Hi Nick:
> >There is a proposed amendment to IEC/EN 60950-1 requiring a test of the
> >protective conductor network at *prospective short-circuit current* for
> >the time it takes for the mains circuit protective device to operate.
> >The details are controversial at present, because the test currents
> >appear not to have taken into account the differences between
> >prospective short-circuit currents in different wiring systems and
> >supply voltages. Given that reservation, the lowest test current is 200
> >A.
> >
> >The amendment is aimed at protective conductors which are surface or
> >internal traces of multi-layer printed boards. It is said that such
> >traces have failed in the field under high-current fault conditions.
> >--
Before we proceed with such a test, we must first
determine
1- where basic insulation could fail to a grounded
part;
2- the impedance of the failure, which must be less
than that of the source impedance (otherwise,
the fault itself limits the current);
3- the path from the point of the fault as only this
path need be tested (not all ground circuit paths
are subject to a basic insulation fault and there-
fore are not subject to the test).
> Is the proposal to replace the existing test in the standard or to
> add an additional test only for certain special circumstances?
>
> Is there any evidence that this test would actually result in a
> significant number of poorly designed products which currently pass
> the requirements of the standard being rejected?
Yes.
Having done this test, a conventional PWB ground path
that passes the 25-amp, 2-minute test MAY not pass
this test.
A PWB ground path will pass the test if the cross-
sectional area of the trace and its connections is
equal the cross-sectional area of the required
ground wire.
> The existing test has its faults but it is easy to do with some very
> cheap apparatus. It strikes me that the cost of doing a test at 200+A
> is potentially very substantial. If the result of an amendment to the
> standard is that significant numbers of self-certified products which
> have not been properly tested in this aspect of their design reach
> the market, then the net result will actually be a significant
> reduction in the safety of end users.
In reality, basic insulation seldom fails.
If the basic insulation does fail, the failure is
likely to be a point-contact, not a large-area
contact. A point contact is a relatively high
resistance, and limits the current.
The basic idea of the test is valid. However, it
may be superfluous and inappropriate for practical
application in many kinds of electronic equipment.
> A cynic's view might also be that an amendment of this nature would
> suit the test labs and larger manufacturers fine, since they will be
> able to justify the cost of the apparatus required, whereas smaller
> manufacturers (and yes, small consultancy companies like mine) will
> not.
The apparatus is quite simple. I would guess that
many labs, if they do high-power testing, already
have the necessary equipment.
> OK, I admit I'm putting two and two together and getting about seven
> but I believe one should get one's retaliation in first in these
> circumstances! Any amendment along the lines suggested should be
> prepared to sacrifice a fair degree of technical accuracy against the
> need for the test to be cheap, quick and easy to perform.
>
> Nowadays, standards writing should not just about getting accuracy
> and repeatability in testing but should also take into account the
> need to ensure that the requirements (and hence the tests) are
> actually possible to apply in the real world, and not just by people
> at specialist test houses.
Well... I would state this a bit differently. A
standards-writing organization should apply the
scientific method to all new requirements.
Best regards,
Rich
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