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I have a problem understanding the installation of surge arresters in C&I(Control and Instrumentation) protection. The attached picture says that SPDs(Surge Protection Devices) are installed in the line path. what does it mean? what is the difference with SPD Installation for Power lines? enter image description here

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Surge Diverters (SPD in parallel,left image) are protection devices typically connected in parallel to the load. Metal Oxide, Varistors,Gas Arrestors and Spark Gaps, etc. are used in these devices. They provide only shunt diversion and are commonly used at the point of entry to a facility. The offer only coarse protection.

In other words: Only the voltage is diverted but a surge current to the load is not controlled.

The picture shows a 800 kV surge diverter (Siemens)

800 kV surge arrester Surge filters (SPD in series, right image) are typically secondary series connected devices that combine shunt and series elements to provide fine protection against surges. You find them mostly further downstream, closer to the load. They contain the same technology elements as the surge diverters but also utilize a combination of inductors and capacitors. The addition of these 2 additional elements add the ability to control or block also the surge current.

In other words: They divert the voltage and control current.

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  • \$\begingroup\$ sorry it suddenly came into my mind. so what's the difference between control signals and power lines? why do we use power line surge arresters in parallel with the comsumers? But for measurement and control signal protection we connect arresters in series with consumer devices \$\endgroup\$ – MINA ASHTIANI Oct 2 '16 at 11:18
  • \$\begingroup\$ A series surge diverter is only need if the surge can damage the downstream device. On power lines this is not the situation. There you have only high voltage switches and high voltage transformes. The first one can not be damaged by an incomming surge stream at all. and the transformer is already an inductor so a protector in in itself \$\endgroup\$ – Decapod Oct 2 '16 at 13:01

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