For one of my analog telephone systems, I have a problem with respect to
EN60950 and TNV circuits which I hope someone can help me with.
The complete system is categorized as Class II equipment which consists of a
separate power supply connected to our analog telephone network interfaces.
The interfaces are enclosed in a plastic housing.
The power supply consists basically of a transformer that has reinforced
insulation with minimum 3KV dielectric withstand voltage. It would be
connected to an AC supply of 220 Vrms, 50 Hz. The AC mains voltage is then
stepped down to a lower AC voltage of maximum 28 Vrms. This lowered AC voltage
is then fed to and terminates on our system which then converts it via a full
wave rectifier circuit to a maximum of 40 VDC output.
Since my DC output is fed from a transformer with reinforced insulation and the
maximum output is 40 VDC, I consider this a SELV circuit, actually an unearthed
SELV circuit since we do not connect to AC ground on our power cord, two-prong
only.
This SELV circuit is used to power my analog TNV circuits which connect
directly to the analog Telecommunication Network. From my understanding of EN
60950, my TNV circuits would be classified as TNV3, since it is exposed to any
surges from the telecommunication network whose characteristics may or may not
be known. The working voltages would thus be 70.7 VAC peak and 120 VDC which
exceeds the limits for a SELV circuit. This is why I do not call them TNV-2?
I wonder if I am correct. I also wonder if I could consider my TNV interfaces
as TNV-1?
EN60950 states that between TNV circuits that exceed the limits of SELV and
unearthed SELV circuits, I must have Basic Insulation. Using the secondary
circuit clearance table requirements for mains up to 300V, Pollution degree 2,
and the creepage table requirements for Material Group III at a working
voltage of 120 VDC I get:
Clearance : 1 mm
Creepage : 1.5 mm
I have no problems with clearance, assuming I correctly chose it. The problem
is with creepage on my PCB.
The terminals and circuitry, Tip/Ring, that connect directly to the
telecommunication network are isolated from my SELV circuit by a transformer
that provides Basic Insulation.
The ground of my SELV circuit, in some locations, comes close to the Tip/Ring
terminals that connect directly to the telecommunications network, The track
separation, creepage, between my SELV ground PCB track and the Tip/Ring pad is
in some cases as low as 0.9 mm, which would thus fail the 1.5 mm creepage
requirement.
I have done dielectric tests as per section 6.4.2.2,that is, a) 1.5 KVAC
between the shorted telecom network connectors and telephones and 1 KVAC for
cases b) and c) with no problems.
As can be seen then I do not meet the creepage requirements between TNV and
unearthed SELV, but I do meet the electric strength requirements.
I am not sure where I stand now with my system. Can someone help me? Is there
anything I can do, apart from redesigning the PCB, to overcome the creepage
problem? I also have systems that are already built and in stock. What can I
do with these?
Any help would greatly be appreciated as soon as possible.
Regards
Joe Schembri
[email protected]
Tel: (905)264-1505
FAX: (905)264-1510
Joe Schembri
TCR
1 Whitmore Rd. #21-23
Woodbridge, Ontario, Canada
L4L 8G4
Phone: +1 (905) 264-1505
Fax: +1 (905) 264-1510
e-mail [email protected]
