Danfoss Native BACnet VLT HVAC Drive Operating Instructions Manual

Contents

3.2 Bus Cabling3.2.1 EMC PrecautionsThe following EMC precautions are recommended to achieveinterference-free operation of the BACnet network.Additional EMC information is available in the VLT® HVACDrive Design Guide, MG.11.Bx.yy. Please also consult theBACnet master manual for further installation guidelines.Ensure compliance with relevant national and localregulations, for example in protective earth connection.3.2.2 Single Ground ShieldingFor installing the bus cable on MS/TP, two differentstrategies can be followed, Single ground of shield andmultiple ground of shield. Each strategy has bothadvantages and disadvantages. The following chapterexplains the different between the two strategies. The singleground shield is specified in the ANSI/ASRAHE 135-2004standard. The solution benefits by having only one groundconnection of the shield, by doing so the possibility forground loop of equalizing current is heavily reduced. Inthese systems the shield of the MS/TP cables has to beisolated from ground at all stations, except one. At eachstation the shield from the two cables has to be connectedwith each other, and isolated from ground. The best solutionfor this has been proven to be the use of shrink tubes. Thesingle ground shielding is a good approach where thesystem uses long bus cables. If two buildings have to beconnected over the same MS/TP bus cable, the use of fibreoptic has to be considered. This will prevent that a lightningstroke will be carried from one building to another, andproblem with difference in earth potential can be neglected.3.2.3 Multiple Ground ShieldingIf the distance between the individual drives is limited (e.g.inside a cabinet or in one control room) Danfossrecommends connecting the screen to ground at both endsof the bus cable. This ensures the maximum protection fromEMC noise. Connecting the screen at each end will requirethat each BACnet device has the same earth potential or anequalizing current will flow in the screen of the cable andcause disturbance and poor performance of the system. Lowimpedance to ground connection of the screen can beachieved by connecting the surface of the screen to ground,by means of a cable clamp or a conductive cable gland. TheFC 100 Series supplies various clamps and brackets to enablea proper ground connection of the BACnet cable screen.Danfoss recommends to connect the screen to ground atboth ends of the bus cable. This ensures the maximumprotection from EMC noise. Connecting the screen at eachend will require that each BACnet device has the same earthpotential or else an equalizing current will flow in the screenof the cable and cause disturbance and poor performance ofthe system. Where this is not possible, the screen can beisolated from the chassis of the drive by use of shrink-tubing.It must be pointed out that the routing of the BACnet cablemust be established with a maximum distance to othercables such as mains, motor cable, etc.How to Install VLT® HVAC Drive Native BACnet Operating Instructions10 MG.14.C1.02 - VLT® is a registered Danfoss trademark33