The ideal use for safety and general purpose switches and relays depends almost exclusively on the type of application they are to be installed in. While both have their uses, keep in mind that safety switches and relays can be used in non-safety related applications, but general switches and relays should never be used in applications where safety is required. Having said that, there are many differences between safety switches and standard relays.
The Basics of Safety Switches
In order for a switch to be considered a safety switch, it must satisfy three conditions:
- Pulling or pressing the actuator should turn the switch on or off. There are no self-resetting contacts.
- The switch must have a direct opening mechanism.
- There must be a safe distance kept between the contacts.
- The switch must have have redundant inputs and outputs as well as with internal self-checking.
“A direct opening mechanism is one that forces open a welded contact by the direct force applied by the actuator. Safety components with direct opening mechanism (forcibly opened contacts) display the following mark on their labels and in their data sheets:”
How Safety Switches Work
The look of the direct opening mechanism inside safety switches will be different depending on the type of action performed. In snap-action switches, a cam mechanism is used to pry apart the welded contact. In slo-action switches, a direct plunger design frees the welded contact.
Direct opening mechanisms help reduce failure when in a hazardous state. The redundant inputs and outputs are there to ensure there is a backup in case one fails, thus further limiting the chances of a malfunction occurring.
The Basics of General Purpose Switches
General purpose switches are non-safety related switches that users operate by pulling or pressing an actuator. They are used to determine if an object is present or absent in non-safety related applications. Unlike safety switches, they do not have a direct opening mechanism to counter contact welding, nor do they have redundant inputs and outputs. The outputs are generally double pole double throw (DPDT) or single pole double throw (SPDT).
When working with non-safety related applications, users can select either a normally open (NO) or normally closed (NC) contact, depending on the application used.
The Basics of Safety Relays
In order for a relay to be considered a safety relay, it must satisfy three conditions:
- The relay must include a direct opening mechanism.
- There must be a safe distance maintained between contacts.
- The relay must have redundant inputs and outputs as well as internal self-checking.
The direct opening mechanism of a safety relay differs slightly from that of a safety switch. According to Omron:
“If no contact becomes welded, all NC contacts will maintain a minimum distance of 0.5 mm when the coil is not energized. Likewise if an NC contact becomes welded, all NO contacts will maintain a minimum distance of 0.5 mm when the coil is energized.” This is a requirement of EN50205 and must have the following mark on the label:”
How Safety Relays Work
Safety relay actuators are mechanically linked together close enough to the contacts to promote adequate stability in the system. They ensure that “fail safe mode” will activate when power is lost by holding all the contacts in the position they were in when the power failure occurred. Redundant inputs and outputs ensure added stability and decrease the chances of malfunction from occurring.
The Basics of General Purpose Relays
General purpose relays do not have mechanically linked contacts, nor are the contacts redundant. These relays also do not force the contacts open using a direct opening mechanism when they weld shut. Like general switches, it doesn’t matter if NO or NC contacts are used.
The Right Switch for Your System
Whether the relays and switches are safety rated or not, make it an Omron product supplied by AMMC. Contact us today to get the right part for your application.