Danfoss VLT 2800 Design Manual

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Danfoss VLT 2800 Design Manual Manual pdf 129 page image

5 All about VLT 28005.1 Special Conditions5.1.1 Galvanic Isolation (PELV)Achieve PELV (Protective Extra Low Voltage) isolation byinserting galvanic isolators between the control circuitsand circuits that are connected to the mains. Thefrequency converter is designed to meet the requirementsfor protective separation with necessary creepage andclearance distances. These requirements are described instandard EN 50 178. It is also a requirement that the instal-lation is carried out as described in local/nationalregulations regarding PELV.All control terminals, terminals for serial communicationand relay terminals are safely separated from the mainspotential, that is, they comply with the PELV requirements.Circuits that are connected to control terminals 12, 18, 19,20, 27, 29, 33, 42, 46, 50, 53, 55 and 60 are galvanicallyconnected to one another. Serial communicationconnected to fieldbus is galvanically isolated from thecontrol terminals, although this is only a functionalisolation.The relay contacts on terminals 1-3 are isolated from theother control circuits with reinforced/double isolation, thatis, PELV is observed, even though there is mains potentialat the relay terminals.The circuit elements described below form the safe electricseparation. They fulfil the requirements for reinforced/double insulation and associated testing according to EN50 178.1. Transformer and optical separation in voltagesupply.2. Optical isolation between Basic Motor Controland control card.3. Isolation between the control card and the powerpart.4. Relay contacts and terminals relating to othercircuits on the control card.PELV isolation of the control card is guaranteed under thefollowing conditions:• TT network with maximum 300 Vrms betweenphase and ground.• TN network with maximum 300 Vrms betweenphase and ground.• IT network with maximum 400 Vrms betweenphase and ground.To maintain PELV all connections made to the controlterminals must be PELV, for example, thermistor must bereinforced/double insulated.5.1.2 Ground Leakage Current and RCDRelaysGround leakage current is primarily caused by the parasiticcapacitance between motor phases and motor cablescreen. When an RFI filter is used, this contributesadditional leakage current, as the filter circuit is connectedto ground through capacitors.The size of the ground leakage current depends on thefollowing factors, in order of priority:1. Length of the motor cable.2. Motor cable with/without screen.3. High switching frequency.4. RFI filter used or not.5. Motor grounded on site or not.The leakage current must be handled carefully duringhandling/operation of the frequency converter if (bymistake) the frequency converter has not been grounded.WARNINGSince the leakage current is > 3.5 mA, reinforcedgrounding must be established, as this is required toensure compliance with EN 50178. The cable cross-section must be at least 10 mm2 or 2 rated ground wiresthat are terminated separately.WARNINGDo not use RCD relays (type A) that are not suitable forDC fault currents from 3-phase rectifier loads.If RCD relays are used, they must be:• Suitable for protecting equipment with a DCcomponent in the faulty current (3-phase bridgerectifier).• Suitable for a pulse-shaped, brief discharge onpower-up.• Suitable for a high leakage current (300 mA).See chapter 3.4.10 Ground Connection for furtherinformation.All about VLT 2800 Design GuideMG27E402 Danfoss A/S © Rev. May/2014 All rights reserved. 1275 5