Application Note. Table 1. Test requirements by equipment type

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1 UL1459 and FCC Part 68 Requirements Application Note Problem/Solution Subscriber equipment, also known as customer premise equipment (CPE), includes any equipment that is connected to the telecommunications network and located at a customer s site. Examples of this type of equipment are station sets, fax machines, answering machines, modems, and PBX systems. This equipment is prone to hazards caused by lightning surges, power cross, and power induction. If left unprotected from these hazards, CPE may fail to operate or may risk the safety of subscribers and maintenance personnel. Agency requirements such as UL1459 and FCC Part 68 are used to simulate field conditions and set a minimum performance standard for CPE. A PolySwitch resettable overcurrent device may be used in conjunction with a SiBar thyristor surge protector to assist equipment manufacturers in meeting these agency requirements. CPE Industry Specifications: UL1459 and FCC Part 68 Standards covering either network or customer premise equipment can be divided by intended application, either primary protector or secondary protector, and the geography in which they apply, either North America or Rest of World (Table 1). This note describes methods that can be used to meet the standards for secondary of subscriber Table 1. Test requirements by equipment type North America Subscriber Premises Primary Protector Secondary Protector Terminal Equipment Telco Premises Primary Protector Network Equipment Bellcore GR974 Bellcore GR1089 premises equipment in North America, specifically UL1459 and FCC Part 68. Special attention will be given to solutions involving resettable overvoltage and overcurrent devices. FCC Part 68 Standards Lightning tests for CPE is governed by the FCC Code of Federal Regulations 47 part 68 (FCC Part 68). Table 2 provides further detail on the actual tests. The intent of the prescribed surge tests is to ensure that network operation will not be adversely affected by any equipment connected to it. Recent changes to the FCC specification include an additional set of surge tests, after which the equipment must be operational. FCC CFR, Part 68 UL 497 UL 497A UL1459 ITU K.17 Rest of World ITU K.20 ITU K.21 ITU K.28 UL1459 Standards The AC testing of CPE is specified by Underwriters Laboratories in the UL1459 Standard of Safety for Telephone Equipment. Section 50A of the UL1459 document defines allowable failure modes for the equipment being tested. A summary of these tests is included in Table. 94 UL and FCC Requirements Raychem Circuit Protection Devices

2 Table 2. FCC Part 68 lightning surge requirements Spec type Primary Waveform Voltage Current No. of Test Note and level? (µs, open circuit) (V, open circuit) (A, short circuit) hits results Lightning Type A Metallic Not specified 10/ A Longitudinal Not specified 10/ A 1 Lightning Type B Metallic No 9/ B Longitudinal No 9/ B 1 Test Results: A = Failure OK if line connected to ground and failure is detectable. B = Operational, no permanent open or short. NOTES: 1 = Surge longitude is tip-ring pair to each grid. Table. UL1459 power induction and contact requirements Spec type Primary Volts (rms) Current (rms) Duration Num. of Test Note and level? (open circuit) (short circuit) hits results Power Induction L2 No s 1 A L No min 1 A L4 No 600 max min 1 A 2 M2 No s 1 A 1 M No min 1 A 1 M4 No 600 max min 1 A 1, 2 Power Contact L1 No s 1 A L5 No min 1 A M1 No s 1 A 1 Test Results: A = Must protect the 1.6 A line simulator fuse and must not cause fire to the cheese cloth. NOTES: 1 = Surge applied to tip-and-ring pair simultaneously. 2 = Run test at most damaging voltage in range V rms. Raychem Circuit Protection Devices UL and FCC Requirements 95

3 Application Details The typical overvoltage and overcurrent circuits are shown in Figure 1 for un-grounded systems and Figure 2 for grounded systems. The series overcurrent should provide resettable fuse, mainly against power cross events. Parallel overvoltage should provide resettable overvoltage. Surge tests may be either metallic, defined as applying the surge between either tip or ring and ground, or longitudinal, defined as applying the surge between both tip and ring lines tied together and ground. In an ungrounded system, the longitudinal test should not cause the overvoltage or overcurrent to operate. For grounded systems, a voltage above the threshold of the overvoltage will cause either the between the tip and ground, or the ring and ground, or both, to activate. Note that the third overvoltage protector shown for grounded systems in Figure 2 is optional, but if included can provide increased in the case of tip-ring faults. When an overvoltage is applied between tip and ring at the input, the voltage across the overvoltage device will increase until the overvoltage device begins to operate (clamp or fold back). With the overvoltage device in the activated state, current is conducted Figure 1: Ungrounded CPE Design with Typical Protection Load Figure 2: Grounded CPE Design with Typical Protection (optional) Load 96 UL and FCC Requirements Raychem Circuit Protection Devices

4 through the overcurrent devices, and diverted around the circuit to be protected. For short-duration surges such as lightning, the overcurrent devices should be selected such that it does not interrupt current, so the circuit can immediately return to normal operation when the overvoltage event passes. For longer overvoltage events such as AC power cross or power induction the overcurrent protector operates, protecting the end equipment, wiring, and overvoltage devices. For voltages below the threshold of the overvoltage device, or when faults in the circuit to be protected occur, an excessive amount of current could be drawn from the power source. In this case, the overcurrent operates to prevent damage to the wiring or circuit. The Coordinated, Resettable Solution PolySwitch resettable fuses are positive temperature coefficient (PTC) devices that increase significantly in resistance ( trip ) in response to an overcurrent surge. SiBar TSP devices are silicon crowbar devices that shunt from a high to a low impedance in response to an overvoltage surge, such as those caused by lightning, power cross, and power surge. For lightning surges, the SiBar device will crowbar to a low impedance, diverting current around the protected circuit and preventing excessive voltages from appearing at the terminals of the device to be protected. The surge current capability of the SiBar device must be considered when designing to protect against a lightning surge. Four waveforms are specified by FCC Part 68 and detailed in Table 2. The 10/160 and 10/560 waveforms apply to both open-circuit and short-circuit conditions. For these two surges the FCC Part 68 requires only that a hazardous failure not occur. The equipment does not have to be operational after these tests. The FCC has recently adopted additional requirements to ensure that equipment remains operational after exposure to a reduced level of lightning surges. The two 9/720-µs waveform surges are defined by specifying the circuit of the surge generator. To comply with the FCC Part 68 specification, the equipment must be operational after the tests. The most robust design addresses the worst-case fault currents and waveforms, with the equipment surviving all tests operationally. To survive operationally, the surge current that passes through the SiBar device must be less than or equal to its surge rating. The TVBXXX-050 devices are rated at 70-amps for the 10/560 waveform and 100-amps for the 10/160 waveform, thus additional line impedance is needed to reduce the surge current to below the SiBar TVBXXX-050 rating. The total amount of resistance required can be calculated by first looking at the impedance of the surge generator. An 800 volt open circuit voltage and 100 amp short circuit current implies a source impedance of: R source = V open circuit /I short circuit = 800 V/100 A = 8Ω To reduce the 10/560 current to 70-amps the completed circuit must have a total impedance of: R total = V open circuit /I rating = 800 V/70 A = 11.5 Ω The additional resistance necessary is: R additional = R total - R source = 11.5 Ω - 8 Ω =.5 Ω A 1500 volt open circuit voltage and 200 amp short circuit current implies a source impedance of: R source = V open circuit /I short circuit = 1500 V/200 A = 7.5 Ω To reduce the 10/160 current to 100 amps the completed circuit must have a total impedance of: R total = V open circuit /I rating = 1500 V/100 A = 15 Ω The additional resistance necessary is: R additional = R total - R source = 15 Ω Ω = 7.5 Ω To pass both tests operationally, a minimum of 7.5 Ω must be inserted in the line to reduce the current to within the SiBar device rating. Raychem Circuit Protection Devices UL and FCC Requirements 97

5 For ungrounded systems, the additional resistance can be put in either the tip or ring line as shown in Figure. For grounded systems, the current path can be between tip and ring, tip and ground, or ring and ground. To protect the overvoltage R Additional device from failure in a grounded system, the additional resistance needs to be placed in both tip and ring as shown in Figure 4. The use of a TR RB as an overcurrent device, with a resistance range of 9 to 12 ohms, will provide the necessary Figure : Ungrounded CPE Design with Additional Series Resistance Output resistance to reduce the lightning surges as defined by FCC Part 68 to within the SiBar TSP rating. Using less or no resistance will allow higher currents to pass through the SiBar device which may damage the device and cause it to fail short. This failure mode is not allowed by the FCC, therefore a higher current rated thyristor should be used in these designs. Since TR600 devices are designed to pass the FCC Part 68 requirements without tripping, the use of a TR RB with a TVBXXX-050 as a fully resettable solution for ungrounded and grounded systems as shown in Figures 5 and 6 is recommended. Figure 4: Grounded CPE Design with Additional Series Resistance R Additional (optional) R Additional 98 UL and FCC Requirements Raychem Circuit Protection Devices

6 Figure 5: Suggested Arrangement to Meet FCC Part 68 for an Undergrounded CPE Design TR RB TVBXXX-050 Load Figure 6: Suggested Arrangement to Meet FCC Part 68 for a Grounded CPE Design TR RB TVBXXX-050 Load TVBXXX-050 (optional) TVBXXX-050 TR RB The SiBar device will operate and switch to a low-impedance, lowvoltage state if the voltage during a power cross or power induction fault exceeds the rated breakover voltage of the SiBar device. The current through the device is determined by the surge voltage divided by the total impedance of the circuit. To minimize any damage to the SiBar device, the continuous peak current and duration should be kept within the maximum transient current surge (I TSM ) rating of the device, as shown in Figure 7. The SiBar device will reset back to a highimpedance state when the current through it decreases below its rated hold current (I H ). For short-duration currents (a few AC cycles) above the I TSM rating, the SiBar device can fail short. For long duration currents well below the I TSM, heating can cause the device junction temperature to exceed its maximum rating. Continuous operation above the maximum rated junction temperature will result in device performance degradation, and often eventual device failure. Operation in this region should be prevented. To prevent the current from exceeding the I TSM, additional resistance can be inserted into the line, as was done to limit the lightning surge current. However, the additional resistance required is typically greater than allowed by the resistance budget of a design. Raychem Circuit Protection Devices UL and FCC Requirements 99

7 Figure 7: I TSM for a TVBXXX-050 Device 100 I TSM (A peak) Time (sec) Another way to prevent current from exceeding the I TSM curve is to ensure that the overcurrent device responds to any current before the maximum duration specified by the I TSM curve is exceeded. This requirement implies that the time to trip curve of the overcurrent device always falls below the I TSM curve. A PolySwitch TR RB device will operate when the fault current exceeds twice its rated hold current. As the SiBar device conducts a large current in response to a power cross condition, the TR RB device will trip, protecting the SiBar device from damage. The TR RB device will remain in a tripped, high impedance state until the fault condition passes and the supply current is turned off. PolySwitch device superimposed. For any given fault current, the TR RB device will trip and protect the SiBar device before any damage occurs. Device Selection Choose the SiBar TVB series and PolySwitch TR RB devices for a coordinated, resettable solution to assist CPE manufacturers meet the specification requirements of UL1459 and FCC Part 68. Select a SiBar device with a rated off-state voltage closest to but greater than the system s peak operating voltage. Figure 8 shows the I TSM curve for the TVBXXX-050 devices with the maximum time to trip curve for a TR RB 100 UL and FCC Requirements Raychem Circuit Protection Devices

8 100 Figure 8: I TSM for a TVBXXX-050 Device and Maximum Time to Trip of a TR RB I TSM for TVBXXX-050 Max. time to trip for TR I TSM (A peak) Time (sec) Raychem Circuit Protection Devices UL and FCC Requirements 101