1. Field of the Invention
This invention relates to surge protection for components in a telecommunications system. Particularly, protection is required against AC surges which can arise due to downed AC power lines or other such occurrences. A surge protector for such events is generally known as a surge protection resistor.
2. Related Art
Originally, such protectors comprised a wire wound resistor. However, an overload could cause the resistor to break down in a manner which was a fire hazzard, because of the materials used for manufacture. Wire wound resistors were replaced by a thick film resistor printed onto a surface of a ceramic substrate. The substrate is at least 0.1 inches thick and the resistor is a continuous layer. To trim the value of the resistor, the layer of resist material is abraded locally, the abrasion extending through the layer to the substrate.
By developments in materials and process, it has been found possible to reduce the thickness of the substrate to 0.06 inches, but disadvantages, such as 100% testing, resulted. Further development enabled some easing of test requirements, but the 0.06 inches thickness is still required.
The thickness of the substrate is set by the liability of the substrate to disintegrate under surge conditions, which heat up the protector. A thin substrate will disintegrate at a lower surge voltage level than a thicker substrate. This effect is amplified by variation in the heating of the substrate over the substrate area, particularly arising from the results of trimming.