1、INTERNATIONAL TELECOMMUN CATION UN ION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU PROTECTION AGAINST INTERFERENCE K.36 (05/96) SELECTION OF PROTECTIVE DEVICES ITU-T Recommendation K.36 (Previously “CCIlT Recommendation”) FOREWORD The ITU-T (Telecommunication Standardization Sector) is a p
2、ermanent organ of the International Telecommunication Union (IT). The IT-T is responsible for studying technical, operating and tariff questions and issuing Recommen- dations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Con
3、ference (WTSC), which meets every four years, establishes the topics for study by the IT-T Study Groups which, in their turn, produce Recommendations on these topics. The approval of Recommendations by the Members of the ITU-T is covered by the procedure laid down in WTSC Resolution No. 1 (Helsinki,
4、 March 1-12, 1993). IT-T Recommendation K.36 was prepared by ITU-T Study Group 5 (1993-1996) and was approved under the WTSC Resolution No. 1 procedure on the 8th of May 1996. NOTES 1. telecommunication administration and a recognized operating agency. 2. follows: In this Recommendation, the express
5、ion ?Administration? is used for conciseness to indicate both a The status of annexes and appendices attached to the Series K Recommendations should be interpreted as - - an annex to a Recommendation forms an integral part of the Recommendation; an appendix to a Recommendation does not form part of
6、the Recommendation and only provides some complementary explanation or information specific to that Recommendation. O ITU 1996 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, w
7、ithout permission in writing from the ITU. 1 2 8 9 10 11 CONTENTS Introduction Scope Definitions Characteristics of protective components . Origin of overvoltages and overcurrents Strategies for protection of telecommunication systems Desirable electrical characteristics . 7.1 Normal operation of th
8、e system 7.2 SPD operating conditions . 7.3 Current-limiting devices . 7.4 Isolating devices . Failure modes . 8.1 Clamping devices 8.2 Current-limiting device . Location and mounting of SPDs Safety aspects . General aspects on costs for installation and maintenance Recommendation K.36 (05/96) Page
9、1 1 1 1 3 3 4 4 4 6 7 7 7 8 8 8 9 I STD-ITU-T RECMN K.3b-ENGL L77b m 48b257L Ob2LBOb ; I Large MOV (limiting) Zener 100 ps Small (limiting) Voltage Current Normal withstand- Resis!?e capability ,?Ec; failure Life time ability stability 1 second mode 650 V Medium 3A 500pF Open High circuit 650 V LOW
10、10 A 1 PF Open High circuit Not Good As - Open - applicable specified circuit 650 V Good 5A - Short - circuit Stability of Normal Life time at limiting 1 y; 1 zgi 1 fkikil 1 rated puise 1 voltage current Medium 1 kA/ps I 500pF 1 Short 1 Low I circuit Short For definitions of different components, se
11、e Recommendations K. 12, K.28 and K.30. 2 Recommendation K.36 (05196) STDaITU-T RECMN K-3b-ENGL L77b 48b257L ObZLB08 b87 5 Origin of overvoltages and overcurrents Recommendation K. 1 1 classifies the sources of electrical stress as being: - direct lightning strikes; - nearby lightning discharges; -
12、induction from fault currents in power networks including traction lines; - direct contacts with mains distribution networks; - rise of earth potential. In addition to the above-mentioned threats, there are sources like: - transients with extreme rate of rise due to electrostatic discharges and burs
13、ts related to current switching; - composite surges combining lightning and a.c. follow-on currents. 6 Strategies for protection of telecommunication systems It is important for manufacturers and operators of telecommunication systems to consider the level of overvoltage protection in an early state
14、 of the design. Disregarding the need of surge protective devices will normally cause excessive costs for additional protection measure, when new equipment has been implemented. There are different strategies for selection of protective devices depending on the prospective electromagnetic environmen
15、t for the installation but also on aspects of practicality. It may be convenient for both equipment installers and users to have mobile desktop equipment provided with a complete built-in protection system, able to resist severe interference from lightning discharges. This ensures that the equipment
16、 will be independent of any protection measures that may or may not exist at the building or equipment-cable interface. Permanent installations of telecommunication equipment should be provided with, or at least have the option to be provided with, protective elements at the cable entrance of a buil
17、ding or cabinet. Primary SPDs, installed at the MDF or. separate terminal blocks, shall divert surges of high peak current, charge and specific energy. They may consist of single components or more complex hybrid or two-port devices. Protection against Electrical Fast Transients (EFT) and Electrosta
18、tic Discharges (ESD) should always be remembered. With respect of surges on incoming lines, the resistibility of switching centres, remote sites, etc., might in certain environment rely on high quality primary SPDs with well-specified protection levels, only provided that the telecommunication equip
19、ment is placed inside a volume, well-shielded from external fields, and without any internal sources of interference. An advantage with this concept is that future generations of the equipment, or parts thereof in such cabinets, may not require additional test procedures. Recommendations K.20, K.21
20、and K.22, for equipment in switching centres and subscriber premises, assume a certain inherent resistibility of the equipment itself. The equipment resistibility level is chosen to meet requirements as defined in Recommendations K.20, K.21 and K.22. In such areas, the need for primary SPDs can be d
21、isregarded. However, the electronic circuits should be provided with protection coordinating elements on the line side. This gives the user a flexibility to use the equipment in a harsher EM environment requiring primary protection. See also Recommen- dation K. 1 1. Recommendations K.20 and K.2 1 gi
22、ve detailed information of the coordination of protective components. Recommendation K.36 (0996) 3 - - STD*ITU-T RECMN K-3b-ENGL 177b m 48b2571 Ob21807 515 m 7 Desirable electrical characteristics 7.1 Normal operation of the system Under normal operation, SPDs shall have negligible effect on the sys
23、tem transmission, signalling or switching performance. Voltage SPDs with high capacitance, e.g. varistors, should be carefully matched to avoid unbalance. The selected SPDs shall be transparent to all relevant signals for transmission, ringing, alarms and for power supply voltage in the telecommunic
24、ation network. There should be an operating margin of the protection level with respect to the maximum transmission signals and supply voltage, considering the behaviour of the SPDs within the full temperature range for the equipment to be protected. At the d.c. voltage of the system, normally -48 V
25、, the SPD shall not load the system at maximum signal and temperature conditions. Some of the special digital services being introduced have d.c. voltages a lot higher than -48 V and account needs to be taken if one protector design is to be used in all situations. In metallic pair cables the immuni
26、ty against external interference depends on the balance about earth to the system. This balance must not be disturbed by high and unstable capacitance values of SPDs. The capacitance for varistors and zeners is a function of the applied system d.c. voltage. A good system balance also requires precis
27、e and stable values for series resistive components used as coordinating and current limiting devices. An SPD shall be able to restore to its off-state level after transients or 50/60 Hz overvoltages of limited duration. This parameter is expressed by the holding current of a solid state type SPD or
28、 by the hold-over voltage for a gas discharge tube. The holding current level should be chosen for the worst condition at maximum d.c. voltage and different circuit loads. Consideration should also be given as to the maximum current that the equipment can supply to the line. SPDs shall meet specifie
29、d climatic requirements for the intended application. Special concern should be given to SPDs placed in cabinets of the outside plant, where the temperature and humidity may vary between extreme values. Bad insulation resistance of SPDs may interrupt or destort the transmission signals. 7.2 SPD oper
30、ating conditions SPDs shall have a fast response time. All SPDs respond very fast with negligible time delay. The time delay of gas discharge tubes is generally of less importance for the protection efficiency than its current handling capability. SPDs used as secondary protective devices shall have
31、 a well-defined clamping voltage. The clamping levei shall be chosen with respect to the withstandability of the circuits to be protected and to the maximum operating voltage of the system. Generally, there are no advantages to choose the lowest possible clamping voltage. An operating margin to the
32、circuit withstandability will eliminate unnecessary operation of the SPDs, which would repeatedly interrupt the ongoing data transmission. The SPD shall have a capability to survive the expected single surges without being destroyed. It shall be able to protect circuits against repetitive transients
33、 from lightning discharges and induced 50/60 Hz overvoltages for time periods specified in Recommendation K.20. The selection of suitable characteristics shall also facilitate the coordination with other SPDs upstream or downstream in the system. Coordinating impedances shall resist relevant energy
34、and voltage stresses without degradation. 7.2.1 Voltage switching devices SPDs used as a primary protection and for the protection of cables in the outside plant are exposed to the highest threat from lightning and from power induction due to earth faults in the power systems. Protective devices con
35、taining components with switching characteristics develop less heat during the discharge process than voltage limiting SPDs, due to a low residual voltage. 4 Recommendation K.36 (05/96) STD-ITU-T RECMN K-3b-ENGL L77b m 98b2.571 Ob21810 237 m 7.2.1.1 Gas discharge tubes Gas discharge tubes are the mo
36、st robust switching component and can survive lightning transients of many kA for a duration of hundreds of p, and several amperes a.c., for a second or more during a power system fault condition. The breakdown voltage of gas discharge tubes is sensitive to the voltage rate of rise and may for light
37、ning induced impulses be twice the value for 50/60 Hz overvoltages. A gas tube is, like all spark gaps, a tough SPD with large operating tolerances. Gas discharge tubes may not be suitable for protection of sensitive circuits inside equipment, due to the weakness mentioned above, but also to its abi
38、lity to create very fast transients during its breakdown event, that can cause interference in nearby, badly shielded, circuits. They should preferably be used as primary SPDs, especially at highly exposed places like rural installations for subscribers or other remote sites, where their capability
39、to handle large energies is important. Gas discharge tubes exposed to many greater currents tend to increase their d.c. firing voltage due to electrode erosion, which will increase the gap spacing. Some gas discharge tubes contain beta-emitting radioactive isotopes in order to minimize the statistic
40、al time delay. Such components may conduct extremely fast also at very steep wave fronts. The effect will be reduced after some years depending on the half-life of the radioactive material. 7.2.1.2 Solid state devices (thyristors) Solid state switching devices are mainly used as secondary protection
41、 on printed circuit boards or as part of a hybrid protection unit. SPDs in the thyristor family have lower peak current resistibility compared with gas tubes but can handle some hundreds of amperes for the same surge duration. This current capability may be high enough to accept them as primary prot
42、ective components at the MDF or elsewhere in rather exposed areas. The development of thyristor type protective devices is proceeding very fast and components also for highly exposed sites like rural subscribers installations may be available on the market. Compared with gas discharge tubes, solid s
43、tate devices have a weil-specified breakdown voltage that are independent of the rate of rise of voltage, du/dt. However, semiconductor SPDs are sensitive to rapidly increasing currents. The simple p-n junctions may develop so- called “hot spots” that increase until they cause burnout of the device.
44、 Thyristor switching devices can be damaged by anode currents with high rate of rise. “Hot spots” may form when the junction area is not given time enough to conduct uniformly. Thyristor diodes behave in their initial state as a voltage limiting device before switching to a lower limiting voltage ta
45、kes place. During this transition time, the operating voltage of the thyristor diode is dependent on the di/dt of the surge and can rise to levels significantly above the nominal clamping voltage. This behaviour may be responsible for many unexpected damages on line card circuits. The user of such S
46、PDs should require detailed information from the SPD manufacturer concerning this characteristic. The switching of thyristors can be initiated in different ways. Thyristors without a gate are self-triggering, i.e. the switching takes place when the anode current is above a threshold value or due to
47、rapidly increasing voltage. The maximum limiting voltage of thyristor overvoltage protectors is set in manufacture: on devices which have a gate terminal the inherent protection level may be lowered by gate control. Thyristors with a gate can be turned on by applying a pulse created by a voltage dro
48、p in a series impedance, often integrated in the SPD. Thyristors are switched off when the current drops to a value below its holding currents. Too low a holding current will keep the SPD in its on-state causing a latch-up problem. Recommendation I636 (05/96) 5 STDgITU-T RECMN K.3b-ENGL 177b 48b2.57
49、1 ObZLBL1 173 7.2.2 Voltage limiting devices Examples of voltage limiting devices are varistors, zener diodes and forward diodes. This type of SPD does not switch to lower voltages in the conducting phase, but limits the overvoltage to a level that is nearly constant for all currents. 7.2.2.1 Varistors Varistors based on Metal Oxide Material (MOV) are widely used on power supply circuits, where their capability to quench follow-on currents is important. They are also used in telecommunication applications, where some of their characteristics are advantageous, e.g. they do not crea
