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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] Furthermore, there are many variants of safety relays, which are suitable for various machinery applications.
Today, microprocessor-based relays can perform many protective functions in one device. [1] When one device performs several protective functions, it is typically denoted "11" by the standard as a "Multifunction Device", but ANSI Device Numbers are still used in documentation like single-line diagrams or schematics to indicate which specific ...
External Device Monitor (EDM). The device issuing the OSSD signals may have an EDM input. The EDM is used to verify that the controlled device (safety relay) did indeed open when the OSSD signals were dropped. The safety relay has normally closed contacts, which close when the relay is de-energized, thereby turning on the EDM input.
A number of relays could be used together to carry out complex functions (relay logic). The principle of relay logic is based on relays which energize and de-energize associated contacts. Relay logic is the predecessor of ladder logic, which is commonly used in programmable logic controllers.
In a digital/numeric relay, many functions are implemented by the microprocessor programming. Any one numeric relay may implement one or all of these functions. A listing of device numbers is found at ANSI Device Numbers. A summary of some common device numbers seen in digital relays is: 11 – Multi-function Device; 21 – Distance; 24 ...
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These relays convert voltage and currents to digital form and process the resulting measurements using a microprocessor. The digital relay can emulate functions of many discrete electromechanical relays in one device, [26] simplifying protection design and maintenance. Each digital relay can run self-test routines to confirm its readiness and ...
Digital protective relays are primarily IEDs, using a microprocessor to perform several protective, control and similar functions. A typical IED can contain around 5–12 protection functions, 5–8 control functions controlling separate devices, an autoreclose function, self monitoring function, communication functions etc. [citation needed]