How to Select Safety Relays
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Safety relays are electromechanical switching devices designed to prevent failure in critical switching operations.
Understanding Safety Relays
Relays are designed to isolate circuits or control a high powered circuit using a low power signal. Simple relays contain a magnetic core wrapped in a wire coil, a movable armature attached to an iron yoke, and at least one set of contacts. When an electric current passes through the coil it generates a magnetic field that activates the armature, moving the contacts to make or break a connection.
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Safety relays have coils and contacts that are designed to be positively driven. A positively driven contact system ensures that the normally open (NO) and normally closed (NC) contacts can never be simultaneously opened or closed, eliminating the potential for faulty operation. Some safety relays feature built-in monitoring equipment in order to detect faults before they occur.
These features make safety relays suitable for use in critical switching applications, such as an emergency machine stop (E-stop). If a switch or relay fails to operate in one of these applications, mechanical or physical human damage could result.
Safety ratings are unique specifications to consider when selecting a safety relay. As shown below, products may be rated as one of four categories or types as defined by the EN954-1 standard. Buyers must determine their application's safety needs in advance and select a product with at least the minimum determined rating. Higher safety ratings typically carry a higher cost.
Cat 1 devices may fail to operate after a single fault. To compensate, these products are designed using well-established components and principles to reduce the incidence of a fault.
Cat 2 devices may experience loss of function if a fault appears between two test cycles.
Cat 3 devices are guaranteed to operate in the case of a single fault.
Cat 4 relays can maintain normal operation in case of multiple faults.
The GlobalSpec SpecSearch database contains information about both single and dual channel safety relays.
Single channel relays are capable of accepting a single normally closed (NC) input and are unable to detect short circuit or monitored input failure in a safety circuit.
Dual channel devices can accept two inputs and have two independent circuits per input. Dual channel relays continually operate even in the event of a single circuit failure, and can detect wiring faults and input failure.
Safety relay specifications include mounting and contact information.
Safety relays may be mounted using a number of different methods.
Bracket (or flange) mounted relays are equipped with a flange for mounting. The flange is typically installed by bolting the device to a matching flanged which is then welded to a corresponding wall.
DIN rail mounted devices are equipped with a fastener capable of mounting on a DIN rail. DIN rails are mounting devices standardized by the Deutsches Institut fur Normung (DIN).
Panel mount relays mount directly to an electrical panel.
PCB relays are mounted on printed circuit boards (PCB) using through hole (THT) contacts or surface mount technology (SMT).
Socket relays are mounted to PCBs using pin sockets.
Specifications about contacts, including contact orientation and maximum ratings, are important to consider when selecting safety relays.
Contact orientation refers to the switch's position when a relay coil is not energized. As its name implies, a normally open (NO) switch is open in a resting, non-energized position; when current is passed through the relay, the switch then closes. A normally closed (NC) switch is therefore reversed: closed at rest, and open when energized. Changeover switches contain both NO and NC contact types.
Contacts are frequently rated to accept a maximum amount of current allowable under specified heat dissipation and ambient conditions. Maximum current is sometimes referred to as maximum switching voltage (expressed in volts) or maximum switching current.
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