In industrial and electrical control systems, AC contactors are the key devices that control the opening and closing of the circuit. The NO and NC markings on contactors typically confuse most novice electricians. This article will discuss the meanings of the two key touchpoints from the perspectives of basic definitions, function analysis, practical application, and common misconceptions, allowing readers to gain fundamental knowledge within a limited time.
Definition of NO and NC: Contact state under normal conditions
NO (Normally Open) refers to the contact point that is in a disconnected state when the contactor coil is not energized. When the contactor is powered on, the normally open contacts close and the circuit conducts.
NC (Normally Closed) refers to the contact point that is in a closed state when the coil is not powered on. When the contactor is powered on, the normally closed contacts open and the circuit is cut off.
This naming convention is based on English abbreviations:
N（Normal）： Normal state, that is, the state when the coil is not powered on.
O（Open）： to break off, C（Close）： close.
For example, a contactor labeled as “13-14 (NO)” indicates that it is a pair of normally open contacts that will only close when the coil is energized
The three core functions of NO and NC
1. Implementation of control logic
Normally open contact (NO): Used for self-locking circuits. For example, in motor start control, the NO contact is connected in parallel at both ends of the start button. After the contactor is closed, the circuit is maintained conductive through the NO contact to achieve continuous operation
Normally closed contact (NC): Used for interlock protection. For example, in the forward and reverse circuit of a motor, the NC contacts are connected in series in the coil circuit of another contactor to prevent short circuits caused by simultaneous electrification of both
2. Status signal transmission
The NO contact can be connected to the running indicator light (such as the red light), which lights up when the contactor is closed;
The NC contact can be connected to a stop indicator light (such as a green light), which displays the device’s stop status when the contactor is powered off
3. Security protection mechanism
NC contacts are often used in fault power-off circuits in protective devices such as thermal relays. When the device is overloaded, the NC contact opens, cutting off the control circuit to protect the device
How to distinguish between NO and NC
1. Alphabet identification method
The ‘O’ of NO corresponds to the English word ‘Open’, which means normally disconnected;
The “C” of NC corresponds to “Close”, which means normally closed
2. Contact numbering rules
Normally closed contacts: numbered 11-12, 21-22, etc., ending with “1-2”;
Normally open contacts: numbered 13-14, 23-24, etc., ending with “3-4”
3. Multimeter testing method
In the power-off state of the contactor:
If the resistance between two contacts is close to 0 Ω, it is NC;
If the resistance is infinite, it is NO
4. Physical structure observation method
Open the contactor and observe the contact position:
The normally closed contact is in a closed state when the coil is not powered on;
Normally open contacts are in a separated state
Common misconceptions in practical applications
1: Confusing main contacts with auxiliary contacts
The main contacts of the AC contactor are all normally open (used for making and breaking the main circuit), while the auxiliary contacts are divided into NO and NC (used for control logic)
2: Incorrect wiring leads to interlock failure
In forward and reverse circuits, if NC contacts are not used correctly for interlocking, it may cause both contactors to simultaneously engage, leading to a short circuit fault
3: Ignore contact capacity limitations
The auxiliary contact current is usually 5-10A. If directly used for high-power loads, it may cause burnout and needs to be extended through an intermediate relay
Basic electrical and electronic symbols for electronic components
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