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/na-
tional regulations on PELV supplies.
In VLT 6000 HVAC all control terminals as well as ter-
minals 1-3 (AUX relay) are supplied from or in con-
nection with extra low voltage (PELV).
Galvanic (ensured) isolation is obtained by fulfilling re-
quirements concerning higher isolation and by provid-
ing the relevant creapage/clearance distances. These
requirements are described in the EN 50178 standard.
For additional information on PELV see RFI switching.
Galvanic isolation
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 three locations
(see drawing below), namely:
-
Power supply (SMPS) incl. signal isolation of
U
DC
, indicating the intermediate current volt-
age.
-
Gate drive that runs the IGTBs (trigger trans-
formers/opto-couplers).
-
Current transducers (Hall effect current
transducers).
NOTE: VLT 6002-6072, 525-600 V units do not meet
PELV requirements in accordance with EN 50178.
Earth leakage current
Earth leakage current is primarily caused by the ca-
pacitance between motor phases and the motor cable
screen. When an RFI filter is used, this contributes ad-
ditional leakage current, as the filter circuit is connec-
ted to earth through capacitors. See drawing on the
following page.
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 earth-
ed.
NB!
Since the leakage current is > 3.5 mA,
rein-forced earthing must be established,
which is required if EN 50178 is to be
complied with. Never use ELCB relays
(type A) that are not suitable for DC fault
currents from three-phase rectifier loads.
If ELCB relays are used, they must be:
-
Suitable for protecting equipment with a di-
rect current content (DC) in the fault current
(3-phase bridge rectifier)
-
Suitable for power-up with short pulse-sha-
ped charging current to earth
-
Suitable for a high leakage current (300 mA).
VLT
®
6000 HVAC Series
MG.61.B5.02 - VLT
®
is a registered Danfoss trademark 195
All about VLT 6000
HVAC
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