All About Vlt 2800; Special Conditions; Galvanic Isolation (Pelv); Earth Leakage Current And Rcd Relays - Danfoss VLT 2800 Manual

Special conditions

Galvanic Isolation (PELV)

PELV (Protective Extra Low Voltage) insulation is ach-
ieved by inserting galvanic isolators between the con-
trol circuits and circuits that are connected to the mains
potential. The VLT is designed to meet the require-
ments for protective separation by means of having the
necessary creepage and clearance distances. These
requirements are described in standard EN 50 178. It
is also a requirement that the installation is carried out
as described in local/national regulations regarding
PELV.
All control terminals, terminals for serial communica-
tion and relay terminals are safely separated from the
mains potential, i.e. they comply with the PELV re-
quirements. Circuits that are connected to control ter-
minals 12, 18, 19, 20, 27, 29, 33, 42, 46, 50, 53, 55
and 60 are galvanically connected to one another. Se-
rial communication connected to fieldbus is galvani-
cally insulated from the control terminals, although this
is only a functional insulation.
The relay contacts on terminals 1 - 3 are insulated from
the other control circuits with reinforced/double isola-
tion, i.e. PELV is observed for these, even though
there is mains potential at the relay terminals.
The circuit elements described below form the safe
electric separation. They fulfill the requirements for re-
inforced/double insulation and associated testing pur-
suant to EN 50 178.
1. Transformer and optical separation in voltage
supply.
2. Optical insulation between Basic Motor Con-
trol and control card.
3. Insulation between the control card and the
power part.
4. Relay contacts and terminals relating to other
circuits on the control card.
PELV insulation of the control card is guaranteed un-
der the following conditions:
TT network with maximum 300 Vrms be-
-
tween phase and earth.
- TN network with maximum 300 Vrms be-
tween phase and earth.
- IT network with maximum 400 Vrms between
phase and earth.
In order to maintain PELV all connections made to the
control terminals must be PELV, e.g. thermistor must
be reinforced/double insulated.
134

Earth leakage current and RCD relays

Earth leakage current is primarily caused by the par-
asitic capacitance between motor phases and motor
cable screen. When an RFI filter is used, this contrib-
utes additional leakage current, as the filter circuit is
connected to earth through capacitors.
The size of the earth leakage current depends on the
following 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 earthed 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, re-
inforced earthing must be established, as
this is required to ensure compliance with
EN 50178. The cable cross section must
be at least 10 mm
that are terminated seperately.
NB!
Never use RCD relays (type A) that are
not suitable for DC fault currents from 3-
phase rectifier loads.
If RCD relays are used, they must be:
- Suitable for protecting equipment with a DC
content in the faulty current (3-phase bridge
rectifier)
Suitable for a pulse-shaped, brief discharge
-
on power-up
Suitable for a high leakage current (300 mA)
-
See the section entitled Earth connection for further in-
formation.

Extreme operating conditions

Short circuit
The frequency converter is protected against short cir-
cuits on motor terminals U, V, W (96, 97, 98). A short
circuit between two motor terminals would lead to an
overcurrent in the IGBT module, which means that all
transistors in the IGBT module would independently
cut out.
MG.27.E2.02 - VLT is a registered Danfoss trademark
2
or 2 rated earth wires