NC of relay protection devices

NC refers to the "Normally Closed" contact in a protective relay, which remains closed under normal conditions and opens when the relay operates.

Definition and Function

In relay protection systems, NC (Normally Closed) contacts are electrical contacts that remain closed when the relay is in its normal, unenergized state. When the relay detects a fault or abnormal condition, the NC contact opens, interrupting the circuit it controls. This is commonly used for alarm circuits, interlocks, or control logic to indicate a fault or to de-energize equipment safely.

Role in Protective Relays

Protective relays often have multiple contacts, including NO (Normally Open) and NC (Normally Closed), to perform different control and signaling functions. For example:

• Alarm circuits: An NC contact can open to trigger an alarm when a fault occurs.
• Trip circuits: NC contacts may be used in series with control circuits to ensure that a breaker trips when the relay operates.
• Interlocks: NC contacts prevent certain operations unless the relay is in a safe state.

ANSI/IEEE Device Numbering Context

In North America, protective relays are identified by ANSI/IEEE C37.2 device numbers, and suffixes or contact designations clarify their application . For instance:

• 50N: Instantaneous overcurrent relay for neutral protection.
• 51G: Time overcurrent relay for ground faults.
• 86T: Lockout relay for a transformer. While the device number identifies the relay function, the NC contact is part of the relay's internal wiring, often shown in schematics with the standard NC symbol. Suffixes like N or G indicate connection points (neutral or ground), but the NC designation specifically refers to the contact state in the relay's control logic .

Practical Considerations

• Wiring diagrams: NC contacts are depicted as a line connecting two terminals with a break that opens when the relay operates.
• Safety: Using NC contacts ensures that circuits fail safely; for example, alarms or trips are activated automatically if the relay detects a fault.
• Microprocessor relays: Modern relays may combine multiple functions, but NC contacts are still used internally or for external signaling. In summary, NC contacts in relay protection devices are essential for safe operation, signaling, and control, and their function is integrated with ANSI/IEEE device numbering to clearly indicate the relay's role in the system .