Electrical Safety of Dc Telephone Systems

Moe Lamothe

Proper classification is crucial to the electrical safety of telephone systems.

Preventing shock or fire is the primary goal behind many electrical safety standards. The electrical safety of telephone interfaces must be addressed because these interfaces produce voltage levels severe enough to present either a shock or a fire hazard.

Several standards address this issue, so confusion over the proper classification of dc-powered telephone equipment with respect to the power interface is common. This article discusses how to properly classify telephone interfaces when considering the potential electrical shock hazard, as outlined in several key standards and guidance reports: International Electrotechnical Commission (IEC) 60950, 3rd ed., IEC Technical Report (TR) 62102, and European Telecommunications Standards Institute (ETSI) 300 001 and 300-132-2.^1–4

Power Requirements

For clarity, voltage levels are referenced in this article as positive voltages rather than as a mix of positive and negative voltages. Telephone equipment used in central offices is typically dc-powered at either 48 or 60 V dc (72 V dc with battery float voltages). The dc source must be classified properly when examining the equipment for safety.

North America has long used a standard of 48 V dc for the feed. Even considering the battery float voltage, the measured voltage still falls below the 60-V-dc hazardous voltage limit. For this reason, 48-V-dc systems are often considered separated extra low voltage (SELV). As defined in standards such as IEC 60950, such systems are classified as a telephone network voltage, TNV-2 or TNV-3.

Europe is also moving toward a standard of 48 V dc, but many telephone installations still use a 60-V-dc supply voltage. The float voltage level of 72 V dc increases the voltage to what could be considered a hazardous level.

Because the dc voltage is typically derived from an ac source, basic insulation from the ac source usually makes the dc supply a secondary circuit. In this situation, the dc power feed would be properly classified as TNV-2 or TNV-3, depending on the network environment. The network environment is defined as the external conditions to which the telephone system is exposed.

Circuits

IEC 60950, 3rd ed., provides clear definitions of TNV circuits. Both TNV-2 and TNV-3 are particularly relevant to dc-powered telephone systems. According to IEC 60950, a TNV circuit is a circuit in the equipment to which the accessible contact area is limited. It is designed and protected so that—under normal operating and single-fault conditions—the voltages do not exceed specified limit values. A TNV circuit is considered to be a secondary circuit as defined in IEC 60950. TNV circuits are classified as TNV-1, TNV-2, or TNV-3.

TNV-2 Circuit. This is a TNV circuit whose normal operating voltages exceed the limits for a SELV circuit under normal operating conditions. These circuits are not subject to overvoltages from telecommunication networks.

TNV-3 Circuit. This is a TNV circuit whose normal operating voltages exceed the limits for a SELV circuit under normal operating conditions. Overvoltages from telecommunication networks are possible for TNV-3 circuits.

Standards Development

Historically, the electrical safety of telephone systems was determined by the Bell System (AT&T), and later by the regional Bell companies. With industry deregulation in North America, CSA International (CSA) and Underwriters Laboratories Inc. (UL) began developing electrical safety standards to ensure operator and user safety. These standards were crucial because the Bell operating companies no longer had close control of the installed products.

In the early 1980s, the first electrical safety standard developed by CSA was CSA C22.2 No. 0.7, "Equipment Electrically Connected to a Telecommunication Network."⁵ This standard was later replaced by CSA C22.2 No. 225, "Telecommunication Equipment."⁶ This was a stand-alone standard rather than a reference standard for other safety standards. Section 6 of CSA C22.2 No. 60950 (and the preceding CSA 22.2 No. 950) also included telephone-interface requirements different from those contained in C22.2 No. 225. No. 225 has now been dropped in favor of the binational (Canada and United States) C22.2 No. 60950.⁷

In the late 1980s, UL developed UL 1459, which contained many new requirements loosely based on both the Bell and CSA C22.2 No 225 requirements.⁸ Many additional requirements introduced into UL 1459 were eliminated when UL 60950 (and the preceding UL 1950) became the standard of choice. This binational standard includes hazard tests for telephone interfaces that are not included in the equivalent international IEC 60950 standard.

In 1991, Europe published EN 41003, "Particular Safety Requirements for Equipment to be Connected to Telecommunication Networks."⁹ This standard was used for telephone interface safety in Europe. The requirements in this standard were later folded into as the now-familiar section 6 of IEC 60950. Although IEC 60950 does not yet contain hazard tests for evaluating interfaces for telephone equipment, there is an indication that such tests will be included in future versions.

Special Requirements Clause NAB.3 of CSA C22.2 No. 60950/UL 60950 identifies requirements that apply to equipment where the earthed conductor of the dc supply is connected to the equipment-earthing conductor or to the frame of the equipment. For equipment connected to a dc-power system earthed at the equipment location, the earthed conductor of the dc supply can be connected to the equipment-earthing conductor and to the frame of the unit as long as all of the following conditions are met: The equipment connects directly to the point of earthing of the dc system. Bus bars, bonding jumpers, and terminals are provided for connection of the equipment-earthing conductors and the earthing-electrode conductor (by permanent wiring methods) to one of the dc-supply conductors. This hardware must be constructed and sized in accordance with UL 891 (Dead-Front Switchboards) and CSA C22.2 No. 31 (Switchgear Assemblies).10–11 The dc-supply conductor can be earthed in more than one piece of equipment as long as all equipment is located in the immediate area (within the earthing window) of the earthing point of the dc system. The equipment connects to the dc source by permanent wiring methods. No disconnecting device is located in the earthed dc-circuit conductor between the point of connection to the supply and the point of connection to the earthing electrode and equipment-earthing conductors. The equipment is permanently marked with instructions for proper earthing and bonding of the system and equipment. Instructions must be near to and visible from the field wiring terminals. They must be worded for either equipment that has provisions to connect the earthed conductor of a dc-supply circuit to the earthing conductor at the equipment, or for equipment that has the earthed conductor of a dc-supply circuit connected to the earthing conductor at the equipment. When connections are not conventional, installation instructions must be provided for field assembly of earthing and bonding conductors.

IEC Technical Report 62102

Several classifications of interface equipment can be connected to information and communications equipment. "Classification of Interfaces for Equipment to be Connected to Information and Communications Technology Networks," IEC TR 62102 was issued in March 2001. This technical report addresses various telephone-interface signals and battery-feed options. It classifies circuits as either Network Environment 0 or Network Environment 1. If the network environment can be classified as 0, then the equipment installed can be evaluated to TNV-2 requirements; Network Environment 1 requires equipment evaluated to TNV-3.

The following list summarizes the requirements for Network Environment 0:

European Requirements

For Europe, telephone-interface requirements are specified in ETSI standards. Although there are still many country-specific deviations, ETSI standards are intended to minimize the differences and reduce duplicated testing.

ETSI 300 001: "Attachments to Public Switched Telephone Network (PSTN); General Technical Requirements for Equipment Connected to an Analogue Subscriber Interface in the PSTN." This standard specifies the requirements for equipment that is connected to the PSTN. For this standard, the dc supply may be a 48- or 60-V battery. Such equipment, which often has ring voltage present, is typically supplied with dc operating power via the telephone line. In this case, the supply source does not meet the Network Environment 0 requirements. Because it falls under Network Environment 1, it must be evaluated as a TNV-3 supply, regardless of the input voltage rating. Even if the equipment uses 60-V dc power (making an operating voltage of 72 V dc), it must still be evaluated as a TNV-3 circuit. It can't be considered a TNV-2 circuit even if the supply voltage is less than 60 V dc.

ETSI 300 132-2: "Equipment Engineering (EE); Power Supply Interface at the Input to Telecommunications Equipment; Operated by Direct Current (dc)." This standard specifies the requirements for systems where the battery power is supplied by an in-plant dc power source (such as a battery bank), or an ac power supply with a dc output. The dc supply may be a 48- or a 60-V battery. In this case, the supply source does meet Network Environment 0 requirements because it is not normally exposed to an outside-plant source. As a Network Environment 0, it can be evaluated as a TNV-2 supply. Even if the equipment uses 60-V dc power (making an operating voltage of 72 V dc), it is still evaluated as a TNV-2 circuit because the dc supply is a secondary circuit.

North American Requirements

The North American version of IEC 60950 (CSA C22.2 No. 60950/UL 60950) includes a clause that provides special considerations for equipment connected to a centralized dc power system (dc mains voltages). For such equipment, the dc mains voltages are considered secondary circuits and are characterized as either SELV circuits or hazardous-voltage circuits, depending on the maximum operating voltage (including the float voltage) of the source. When applying insulation requirements, dc mains voltages are treated as a particular type of circuit depending on the voltage level: SELV circuit (up to 60 V), TNV-2 circuit (>60 V, up to and including 80 V), or hazardous voltage (>80 V). These circuits are not subjected to TNV-circuit current limits when providing power for centralized dc equipment. However, they must meet appropriate current limits when connected to a telecommunications network.

It is important to note the use of the words characterized and treated in these requirements. They indicate clearly that although voltages may be considered hazardous, they are not necessarily evaluated as hazardous voltages.

Centralized Dc Power Systems

A centralized dc power-distribution system consists of open batteries, charger and rectifier circuits, and primary and secondary distribution equipment to provide power to equipment loads. Systems rated not less than 48 V have one point directly earthed. Protective earth conductors connect the exposed conductive parts of the installation to that point. Systems rated less than 48 V may have one point directly earthed.

Two types of systems are recognized based on the arrangement of earthed and protective earth (earthing) conductors:

General Requirements. A centralized dc power system is considered to be a secondary circuit. Equipment to be connected to these systems must comply with the requirements in CSA 22.2 No. 60950/UL 60950. Requirements include:

Conclusion

Dc equipment is usually evaluated as having an input that is classified as TNV-2 because the source can be reasonably protected from overvoltages. Equipment such as a single-line telephone is usually evaluated as having a TNV-3 input because the dc source is often exposed to potential overvoltages.

Because the input for dc-powered telephone equipment meets TNV-2 or TNV-3 requirements as defined in the safety standards, it would be highly unlikely that such equipment would be evaluated as having Class I input. To ensure the safety of dc-powered equipment, proper classification is imperative.

References

1. IEC 60950, 3rd ed., "Safety of Information Technology Equipment," International Electrotechnical Commission (IEC), Brussels, 1999.

2. IEC Technical Report 62102, "Classification of Interfaces for Equipment to be Connected to Information and Communications Technology Networks," IEC, Brussels, 2001.

3. ETSI 300 001, "Attachments to the Public Switched Telephone Network (PSTN); General Technical Requirements for Equipment Connected to an Analogue Subscriber Interface in the PSTN," European Telecommunications Standards Institute, Brussels, 1998.

4. ETSI 300-132-2, "Environment Engineering (EE); Power Supply Interface at the Input to Telecommunications Equipment; Part 2: Operated by Direct Current (dc)," ETSI, Brussels, 2001.

5. CSA C22.2 No. 0.7, "Equipment Electrically Connected to a Telecommunication Network," Canadian Standards Association (CSA), Toronto.

6. CSA C22.2 No. 225-M90, "Telecommunication Equipment," CSA, Toronto.

7. C22.2 No. 60950, "Safety of Information Technology Equipment," CSA, Toronto.

8. UL 1459, "Standard for Safety of Telephone Equipment," Underwriters Laboratories Inc., Northbrook, IL.

9. IEC/EN 41003, "Particular Safety Requirements for Equipment to be Connected to Telecommunication Networks," IEC, Brussels.

10. UL 891, "Standard for Safety for Dead-Front Switchboards," UL, Northbrook, IL, 2000.

11. CSA C22.2 No. 31, "Switchgear Assemblies," CSA, Toronto.

Moe Lamothe is the president of M.A. Lamothe & Associates Inc. (Georgetown, ON, Canada). He can be reached at moe@lamothe-approvals.com.