Hi Jim:
You have posted some good questions!
With advice from colleague Lal Bahra:
In IEC 60950-1, Figure F.12 specifies Point A
for the air gap (in 2.1.1.1).
Figure F.12 specifies Point B for both clearance
(in 2.10.3) and creepage (in 2.10.4).
Figure F.12 seems to distinguish between an
"air gap" and a "clearance."
I conclude that there are THREE dimensional
requirements for the probe: (1) air gap
>from the probe tip to the hazardous voltage
conductor; (2) clearance from the probe-to-
enclosure contact point to the hazardous
voltage conductor; (3) creepage from
the probe-to-enclosure contact point to the
hazardous voltage conductor.
2.1.1.1 invokes "no contact" from Point A
(for voltages up to 1000 ac and 1500 dc)
and a clearance dimension from Point A for
the *air gap* (for voltages exceeding
1000 ac and 1500 dc).
However, when reading 2.10.3.1, clearances are
between hazardous voltage and an accessible
conductive part of an enclosure. Clearance to
the accessibility probe is not mentioned,
although the compliance statement invokes Annex
F, which includes the probe. The third dashed
compliance paragraph (of 2.10.3.1) invokes the
probe for enclosures of insulating material,
but not for enclosures of conductive material.
The minimum clearances of 2.10.3.1 would be
applied from the hazardous voltage part to
Point B on the probe.
Clearance would not apply to the probe applied
to an enclosure of conductive material. This
is reasonable since 2.10.3.1 specifies the
clearance between the hazardous voltage and
the conductive enclosure.
Likewise, 2.10.4, creepages, invokes the probe
only in the compliance statement.
This, too, would only apply to enclosures of
insulating material.
If we consider a 250 V mains circuit, the
dimensions to the probe would be:
Point A, air gap: No contact.
Point B, clearance: 4 mm (insulating enclosure)
0 mm (conductive enclosure)
Point B, creepage: 4 mm (insulating enclosure)
0 mm (conductive enclosure)
You are correct that application of the criteria
for Point A and Point B make the answer quite
clear.
One of the difficulties is that probe requirements
are in the compliance statements and in Figure
F.12. Another difficulty is that we have an air
gap and a clearance, which are not the same line.
Furthermore, the accessibility requirements do
not include all of the dimensional requirements
>from hazardous voltage to the probe, i.e., they
do no include the clearance and creepage
dimensions.
In answer to your questions:
> Q1: So what is our requirement?
>
> a) Prevent contact (only) of the 3 different probes in 2.1.1.1
> b) Item a) plus clearance and creepage per 2.10.3 and 2.10.4 and F.12?
b.
> Q2: If the answer to Q1 is b), then given that it's hazardous
> voltage, do we require Reinforced creepage and clearance distances?
Yes.
> Q3: What Pollution Degree do we use, given that when closed, the
> connector is certified as IP54 (which gives us PD I in my
> opinion) but when open the connector is exposed to the environment
> (outdoor product). Since PD relates to contamination, I would be
> willing to apply PD I since the connector spends most of its life
> closed, but what about during/before installation or during servicing?
> It's open, but should that short duration exposure count?
Pollution usually is a long-term effect.
I presume normal operation is with the
connector pair mated, in which case the
pollution degree should be based on the
mated condition.
> Q4: If the answer to Q1 is a), what is the justification for only
> requiring no contact as opposed to enforcing creepage and clearance?
> Is it related to the fact that the person's finger will only be there
> briefly, making it unlikely that breakdown of the small gap between
> the finger and the live part would occur?
Accessibility is considered transitory
(as compared to permanent separation of
conductors). One rationalization is
that the coincidence of a transient over-
voltage and accessibility is unlikely.
Another rationalization is that if a
transient should occur, the duration of
the transient shock voltage is non-injurious.
Another point is that, according to Paschen's
Law, air insulation (at normal temperature
and pressure) does not break down below about
300 volts rms.
Having said all this, Pete Perkins provided
me with a copy of CTL Decision Sheet 439
which states, simply, for voltages up to
1000 V ac and 1500 V dc, no contact.
Best regards,
Rich
e-mail: [email protected]
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