Monday, 29 July 2013

A common question we face is the considerations needed when earthing drive systems. When arranging the earth connection of a drive system, the safety aspects as well as the EMC requirements of the installation need to be satisfied.

Traditionally, earthing is aimed at providing electrical safety to protect users from electrical shock and to limit material damage in the case of electrical fault. However, to prevent electromagnetic interference, there are some further considerations.

Electro-magnetic compatibility, or EMC, is the ability of equipment to operate without disturbing, or being disturbed by, other equipment nearby. Disturbances can take the form of either radiated emissions or conducted emissions. In the case of earthing, the concern is that conducted emissions may propagate to other equipment via the earth bond.

The general objective for an EMC compliant earthing system is to equalise the potential of all the equipment. This must be borne in mind when planning an installation, to ensure that the materials used will offer low impedance at high frequency. The generally available means are stranded or braided copper conductors. Plating will improve the conductive characteristics. Screening of cables is also important. For an industrial installation, galvanised steel wire armour is usually adequate.

In some drives with active rectifier, common mode filters are used to compensate for the lack of a neutral conductor. These should be installed in accordance with the drive manufacturer’s instructions.

As the drive is the end of a sub-circuit in an installation, the minimum sizing for protective conductors will depend on the fuse or circuit breaker feeder. When it comes to the motor, it may well need a local grounding conductor, in case of cable failure, but the most important thing is the equalising of the potential, which is most often achieved by a symmetrical screened cable. This should be routed back to the drive and also to the transformer.

For EMC purposes, the principle is to return the “noise” to its source, so as the source is the drive module, and the noise propagates down the cable, the appropriate low impedance path for high frequencies to return should be the cable screen and reduce the area enclosed by loops with the general earth.

Finally, it is important to ensure that any cable tray used should have good electrical bonding between sections, and to the grounding terminals of the drive.

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