What is Normally Open (NO):
NO in relay is nothing but a Normally Open. It is a two-terminal device and which is used in electrical and electronics circuits for controlling the one or more circuit.
Here Normally open indicate the contact will be in open by default. It means, if you connect it in a circuit, by default, it won’t allow you to pass the current. Look at the image of NO symbol,
To make current flow, it requires some action like manual holding (ex: push button) or contactor holding (contactor energize) and relaying action (mechanism activated). During the action, the normally open contacts will be changed to close. Then the current flow starts.
If you remove the action, the Normally open contacts will come to its normal state (Default position).
Look at the above image, the terminal T1 and T2 is connected to a lighting circuit. At starting the contacts is in open circuit and the current flow through the circuit is zero.
When you press the contact (or use contactor switching), the current flow starts and the light glows.
While releasing the action, the contact will come to a normal state.
The NO contacts design will be varied by different manufacture. i.e if you have Siemens NO contact means, the contact works with the Siemens switch gears only.
Example:
- 3SB5400-0B Siemens make Push-button NO,
- 3TX4010-2A 1S/1NO – Contactor NO.
- SPST Switches at NO position.
How to check NO contacts:?
- Separate the NO from the circuit
- Take a multimeter or continuity tester.
- Put them in a continuity mode.
- Touch the NO terminal with the meter lead.
- If you get infinite ohm or OL means, NO is in good condition.
- If you get peep sound or zero ohms’ resistance, the NO is not in working condition.
Application:
- Electrical Motor starters, Lighting control circuits
- Cooling circuits.
- ON push buttons
What is Normally Closed (NC):
NC is nothing but a Normally closed. NC is the reverse function of NO. By default, the switch contacts will be in a closed condition and which allows the current flow.
By triggering the contacts with some action, the closed contacts become open circuit. NC interrupts the current flow. When releasing the action, the contacts will come to the Normally closed state.
Example:
- Auxiliary contact blocks 3TX4 001-2A 1NC Siemens
- Siemens 3SB34 20-0C Switch Block
- SPST switch under NC position.
How to Check NC Contacts:
- Separate the NC from the circuit
- Take a Multimeter
- Put it in continuity test mode.
- If you get peep sound or zero ohms’ resistance, the NC contact is working fine.
- If you get OL, then the NC contacts will be treated as Not working.
Application:
- Stop Pushbutton
- Emergency Button
- Off Indication lamps
- Thermal overload relays and motor protection circuits
- Limit switches
I think the key to understanding N.O and NC is to define normal. And the best definition I’ve heard is:
The normal state of any device is “with no external influences acting upon it”. Whether installed in a circuit or sitting in a box, on a shelf, in a stock room; normal is the same.
Examples of external influences could be a finger pressing a button, power across a relay coil, a magnet near a magnetic contact, etc.