Hi Pierre:
Here is an ASCII block diagram of the construction the way
I interpret your message:
+-----------+
| |
| dc-dc |
12 V dc --->| converter |---+
| | |
| | |
+-----------+ |
+----> 300 V dc
+-----------+ |
| | |
| | |
230 V ac -->| rectifier |---+
| |
| |
+-----------+
(Note that this construction is the same as a UPS.)
You mention that, when operating from 12 V dc, the only
"insulation" between the 300 V dc and the 230 V ac mains
plug is the diode rectifier.
As a general rule, a semiconductor is not relied upon for
insulation for the purposes of protection against electric
shock (unless specifically rated as an insulator).
When operating from 12 V dc, an electric shock hazard may
exist BETWEEN THE POLES of the 230 V plug.
When operating from 12 V dc, a leakage current shock hazard
MAY exist from either pole of the 230 V plug to ground
(DESPITE the isolation of the 300-V circuits from ground).
The conventional safety approach to this construction is
to use a transfer relay between the two 300-V sources:
+-----------+
| |
| dc-dc |
12 V dc --->| converter |---+
| | |
| | |
+-----------+ \
o--> 300 V dc
+-----------+ /
| | |
| | |
230 V ac -->| rectifier |---+
| |
| |
+-----------+
This construction eliminates the need for safety isolation
in the dc-dc converter. (Note that a similar problem,
leakage current shock hazard, MAY exist from either pole of
the 12 V dc plug to ground. In the absence of a transfer
relay, a good safety approach would be to use a safety
isolating transformer construction in the dc-dc converter
as well as conventional pri-sec spacings and insulations.)
Best regards from San Diego,
Rich
-------------------------------------------------------------
Richard Nute Product Safety Engineer
Hewlett-Packard Company Product Regulations Group
AiO Division Tel : +1 858 655 3329
16399 West Bernardo Drive FAX : +1 858 655 4979
San Diego, California 92127 e-mail: [email protected]
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