Safety relay

Last updated

Safety relays are devices that generally implement safety functions. [1]

Contents

History

Relays and contactors were used to control plant and machinery in the early days of control technology. In the event of a hazardous situation, the actuator was simply isolated from the energy supply. This type of protection system could be manipulated in the event of a malfunction, disabling the protective function. Special relay circuits, such as the three-contactor combination, were the first designs to come out of deliberations into how this could be avoided. These device combinations led to the development of the first safety relay from the German automation manufacturer Pilz, the PNOZ. [2] [1] [3] [4]

Description

In the event of a hazard, the task of safety functions (e.g. E-STOP, safety gate or standstill monitoring) is to use appropriate measures to reduce the existing risk to an acceptable level. These many safety functions include:

As such, safety relays monitor a specific function. When connected to other safety relays, they guarantee full monitoring of a plant or machine. [1] They meet the requirements of EN 60947-5-1, EN 60204-1 and VDE 0113-1. [5]

Design and function

The design technology is the main difference between the safety relays:

Safety relays must always be designed in such a way that, if wired correctly, neither a fault in the device nor an external fault caused by the sensor or actuator will lead to the loss of the safety function. [6]

A normal relay uses a wire coil and the mechanical movement of the metal contacts to switch the load on and off. The metal contacts may weld shut after repeated operation cycles. If this happens, the machine would continue running if the operator pressed the emergency stop pushbutton. This would be hazardous for the operator. For this reason, many European, American, national and international norms and safety standards prohibit the use of simple relays or contactors on hazardous machines. [7]

The typical design of a first-generation safety relay is based on the classic three-contactor combination. The redundant design ensures that wiring errors do not lead to the loss of the safety function. Two relays (K1, K2) with positive-guided contacts provide the safe switching contacts. The two input circuits CH1 and CH2 each activate one of the two internal relays. The circuit is activated via the start relay K3. There is another monitoring circuit between the connection points Y1 and Y2 (feedback loop). This connection is used to check and monitor the position of actuators, which are activated or shut down via the safety contacts. The device is designed in such a way that any faults in the input circuit are detected, e.g. contact welding on an emergency off/emergency stop pushbutton or on one of the safety contacts on the output relay. The safety device stops the device switching back on and thereby stops the activation of relays K1 and K2. [1]

Related Research Articles

Programmable logic controller Programmable digital computer used to control machinery

A programmable logic controller (PLC) or programmable controller is an industrial computer that has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, machines, robotic devices, or any activity that requires high reliability, ease of programming, and process fault diagnosis. Dick Morley is considered as the father of PLC as he had invented the first PLC, the Modicon 084, for General Motors in 1968.

Relay Electrically-operated switch

A relay is an electrically operated switch. It consists of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals. The switch may have any number of contacts in multiple contact forms, such as make contacts, break contacts, or combinations thereof.

In engineering, a fail-safe is a design feature or practice that in the event of a specific type of failure, inherently responds in a way that will cause minimal or no harm to other equipment, to the environment or to people. Unlike inherent safety to a particular hazard, a system being "fail-safe" does not mean that failure is impossible or improbable, but rather that the system's design prevents or mitigates unsafe consequences of the system's failure. That is, if and when a "fail-safe" system fails, it remains at least as safe as it was before the failure. Since many types of failure are possible, failure mode and effects analysis is used to examine failure situations and recommend safety design and procedures.

Ladder logic was originally a written method to document the design and construction of relay racks as used in manufacturing and process control. Each device in the relay rack would be represented by a symbol on the ladder diagram with connections between those devices shown. In addition, other items external to the relay rack such as pumps, heaters, and so forth would also be shown on the ladder diagram.

<span class="mw-page-title-main">Circuit breaker</span> Automatic circuit protection device

A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the risk of fire. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset to resume normal operation.

Mercury switch Type of electrical switch

A mercury switch is an electrical switch that opens and closes a circuit when a small amount of the liquid metal mercury connects metal electrodes to close the circuit. There are several different basic designs but they all share the common design strength of non-eroding switch contacts.

Kill switch Safety mechanism to quickly shut down a system

A kill switch, also known as an emergency stop (E-stop), emergency off (EMO) and as an emergency power off (EPO), is a safety mechanism used to shut off machinery in an emergency, when it cannot be shut down in the usual manner. Unlike a normal shut-down switch or shut-down procedure, which shuts down all systems in order and turns off the machine without damage, a kill switch is designed and configured to abort the operation as quickly as possible and to be operated simply and quickly. Kill switches are usually designed to be noticeable, even to an untrained operator or a bystander.

Push-button Device to create an electronic circuit

A push-button or simply button is a simple switch mechanism to control some aspect of a machine or a process. Buttons are typically made out of hard material, usually plastic or metal. The surface is usually flat or shaped to accommodate the human finger or hand, so as to be easily depressed or pushed. Buttons are most often biased switches, although many un-biased buttons still require a spring to return to their un-pushed state. Terms for the "pushing" of a button include pressing, depressing, mashing, slapping, hitting, and punching.

Fire alarm control panel Controlling component of a fire alarm system

A fire alarm control panel (FACP), fire alarm control unit (FACU), fire indicator panel (FIP), or simply fire alarm panel is the controlling component of a fire alarm system. The panel receives information from devices designed to detect and report fires, monitors their operational integrity and provides for automatic control of equipment, and transmission of information necessary to prepare the facility for fire based on a predetermined sequence. The panel may also supply electrical energy to operate any associated initiating device, notification appliance, control, transmitter, or relay. There are four basic types of panels: coded panels, conventional panels, addressable panels, and multiplex systems.

Redundancy (engineering) Duplication of critical components to increase reliability of a system

In engineering, redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the form of a backup or fail-safe, or to improve actual system performance, such as in the case of GNSS receivers, or multi-threaded computer processing.

Actuator Sensor Interface is an industrial networking solution used in PLC, DCS and PC-based automation systems. It is designed for connecting simple field I/O devices in discrete manufacturing and process applications using a single two-conductor cable.

Building automation Branch of automation

Building automation is the automatic centralized control of a building's HVAC, electrical, lighting, shading, Access Control, Security Systems, and other interrelated systems through a Building Management System (BMS) or Building Automation System (BAS). The objectives of building automation are improved occupant comfort, efficient operation of building systems, reduction in energy consumption, reduced operating and maintaining costs, increased security, historical performance documentation, remote access/control/operation, and improved life cycle of equipment and related utilities.

Fire alarm system A system, that works using multiple devices to warn of an emergency.

A fire alarm system warns people when smoke, fire, carbon monoxide or other fire-related emergencies are detected. These alarms may be activated automatically from smoke detectors and heat detectors or may also be activated via manual fire alarm activation devices such as manual call points or pull stations. Alarms can be either motorized bells or wall mountable sounders or horns. They can also be speaker strobes which sound an alarm, followed by a voice evacuation message which warns people inside the building not to use the elevators. Fire alarm sounders can be set to certain frequencies and different tones including low, medium, and high, depending on the country and manufacturer of the device. Most fire alarm systems in Europe sound like a siren with alternating frequencies. Fire alarm electronic devices are known as horns in the United States and Canada and can be either continuous or set to different codes. Fire alarm warning devices can also be set to different volume levels.

In the design of electrical power systems, the ANSI standard device numbers identifies the features of a protective device such as a relay or circuit breaker. These types of devices protect electrical systems and components from damage when an unwanted event occurs, such as an electrical fault. Device numbers are used to identify the functions of devices shown on a schematic diagram. Function descriptions are given in the standard.

Relay logic is a method of implementing combinational logic in electrical control circuits by using several electrical relays wired in a particular configuration.

Electric power system Network of electrical component deployed to generate, transmit & distribute electricity

An electric power system is a network of electrical components deployed to supply, transfer, and use electric power. An example of a power system is the electrical grid that provides power to homes and industries within an extended area. The electrical grid can be broadly divided into the generators that supply the power, the transmission system that carries the power from the generating centers to the load centers, and the distribution system that feeds the power to nearby homes and industries.

Numerical relay System using algorithmic detection of electrical faults

In utility and industrial electric power transmission and distribution systems, a numerical relay is a computer-based system with software-based protection algorithms for the detection of electrical faults. Such relays are also termed as microprocessor type protective relays. They are functional replacements for electro-mechanical protective relays and may include many protection functions in one unit, as well as providing metering, communication, and self-test functions.

Partial stroke testing is a technique used in a control system to allow the user to test a percentage of the possible failure modes of a shut down valve without the need to physically close the valve. PST is used to assist in determining that the safety function will operate on demand. PST is most often used on high integrity emergency shutdown valves (ESDVs) in applications where closing the valve will have a high cost burden yet proving the integrity of the valve is essential to maintaining a safe facility. In addition to ESDVs PST is also used on high integrity pressure protection systems or HIPPS. Partial stroke testing is not a replacement for the need to fully stroke valves as proof testing is still a mandatory requirement.

Protective relay Relay device designed to trip a circuit breaker when a fault is detected

In electrical engineering, a protective relay is a relay device designed to trip a circuit breaker when a fault is detected. The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detection of abnormal operating conditions such as over-current, overvoltage, reverse power flow, over-frequency, and under-frequency.

Pilz (company)

Pilz GmbH & Co. KG is a German automation technology company in Ostfildern, Germany. In addition to the head office in Germany, Pilz is represented in 42 subsidiaries and branches on all continents.

References

  1. 1 2 3 4 Safety Compendium, Chapter 4 Safe control technology, p. 115
  2. ↑ "Dokument DE4033801C1 | Patent specification Emergency stop relay". patent-de.com. Retrieved 2016-11-30.
  3. ↑ Practical Machinery Safety (Google eBook), David Macdonald, Newnes, 16.07.2004, Chapter 5: Emergency-stop monitoring and the safety relay, p. 112 et sqq.
  4. ↑ Pilz GmbH. "Safety relays PNOZ - Pilz INT". pilz.com. Retrieved 2016-11-30.
  5. ↑ Practical Machinery Safety (Google eBook), David Macdonald, Newnes, 16.07.2004, Chapter 5: Emergency-stop monitoring and the safety relay, p. 113 et sqq.
  6. ↑ Safety Compendium, Chapter 4.1, Safety relays p. 116
  7. ↑ Safety compendium, Chapter 4.1, Safety relays p. 118
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.