Why do we need a fuse upstream of SDPs?

Защищаемый объект - Protected facility

While air spark gap arresters have expanded the operating range of the lightning current and successfully dealt with large follow short-circuit currents, they still do not ensure an absolute reliability. Any technical device can fail at any point. And not in all cases disconnection of the damaged electrical circuit may be entrusted to the electromechanical upstream breaker. And the point is not only in its relatively slow action. When the breaker is actuated, it cuts off power supply of the protected equipment and stops its normal operation. This is often unacceptable. The solution is obvious: a power fuse should be installed in the SDP circuit.

It is possible to choose the fuse that can pass the required pulse current of the lightning. There is a different issue. What will the fuse do upon the follow short-circuit current exposure? The lightning current in Russia is determined by the Prophet Elijah but not a human, and the short-circuit current is determined by the transformer that has been installed at the substation by the personnel. They can hardly align these currents.

If we choose a fuse with some reserve in terms of the pulse current, it may fail disconnecting the short-circuit current. The choice made by the short-circuit current may lead to false triggering of the fuse from the pulse lightning current, which would not have be dangerous for the SDP. We have to choose the lesser of two evils.

Read more details in the article by Prof. E. M. Bazelyan "SDPs Unvarnished".

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