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the primary protector operating voltage. Once this has been achieved, the primary protector will shunt away the majority of the fault current from the equipment. The protection in the equipment needs to support the fault voltage until the primary protector has operated. Types of Protection The primary and secondary protector each can be a switching device or a clamping device, which means four protection combinations are possible. For example, a gas discharge tube (GDT) primary protector is a switching device and a transient voltage suppressor (TVS) secondary protector is a clamping device. The opposite could be true with a clamping device as primary protector and a switching device such as a thyristor as the secondary protector. Finally, the primary and secondary protectors both can be clamping devices or both can be switching devices. ATIS-0600338.2010 covers all four combinations of technologies defined as primary and secondary protectors. A typical telecom application is shown in Figure 3 with a 5-pin protector and a typical line card layout. This solution makes use of switching devices as both the primary and secondary protection. The 5-pin in the main distribution frame (MDF) provides primary protection per GR-974-CORE. Secondary protection is designed per GR-1089-CORE and consists of two 1.25A telecom fuses and suitably rated 100 A 10/1000 and 500 A 2/10 thyristors. The secondary protector ideally should match or exceed the fuse current ratings. In this example, the 1.25A fuse is the series coordinating element. Theory of Current-Type Coordination The interaction of components needs to be considered as in Figure 4, which shows various circuit elements in a current- type switching coordination. Analysis of this circuit aids in understanding how the various components involved in a protection solution work together when tested with a surge generator. Figure 4 shows the test generator with voltage V G and internal resistance etc. The voltage V S required to operate the secondary protector is developed across the load, R L . The primary protector operating voltage is defined as V P and the voltage developed across the coordination element is identified as V Z . The current through the coordination element is I S . Since this application uses a crowbar type device for the secondary protector, V Z will be approximately equal to V P when this operates. Selecting the Optimal Coordination Element, Z The coordination element can be cable impedance or traditional components such as ceramic and polymer PTCs, electronic current limiters (ECLs), fuses, and line feed resistors (LFRs). With cable coordination, the longitudinal time delay relies on the cable having a propagation speed of 9ns/m (18ns/m return). It has been established that a cable length of 100ft (30m) or more is normally required to ensure surge coordination. Polymer PTC (PPTC) thermistors increase in resistance with an increase in ambient temperature. PPTC resistance normally does not | 21 | Figure 1: Voltage-Type Coordination Figure 2: Current-Type Coordination Figure 3: Typical Solution Figure 4: Typical Solution