Download AGENDA ITEM:____

TR41.7.1-04-05-04-L-TAG108-Hershey-MT1-Boksiner-1
TAG108(Secretary)SD-1
March 21, 2003
TAG108-(Hershey-MT1-Boksiner])
May 2004
AGENDA ITEM
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Proposal to amend IEC 60950-21
Clause 6.2.1, maximum power limitation for RFT-V circuits
______________________________
Impact:
Present requirement is more stringent than intended
Background:
Recent communications between Randy Ivans of UL, Jim Wiese of ADTRAN and Telcordia indicate
that the present wording of 60950-21 regarding power limitations for RFT-V circuits does not
adequately reflect the intent to mirror GR-1089-CORE and Class 3 power limitation from the National
Electrical Code. The discrepancy between 60950-21 and the original requirements is due to vague
wording. This proposal aims to initiate a revision of 60950-21.
Additional refinement to this proposal is expected after discussions at the TR41.7.1 meeting.
Rationale:
The present text in clause 6.2.1 relating to maximum power limitation is vague. being interpreted in a
manner that does not match the intent .
The present wording of 60950-21, clause 6.2.1. states:
Under normal operating conditions, an RFT-V CIRCUIT shall comply with the following:


the steady state open circuit voltage between earth and each conductor that normally connects
to a TELECOMMUNICATION NETWORK shall not exceed

140 V d.c., or

200 V d.c., provided that a monitoring and control device is used that limits the current to
earth in accordance with 6.2.3;
the maximum power that can be supplied to any load connected to the TELECOMMUNICATION
NETWORK shall be limited to 100 W after 1 s;
The last dash is the power limitation requirement adapted from Telcordia GR-1089-CORE Section 7.6
that states:
Sources that may be applied to communication wiring shall be limited to a rating not exceeding 100
volt-amperes.
This limitation was intended to be the same as Class 3 power limits in the National Electrical Code
(NEC). Although NEC Class 3 power limits do not apply to the TELECOMMUNICATION NETWORK, the Class 3
power limitation was applied to RFT-V circuits to help reduce fire hazard from this type of circuit. The
NEC class 3 limits for dc voltages for inherently limited sources are described in the following table and
definitions
841012002 PagePage 1 of 2
TR41.7.1-04-05-04-L-TAG108-Hershey-MT1-Boksiner-1
TAG108(Secretary)SD-1
March 21, 2003
:
Power limitations
VAmax (volt-amperes)
—
Current limitations
Imax (amperes)
150/Vmax
Power source maximum
nameplate rating
VA (volt-amperes)
100
Current (amperes)
100/Vmax
Vmax, Imax, and VAmax are determined with the current-limiting impedance in the circuit (not
bypassed) as follows:
Vmax: Maximum output voltage regardless of load with rated input applied.
Imax: Maximum output current under any noncapacitive load, including short circuit, and with
overcurrent protection bypassed if used. Where a transformer limits the output current, Imax limits
apply after 1 minute of operation. Where a current-limiting impedance, listed for the purpose, or as
part of a listed product, is used in combination with a nonpower-limited transformer or a stored
energy source, e.g., storage battery, to limit the output current, Imaxlimits apply after 5 seconds.
VAmax: Maximum volt-ampere output after 1 minute of operation regardless of load and overcurrent
protection bypassed if used.
In GR-1089 and the NEC the power rating refers to power delivered under normal conditions of the
source, load and the intervening telecommunication network, while VAmax refers to power delivered
under any possible load, including fault conditions. Nether the NEC Class 3 for inherently limited
sources nor GR-190-CORE specifiy VAmax. The NEC refers to the maximum current, Imax, rather than
VAmax. Under any condition, the power could theoretically go up to 150 VA although Imax and Vmax do
not necessarily occur at the same point on the load line. In fact they are unlikely to occur on the same
point in the load line.
The present language of IEC 60950-21 is confusing. The sentence uses the words “the maximum
power that can be supplied to any load” and seemingly describes VAmax. However, the requirements
are introduced by “Under normal operating conditions” so it could refer to the nominal rating. It seems
that certain test laboratories are interpreting the 100W limit to be VAmax restricting the RFT technology
that has been used for decades. Improved wording based on NEC table is proposed.
Proposal:
Modify the.second dash of Section 6.2.1 as follows:

the maximum power that can be supplied to a normal any load connected to the
TELECOMMUNICATION NETWORK under normal conditions shall be limited to 100 W after 1 s;
Add a new .dash to Section 6.2.2 as follows:

the maximum current supplied by the RFT-V circuit to any any noncapacitive load, including
short circuit, shall be limited to 150/Vmax, where Vmax is the maximum output voltage regardless
of load.
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