Danfoss VLT HVAC Design Manual

Also see for VLT HVAC: Instructions Operating instructions Instruction Operating instructions Instruction manual

Contents

Danfoss VLT HVAC Design Manual Manual pdf 117 page image

5.7.3 Earthing of Screened/Armoured Control CablesGenerally speaking, control cables must be braided screened/armoured and the screen must be connected by means of a cable clamp at both ends tothe metal cabinet of the unit.The drawing below indicates how correct earthing is carried out and what to do if in doubt.a. Correct earthingControl cables and cables for serial communication must be fit-ted with cable clamps at both ends to ensure the best possibleelectrical contact.b. Wrong earthingDo not use twisted cable ends (pigtails). They increase thescreen impedance at high frequencies.c. Protection with respect to earth potential between PLCand frequency converterIf the earth potential between the frequency converter and thePLC (etc.) is different, electric noise may occur that will disturbthe entire system. Solve this problem by fitting an equalisingcable, next to the control cable. Minimum cable cross-section:16 mm 2 .d. For 50/60 Hz earth loopsIf very long control cables are used, 50/60 Hz earth loops mayoccur. Solve this problem by connecting one end of the screento earth via a 100nF capacitor (keeping leads short).e. Cables for serial communicationEliminate low-frequency noise currents between two frequencyconverters by connecting one end of the screen to terminal 61.This terminal is connected to earth via an internal RC link. Usetwisted-pair cables to reduce the differential mode interferencebetween the conductors.5.8.1 Residual Current DeviceYou can use RCD relays, multiple protective earthing or earthing as extra protection, provided that local safety regulations are complied with.If an earth fault appears, a DC content may develop in the faulty current.If RCD relays are used, you must observe local regulations. Relays must be suitable for protection of 3-phase equipment with a bridge rectifier and fora brief discharge on power-up see sectionEarth Leakage Current for further information.VLT® HVAC Drive Design Guide 5 How to InstallMG.11.B9.02 - VLT® is a registered Danfoss trademark 1175