When a relay contact is Normally Closed (NC), there is a closed contact when the relay is not energized. In either case, applying electrical current to the contacts. Triton's Relays: Normally Open vs. Normally Closed - What does that mean, and how are they used? The eight electro-mechanical relays in the Triton controller. I Think there is no any Normally Close type of Relay. All relays have both Normally Close as well as Normally Open Contacts. Normally Close: its Potential free Why are normally open and normally close switches used in relay?
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If the set of contacts was closed when the relay was de-energized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a normally closed relay, approximately half as strong as the magnetic force, to its relaxed position.
Usually this force is provided by a spring, but gravity is also used normally closed relay in industrial motor starters. Most relays are manufactured to operate quickly.
In a low-voltage application this reduces noise; in a high voltage or normally closed relay application it reduces arcing. When the coil is energized with direct currenta diode is often placed across the coil to dissipate the normally closed relay from the collapsing magnetic field at deactivation, which would otherwise generate a voltage spike dangerous to semiconductor circuit components.
Normally-open-and-normally-closed-contacts - MikroElektronika
Such normally closed relay were not widely used before the application of transistors as relay drivers, but soon became ubiquitous as early germanium transistors were easily destroyed by this surge. Some automotive relays include a diode inside the relay case.
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If the relay is driving a large, or especially a reactive load, there may be normally closed relay similar problem of surge currents around the relay output contacts. In this case a snubber circuit a capacitor and resistor in series across the contacts may absorb the surge.
Suitably rated capacitors and the associated resistor are sold as a single packaged component for this commonplace use. If the coil is designed to be energized with alternating current ACsome method is used to split the flux into two out-of-phase components normally closed relay add together, increasing the minimum pull on the armature during the AC cycle.
Where to get normally closed relay? Or something like it
Typically this is done with a small copper "shading ring" crimped normally closed relay a portion of the core that creates the delayed, out-of-phase component,  which holds normally closed relay contacts during the zero crossings of the control voltage.
Materials with low contact resistance may be oxidized by the air, or may tend to "stick" instead of cleanly parting when opening.
Contact material may be optimized for low electrical resistance, high strength to withstand repeated operations, or high capacity to withstand normally closed relay heat of an arc. Where very low resistance is required, or low thermally-induced voltages are desired, gold-plated contacts may be used, along with palladium and other non-oxidizing, semi-precious metals.
Silver or silver-plated contacts are used for signal switching. Mercury-wetted relays make and break circuits using a thin, self-renewing film of liquid mercury. normally closed relay
Relay - Wikipedia
For higher-power relays switching many amperes, such as motor circuit contatctors, contacts are made with a mixtures of silver and cadmium oxide, providing low contact resistance and high resistance to the heat of arcing.
Contacts used in circuits carrying scores or hundreds of amperes may include additional structures for heat dissipation and management of the arc produced when interrupting the circuit. This protects the receiver normally closed relay the high power of the transmitter.
Such relays are often used in normally closed relay which combine transmitter and receiver in one unit.
Where to get normally closed relay? Or something like it | Electronics Forums
The relay contacts are designed not to reflect any radio frequency power back toward the source, and to provide very high isolation normally closed relay receiver and transmitter terminals. The characteristic impedance of the relay is matched to the transmission line impedance of the system, for example, 50 ohms.
Contactor A contactor is a heavy-duty relay with higher current ratings,  used for switching electric motors and lighting loads.
Continuous current ratings for common contactors range from 10 amps to several hundred amps. High-current contacts normally closed relay made with alloys containing silver.
The unavoidable arcing causes the contacts to oxidize; however, silver oxide is still a good conductor.