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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