Hi Darrell:
> I have in the past worked on a UPS project where we provided double
> insulation from all hazardous voltages to the operator interface, and a
> three pronged line cord with the green wire ground bonded to the chassis.
> In essence we had both types of protection which we thought was good.
> However, we received some confilcting views on this, especially concerning
> Europe. Some people said we could not have both. I do not remember the
> reasoning why. Is there anything prohibiting simultaneous protection
> schemes?
We routinely use double-insulation in our grounded
products. We do this because we KNOW that some of
our products will be used without a ground. (Our
ground wire is for EMC purposes, not for safety
purposes.)
It is almost impossible to build a grounded product
(with an accessible secondary circuit) without using
double- or reinforced insulation at a number of points
within the product.
If you use a detachable power cord and an IEC 320
appliance coupler, then you have reinforced insulation
in the coupler.
If you use an operator-accessible fuseholder, then
you have reinforced insulation. If you use a plastic
power switch, then you have reinforced insulation.
Socket-outlets and wall switches are reinforced
insulation.
Within the product, primary and secondary wires are
separated by supplemental air or solid insulation:
the primary wire has the basic insulation and the
secondary wire (if rated the same as the primary wire)
has the supplmental insulation. (If not rated the
same, then you are using air insulation for the
supplmental insulation.)
Between bare parts of primary circuits and bare parts
of secondary circuits, you are using reinforced air
insulation (a nonsense concept).
The primary-secondary coils of switching-mode
transformers are almost always separated by double
or reinforced insulation. (A grounded barrier in a
switching mode transformer reduces its performance.)
The power supply printed wiring board necessarily uses
air and solid reinforced insulation between primary and
secondary.
A true grounded product would have a solid metal grounded
barrier between all of the primary and all of the
secondary. This is hard to imagine.
Examples of products that are grounded and do not use
double or re-inforced insulation are a breaker panel,
a metal conduit, a washing machine, a refrigerator, and
similar construction that has a continuous grounded metal
enclosure for ALL circuits.
So, a combination of grounded and double-insulated
construction is common, although often not recognized.
The IEC forces the product into either the Class I
(grounded) camp or the Class II (double-insulated) camp
without recognizing or evaluating the Class II
construction within the Class I product! The two
schemes can and should be recognized as co-existing in
the same product. Indeed, a Class I construction can
be employed within a Class II product -- without
grounding the product. (If you are interested, ask me
how.)
Neither is better than the other; they each provide
equal protection against electric shock.
Best regards,
Rich
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