{"id":6822,"date":"2026-11-13T00:00:00","date_gmt":"2026-11-13T00:00:00","guid":{"rendered":"https:\/\/chac-electric.com\/?p=6822"},"modified":"2026-11-12T08:06:18","modified_gmt":"2026-11-12T08:06:18","slug":"normally-open-contact-symbol-no-vs-nc","status":"publish","type":"post","link":"https:\/\/chac-electric.com\/de\/blog\/normally-open-contact-symbol-no-vs-nc\/","title":{"rendered":"Normally Open Contact Symbol: NO vs NC Explained"},"content":{"rendered":"
\n

Imagine a maintenance technician replacing a relay in a stopped conveyor. The replacement has the same coil voltage, but the interlock behaves differently because the selected auxiliary contact is normally open instead of normally closed. The problem is a mismatch in contact function, not evidence that the new relay is defective.<\/p>\n

Zusammenfassung:<\/strong> A normally open contact symbol<\/strong> describes a contact that is open in its specified normal, unactuated state. A normally closed contact is closed in that state. For an ordinary non-latching relay, normal generally means the coil is de-energized. Physical contact state must also be distinguished from the true or false evaluation of a PLC instruction.<\/p>\n

What does normal mean for a relay or push-button contact?<\/h2>\n

Normal does not mean that the machine is producing, that power is available or that the operator considers conditions safe. It is a reference state used to make diagrams consistent. On a simple spring-return push button, that state is with the button released. On an ordinary electromagnetic relay, it is normally with the coil de-energized.<\/p>\n

Latching relays, maintained selectors and devices with a defined reset position need additional care. Removing coil power does not necessarily return a latching contact to one unique state. Read the device description and drawing notes before applying the simple de-energized rule to every component.<\/p>\n

Autodesk’s IEC-60617 relay library identifies normally open contacts, normally closed contacts, relay coils and maintained-position or delayed contacts separately. This is important because a small qualifier can change the meaning of an otherwise familiar symbol. A time-delayed contact should not be interpreted as an instantaneous auxiliary contact.<\/p>\n\n\n\n\n\n\n\n
Ordinary non-latching relay condition<\/th>\nNO contact<\/th>\nNC contact<\/th>\nInterpretation<\/th>\n<\/tr>\n<\/thead>\n
Coil de-energized<\/td>\n\u00d6ffnen<\/td>\nGeschlossen<\/td>\nNormal reference condition<\/td>\n<\/tr>\n
Coil energized and mechanism operated<\/td>\nGeschlossen<\/td>\n\u00d6ffnen<\/td>\nActuated condition<\/td>\n<\/tr>\n
Coil power removed and mechanism returned<\/td>\n\u00d6ffnen<\/td>\nGeschlossen<\/td>\nReturn to normal, assuming a functioning spring-return device<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

This table describes intended steady-state behavior. It does not specify switching time, contact bounce, break-before-make sequencing or a safety performance level. Those details require the selected device’s data and, where relevant, a validated safety design.<\/p>\n

\"Normal<\/figure>\n

How to identify NO and NC contacts on a wiring diagram<\/h2>\n

In a common ladder representation, a normally open contact appears as two separated parallel contact lines. A normally closed representation adds a diagonal mark. IEC-style schematic contacts can instead resemble a movable contact approaching a fixed contact. The normally closed contact symbol<\/strong> therefore cannot be identified solely by memorizing one image from a different drawing library.<\/p>\n

Use the project legend first, then follow the component tag. A contact assigned to relay K1 should be traced to the K1 coil and to the device contact arrangement. K1 is an illustrative tag, not a universal coding requirement. Cross-references may use sheet numbers, grid positions or contact-location tables.<\/p>\n

A symbol for normally open contact says what the contact does in its reference state. It does not specify how much current it can switch. Main power contacts and auxiliary contacts can have different ratings, even when both are normally open and belong to the same contactor.<\/p>\n

When drawings are copied into purchasing documents, preserve the contact qualifier, tag and reference text. A cropped image that excludes a timer mark or contact identifier can turn a clear schematic into an ambiguous replacement request.<\/p>\n

Why a PLC contact instruction may differ from the physical switch<\/h2>\n

A physical switch contact creates or interrupts an electrical path. A PLC ladder instruction evaluates a stored bit or expression. Although the graphics may look similar, the instruction is not necessarily a drawing of the field switch.<\/p>\n

For example, a physically normally closed stop contact can energize a PLC input during a healthy condition. An instruction that evaluates whether that input bit is 1 can then be true. Choosing an instruction because its graphic resembles the physical contact can produce the opposite behavior from the intended logic.<\/p>\n\n\n\n\n\n\n\n\n
Artikel<\/th>\nWas es beschreibt<\/th>\nWas zu \u00fcberpr\u00fcfen ist<\/th>\n<\/tr>\n<\/thead>\n
Physical NO or NC contact<\/td>\nConductive state in the specified reference condition<\/td>\nContact arrangement and actuator state<\/td>\n<\/tr>\n
PLC input channel<\/td>\nElectrical input interpreted by the controller<\/td>\nInput type, voltage and wiring documentation<\/td>\n<\/tr>\n
Input bit<\/td>\nStored input status, often represented as 0 or 1<\/td>\nAddress mapping and any inversion or processing<\/td>\n<\/tr>\n
Ladder contact instruction<\/td>\nA test of a bit or logical condition<\/td>\nThe controller vendor’s instruction semantics<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Keep these four descriptions separate during troubleshooting. Record the physical actuator position, the input indication, the controller bit and the instruction result. That short sequence helps locate whether the mismatch is mechanical, electrical, a mapping issue or a software assumption.<\/p>\n

A normal control input is not automatically a safety-rated input. A basic ladder example also does not establish a suitable emergency-stop architecture, diagnostic coverage or fault response. Those functions need their own risk assessment and applicable safety requirements.<\/p>\n

\"Separate<\/figure>\n

How start and stop contacts control a motor without isolating its supply<\/h2>\n

A simple momentary start push button often uses an NO contact that closes when pressed. A simple stop circuit often uses an NC contact that opens when pressed. These examples explain the logic, but the complete machine may use interlocks, monitored devices or a safety controller that changes the required arrangement.<\/p>\n

OSHA 29 CFR 1910.147 states that push buttons, selector switches and other control-circuit devices are not energy-isolating devices. Stopping a motor through a contactor therefore does not by itself establish an isolated work condition. The separate disconnecting equipment and site procedure still matter.<\/p>\n

For a practical distinction between switching families, compare the PXD6-125 isolator<\/a> with an AC contactor. They should be selected for their documented functions, rather than treated as interchangeable ways to draw an open circuit.<\/p>\n

Specify coil supply and NO\/NC contact requirements before ordering<\/h2>\n

A replacement request should state the exact model, coil supply, required number of NO and NC auxiliary contacts, switching duty and mounting arrangement. State whether the required contact is built into the device or supplied as a separate accessory. Do not rely on a family photograph to determine what is included.<\/p>\n

Der CHAC CQC6 contactor range<\/a> includes model and accessory information that should be checked against the application. Confirm the contact block designation and terminal diagram for the exact ordered combination. A matching coil voltage alone is insufficient.<\/p>\n

For maintenance teams, a useful spare-parts record combines the approved part number, coil data, contact arrangement and drawing revision in one entry. This reduces repeated interpretation during a breakdown. It also makes a proposed alternative easier to assess without assuming that every device in a similar housing has identical contacts.<\/p>\n

Common questions about normally open and normally closed contacts<\/h2>\n

What does a normally open contact symbol mean?<\/h3>\n

It means the contact is open in its specified normal or unactuated state. Actuation is intended to close it, subject to the operating behavior of the particular device.<\/p>\n

How does a normally closed contact symbol differ from a normally open symbol?<\/h3>\n

It indicates a conductive contact in the normal state, with opening on actuation. The graphical difference depends on the drawing convention, so use the legend rather than assuming every schematic uses the same diagonal mark.<\/p>\n

Does normal mean the relay coil is de-energized?<\/h3>\n

For a typical non-latching electromagnetic relay, yes. Latching, maintained-position and specialized devices can require a different stated reference condition, which should be identified in the documentation.<\/p>\n

What happens to a normally open contact when its relay coil is energized?<\/h3>\n

It closes when the relay mechanism operates as intended. This description assumes the correct coil supply and a functioning device, and does not define the contact’s switching delay or load rating.<\/p>\n

How can you tell whether a contact is normally open or normally closed?<\/h3>\n

Check the manufacturer’s contact diagram and the project legend in the defined reference state. Any physical continuity testing should be performed under an appropriate safe isolation procedure, with external circuit paths accounted for.<\/p>\n

How is a normally open push button shown in a wiring diagram?<\/h3>\n

It uses an NO contact symbol with a push-button actuation indication or designation. The normal state is generally the released condition for a momentary spring-return button.<\/p>\n

Does a normally open PLC contact instruction always describe a physical NO switch?<\/h3>\n

No. A PLC instruction evaluates a logical condition or bit, which may be associated with many kinds of physical devices. Read the input mapping and the controller’s instruction definition separately.<\/p>\n

Verweise<\/h2>\n