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Methods of protecting telecommunications equipment

When telecommunications equipment is damaged by irregular voltage, this is because parts that should have been insulated have had their insulation destroyed by the irregular voltage, allowing overcurrent to enter the equipment. Telecommunications wires are generally a two wire pair. Between these two wires there is the same geometrical state with regard to the earth, thunderclouds, transmission lines, etc., so that if there is irregular voltage from lightning or the transmission lines, etc., the same voltage is applied to both wires in the same phase. If this voltage is greater than the withstand voltage of the telecommunications equipment, the insulation of the section with the lowest withstand voltage will be destroyed and overcurrent from the irregular voltage, or from the supply voltage (for example, even a very slight current in the reverse direction of a semiconductor element will be overcurrent) enters and components on that path are damaged. If there is insulation damage between one wire and the ground, line voltage is also generated momentarily, and for telecommunications equipment with low withstand voltage, those parts with low line withstand voltage will be damaged. Protective devices are used in order to protect telecommunications equipment with low withstand voltage from damage due to irregular voltage, by preventing the application of irregular voltage in excess of the equipment’s own withstand voltage. There are mainly three types of protective devices and their operating functions.
(1)  Insulation type:   These devices use insulating transformers and neutralizing transformers, etc., to insulate and protect circuitry from irregular voltage.
(2) Discharge type:   These protective devices use surge suppressing devices, such as arrester tubes and varistors, etc., to lower circuit impedance against irregular voltage, suppressing irregular voltage from being applied to the protected equipment.
(3) Open type:   These protective devices prevent damage from irregular current to the protected device by closing and opening circuits in fuses and heat coils, etc.

Insulation type protective devices are the surest way to protect equipment from irregular voltage, up to an expected voltage level. Insulation type protective devices have no consumable parts and so last a long time without maintenance. However, they can be big and comparatively expensive, so they tend to be used for the protection of substations and wireless relay stations on mountain tops, etc., where telecommunications equipment has been located in places where extremely severe irregular voltage may be applied, or for the protection of telecommunications circuits that require a high degree of reliability.

Open type protective devices are compact and cheap and can provide a significant level of protection in combination with other suitable protective devices, so they tend be used fairly often in the protection of general telecommunications equipment

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