Hi Klaus,
I apologize for the delay in responding. If I've given you
too much info, then default right down to the end of my post
and make your assessment of my advice.
You asked a very good question. I work in the
telecommunications industry doing approvals to these
agencies. Telco spacings has always been a mystery to me.
I asked this question of several standards and agency
engineers and supervisors causing quite a stir. The bottom
line is no one seems to have anything really definitive.
It's a question that has many sides to it.
I'll try to make it simple with some explanation behind
my reasoning for it.
PSTN - Public Switched Telephone Network.
Your concern would be FCC Part 68 surge testing survival
and UL-1950 3rd Edition safety testing since this UL
standard also covers telephone products.
FCC Part 68 won't tell you a damn thing about spacing.
It's been described as an extremely selfish test. It only
focuses on performance and not construction. In other words,
if your board survives the test - fine. They won't TELL you
how to construct the board to pass the test. The FCC does
abnormal testing called "surge" testing. It is an attempt
to simulate lightning attacks to the telephone line.
UL-1950 focuses on BOTH construction (how you made the thing)
and performance (testing the thing). The performance testing
side of things with the FCC and UL will test under normal
conditions and abnormal conditions. The PRIMARY concern for
both the FCC and UL testing for direct connection of telephone
equipment to the line is to protect the telephone line and the
end user.
Sorry for the long background but thought it was necessary.
The problem starts getting deep, unfortunately.
It begins with the different way the US views what the T1 telephone
equipment connects to for power as opposed to the way Europe views what
the E1 telephone equipment connects to for power.
Since US AC powered equipment plugs into the wall outlet,
it has the potential of hitting 120vac + 10% overhead = 132 volts.
It also has the potential of shorting from the primary
side to the T1 line. Something which the agencies
want to protect. That's the primary purpose of these
standards - protecting the line.
Goto UL-1950, 3rd Edition,
page 77, Table 6 "Minimum Creepage Distances.
132 volts translates to the 150 volt row.
Go across to "Pollution Degree 2", "Material group
IIIa + IIIb" and you will find a spacing of 1.6 mm.
That's for BASIC insulation or in this case from the
TNV (Telephone Network Voltages) circuit to GROUND.
Double that for REINFORCED insulation for TNV to
SECONDARY to get 3.2mm.
Also in UL-1950 is specified a 0.4 mm spacing through the pcb
material for inner layers. That specification is found on page 78,
Section 2.9.4.1 to be exact.
In Europe, AC equipment is powered with 230 vac. So if there
is a single fault from primary to the TNV stuff it's capable
of hitting 230vac +10% or roughly 250volts. Go to EN-60950,
Table 6, page 57 and find 2.5mm under the same column headings
as with the US standard UL-1950. Double it for REINFORCED
and you come up with 5.0mm. Some people I've heard rough
this spacing up to 6.0mm.
Thus, the big difference between US and Europe and why so
many US companies get zapped with this. US line transformers
are only good to about 1,000 volts. Europe tests to 3,000 volts.
Ok, so if you've stayed with me this far, my *advice* to
you is this (I'm sure there are alot of people out there
with some good advice also) -
To ensure world wide marketing availability be it US
or Europe without a hitch, go with
Inner Layer Spacings:
0.4mm min.
Outer Layer Spacings:
2.5mm TIP-to-Ground min.
2.5mm RING-to-Ground min.
6.0mm TIP-to-Any trace on other side of transformer min.
6.0mm RING-to-Any trace on other side of transformer min.
Sorry for the lengthy post everyone.
Regards, Doug
----------
From: Klaus LOIBNER <[log in to unmask]>
To: [log in to unmask]
Subject: [TN] Track spacing for FCC requirements
Date: Friday, January 23, 1998 1:48 AM
What is the required spacing on both inner layers and outer layers to
fulfill the necessary requirements for telecommunications equipment (i.e.
PSTN-interfaces).
Which standards have to be taken into account.
Any input would be most appreciated.
Klaus A. Loibner
Frequentis
[log in to unmask]
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