The SDP's principle of operation is very simple. You should bypass the protected equipment with a fast-acting automatic circuit breaker. In a normal operating mode, the breaker is open and does not affect the electrical circuit in any way. In case of a surge, it should trigger by shorting the circuit and thus avoid surge getting to the protected facility. It should do that for several fractions of a microsecond. This is the time required for the lightning current to grow, which causes electromagnetic interference. It is clear that no electromechanical device can be used because its triggering time is several milliseconds. We have to choose between an air-gap spark discharger and reduced-pressure gas-filled dischargers. Even semiconductor devices with a closed p-n junction are suitable. All of these are called switching-type SDPs. In fact, they make a short circuit in the circuit, at the input of the protected equipment (shown in figure), and thereby avoid the hazardous surge getting into the equipment.
The main task has been accomplished although with some difficulties. Short circuit is not an easy thing to cope with. The equipment is protected but its normal operation is disturbed. Short circuit should be removed by an upstream breaker that will completely interrupt the power supply. We cannot out that the SDP itself will be destroyed by the short-circuit current. Since we have to stick our (in particular, the SDP's enclosure's) neck out, the work can be hardly considered to be well organized in terms of technology.
We should search for an essentially new solution.
Подробнее читайте в статье профессора Э. М. Базеляна "УЗИП без рекламной ретуши".