5.2 Special Conditions
5.2.1 Galvanic Isolation (PELV)
PELV (Protective Extra Low Voltage) isolation is achieved by inserting galvanic isolators between the control circuits and circuits that are connected to
the mains potential. These separators are designed to meet the requirements for increased separation by means of having the necessary creepage and
air clearance. These requirements are described in standard EN 50 178. It is also a requirement that installation is carried out as described in local/national
PELV regulations.
All control terminals, terminals for serial communication and relay terminals are safely separated from the mains potential, i.e. 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 galvanically connected to one
another. If switch S100 is opened the potentials of the group 18, 19, 20, 27, 29, 33 are separated from all other input/output. In that case terminal 12
cannot be used for supply for the digital inputs on these terminals.
Serial communication connected to terminals 67 - 70 is galvanically isolated from the control terminals, although this is only a functional isolation.
The relay contacts on terminals 1 - 3 are isolated from the other control circuits with increased isolation, i.e. PELV is observed for these, even though
there is mains potential in the relay terminals.
The circuit elements described below form the safe electric separation. They fulfill the requirements for increased isolation and associated testing pursuant
to EN 50 178.
1.
Transformer and optical separation in voltage supply.
2.
Optical isolation between Basic Motor Control and control card
3.
Isolation between the control card and the power part.
4.
Relay contacts and terminals relating to other circuits on the control card.
PELV isolation of the control card is guaranteed under the following condition:
-
There may be max. 300 V between phase and earth.
A motor thermistor connected to terminals 31a-31b must be double isolated to obtain PELV. Danfoss Bauer provides double isolated thermistors.
See also the section
Diagram
in the Design Guide.
5.2.2 Earth Leakage Current and RCD Relays
Earth leakage current is primarily caused by the capacitance between motor phases and 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 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
earthed.
NB!
Since the leakage current is > 3.5 mA, reinforced earthing must be established, as this is required to ensure compliance with EN 50178.
Never use ELCB relays (type A) that are not suitable for DC fault currents from 3-phase rectifier loads.
5 All About FCD 300
VLT
®
Decentral FCD 300 Design Guide
126
MG.90.S1.02 - VLT
®
is a registered Danfoss trademark
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