VLT® 5000 Crane
Appendix
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Galvanic Isolation (PELV)
PELV offers protection by way of extra low voltage.
Protection against electric shock is considered to be
ensured when the electrical supply is of the PELV
type and the installation is made as described in
local/national regulations on PELV supplies.
In VLT 5000 Crane all control terminals as well as
terminals 01-03 (AUX relay) are supplied from or in
connection with extra low voltage (PELV).
Galvanic (ensured) isolation is obtained by fulfilling
requirements concerning higher isolation and
by providing the relevant creapage/clearance
distances. These requirements are described
in the EN 50178 standard.
The components that make up the electrical
isolation, as described below, also comply with the
requirements concerning higher isolation and the
relevant test as described in EN 50178.
The galvanic isolation can be shown in five locations
(see drawing below), namely:
1. Power supply (SMPS) incl. signal isolation of U
DC
,
indicating the intermediate current voltage.
2. Gate drive that runs the IGBTs (trigger
transformers/opto-couplers).
3. Current transducers (Hall effect current transducers).
4. Opto-coupler, brake module.
Galvanic isolation
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Earth leakage current
Earth leakage current is primarily caused by the
capacitance between motor phases and the motor
cable screen. When an RFI filter is used, this
contributes additional leakage current, as the filter
circuit is connected to earth through capacitors.
The size of the leakage current to the ground depends
on the following factors, in order of priority:
1. Length of motor cable
2. Motor cable with or without screen
3. Switching frequency
4. RFI filter used or not
5. Motor grounded on site or not
The leakage current is of importance to safety during
handling/operation of the frequency converter if (by
mistake) the frequency converter has not been earthed.
NB!:
Since the leakage current is >3.5 mA, reinforced
earthing must be established, which is required
if EN 50178 is to be complied with. For
3-phased frequency converters, only fault current
relays which are suitable for protection against DC
currents (Din VDE 0664) are to be used. RCD fault
current relays type B comply with these requirements
according to the norm IEC 755-2.
The following requirements must be complied with:
-
Suitable for protecting equipment with a
direct current content (DC) in the fault
current (3-phase rectifier)
-
Suitable for power-up with short pulse-shaped
charging current to earth
-
Suitable for a high leakage current.
MG.50.N4.02 - VLT is a registered Danfoss trademark
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