Safety controllers: a primer

Clearly, interest in providing safer workplaces is growing among machinery manufacturers and end-users. This interest has been stimulated by new and emerging safety standards and guidelines (such as the European Machinery Directive, ANSI B11.19, ANSI B11.20, ANSI/RIA 15.06, OSHA National Emphasis Programs), and by increased awareness of the benefits of a safer work environment.

09/01/2002


For an expanded version of this article, click here .

Clearly, interest in providing safer workplaces is growing among machinery manufacturers and end-users. This interest has been stimulated by new and emerging safety standards and guidelines (such as the European Machinery Directive, ANSI B11.19, ANSI B11.20, ANSI/RIA 15.06, OSHA National Emphasis Programs), and by increased awareness of the benefits of a safer work environment.

The level of interest is marked by a growing use of 'safety-approved' machine guarding components and by a greater awareness of the importance of risk assessment by the equipment designer and end-user.

Those involved in workplace protection and machinery guarding have been influenced by a growing body of standards and regulations. In particular, decision-tree models (such as EN 954), and more recently publication of ANSI's risk assessment guidelines (ANSI Technical Report B11.TR3), have prompted OEMs and end-users to recognize the importance of ensuring reliable operation of safety circuits through fault monitoring and detection by using 'safety controllers.'

Safety controllers defined

Safety controllers are electromechanical or microprocessor-based monitoring devices installed between machine guarding input devices and the machine's primary stop controls, such as motor contactors or control relays. These controllers typically contain redundant, self-checking safety system monitoring circuits and positive-guided output relays, commonly called 'safety relays'.

Each controller is designed to detect faults in the safety circuit's components and interconnection wiring, as well as in its own internal monitoring circuits and output relays. In addition, it senses the actuation of a machine guard interlock/E-stop switch.


This basic safety contorl system simply and inexpensively provides fault detection plus cross-monitoring and self-checking using a safety controller ('black box'). The safety controller achieves the same function as a hard-wired circuit consisting of three positive-guided relays and more thatn 40 wiring points. Source: Schmersal

If the controller detects a fault or open machine guard, it disables the output signals, stops the machine, and/or keeps the machine from restarting until the fault has been corrected. Units are available to satisfy a broad range of Stop Category 0/1 application requirements.

Detect fault in safety circuit

Depending on their design, safety controllers can detect various types of faults. These include:

  • Welded or stuck interlock/E-stop switch contacts;

  • Misaligned guards;

  • Open circuit in interconnection wiring;

  • Short-to-ground faults in interconnection wiring;

  • Fault in the controller's monitoring circuits;

  • Welded or stuck contacts in the controller's safety relays;

  • Insufficient operating voltage to the controller;

  • Capacitive/inductive interference on the controller's inputs; and

  • Welded or stuck contacts in the controlled primary machine stop element.

Some microprocessor-based safety controllers also feature integrated system diagnostics with LED displays, which indicate fault type and location, speeding troubleshooting and minimizing machine downtime.

Ensuring that a safety system will perform requires ability to detect safety circuit/component faults, then shut down the machine until the fault has been corrected. Safety controllers heighten safety system reliability and reduce the possibility of worker injury.

For more information, visit www.schmersalusa.com

Recommended uses

Numerous applications exist in which the use of safety controllers is recommended. These include, but are not limited to applications where:

  • Assessed risk of injury is relatively high. For example, applications assessed as Safety Category 3 or 4 using EN954, or medium or high assessed risk levels using ANSI B11.TR3 guidelines;

  • Safety system inputs are from coded-magnet (reed switch-based) sensors;

  • The designer wishes to satisfy ANSI 'control reliability' requirements; and

  • Relatively low level of assessed risk exists for which the user wishes to heighten the overall reliability of the system.


Author Information


Maurizio Lauria is an application engineer with Schmersal Inc., Elmsford, NY.

Comments? E-mail jkatzel@reedbusiness.com


Read an expanded version of this article, including information on standards and typical components, online:



No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Salary and career survey: Benchmarks and advice; Designing controls; Remote data collection, historians; Control valve advances; Hannover Messe; Control Engineering International
System integration: Best practices and technologies to help; Virtualization virtues; Cyber security advice; Motor system efficiency, savings; Product exclusives; Road to Hannover
Collaborative robotics: How to improve safety, return on investment; Industrial Internet of Things, Industrie 4.0: World views; High-performance HMI, Information Integration: OPC and OMG
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
PLCs, robots, and the quest for a single controller; how OEE is key to automation solutions.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Getting to the bottom of subsea repairs: Older pipelines need more attention, and operators need a repair strategy; OTC preview; Offshore production difficult - and crucial
Digital oilfields: Integrated HMI/SCADA systems enable smarter data acquisition; Real-world impact of simulation; Electric actuator technology prospers in production fields
Special report: U.S. natural gas; LNG transport technologies evolve to meet market demand; Understanding new methane regulations; Predictive maintenance for gas pipeline compressors
click me