In a numerical cross-reference system, which contacts are underlined?

• A. Normally open
• B. Normally closed
• C. Always connected
• D. Only used for grounding

Answer: (B)

In a numerical cross-reference system, underlined contacts represent normally closed contacts. This is a convention used in electrical schematics and diagrams to easily identify and differentiate between types of contacts within the system.

Normally closed contacts are those that allow current to flow in their default state when no external force or condition is applied. This means that unless the circuit is interrupted either by the actuation of a switch or the triggering of a condition (like the activation of an alarm), these contacts will remain in a closed state, enabling current flow.

The convention of underlining normally closed contacts helps engineers and technicians quickly identify their function in a system, especially when troubleshooting or designing circuits. Being able to differentiate between normally open (where contacts are only closed under certain conditions) and normally closed contacts is critical for understanding how a circuit operates. This system enhances clarity and efficiency in electrical design and analysis.