NCCER IME&I Practice Exam – Complete Preparation Resources 2026

In a 3-wire control push button circuit, the auxiliary holding contact is wired in series with the start button. This configuration allows the holding contact to maintain the circuit's operation once the start button has been pressed.

When the start button is activated, it closes the circuit and energizes the coil of the control relay or contactor, which then engages the auxiliary holding contact. The holding contact remains closed as long as the relay is energized, enabling the circuit to stay closed even after the start button is released. This feature is critical in control circuits because it ensures that the equipment remains operational until it is intentionally stopped by the stop button, which is typically wired separately in the circuit.

In contrast, if the holding contact were wired in parallel with the start button, the operation would differ significantly. Such a configuration would not effectively maintain the circuit once the start button is released, leading to a loss of control and requiring continuous pressing of the start button for operation. This demonstrates the importance of proper wiring in control circuits to ensure reliable and safe operation.