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Question 4/5

Question 4/5 – Resistibility and safety in telecommunications
(Continuation of Questions 4 and 11/5)


New types of equipment (examples are routers, set-top units, cable modems, ONTs, Ethernet type interfaces, xDSL modems, and WLL) are appearing in the network. Also, different types of equipment are being interconnected at customers' premises and the requirements for the internal interfaces and the effects of these interconnections need to be considered.

At least four major changes have occurred which affect the requirements of the existing resistibility recommendations.

  1. The access network is rapidly changing from a traditional exchange (switching centre) which may be kilometres from the customer to the switching equipment being close to the customer with more pervasive wireless nodes and optic fibre. As an example, digital circuits (e.g. Ethernet) are being used, high-speed bonded data services (xDSL) are being introduced, and voice can now be transmitted over the Internet and routers with packet-switching that competes with conventional switching systems.
  2. POTs, data and video services (FXO/FXS) are being generated in the customer building and Remote Terminals. The customer may want to route these services to other buildings. The ports connecting to these services are typically designed for connection to internal cables only.
  3. The environment where the equipment is housed is also changing along with storm patterns that are changing (fewer lightning storms but more severe when they do occur). Traditionally switching equipment has been installed in a controlled environment with the protection installed in an MDF. With miniaturization, green efforts (resulting in equipment being temporarily turned off for power saving features), lower operating voltages (once again another green effort) and changing technologies is causing bulky rack-mounted equipment to be replaced by small equipment, which may be installed on a wall, in a shelter, a Remote Terminal, or underground resulting in a high exposure environment along with more sensitive line drivers/telecommunications equipment. Withstand ratings of this new generation of products should be reviewed (equipment resistibility may need to be improved).
  4. Many countries have regulations regarding grades of service where the operator can be fined if an adequate grade of service is not provided. Life line capabilities must also be considered for emergency situations during natural disasters and other calamities.

These changes require study to determine what steps are required to ensure that the recommendations fit the needs of the operators and the user of the telecommunication network.

This Question impacts climate change issues by ensuring that adequate resistibility requirements for equipment and protective measures for installations reduce the unnecessary replacement of equipment and reduces the required operating voltages/currents. By reducing damages less fossil fuels are burnt (personnel do not have to drive to a site to repair equipment) and less manufacturing is required (less replacement equipment is needed).

Study Group 5 has also produced safety recommendations which provide key information to the telecommunications industry on the:

  • Safety requirements for telecommunication equipment;
  • Safe working practices for network operator personnel.

This question will provide the route for the maintenance of these recommendations. The question will determine when these documents need updating or amending and ultimately provide guidance on when they can be deleted and when a new safety subject needs to be studied.

Furthermore, the evolution of the power architecture of present and future ICT systems leads to the need to study its impact on the resistibility and safety of telecommunication equipment.

The following Recommendations and Handbooks, in force at the time of approval of this Question, fall under its responsibility:

  • K.20, K.21, K.44, K.45, K.50, K.51, K.64, K.75, K.89 and K.90
  • Resistibility Guide.


The objective of this question is to produce resistibility requirements for equipment installed in telecommunications centres, in outdoor enclosures in the access and trunk networks and at customer premises. The sources of overvoltages and overcurrents that may cause damage to the equipment include lightning, power induction and mains power contacts. The types of interfaces include symmetric pair, coaxial, dedicated power feeds and mains power ports.

Study items to be considered include, but are not limited to:

  • Acceptable period between surges during surge testing and power fault testing of ports (K.20, K.21, K.44 and K.45);
  • Review equipment resistibility of other ports (e.g. customer side of ONTs) compared with typical analogue telecommunication ports (K.20/21) for potential exposures currently unaccounted for;
  • Review of Recommendation K.85 (under Question H) for correct equipment resistibility in light of any new field studies or data;
  • Determine equipment resistibility taking into account the effects of new equipment port types connecting to new and different services e.g. mains, symmetric pair telecommunications, services derived within the building, CATV broadband, WLL etc.;
  • The EPR test for external coaxial cable ports. It is necessary to consider the impact of networks on the test voltage and generator impedance;
  • The protection of mains ports (both the tested and untested ports). It is necessary to check the coordination between the primary protector and equipment SPCs/SPDs;
  • The protection of non-earthed equipment with SPCs (that bridge the safety isolation and not allowed by IEC 60950-1);
  • Review USB 3.0 implementations for correct equipment resistibility levels and recommendations;
  • Review Ethernet isolation requirements and benefits against CDEs (Cable Discharge Events), ESD, EFTs, and surges vs. implementing common mode protection and/or transverse protection. Also consider the new PoE non-IEEE 802.3 compliant versions that have up to 100W specified for delivery;
  • Review current field installations and best practices for correct resistibility levels and recommendations;
  • Review green field/ new market/ rising economies for proper resistibility levels and recommendations (may already be covered in K.44 for all modes);
  • Review impact of wireless growth/changes on wired networks and their susceptibility (example: distributed base stations vs. typical base stations);
  • Risk of people injury due to overvoltages and overcurrents on telecommunication lines due to lightning in collaboration with Question B/5;
  • Investigate whether IEC 60950-1 and IEC 62368-1 take into account network operator requirements;
  • Investigate whether IEC TR 62102 takes into account the normal field situation;
  • Effects of induced voltages by electric power and railway lines in normal conditions on safety voltage limits on telecommunication lines;
  • Specifically review Figures A.6.2.1 through A.6.2.3 for potentially less complicated method (consider connecting coax as in regular use and injecting current onto shield as described in IEC 61000-4-6 for example);
  • The safety aspects of DC Remote Power Feeding System for RRU should be considered;
  • The introduction of surge suppressors requirements into the latest IEC 60950-1 standard especially regarding the scope of VDR which used in a primary circuit and to bridge the reinforced/double insulation;
  • The update of Recommendations K.50, K.51, K.64 and K.75 taking into account IEC 62368-1;
  • The possible update of K.50 and K.64 and or a new recommendation to include the power supply system;
  • New voltage protection guide topic;
  • Wire current carrying capabilities specifically as applied to new higher power Ethernet – not just fusing of wire but damage thereof;
  • Investigate the use of lightning isolation transformers (LITs) for overvoltage protection;
  • Current “under study” items in the K.20 and K.21 topics regarding EPR coax testing and other topics will be resolved during this new study period.


Tasks include, but are not limited to:

This question will review existing K-series safety recommendations to ensure that they remain accurate and appropriate to the telecommunications industry and environment. It will ensure that changes in the environment or in technology (for example the introduction of new broadband systems) will produce an updated version to ensure that these documents remain current and valid.

This question may recommend further areas of study.

Study items to be considered include, but are not limited to:

  • Risk of people injury due to overvoltages and overcurrents on telecommunication lines due to lightning in collaboration with Question 5/5 (high priority);
  • Investigate whether IEC 60950-1 and IEC 62368-1 take into account network operator requirements. (high priority);
  • Investigate whether IEC TR 62102 takes into account the normal field situation (medium priority);
  • Effects of induced voltages by electric power and railway lines in normal conditions on safety voltage limits on telecommunication lines (medium priority) (60 volt ac continuous induction on the 48 volt existing supply currently allowed);
  • Review the Resistibility guide to ensure that compliance testing is done correctly. Due to cost constraints short cuts are often taken resulting in incomplete testing. As a result incorrect conclusions maybe drawn re compliance;
  • Maintenance and enhancement of the existing recommendations K.20, K.21, K.44, K.45, K.50, K.51, K.64, and K.75;
  • Maintenance and enhancement of Recommendation K.89 on reduction of risk of injury to people (telecom personnel and user of the telecom network) due to overvoltages and overcurrents on telecommunication lines due to lightning;
  • A new Guide on applying the IEC 60950-1/-21 and 62368-1 standards to network telecommunication equipment and IEC TR 62102 for classifying equipment interfaces, if necessary;
  • Maintenance and enhancement of Recommendation K.90. This includes evaluation techniques and working procedures for compliance with limits to power-frequency magnetic field exposure of network operator personnel;
  • Prepare the new Overvoltage protection guide.

An up-to-date status of work under this Question is contained in the SG 5 work programme (use URL as shown in the table below).




  • K.11, K.12, K.27, K.28, K.35, K.36, K.66, K.67, K.71 and K.85


  • 2, 3, 5, 6, 8, 9, 10 and 12/5

Study Groups:

  • none

Standardization bodies:

  • IEC TC 108, IEC TC 81, IEC SC 37A, IEC 37B, IEC SC 77B WG13, CENELEC TC 81X, ETSI ERM, IEC TC 108, ETSI, STP 60950-1/62368-1