NC SPST Relay – Normally Closed Relay

The NC Normally-Closed SPST (Single Polse Single Throw) relay has an initial closed circuit state when no current is applied to its internal coil therefore it connects the power to the load (fan, light bulb, etc). When voltage is applied to the coil of the relay the internal switch goes to the open position and disconnects the power from the load. It is also called Form B.

Video presentation of how the Normally-Closed SPST relay works

Check out the picture below to better understand how this process takes place.

NC normally closed relay

Normally-Closed Single Pole Single Throw SPST relay

In figure 1 no DC voltage is applied to points A and B therefore no current flows through the coil of the relay, the contact stays in the close position and the fan will be switched ON because it is connected to the 220V.

In figure 2 we apply a DC voltage (let’s say 12V) to the coil (points A and B). Now the coil gets energised and it switches the contact to the opened position and the fan will be switched OFF because no current can flow from the 220V.

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