 300mm). Secondly, ensure that all other cables are separated adequately from the signal or control cables by at least 200mm, to avoid interference.

When control cables must cross power cables, make sure that the crossover angle is as near to 90 degrees as possible. To prevent cross-coupling, it is also important to avoid mixing pairs with different signal types – 110V AC, 230V AC, 24V DC, analogue or digital. For example, an unsuppressed relay coil switched at 110V can transmit a surprisingly large transient pulse into an adjacent 24V DC or analogue signal line.

Cable trays should have good electrical bonding between each section and to the grounding electrodes. Stainless-steel or aluminium tray systems can be used to improve the local equalisation of potential and reduce the problem of corrosion.

If you are using plastic trunking, secure it directly to the installation plates or framework. To avoid antenna affects, you should not run spans in mid-air.

It is advisable to use twisted-pair wires with equalising conductors wherever possible to avoid differential mode disturbances, which can lead to spurious signals. In some installations, it may be beneficial to use ferrite rings to avoid common-mode disturbances. These disturbances do not distort the signal itself, but can disturb the receiving device. Installers should endeavour to keep wires twisted and as close to the terminal as possible.

You should avoid unused wires in cables. They should be connected to ground or to another signal. There is no right or wrong way of doing things, but pump manufacturers, contractors or end-users should double-check any system they are proposing. With thousands of installations in the UK alone, it is fair to say that we have seen a lot of problematic applications and this has moulded our best engineering practice.




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