Status Indication of Surge Protective Devices

Status Indication of Surge Protective Devices

Thermal Disconnectors 

Some nonlinear elements of SPDs can be stressed during several operations especially with surge current amplitudes which are close to the maximum discharge capability of the SPD or even above. Especially MOVs show a typical ageingbehavior when they are frequently stressed by high surge currents. This aging process can cause a slow decrease of the initial insulation resistance of the device which yields to an increase of a resistive leakage current. This leakage current can cause a significant temperature rise of the MOV which also may cause a danger of fire. For this reason, these SPDs have to be equipped with a thermal disconnector which disconnects the device from the power supply in case a critical temperature isexceeded.

In spark gap based lighting current arresters usually the trigger circuit contains a thermal disconnector to prevent overheating of the trigger electronics. These devices also provide an optical indication. It is state of the art that nearly all SPDs with status indication are available with an additional remote contact to bring the status information into the control system of the facility. Thermal disconnection of SPDs in caseof their end of life with optical indication and remote signaling is the most common way of function monitoring used by nearly all existing state of the art SPDs. It is well known that after a thermal disconnection these SPDs do not provide protection any more so since many years there are different approaches known to minimize or eliminate this risk of loss of protection.

Two stage thermal disconnectors

Some SPDs provide a two stage thermal disconnector that operates at different temperature levels. If a degradation of the MOV is in progress, the temperature starts to rise and a first low temperature switch is operating. The difference to the common SPDs is that this first switching operation does not disconnect the protective devices from the power mains, it just activates a first remote contact to let the operator know that the end of life of the SPD is soon very likely. Now the operator has some time to exchange the device before its end of life is reached. Finally at the end of life is reached a second thermal disconnector operates and disconnects the device from the power mains. A second remote contact transfers the end of life information to the operator. Some of these SPDs provide an optical indication that shows green in normal operation, yellow after the first disconnection and red in case of end of life.

Function Monitoring of SPDs

Function monitoring means, that the surge protective device contains a system to evaluate the actual state of the protective elements inside. There are different approaches known to perform this kind of monitoring. These approaches are mostly realized in SPDs for measuring, control and regulation (MCR) signal lines. The electrical power transmitted in these signal lines does not always ensure the functionality of a thermal disconnection device. So for applications with very high demands on the signal quality SPDs with extended electronical function monitoring are used. . In this device each protective element has a sensor to accumulate data about the real stress by surge current events. The GDTs are monitored by an optical sensor, sensing the emitted light of the electrical arc inside of the GDT. The intensity and time duration of the light impulse gives information about the energy content of the surge impulse. The SADs are monitored by additional series diodes that show a let-through voltage in case of any leakage current of the SADs. This measured let-through voltage gives a very early indication of a slight degradation of the SADs. All this information is evaluated in an on board microprocessor and the status information is transformed into an optical information.

Green means OK, yellow means a soon end of life so the exchange of the arrester is recommended and red means out of order and no more protection.