From: "Patterson,Gordon" <[email protected]>
Subject: RE: telco spacing, listings, etc.
Doug,
It may be wiser to treat insulation between traces on pcb
inner layers as pollution degree 1 rather than solid
insulation. This precludes any question about "voids" in the
"solid" insulation produced by heat curing.
Gordon
Gordon Patterson
Racal Datacom, Inc.
Dear People,
Well, I'll be the first to say it -
I must be the dumbest person around.
After having several very patient souls email me
off-line with suggestions I'm still in a quandary.
That is absolutely no reflection on these good people.
If my product connects to a dry T1 line, the signals
on the T1 line are 3 volt signals. I have a very
technical report detailing results from the
signal mask. In some cases, the report even specifies
milli-volts. All digital. No analog.
Someone did mention (off-line) that the US views the
T1 line as SELV with the capability of hitting 150 volts.
So, you go to Table 6, UL-1950 and lo n' behold you get
1.6 mm for Basic. That's (TIP or RING) -> GND and
(TIP or RING) -> the other side of the line isolation
transformer. Imposing SELV Basic insulation requirements
thus causes testing to be done at 1,000 volts (Table 18).
He went on to say that Europe considers the E1 line
as NOT SELV with the capability of hitting 250 volts.
They impose primary voltage standards. Thus, you go
over to Table 6, EN-60950 and lo n' behold you get 2.5 mm.
BUT, that's only for That's (TIP or RING) -> GND.
Spacing to traces on the other side of the line
isolation transformer are doubled for reinforced
insulation and that is 5.0mm. Imposing primary
voltage requirements thus causes testing to be done
at 3,000 volts (also Table 18).
Both standards agree of 0.4mm for inner layers.
Now, standards and requirements for primary and secondary
traces outside of telco? No problem.
But, I'll be damned if I can get someone to tell me (no insult
intended to anyone - maybe it's my stupidity) the rationalization
to "working voltage" of the T1 or E1 line that gets me some
resolution to Table 6 in either standard. And I'll be damned again
if I can find anything in my UL, FCC or CTR reports
or standards that mention anything close to 150 volts for
domestic stuff or 250 volts for European.
Didn't mean to offend anyone that has helped so far.
The discussion both on and off line has been an eye opener.
Just very frustrated. I'm trying to define and write up
some agency design criteria for engineering.
And don't be afraid to email me off-line, either.
mailto:[email protected]
Regards, Doug
