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change during a surge, and therefore the PPTC minimum resistance can be used for the coordination impedance. PPTC thermistors are chosen based on the temperature, resistance, and time to trip. Ceramic PTCs (CPTCs), on the other hand, will see their resistance reduce as ambient temperature increases. The lowest resistance typically can be around 85˚C before entering the trip point, above which the resistance increases exponentially. The CPTC resistance also will decrease by 70% or more during a surge, depending on the CPTC and the surge applied to it. The graph below shows a 55Ω CPTC looking like 16Ω, where the secondary overvoltage protector current needs to be considered. CPTC thermistors need to consider the minimum resistance for impedance coordination calculation. The key factors are also the maximal voltage across the CPTC during a surge and the time between surges. Bourns ® TBU ® High-Speed Protectors (HSPs) are an optimal electronic current limiter based solution for POTS or dry- line data interfaces such as VDSL2 that need capacitance coupling coordination. Bourns TBU HSPs quickly react to surges to provide a high impedance interface that helps to ensure coordination while also limiting the let-through current to the secondary overvoltage protector. Because of their high impedance in their tripped state, TBU HSPs must be limited to the data sheet maximum, as illustrated in the figure below using the Bourns model TBU-CA-085-500-WH as an example. | 22 | Digi ® XBee ™ Cellular LTE Cat 1 CDDFN10-3304NA TVS/STEERING DIODE ARRAY • Designed to protect high-speed data ports from ESD transients • Uses a 3.3V TVS diode with steering diodes to provide protection for four high-speed data lines • Easy to handle with standard pick and place equipment LEARN MORE 4 Figure 5: CMF-RL55-0-A1 Resistance During 8/20µs Generator Resistance = 2Ω LFRs are made of a ceramic substrate with the resistor inked onto its surface. They are designed specifically to withstand surge. The LFR can provide a tight ratio-matched tolerance of 1%, and 0.2% is possible. Due to its construction, an LFR also has an exceptional resistance versus temperature characteristic, where a change of just 6mΩ/˚C is possible. AC protection commonly is done with a thermal link fuse on the ceramic. It will open should the ceramic substrate temperature increase due to power being dissipated in the resistance. Defining the Primary Protector Operating Voltage, V P To address the numerous environments in which telecommunications protection must operate, three voltage levels are specified in GR-974-CORE. This aids in selecting the appropriate technology for the protection solution. High voltage limiting protects to 1000V and typically includes just a GDT. Figure 6: TBU ® HSP tripped resistance is 100KΩ. No blind spots in the current-type coordination.

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