Hi Brian:
> What I've always wondered about, at least for class 1 construction, is just
> what is really being tested by the pri/sec hi-pot, when the customer chooses
> to ground the power supply's return.
(These comments presume the secondary is SELV.)
When you use a ground for safety purposes, the
ground circuit must be capable of carrying the
fault current. (This construction is called
"bonding.")
Typically, the construction of the secondary
ground circuit does not meet the requirements
of bonding. Consequently, the ground cannot
be relied upon for safety.
Therefore, pri-sec insulation must be double/
reinforced.
So, the pri-gnd insulation is basic and is
backed up by grounded/bonded parts. And, the
pri-sec insulation is basic backed up by
supplementary (or is reinforced).
(Most switching-mode transformers don't use a
grounded shield between pri-sec as this
reduces coupling and does other things that
reduce the effectiveness of the transformer.
Therefore most switching-mode transformers
use double or reinforced insulation between
pri-sec.)
By the way, another reason for not using the
secondary ground for safety is that the sec
winding may not be capable of carrying the
fault current, and will open. The problem is
that fault current is passing through the part
of the winding that is grounded, that part of
the winding opens, and the high side of the
SELV secondary is now connected to primary.
> Also,as some of our output-to-chassis "Y" caps are just 100V decouple caps,
> and because SELV to P.E. spacing and insulation requirements would never
> withstand reinforced test levels, how does the end-use installation ever get
> pri/sec hi-pot to pass without removing the unit from the chassis?
There is no requirement for SELV-to-PE spacings
and insulation. After all, both the SELV and
the PE are accessible parts and need not have
any safety insulation or spacings between them.
If your SELV is floating, then the pri-sec hi-pot
test is simply pri-sec. No connection to chassis
or ground. This will, however, damage your sec
decoupling caps because there will be a capacitive
voltage divider from primary to Y-cap to chassis
to decoupler cap to sec. To prevent overvoltage
of the decoupler, either short it out or open it.
Best regards,
Rich
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