Board Mountable
DC-DC Converters
IMS 7 Series
Edition 6/4.2000
15/16
AC-DC
front
end
DC-DC
con-
verter
Mains
Fuse
Battery
Earth
connection
Suppressor
diode
SELV
Earth
connection
+
–
~
10004
Fig. 27
Schematic safety concept. Use fuse, suppressor diode
and earth connection as per table: Safety concept leading
to an SELV output circuit.
Table 16: Insulation concept leading to an SELV output circuit
Conditions
Front end
DC-DC converter
Result
Supply
Minimum required grade
Maximum
Minimum required safety
Measures to achieve the
Safety status of
voltage
of isolation, to be provided DC output
status of the front end
specified safety status of the
the DC-DC
by the AC-DC front end,
voltage
output circuit
converter output
including mains supplied
from the
circuit
battery charger
front end 1
Mains
Basic
≤60 V
Earthed SELV circuit 2
Operational insulation, pro-
SELV circuit
≤250 V AC
vided by the DC-DC converter
ELV circuit
Input fuse 3 output suppressor
Earthed SELV
>60 V
Hazardous voltage
diode(s) 4, and earthed
circuit
secondary circuit
output circuit(s) 2
Double or reinforced
≤60 V
SELV circuit
Operational insulation, pro-
SELV circuit
vided by the DC-DC converter
>60 V
TNV-2 circuit
Supplementary insulation,
Double or reinforced insu-
based on the maximum input
lated unearthed hazardous
voltage, provided by the
voltage secondary circuit 5
DC-DC converter
1 The front end output voltage should match the specified input voltage range of the DC-DC converter.
2 The earth connection has to be provided by the installer according to the relevant safety standard, e.g. IEC/EN 60950.
3 The installer shall provide an approved fuse (type with the lowest rating suitable for the application) in a non-earthed input line directly
at the input of the DC-DC converter (see fig.
Schematic safety concept). For UL’s purpose, the fuse needs to be UL-listed. See also
Input Fuse.
4 Each suppressor diode should be dimensioned in such a way, that in the case of an insulation fault the diode is able to limit the output
voltage to SELV (<60 V) until the input fuse blows (see fig.
Schematic safety concept).
5 Has to be insulated from earth by basic insulation according to the relevant safety standard, based on the maximum output voltage
from the front end.
Safety of operator accessible output circuit
If the output circuit of a DC-DC converter is operator acces-
sible, it shall be an SELV circuit according to IEC/EN 60950
related safety standards
The insulation concept table below shows some possible
installation configurations, compliance with which causes
the output circuit of the DC-DC converter to be an SELV cir-
cuit according to IEC/EN 60950 up to a configured output
voltage (sum of nominal voltages if in series or +/– configu-
ration) of 42 V.
However, it is the sole responsibility of the installer to en-
sure the compliance with the relevant and applicable safety
regulations. More information is given in:
Technical Infor-
mation: Safety.