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Safety Relays are devices that implement safety functions. In the event of a hazard, a safety relay will work to reduce to risk to an acceptable level. When an error occurs, the safety relay will initiate a safe and reliable response. Each safety relay monitors a specific function. By connecting them to other safety relays, one can achieve total monitoring of a machine or plant. Safety relays are a simple and efficient way to meet existing safety standards, resulting in safe operation for your personnel and equipment as well as a long service life. Risk reduction should be a priority for any business, to both protect its employees and reduce the possibility of costly accidents or equipment replacement. Generally, if a risk is able to be reduced, then it should be.

The relay switch is classified by their number of poles and throws and they include SPST, SPDT, DPST, DPDT etc.

The input circuit consists of two terminals – positive and Negative potential.

The output circuit consists of three terminals – common (COM), normally-Open (NO), and normally-Closed (NC).

In de-energized condition, the contact is between COM and NC terminals. When the coil is energized, the contact is between COM and NO terminals. This is the basic concept of a relay.

 This normal relay protects both the circuits from damaging each other.

For example, if any issue occurs on the input side, then only the input circuit
will be affected. No damage would happen on the outputs side.
But, in today’s advances in Automation and instrumentation , the safety of the
environment and electrical components is a crucial factor in designing the
system. 
In a normal
relay , as mechanical contacts are used, they may weld or jam with each other
after repeated cycle operation. 
It is a rare condition but if the safety of the panel and system is a main
criterion, then a normal relay would prove dangerous in this situation. It will
happen like the coil is energized, but the contacts have not switched.


So suppose if an emergency stop contact is used at the input side and the
output side is connected to a PLC input, then the contact will not be detected
and the system will thus not stop, even if the switch is pressed.
Many European and American standards avoid the use of a standard relay in their
control panel.
Types of Relay Based on the principle of construction and function, there are different types
of relays categorized as

  1. Electromechanical relay (EMR)

If multiple circuits have to be controlled with a single trigger pulse, then electromechanical relays are the best choice. Electromechanical relays have switch contacts and use electromagnetic force to mechanically open and close the contacts to turn ON/OFF. The advantage is it requires less resistance to switch current and doesn’t need a heat sink. Moreover, they are less prone to electric shock and are fail-safe. The only disadvantage with EMR is wearing out quickly.

  1. Solid State Relay (SSRs)

The advantage of Solid-state relays over electromechanical relay is, they do not have the mechanical moving parts. Instead, they consist of semiconductors and electronic parts such as optocouplers. Solid-state relays turn ON/OFF signals, currents, or voltages electronically by the operation of these electronic circuits. A Solid state relay or MOSFET relay works by the principle of a simple switch. When power is applied to one terminal and control signal is passed through another terminal, the load connected to the relay circuit will turn on and turn off quickly with the help of MOSFET transistor. Then, current will be blocked when the relay is in off state and allows the current when a control signal is triggered.
SSRs are used for both AC and DC switching applications. The DC relays operate using a single MOSFET transistor which allows higher current and AC relays uses two MOSFET transistors to drive current in both directions (positive half cycle and negative half cycle).

  1. Microprocessor/Numerical relay

Uses microprocessor type relays for switching purposes. The electrical faults are detected using software testing algorithms and diagnostics communication etc. The microprocessor detects over voltage and current, frequencies, ground faults, excitation loss of power etc.
Safety Relays in the Field
Safety Relays are commonly used in control devices, such as:

  • Light Curtains: Light Curtains act as a sort of tripwire and are used to protect personnel in the vicinity of moving machinery with the potential to cause harm. When any of the infrared beams generated by the device are broken, a stop signal is sent to the relevant equipment. Light curtains will typically be connected to a safety relay, which will then handle the actual process of removing the motive power from the hazard. Some safety relays may also be equipped with a muting functionality, which enables the temporary disabling of the safety function. For example, when used with light curtains, muting can allow objects to pass through the curtains without tripping the safety relay.
  • Safety Mats: Pressure-sensitive safety relays can be used in conjunction with safety mats to ensure the safety of personnel, and further supplement other safety devices. For example, one may setup a set of light curtains to allow objects to pass through it while the safety mat is activated, allowing access to load or unload a machine. Safety mats are also usable as an independent safety measure. Just like light curtains, they can be configured to initiate a stop command when activated.
  • Three-position Devices: Safety devices such as a three-position device can be essential when troubleshooting an application, and there is a variety of devices in this category. Three-position devices often feature a pressure-sensitive joystick that is held in a particular position to operate, and when the user lets go of the joystick, it will return to the default stop position.
  • Two-hand Control Devices: A two-hand control device is essential for operations that require a high degree of safety consideration. Such devices are crucial when you want to prevent the operator from being able to reach into the hazardous area. When the device is out of reach from the operator controls, a one-handed control device can suffice instead.
  • Magnetic Switches: A magnetic switch is useful in applications where it is crucial for a door or hatch to be closed or for two objects to intersect or be aligned with each other. When contact between the two sensors is lost, an emergency stop signal can be sent to the appropriate relay to safely stop the machine from operating. Magnetic switches are very compact which allows for easy positioning or hiding when used in gates or switches. Because no mechanical contact is needed for operation, a magnetic switch will often tout a long operational lifetime. Water, dirt, and dust do not affect magnetic switches, allowing for their use in a variety of environments and conditions.
  • Emergency Stop Buttons: Emergency Stop buttons (also known as E-stop) is used to stop a machine when it is breaking down if when someone is in danger. All E-stop buttons should be red with a yellow housing. Some emergency stop buttons will also feature an emergency grab wire, allowing interaction with the stop interface even when at a distance from the button itself. The grab wire should also be red for easy identification.
  • Non-contact Safety Sensors: Similar to a magnetic switch, a non-contact safety sensor is ideal to use when it is crucial for segments to be aligned but when they do not have to be in direct contact, unlike with a magnetic switch. Some of these sensors will also allow for multiple sensors to be used in conjunction, allowing for a more precise configuration when necessary.
  • Interlock Safety Switches: An interlock safety switch is used to detect when components are locked together, and can also be used to hold them in place until certain parameters are met, such as completion of an operation. This can be achieved in several different ways, such as a spring lock or locking itself when a specified position occurs.
Relay Applications
Here are some of the important applications of relay that are used in automation, energy, smart grid, telecom, and power industries.
  • Relay isused for ON/OFF applications
  • Capable of switching multiple circuits
  • Coil relays are used for the protection of circuits
  • Controls high power circuit with a low power signal
  • For home appliances such as refrigerators, washing machines
  • For moulding equipment, packing machinery, vending machines
  • They are used in Motor & lighting control
  • Used in Aerospace, Defense, and automotive industries
  • Used in the traffic signal controllers, temperature controllers, heaters
  • When the supply voltage is other than the rated voltage, a set of relays sense the voltage variations and controls the load circuit with the help of circuit breakers.
  • Solid-state relays use power semiconductor devices such as transistors and thyristors to switch currents up to 100 amps.
  • Prevention of non-directional overcurrent protection and direction control of earth faults.

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