Hi Dave:
> the UL report for the PSU, it states that the output is ELV, not SELV,
> because of the connector interfacing the modules to the rack.
I don't see how a connector makes the difference
between ELV and SELV.
The terms "ELV" and "SELV" imply the energy is
derived from a hazardous voltage source, i.e., a
source that could cause electric shock.
ELV has only one safeguard (i.e., Basic Insulation)
interposed between it and the hazardous voltage
source.
+-----------+ +-----------+ +-----------+
| | | | | |
|Hazardous | |Basic | | |
|Voltage |->|Insulation |->|ELV |
| | | | |(not |
| | | | |accessible)|
+-----------+ +-----------+ +-----------+
SELV has two safeguards (i.e., Basic Insulation
PLUS another safeguard, e.g., Supplementary
Insulation, grounded barrier, etc.) interposed
between it and the hazardous voltage source.
+-----------+ +-----------+ +-----------+ +-----------+
| | | | | | | |
|Hazardous | |Basic | |Supplementary | |
|Voltage |->|Insulation |->|Safeguard |->|SELV |
| | | | | | |(accessible|
| | | | | | |part) |
+-----------+ +-----------+ +-----------+ +-----------+
In this modern age, the cost of the supplementary
safeguard is negligible. I cannot understand why
a PSU manufacturer would produce a 48-volt supply
that is *NOT* SELV.
ELV cannot be accessible. This requires that a
supplementary safeguard be interposed between the
ELV circuits and accessible parts.
+-----------+ +-----------+ +-----------+ +-----------+
| | | | | | | |
|Hazardous | |Basic | | | |Supplementary
|Voltage |->|Insulation |->|ELV |->|Safeguard |
| | | | |(not | |(accessible|
| | | | |accessible)| |part) |
+-----------+ +-----------+ +-----------+ +-----------+
However, my understanding of TNV circuits is that
TNV circuits are more-or-less treated as an ELV
circuit.
+-----------+ +-----------+ +-----------+
| | | | | |
| | |Supplementary | |
|TNV |->|Safeguard |->|Accessible |
| | | | |part |
| | | | | |
+-----------+ +-----------+ +-----------+
+-----------+ +-----------+ +-----------+
| | |Basic or | | |
| | |Supplementary | |
|ELV |->|Safeguard |->|Accessible |
| | | | |part |
| | | | | |
+-----------+ +-----------+ +-----------+
If this is the case, then the 48 volts supply may
be ELV without causing undue construction
requirements in the CO.
(Note that ELV allows either Basic or Supplementary
safeguards, but TNV must use a Supplementary
safeguard. This is an interesting way of thinking
about ELV and TNV circuits.)
Best regards,
Rich
ps: Note that one supplemental safeguard is that of
grounding the SELV circuit. See IEC 60950, 2nd,
sub-clause 2.3.3.3. So, if you ground the 48 V
in your equipment, then you change the ELV to
SELV (if you meet the relative impedance
requirements of 2.3.3.3.)
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