1、 INTERNATIONAL TELECOMMUNICATION UNION ITU-T K.30TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (12/2004) SERIES K: PROTECTION AGAINST INTERFERENCE Self-restoring overcurrent protectors ITU-T Recommendation K.30 ITU-T Rec. K.30 (12/2004) i ITU-T Recommendation K.30 Self-restoring overcurrent protec
2、tors Summary Current-limiting protective devices are used worldwide to limit currents conducted by telecommunications wiring during high voltage faults on nearby power lines, due to electric traction systems, and from low voltage mains power contacts. This Recommendation provides performance require
3、ments for positive temperature coefficient (PTC) thermistors and contains information on coordination and application of self-restoring overcurrent protectors. These overcurrent protectors may be used inside the equipment to enable compliance with the requirements of the resistibility ITU-T Recs K.2
4、0, K.21 and K.45. A PTC thermistor is mainly intended to limit overcurrents of relatively long duration and will normally have too slow a response time for switching transients or surges caused by lightning discharges, while semiconductor self-restoring overcurrent protectors (see Appendix II) have
5、a faster response time than the PTC and will also operate for short duration surges. Source ITU-T Recommendation K.30 was approved on 14 December 2004 by ITU-T Study Group 5 (2005-2008) under the ITU-T Recommendation A.8 procedure. ii ITU-T Rec. K.30 (12/2004) FOREWORD The International Telecommunic
6、ation Union (ITU) is the United Nations specialized agency in the field of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a
7、view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics. The approval of ITU-T Recommen
8、dations is covered by the procedure laid down in WTSA Resolution 1. In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with ISO and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conci
9、seness to indicate both a telecommunication administration and a recognized operating agency. Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain mandatory provisions (to ensure e.g. interoperability or applicability) and compliance with the Recommendati
10、on is achieved when all of these mandatory provisions are met. The words “shall“ or some other obligatory language such as “must“ and the negative equivalents are used to express requirements. The use of such words does not suggest that compliance with the Recommendation is required of any party. IN
11、TELLECTUAL PROPERTY RIGHTS ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights,
12、 whether asserted by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation, ITU had not received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementors a
13、re cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. ITU 2005 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. ITU-T Rec. K.30 (12/
14、2004) iii CONTENTS Page 1 Foreword. 1 2 Introduction 1 3 Scope 1 4 Performance parameters . 2 4.1 Environment characteristics . 2 4.2 Electrical characteristics. 2 4.3 Selection of PTC thermistors 6 5 Coordination and application of PTC. 6 Appendix I Examples of characteristics of PTC thermistors 8
15、Appendix II Semiconductor self-restoring overcurrent protectors . 9 ITU-T Rec. K.30 (12/2004) 1 ITU-T Recommendation K.30 Self-restoring overcurrent protectors 1 Foreword Current-limiting protective devices have been used worldwide to limit currents conducted by telecommunications wiring during faul
16、ts from nearby power lines and electric traction systems. These devices are non-resetting, typically heat coils or fuses, placed at the Main Distribution Frame (MDF), at the subscriber network interface or inside communication equipment. Current-limiting devices, positive temperature coefficient (PT
17、C) thermistors, which are self-restoring, have been introduced and are now in use in a variety of applications worldwide. These devices are based on resistors with positive temperature coefficient. This Recommendation presents performance parameters for PTC thermistors. A new generation of self-rest
18、oring overcurrent protectors using semiconductor technology are described in Appendix II. 2 Introduction The purpose of this Recommendation is to provide performance requirements for PTC thermistors to ensure their satisfactory operation in telecommunication networks. This Recommendation also covers
19、 coordination of these devices fitted in the MDF and the equipment. The PTC thermistor is intended for the same application as heat coils or fuses. However, as this device is self-restoring, there is no need for replacement after each operation when the device is used within the range of its self-re
20、storing capability. This allows protection systems to be self-restoring for current limiting as well as voltage limiting. A PTC thermistor is mainly intended to limit overcurrents of relatively long duration and will normally have too slow a response time for switching transients or surges caused by
21、 lightning discharges. PTC thermistors have some characteristics that may limit the use of the device: Due to frequency dependence of some PTC thermistors, the transmission in high frequency (tens of MHz) systems might be influenced. As series resistance on the a and b conductors, the devices can ef
22、fect the balance of the line. An activated device might still let a small current flow in the circuit. This small current can be high enough to generate too high heat dissipation in other devices like secondary overvoltage protection components if careful coordination is not achieved. An activated P
23、TC thermistor may not self-restore on certain communications circuits, which have continuous current flow. 3 Scope This Recommendation applies to current-limiting devices that address the principles of protection against overcurrents in accordance with ITU-T Rec. K.11. The performance parameters des
24、cribed herein are intended as a guide for general-purpose communications circuits. Specific systems, terminal equipment, or environments may have different needs. 2 ITU-T Rec. K.30 (12/2004) 4 Performance parameters 4.1 Environment characteristics PTC thermistors should operate satisfactorily in tem
25、perature and humidity ranges selected for the intended application. The selected temperatures should be a designated range that is within the extreme values of 40 C and +70 C. The selected humidity range should be up to 95% r.h. Tests contained in electrical characteristics, see 4.2, shall be perfor
26、med at room temperature of +25 C. Further testing may be performed at the selected extreme temperatures. However, the response time, the rated current, and the d.c. series-resistant characteristics of the device may differ from those specified at room temperature. Each test contained in 4.2 shall be
27、 performed on previously untested devices. 4.2 Electrical characteristics 4.2.1 Operation of PTC thermistors The PTC thermistor operates to limit the current as an open circuit. The device operates to limit the current by increasing its resistance from a low to a high value. The PTC thermistor is pl
28、aced as a series element in the telecommunication loop. The current-limiting device may be provided in the same package with the primary protector unit on the MDF, at the subscriber network interface or on printed circuit boards of the communication equipment. 4.2.2 PTC thermistor operating characte
29、ristics PTC thermistors are available with a variety of operating characteristics to suit the particular needs of each application. The following characteristics are of particular importance: Response time, TR, is the maximum time required for the PTC thermistor to reduce a given fault current to an
30、 acceptable value that will not cause a damage or safety hazard to the protected load. Transition current, It, is the current level required to cause a PTC thermistor to change state at a given temperature and duration. Rated current, Ir, is the maximum current that a PTC thermistor is able to carry
31、 for a specified period of time. The selected current should be greater than the maximum normal operating current over the operating temperature range. Maximum voltage, Vmax, is the highest voltage that may be applied to the PTC thermistor without altering its performance. Impulse and a.c. life are
32、the number of lightning impulse and a.c. currents and voltages that a PTC thermistor withstands without experiencing a failure mode. End-of-life is defined when the d.c. resistance of the device is not within its specified limit values after the end of current applications or fails to meet the rated
33、 current and response time characteristics. The PTC thermistor should withstand overload surges and a.c. power without causing safety hazard or propagating fire. Table I.1 shows examples of some PTC thermistor characteristics. 4.2.3 Response time, TR The PTC thermistor shall operate within the respo
34、nse time when the specified transition current (see Table I.1) is applied through its terminal. When the device operates, the current shall decrease to an acceptable current value. ITU-T Rec. K.30 (12/2004) 3 The device shall have a resistance within the specified values measured after the source is
35、 disconnected. Restoral of the d.c. series resistance to its specified value shall be measured after a time period selected for the intended application. Test method Figure 1 depicts an example of a circuit that can be used to perform the test. The current source of Figure 1 shall have the transitio
36、n current value as specified in the appropriate category when the current-limiting device is placed in the test circuit. Check that the current decreases to an acceptable current value within the appropriate response time. After the source is disconnected and the device has reached the ambient tempe
37、rature and a specified time has passed, measure the resistance of the PTC thermistor to ensure it is within its specified values. Repeat the above procedure five times for each load current. The repetition rate shall be sufficient to avoid thermal accumulation. Figure 1/K.30 Response time test circu
38、it 4.2.4 Rated current, IrThe PTC thermistors shall carry the specified rated current (see Table I.1) applied simultaneously to each current-limiting device as shown in Figure 2 for the appropriate test period. During the rated current tests, the devices shall have a resistance within the specified
39、values. Test method Figure 2 depicts an example of a circuit that can be used to perform the test. In applications where there is no thermal coupling between devices, only one device need be tested, rather than two devices simultaneously. The d.c. constant current source shall have the rated current
40、 value as specified in the appropriate category when the PTC thermistor is placed in the test circuit. During the rated current tests, measure the resistance of the device to ensure that it is within the specified values. The d.c. series resistance of the device is the quotient of the voltage measur
41、ed across the PTC thermistor divided by the current measured by the ammeter. 4 ITU-T Rec. K.30 (12/2004) Figure 2/K.30 Rated current test circuit 4.2.5 Impulse life The PTC thermistor shall conduct the number of applications of impulse currents specified in the appropriate impulse life without exper
42、iencing a failure. Examples of test pulses are shown in Table I.2. The end-of-life of the device is determined when: its resistance is not within the specified limit values; it fails to pass the rated current and response time test at 25 C. Test method Figure 3 depicts examples of circuits that can
43、be used to perform the test. The generator may be defined as the waveforms for the open circuit voltage and the short circuit current, or as a generator with specified components. After each ten current applications, and when the device has reached the ambient temperature and a specified time has pa
44、ssed, measure the resistance of the devices to ensure that it is within the specified limit values. The impulse repetition rate shall be such as to avoid thermal accumulation. ITU-T Rec. K.30 (12/2004) 5 a) First example of test circuitb) Second example of test circuitPTCUcRLPTC thermistorOpen circu
45、it test voltageLoad resistanceSurgegeneratorFigure 3/K.30 Impulse life test circuit 4.2.6 a.c. life The self-restoring current-limiting mechanism shall conduct the number of applications of the 48-62 Hz sinusoidal currents specified in the appropriate a.c. life without experiencing a failure. Exampl
46、es of test values are shown in Table I.3. The end-of-life of the PTC thermistor is determined when: its resistance is not within its specified limit values; it fails to pass the rated current and response time test at 25 C. Test method Figure 4 depicts an example of a circuit that can be used to per
47、form the test. In applications where there is no thermal coupling between devices, only one device need be tested rather than two devices simultaneously. The open-circuit voltage and the short-circuit current of the generator shall be specified. After each ten current applications, and when the devi
48、ce has reached the ambient temperature and a specified time has passed, measure the resistance of the devices to ensure that it is within the specified limit values. Perform the rated current and response time test at 25 C. The a.c. repetition rate shall be such as to avoid thermal accumulation. 4.2
49、.7 Failure mode test The PTC thermistor shall survive or fail open circuit or high resistance mode when overloaded by a surge or an a.c. voltage. The PTC thermistor shall withstand an application of a 48-62 Hz sinusoidal power contact current for a 15-minute duration with an open circuit voltage source and source resistance specified to be appropriate for the intended application. The PTC thermistor shall withstand an application of an impulse current with an open circuit voltage source and source resistance specified to be appropriate for the in
