Resettable Circuit Protection for Telecom Network Equipment

Lisa Leo

Resettable overcurrent protection devices that meet national and international standards can also enhance the safety and reliability of telecom equipment.

As the integration and complexity of integrated circuits (ICs) for telecommunications have increased significantly over the past several generations, the features contained within these ICs have become more fragile, leaving telecommunication equipment more sensitive to overvoltage and overcurrent hazards. Dependence on telecommunication systems and the heightened competition among telecom operators have also led to an increased demand for reliable network equipment.

The electronic interface to modern public switched telephone network (PSTN) equipment is subject to the same overvoltage and overcurrent stresses that have plagued telecommunication systems since their inception. Legacy network equipment interfaces tolerated these overstresses fairly well, but electronic interfaces are much less robust. Furthermore, the objectives of network protection in the past were primarily to prevent injury and fire, whereas new network protection methods are also expected to prevent damage to valuable equipment.

Overvoltage and overcurrent hazards usually result from lightning, from transients induced by adjacent power lines, from direct contact with power lines, or from malfunctioning subscriber equipment. These hazards could destroy network equipment or cause injury to subscribers and maintenance personnel. Factors such as the rising costs of advanced telecommunication system failures, increased use of unattended equipment in remote locations, and subscribers' high service expectations make the loss of a telephone line unacceptable. To ensure that telephone lines will operate uninterrupted, many telecom equipment manufacturers have turned to resettable overcurrent protection devices, such as the polymeric positive temperature coefficient (PPTC) device, and fold-back devices, such as the thyristor. These devices are designed to enhance equipment safety and reliability.

This article examines the electrical overstresses to which telecommunications systems are exposed and the protection methods commonly used to control exposure. It reviews the Telcordia GR-1089 specification for electromagnetic compatibility and electrical safety, which governs the performance of protective devices and design considerations for communications network equipment in North America. It also discusses relevant recommendations from the ITU-T Telecommunication Standardization Sector of the International Telecommunication Union.

Electrical Overstresses

Lightning strikes or interactions with an ac power network can cause overstresses in the form of overvoltage and overcurrent in telecommunications systems. Lightning surge is the most common source of overstress. Currents can enter suspended cables by direct or indirect strike. They can also penetrate buried cable through ground currents. Because telephone cables often share a pole or common-use trench and ground rod with the ac power system, some level of induced current is almost always measurable on the tip-and-ring conductors. When a fault occurs in the power system, these currents can become very large.

Three types of overstress occur on telecommunications circuits as a result of power system faults:

• Power cross occurs when the power lines make electrical contact with the telephone circuit conductors. A power cross can drive large currents through the telephone cables.
• Power induction occurs when neighboring power lines carry a heavy current due to a fault or switching transient.
• Ground potential rise occurs when high currents caused by a power fault or lightning surge–to-ground result in a significant potential difference between the point of the fault and the ultimate earth ground.

Overstresses occur in two modes: longitudinal and metallic. In longitudinal mode, the overstress is present between tip-and-ring and ground. Longitudinal overstresses are the most common and occur during power induction or power crosses in which both conductors have the same exposure to the hazard. Lightning-induced overstresses are typically longitudinal in the absence of any imbalance resulting from terminating equipment.

Metallic mode refers to an overstress between tip and ring. Metallic overstresses can also be the result of an imbalance in the network, such as when a protector on one side of the line conducts, but one on the other side does not.

Protection Methods

 

Figure 1. Simplified model of the central office end of a subscriber loop.

Line protection networks are traditionally split into primary, secondary, and sometimes tertiary components. Primary protectors have greater energy-handling capacity than secondary or tertiary protectors; however, the activation threshold for primary protection components is often less precise than for secondary protection components. Figure 1 is a simplified model of a conventional central office subscriber loop driven by an electronic interface, showing the location of the various protection components.

Primary protection is the first level of protection from an overstress event occurring in the outside plant. Primary protection devices typically reside in the main distribution frame for central office equipment, and at building entrances. Primary protection is intended to divert all overstresses above a loosely defined threshold away from the protected equipment and into a reliable earth ground. Primary protection is generally the property of the operating company, and specifications for primary protectors provide the minimum level of protection that the telephone company guarantees its customers. Primary protectors always contain overvoltage protection devices and may contain overcurrent protection devices as well.

Secondary protection operates on the residual voltages and currents passed by the primary protection. Secondary protection devices are usually located on the equipment that needs to be protected. The equipment manufacturer is responsible for these devices. The requirements for secondary protection are determined by standards and by customer expectations.

Secondary protection usually consists of both overvoltage devices and current-limiting devices. Overvoltage protection is necessary to prevent shock hazards and damage to the equipment. Current-limiting devices are necessary to prevent damage to the wiring and to the overvoltage devices. In addition, because the secondary protectors usually operate at a lower threshold than the primary protectors, current-limiting devices serve to coordinate the actions of the primary and secondary overvoltage devices.

North America PSTN Equipment: Standards

Telcordia Technologies (Morristown, NJ), formerly Bellcore, publishes the GR-1089 standard and other documents relevant to the overstresses that can appear on the PSTN in North America. Equipment passing the tests in this standard can be expected to operate satisfactorily on the PSTN, even when subjected to the overstresses discussed previously. Table I lists the GR-1089 requirements.

 

Spec Type	Level	Primary Protection?	Waveform (µs, open circuit)	Voltage (V, open circuit)	Current (A, short circuit)	Hits	Test Results	Notes
Lightning	Level 1, surge 1	No	10/1000	600	100	±25	A	1
	Level 1, surge 2	No	10/360	1000	100	±25	A	1
	Level 1, surge 3	No	10/1000	1000	100	±25	A	1, 5
	Level 1, surge 4	No	2/10	2500	500	±10	A	1
	Level 1, surge 5	No	10/360	1000	25	±5	A	1, 2
	Level 2, surge 1	No	2/10	5000	500	±1	B	—
Spec Type	Level	Primary Protection?	Volts (rms) (open circuit)	Current (rms) (short circuit)	Duration (min)	Hits	Test Results	Notes
Power Induction	Level 1, test 1	No	50	0.33	15	1	A	—
	Level 1, test 2	No	100	0.17	15	1	A	—
	Level 1, test 3	No	600 max.	1 (at 600V)	1	60	A	3
	Level 1, test 4	Yes	1000	1.00	1	60	A	4
	Level 2, test 3	No	600	7.00	5	1	B	—
	Level 2, test 4	No	600 max.	2.2 (at 600V)	15	1	B	3
Power Contact	Level 2, test 1	No	120, 277	25.00	15	1	B	—
	Level 2, test 2	No	600	60.00	5	1	B	—
Test Results A = Must continue to operate after test. B = Must not cause fire. Notes 1 = May apply either surges 1, 2, 4, 5 or surges 3, 4, 5. 2 = This test is to be done on 12 tip-and-ring pairs simultaneously. 3 = Run test at 200, 400, and 600 V rms, and just below OV protective device breakover voltage. 4 = Surge applied to tip-and-ring pair simultaneously.

Table I. GR-1089 requirements for overstress protection.

Protective Devices

Protective devices are generally classified as current limiting or voltage limiting. Current-limiting devices are most important in protecting equipment from ac power induction and power faults, during which joule heating can result in a fire hazard or can damage thermally sensitive components. Voltage-limiting devices are intended to prevent dielectric breakdown of component or system insulation, which can cause high currents, arcing, and other potential hazards. Current limiting can be accomplished by using a resistor, fuse, or PPTC device. Resistors are rarely an acceptable solution because an expensive high-power resistor is required. Specially designed fuses can be used, but they are susceptible to nuisance tripping and must be replaced after a fault event. In addition, lightning-robust fuses generally have a higher hold current than PPTC devices, thereby letting through higher levels of fault current.

The preferred solution is an active element, such as a PPTC device, which has low resistance in normal operation and high resistance in fault states. Such devices are self-resetting in that they return to normal operation after the fault has been cleared and power to the circuit has been removed.

Overvoltage protection devices include metal oxide varistors (MOVs), transient voltage suppression (TVS) diodes, and thyristors. Radial-leaded MOVs and their surface-mount versions (sometimes called multilayer varistors) are voltage-clamping devices and tend to be the lowest-cost option. However, MOVs may be inappropriate for high-data-rate circuits because of their relatively high capacitance. TVS diodes, which are also clamping devices, typically have lower capacitance than MOVs, and thus provide a better protection alternative. Thyristors are fold-back devices. These devices provide the best protection levels, but at a modest price premium to MOVs and diodes. Their low on-state voltage allows for smaller form factor devices and minimizes use of valuable printed circuit board (PCB) real estate. The relatively low capacitance of thyristors also allows them to be used on high-data-rate circuits.

Protection Design Example

 

Figure 2. Simplified example of a line card design. The use of an optional resistance, Ropt, and a current-sensing feedback resistor, Rf, is explained in Telcordia GR-1089.

Figure 2 illustrates a line-card design with an electronic network interface and on-board secondary protection. The interface is provided by a subscriber-line interface circuit chip with an automatic line-balancing feature. The line-balancing feature requires a current-sensing resistance in tip-and-ring for operation. The secondary protection consists of a series overcurrent limiter in both the tip-and-ring lines, and a secondary overvoltage-limiting device applied tip-to-ground and ring-to-ground. In this application, a third overvoltage device applied tip-to-ring is recommended to provide improved protection from metallic surges.

PPTC devices meet the power-induction and power-cross requirements of GR-1089 and provide resettable protection. Thyristors meet the lightning requirements of GR-1089 with no additional series impedance. In the example in Figure 2, secondary overvoltage protection is provided with a SiBar SC series thyristor, which offers high energy-handling capability, tight protection voltage specifications, low off-state power dissipation, low capacitance, and small size. The overcurrent protection device is a PolySwitch device. When designing with the PolySwitch TR600 or TS600 devices, an optional 10-W, 2-W resistor (labeled R^_opt in Figure 2) is needed if the circuit is to be subjected to the GR-1089 Level 1, Surge 3 lightning test. However, the 10-W resistor R^_opt can be omitted if the Level 1, Surge 1 and Surge 2 tests are used as allowed by the specification.

The current-sensing resistance is the sum of all the resistances in the feedback loop, which in this case comprises the sum of the feedback resistor, R^_f, the resistance of the TR600 or TS600 device, and any additional resistance in the loop, such as R^_opt. A typical value for the required current-sensing resistance is 100 W. Assuming the nominal resistance of the TR600-160 device is 8 W, the feedback resistor R^_f in this example needs to be 92 W. Because the secondary protector protects R^_f, it does not need to withstand the GR-1089 lightning impulses. Instead, it needs to withstand only the I²t let-through of the current-limiting device. The use of a PPTC device typically results in lower I²t let-through energy than when fuses of equivalent resistance are used. Therefore, smaller, less-expensive resistors can be used in these applications.

Coordinated Protection

Overcurrent protection devices are called on to protect the thyristor under power-induction and power-cross faults when the ac voltage exceeds the thyristor breakover voltage. Bellcore tests such as Level 1, Test 3 and Level 1, Test 4 are representative examples of this situation. To prevent the thyristor from being damaged, it is important to coordinate the time-to-trip performance of the overcurrent protection device with the time-to-damage characteristic of the thyristor, ensuring that the overcurrent device reacts before the thyristor is damaged.

International Standards

Telcordia GR-1089 is the accepted standard for North America. In other parts of the world, network switching and transmission equipment manufacturers must meet the requirements recommended by the ITU-T Telecommunication Standardization Sector of the International Telecommunication Union.

In February 2001, the ITU-T committee agreed on a new set of recommendations, which can be summarized as follows:

• K.20 specifies resistibility requirements and test procedures for telecommunication equipment installed in a telecommunication center.
• K.21 specifies resistibility requirements and test procedures for telecommunication equipment installed in or on a customer premises building.
• K.44 establishes fundamental testing methods and criteria for the resistibility of telecommunication equipment to overvoltages and overcurrents for use by network operators and manufacturers. This recommendation is an overarching recommendation and does not specify either test levels or particular acceptance criteria for specific equipment. The appropriate test levels and test points are contained in the specific product family recommendations (K.20, K.21, and K.45).
• K.45 specifies resistibility requirements and test procedures for telecommunication equipment installed between a telecommunication center and customer premise building.

Following either K.20, K.21, or K.45 is based on the type of grounding employed at the location of the equipment. For grounding recommendations related to K.20, K.21, and K.45 equipment, refer to recommendations K.27, K.31, and K.35, respectively. The new recommendations include lightning, power-induction, and power-contact tests. The recommendations also include both basic and enhanced level tests, with optional higher power-induction levels and a lightning coordination test. Resettable protection is required to meet the enhanced power-contact test.

 

Figure 3. Typical protection system for network equipment.

Figure 3 shows a typical protection system that network equipment manufacturers use to comply with ITU-T K.20 requirements. The thyristor protects the sensitive electronics from fast overvoltage events, including lightning transients. The line feed resistor regulates the steady-state current to the telephone. The 250-V PPTC devices provide current limiting that may be required during power-contact events that have a voltage lower than the fold-back voltage of the thyristor. Additionally, the base resistance of the PPTC device limits the current during events that exceed the fold-back voltage of the thyristor.

When a PPTC device is installed in the circuit, it provides two important advantages. First, it protects the line-feed resistors from overheating. If there is no overcurrent protection during ac sneak current events—in the 200 mA to 1 A range—these line-feed resistors do not fuse open. Typically, they will overheat and cause catastrophic damage to the PCB. Installing a PPTC device helps limit the sneak current and prevents overheating of the line-feed resistor.

Second, network equipment manufacturers and network operators must provide reliable telecommunication service, with minimal loss of system availability and minimal maintenance costs. If nonresettable overcurrent protection is used, the circuit will be out of service, and a service technician must be dispatched to change the line card or subscriber's terminal, even after the overcurrent fault is cleared. If a PPTC device is used, the circuit will reset, and telephone service will resume without need for repair or a service call.

Conclusion

PPTC devices and thyristors help telecom equipment manufacturers meet ITU K series and Telcordia GR-1089 requirements. PPTC devices are used worldwide as overcurrent protection elements in central office switching equipment, digital loop carriers, primary protection modules, subscriber protection equipment, private branch exchanges, and subscriber equipment. A number of newer technologies, such as asymmetric digital subscriber line modems, T1 repeaters, and integrated services digital network lines also utilize PPTC devices for circuit protection. Resettable functionality, small size, and low resistance make them ideal for such applications.

Thyristors are designed to help manufacturers meet ITU and Telcordia lightning surge and overvoltage protection requirements. Their key advantages are small form factor, low on-state power dissipation, and accurate voltage clamping. Their low capacitance also allows them to be used on high-data-rate circuits.

Integrating devices such as these enables manufacturers to enhance equipment safety and reliability. Because the ICs in telecom equipment are so sensitive to overvoltage and overcurrent hazards, it is critical to identify the best methods to control overstress exposure and meet the requirements in today's standards.

Lisa Leo is the telecommunications marketing manager for Tyco Electronics Corp., Raychem Circuit Protection Div. (Menlo Park, CA). She can be reached at lleo@tycoelectronics.com or 650-361-6029.

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