> From: Zhangguoqing (A)
> Sent: Friday, September 13, 2013 17:57
>
> If a transmit equipment can deliver 1.3A current into
> telecommunication network, then in the corresponding
> receive equipment (terminals), 1.3A current is
> available, do you think so?
The 1.3 A limitation is under worst-case loading conditions, and is not
the normal operating current. For any loading condition up to and
including short-circuit, the output current available from the source
cannot exceed 1.3 A.
This concept is not unlike the LPS current limits in §2.5 in that the
intent is to not create a risk of fire in certain common premises wiring
and connection devices.
> My question is why the standard states the transmit
> equipment can deliver 1.3A (max.), but at the same time
> it states the power available from a TELECOMMUNICATION
> NETWORK is limited to 15 VA?
The 15 VA assumption in §1.4.11 is assumed by definition. It is the
assumed power available from an unknown/generic telecommunications
network, which may come from a PBX, a central office, a key system or
other equipment, which the equipment under evaluation has no specific
knowledge of.
Because this is assumed by definition it is not a requirement that applies
to equipment outputs in IEC 60950-1. These requirements are contained
elsewhere in the standard.
If you connect a terminal device to a telecommunications network, the
anticipated power available from the telecommunications network is 15 VA.
By defining this power limitation, "for the purposes of this standard,"
doubt about what the connection is like is ameliorated.
For example, if I were to connect a POTS telephone or other network
(nonPoE) device to a telecommunications network, the POTS device:
• does not need to comply with §4.6.4.3
• does not need a fire enclosure for the connectors, per §4.7.2.2,
7th dashed paragraph
• does not need a fire enclosure for materials rated HB75 or HB40
less flammable (under specified conditions), per §4.7.2.2, 9th dashed
paragraph, 2nd dotted subparagraph
• use the connector material exemption in §4.7.3.3, 3rd paragraph,
5th dashed subparagraph
• etc.
The same exceptions/exemptions can apply to those portions of a fax
machine, computer, answering machine and the like, where there are no
extenuating circumstances caused by other circuits/components in the
vicinity of the TNV circuit.
Regards,
Peter Tarver
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