Electrical Holding Circuits: Normal Open (NO) and Normal Closed (NC) Contacts in Auxiliary Contactor Operations

In the realm of electrical engineering, the use of auxiliary contacts to maintain an electrical holding circuit is a common practice. This article delves into the functionality of these contacts, specifically focusing on the roles of normally open (NO) and normally closed (NC) contacts within auxiliary contactors.

Understanding Normally Open (NO) and Normally Closed (NC) Contacts

Relays, an integrated part of electrical control systems, consist of two main components: the coil and the contacts. The coil functions as the control element, typically an inductor, which, when energized, moves the contacts into specific states. Contacts, in this context, can be either normally open (NO) or normally closed (NC).

NO contacts are not connected to the common terminal by default. They get connected to the common terminal when the relay coil is supplied with sufficient voltage. On the other hand, NC contacts are connected to the common terminal by default and disconnect from it when the coil is energized. These contacts work in direct opposition to each other, providing flexibility in designing electrical circuits to suit specific operational requirements.

Role of Contacts in Holding Circuits

The primary purpose of a holding circuit is to keep an electrical device energized for as long as necessary, even after the initial activation. In such circuits, auxiliary contacts play a crucial role. Let's explore the significance of using NO and NC contacts in these circuits.

Using Normally Open (NO) Contacts for Holding Circuits: One common practice is to use a normally open (NO) contact of an auxiliary contactor in a holding circuit. When the auxiliary contactor is energized, the NO contact remains open, but it does not matter because the circuit is designed to maintain power to the main contactor. Hence, when the auxiliary contactor coil is de-energized, the NO contact opens, causing the main power contactor to drop off, just like a push button.

Using Normally Closed (NC) Contacts for Holding Circuits: If a normally closed (NC) contact of an auxiliary contactor is used in a holding circuit, this would cause the contact to remain open even when the auxiliary contactor is energized. This is not the desired behavior as we aim to maintain the main power contactor engaged when the auxiliary contactor is active. Therefore, using a normally open (NO) contact is more appropriate for holding circuits.

Fail-Safe Mechanisms and Relay Configurations

A fail-safe mechanism in electrical systems typically uses normally open (NO) contacts, as they ensure a safe shutdown in emergencies. For example, a fail-safe system might be designed so that NO contacts remain closed until the auxiliary contactor is de-energized, thereby providing a redundant safety measure.

Additionally, the configuration of relays can vary widely to accommodate different operational needs. Common configurations include:

Two sets of contacts (one NO, one NC) Four front and four back contacts Eight front contacts and eight back contacts

The choice of contact type and configuration depends on the specific requirements of the electrical circuit and the system's operational safety.

Example of Parallel Configuration

Consider a scenario where you need to activate a relay coil and maintain its state using a normally open (NO) push button in parallel with the auxiliary NO contact. When both are in the closed position, the relay coil is energized and locked, ensuring continuous operation. This parallel configuration is a practical and effective way to maintain a circuit.

Conclusion

Understanding the roles and configurations of normally open (NO) and normally closed (NC) contacts in auxiliary contactors is essential for effective electrical holding circuit design. By leveraging the correct contacts and configurations, engineers can create reliable and safe electrical systems that meet the needs of various applications.