Robert & Ilan -
I am breaking my response into parts, to maintain separate
foci for each. The subject line is modified for each, as
appropriate.
Since Robert references Figure 6C (he mistakenly stated 5C)
and overvoltage testing of coax connected equipment
[Customer Premises Equipment (CPE) was not specifically
mentioned], it is assumed the applicable standard is
CSA/UL60950, rather than the referenced IEC version.
> From: Ilan Cohen
> Sent: Sunday, November 30, 2003 10:05 PM
>
> 2) Coax is typically regarded as SELV since it is
> not used for TNV type of interfaces in 60950. For
> example the E1/T1 coax 75Ohm interface is never
> subjected to Overvoltage since it resides with in
> the building only. Same applies to DS3 or other
> interfaces.
OVERVOLTAGES ON COAX:
The above is not strictly true, especially if the equipment
under consideration is a CSU, DSU or similar piece of
equipment.
Coax is routinely used for T1/T3 carrier connected to
exposed (outside) plant. In the early '90s, perhaps one in
50 telecommunications equipment evaluations at UL's Santa
Clara office included coax (or a coax option) for just this
purpose.
There is no doubt that coax *can* be subject to overvoltages
and has been tested as such, particularly under CSA 22.2 No.
225 and UL1459. The practice should still exist under
CSA/UL60950. Short of complying with the construction only
path in Fig. 6C of CSA/UL60950 or presence of an equivalent
mitigating circumstance, there's nothing to allow otherwise.
EQUIVALENT MITIGATING CIRCUMSTANCE (for CPE):
It is normally the case that a CSU (or similar equipment)
will be used with a T1 or T3 carrier line (or 'fractional,'
thereof). However, a number of pieces of equipment I've
seen and evaluated incorporated the functionality and
physical layer isolation normally associated with a CSU/DSU
(or similar equipment) into the CPE. In such cases the
connecting circuit is most definitely TNV-1.
When a CSU/DSU (or similar equipment) is used, there is
still not a 100% certainty that the device is an isolating
type or that it would provide protection against
overvoltages. I say this because when asked the question
some number of years ago, no telecommunication carriers
would state with certainty that all CSU type equipment in
their networks is isolating or that it provides overvoltage
protection. This answer probably reflected liability
concerns, but told its tale, nonetheless.
The only reasonable mechanism to CPE manufacturers is to
either:
1) always provide a CSU/DSU (or similar equipment)
2) build the physical layer isolation and overvoltage
protection into the CPE
3) specify use of a CSU/DSU (or similar equipment) as a
requirement in your installation instructions, including its
isolating nature, its overvoltage protection character, and
that it must be safety certified
As imperfect as Option 3 is, it gets one over the safety
certification hump.
Regards,
Peter L. Tarver, PE
[email protected]
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