The single-pole and multipole surge arresters of the DEHNguard® ... H LI family provide maximum safety and user-friendliness and set new standards in surge protection. With the "Pro-Active Thermo Control" concept, they are the first "intelligent" surge arresters to warn the user of failures of the surge protective device in due time. With this concept, the new SPD monitoring device meets the ever increasing requirements on the availability of electrical systems in industrial, commercial and domestic environments. The three-way visual indication GREEN-YELLOW-RED along with a floating changeover contact for remote signalling provides information on the operating state of the arrester. If there is a risk that the varistor might be irreversibly damaged due to an increased energy load, the yellow indicator and the remote signalling function indicate that the protection module should be replaced, thus preventing that the installation is unprotected!