Earthing of screened/armoured control cablesIn general control cables must be screened/armoured,and the screen must be connected to the unit's metalcabinet with a cable clamp at each end.The drawing below shows the correct way to performthe earthing, and what to do when in doubt.1. Correct earthingControl cables and cablesfor serial communication must be attachedwith cable clamps at both ends to ensuremaximum possible electrical contact.2. Incorrect earthingDo not use twisted screenends that are plaited together (pigtails), asthese increase screen impedance at higherfrequencies.3. Protection with respect to earth potentialbetween PLC and VLTIf the earth potentialbetween the VLT frequency converter andthe PLC (etc.) is different, electric noise mayoccur that will disturb the whole system. Thisproblem can be solved by fitting an equalisingcable, to be placed next to the control cable.Minimum cable cross-section: 16 mm 2 .4. In the event of a 50/60 Hz earth loopIf verylong control cables are used, 50/60 Hz earthloops can arise, and these can interfere withthe whole system. This problem is resolvedby attaching one end of the screen to theearth via a 100 nF capacitor (short pinlength).MG.27.E2.02 - VLT is a registered Danfoss trademark 45Installation
