5.7.3 Grounding of Shielded/ArmoredControl CablesControl cables should be braided, shielded/armored, andthe shield must be connected with a cable clamp at bothends to the metal cabinet of the unit. Figure 5.106 showscorrect grounding examples.Figure 5.106 Grounding Examplesa Correct groundingb Incorrect groundingc Protection from potential between PLC and adjustablefrequency drived 50/60 Hz ground loopse Cables for Serial CommunicationTable 5.84 Legend to Figure 5.106a. Correct groundingControl cables and cables for serial communi-cation are fitted with cable clamps at both endsto ensure the best possible electrical contact.b. Incorrect groundingDo not use twisted cable ends (pigtails). Theyincrease the shield impedance at highfrequencies.c. Protection from potential between PLC andadjustable frequency driveIf the ground potential between the adjustablefrequency drive and the PLC (etc.) is different,electric noise that disturbs the entire system ispossible. Fit an equalizing cable next to thecontrol cable. Minimum cable cross-section: 0.025in2 [16 mm2].d. For 50/60 Hz ground loopsUsing long control cables sometimes causes50/60 Hz ground loops. Connect one end of theshield to ground via a 100 nF capacitor (keepingleads short).e. Cables for serial communicationEliminate low-frequency noise currents betweentwo adjustable frequency drives by connectingone end of the shield to terminal 61. Thisterminal is grounded via an internal RC link. Usetwisted-pair cables to reduce the differentialmode interference between the conductors.5.8 Residual Current DeviceUse RCD relays, multiple protective grounding orgrounding as extra protection in compliance with localsafety regulations.If a ground fault appears, a DC content may develop in thefaulty current.If using RCD relays, observe local regulations. Relays mustbe suitable for protecting 3-phase equipment with abridge rectifier and for a brief discharge on power-up. Seechapter 2.11 Ground Leakage Current for more information.How to Install Design GuideMG16C122 Danfoss A/S © Rev. 2014-01-13 All rights reserved. 1715 5
