Download AGENDA ITEM:____

TAG108(Arli06-MT2-Boksiner)
May 2006
T
AGENDA ITEM
[]
Proposal to amend IEC 60950-21, Clause 6.2.3 and Clause 6.2.1
Clause 6.2.3, Limits with one conductor earthed
Clause 6.2.1 Limits under normal operating conditions
______________________________
Impact:
Proposal provide a more thorough test procedure as present requirement is
not sufficient to limit current to values below 10 mA.
Background:
Clause 6.2.3, Limits with one conductor earthed, of IEC 60950-21 states:
for RFT-V CIRCUITS whose open circuit voltage exceeds 140 V d.c. under normal operating
conditions, the current between the other conductor and earth, measured through a 2 000
2 % resistor, under any external load condition, shall not exceed the relevant line-toearth limit given in Figure 1. Moreover, this current shall not exceed 10 mA d.c. after 10 s.
The intent of this requirement was to limit the current available from an RFT-V power source
exceeding 140 V d.c to earth to 10 mA. However, the present requirement that consists of a
measurement at a single value of test resistance is not sufficient to assure conformance with this
current limitation for non-linear source.
Also, the limitation to 10 mA for power source exceeding 140 V should also apply limits under normal
operating conditions since a person might touch the source under normal conditions. Finally, Figure 1
– Maximum current after a single fault is used as the limit curve in 6.2.3. This figure should not be
used in 6.2.3 as it is not consistent with 10 mA long-term limit.
Rationale:
The present text in clause 6.2.3 was intended to limit the current available from an RFT-V power
source exceeding 140 V d.c to earth to 10 mA. Figure 1 shows the desired characteristics of an RFT-V
circuit.
840959616
Page 1 of 7
TAG108(Arli06-MT2-Boksiner)
May 2006
200
Unacceptable
DC Voltage (V)
180
160
140
120
Acceptable
100
80
0
5
10
15
20
80
100
120
Current (mA)
Figure 1  Desired voltage-current characteristics of RFT-V circuit (w.r.t. earth)
Consider a general measurement circuit for the determination of the circuit voltage-current
characteristics. Figure 2 shows a measurement circuit using a variable test resistor.
RFT-V circuit
Current between the other
conductor and earth
B
E
Test resistor
A
One output
earthed
Figure 2  Measurement circuit to determine voltage-current characteristics of RFT-V circuit
with one output earthed
Suppose the RFT-V circuit under test has an open-circuit voltage greater than 140 V. The current
flowing from the circuit will increase as the test resistor is decreased in value inversely proportional to
the resistor value. However, as the current reaches 10 mA, the monitoring and control device required
by clause 6.2.1 should activate to limit the current. For an open-circuit voltage of 200 V, the highest
voltage allowed for RFT-V circuit, the device should activate at 20 k or the current would continue to
increase beyond 10 mA. Figure 3 shows the behavior of the output current as a function of test
resistance for a circuit that limits the output current to 10 mA at an open-circuit voltage of 200 V d.c.
840959616
Page 2 of 7
TAG108(Arli06-MT2-Boksiner)
May 2006
Current (mA)
100
Monitoring and control device
activates to limit current
to 10 mA
10
1
0.5 0.75 1
2
3
5
7.5 10
20
30
50
75 100
Test Resistance (k)
Figure 3  Load line for circuit that limits the current to 10 mA at an open-circuit voltage of 200
V d.c.
Clause 6.2.3 requires a single measurement at a test resistance value of 2 k. This means that for test
resistance values greater than 2 k the characteristics of the monitoring and control device are not
verified so that the device may not provide the desired current limiting. At an open-circuit voltage of
200 V, test value of 2 k corresponds to a current of 100 mA. Thus, it is possible that an RFT-V may
comply with IEC 60950-21 but allow up to 100 mA current without limitation. Figure 4 shows the load
lines circuits that provide current limiting at 2 and 20 k. Figure 5 compares the characteristics of
circuits that provide true 10 mA current-limiting and potential characteristics of circuits that are tested
at a single value of 2 k .
840959616
Page 3 of 7
TAG108(Arli06-MT2-Boksiner)
May 2006
Current (mA)
100
Circuit compliant
with 6.2.3
10
Circuit truly limited
to 10 mA.
1
0.5 0.75 1
2
3
5
7.5 10
20
30
50
75 100
Test Resistance (k)
Figure 4  Load line for a circuit that limits the current to 10 mA and circuit that complies with
caluse 6.2.3 at an open-circuit voltage of 200 V d.c.
2 k
200
DC Voltage (V)
180
Possible characteristics of RFT-V
circuit verified at 2 k only
160
140
120
Circuit truly limited to 10 mA
100
80
0
5
10
15
20
80
100
120
Current (mA)
Figure 5  Comparison of desired voltage-current characteristics of RFT-V circuit and
possible characteristics of RFT-V circuit tests at 2 k only (w.r.t. earth)
840959616
Page 4 of 7
TAG108(Arli06-MT2-Boksiner)
May 2006
There is no rationale for using 2 k as the only test resistance. Clause A.5.2 states that for
measurements from one hand to both feet (between one conductor and earth), the conventional value
of 2 000 has been used. This value may represent a severe case for a linear source, but it does not
necessarily represent a severe case for a non-linear source such as RFT-V circuit with monitoring and
control device. the test resistance is intended to represent the resistance of a human body in contact
with the source and any other resistance external to equipment that limits the touch current. Typical
body resistance is up to 2 k for large area of contact and up to 10 k for a small area of contact.
Furthermore, other impedances, such as shoes or line resistance, may occur in the circuit. For a
typical telecommunication application for these types of circuits, a resistance significantly greater than
2 k is likely and should be considered.
Based on these considerations, it is necessary to verify performance of RFT-V circuits over a range of
test resistances. Several test resistance between 2 and 20 k should be used. Ione class of RFT-V
circuits used in North American is designed to limit the current to 5 mA, so a test at 40 k should also
be performed.
Also, clause 6.2.3 references Figure 1 of 60950-21 for current limits for durations less than 10 s.
However, this figure corresponds to a long-term current limit of 25 mA. This figure does not
correspond to a limit of 10 mA. It is necessary to scale the figure to correspond to a long-term limit of
10 mA. A new figure addressing limits for RFT-V circuits is needed.
Finally, clause 6.2.3 requires a measurement of the current with the other output earthed, but does not
require a measurement with the other output open-circuited or connected as in service. In reality, a
person may contact one output with the other output open-circuited or in service or earthed.
Proposal:
Modify the second dash of Section 6.2.3 as follows:

for RFT-V CIRCUITS whose open circuit voltage exceeds 140 V d.c. under normal operating
conditions, the current between the other conductor and earth, measured through 2 000 ,
5 000 10 000 20 000 and 40 000 ± 2 % resistors, under any external load
condition, shall not exceed the relevant line-to-earth limit given in Figure 1 Figure XXX.
Moreover, the currents measured through these resistors shall not exceed 10 mA d.c. after
10 s.
840959616
Page 5 of 7
TAG108(Arli06-MT2-Boksiner)
May 2006
Id.c. mA
100
24
10 mA
10
0.01
0.1
1
10
Time s
Figure XXX  Maximum current to earth for RFT-V circuit
840959616
Page 6 of 7
TAG108(Arli06-MT2-Boksiner)
May 2006
Modify the second bullet of second dash of Section 6.2.1 as follows:

200 V d.c., provided that a monitoring and control device is used that limits the current to
earth to 10 mA d.c.for duration longer than 10 ms or in accordance with Figure XXX for
duration less than 10 ms and in case of one output grounded externally to the EUT in
accordance with 6.2.3;
…………………………
………………………….
Compliance is checked by inspection and measurement. Compliance with current line-to-earth current
limitation are measured through 2 000 , 5 000 10 000 20 000 and 40 000 ± 2 % resistors.
840959616
Page 7 of 7