What is the difference between safety and general-use relays?
When looking at Mechanical Relays Panasonic gives the following answer:
A safety relay is equipped with a force guided contact mechanism for enabling the detection of abnormal conditions.
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When ‘b’ contacts are welded, a gap of 0.5 mm or greater is maintained between ‘a’ contacts even if the coil is in an excited condition.
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When ‘a’ contacts are welded, a gap of 0.5 mm or greater is maintained between ‘b’ contacts even if the coil is in a non-excited condition.
Caution:The safety relay only detects abnormal conditions and does not prevent any anomalies.
Here is a snip from the SF Series data sheet that shows how some of the different configurations operate.
Applicable Part Numbers
255-1746-ND 255-1441-ND 255-1440-ND 255-1745-ND 255-3666-ND 255-4134-ND 255-1747-ND 255-4130-ND 1110-2505-ND 255-4135-ND 1110-2506-ND 255-3097-ND 255-4129-ND 255-4136-ND 255-5722-ND 255-5353-ND 255-5349-ND 255-5359-ND 255-5356-ND 255-5355-ND 255-5350-ND 255-5354-ND 255-5351-ND 255-5357-ND 255-3351-ND 255-5720-ND 255-4137-ND 255-3094-ND 255-3095-ND 255-4133-ND 255-5352-ND 255-3096-ND 255-5348-ND 255-5358-ND SFS6-DC12V-ND SFS6-DC48V-ND SFS5-DC48V-ND SFS3-L-DC48V-D-ND SFS5-DC24V-ND SFS5-DC12V-ND SFS5-L-DC12V-D-ND SFS5-L-DC24V-D-ND SFS6-L-DC12V-D-ND SFS6-L-DC24V-D-ND SFS4-L-DC48V-D-ND SFS5-L-DC48V-D-ND SFS6-L-DC48V-D-ND SFS5-L-DC24V-ND SFN4D-DC12V-ND SFS6-L-DC24V-ND SF3-DC12V-ND SF2D-DC48V-ND SFY2-DC18V-ND SFY2-DC21V-ND SFY3-DC18V-ND SFY3-DC21V-ND 255-4131-ND 255-4132-ND SF2D-DC60V-ND SF3-DC48V-ND SFY4-DC18V-ND SFY4-DC21V-ND SFY5-DC18V-ND SFY5-DC21V-ND 255-4138-ND 255-4139-ND 255-4140-ND 255-1748-ND 255-3665-ND SF4D-DC60V-ND SFS4-L-DC24V-ND 255-1412-ND 255-1413-ND 255-5721-ND
SF2D-DC24V SF4D-DC24V SF4D-DC12V SF2D-DC12V SF3-DC24V SFY3-DC5V SF2D-DC5V SFY2-DC24V SFS3-DC24V SFY4-DC12V SFS3-L-DC24V SFS4-DC24V SFY2-DC12V SFY4-DC24V SFS2-DC48V SFS3-DC48V SFS2-L-DC24V SFS6-DC24V SFS4-DC48V SFS3-L-DC24V-D SFS2-L-DC24V-D SFS3-L-DC12V-D SFS2-L-DC48V-D SFS4-L-DC12V-D SFN4D-DC24V SF3-DC5V SFY4-DC5V SFS2-DC12V SFS2-DC24V SFY3-DC24V SFS3-DC12V SFS4-DC12V SFS2-L-DC12V-D SFS4-L-DC24V-D SFS6-DC12V SFS6-DC48V SFS5-DC48V SFS3-L-DC48V-D SFS5-DC24V SFS5-DC12V SFS5-L-DC12V-D SFS5-L-DC24V-D SFS6-L-DC12V-D SFS6-L-DC24V-D SFS4-L-DC48V-D SFS5-L-DC48V-D SFS6-L-DC48V-D SFS5-L-DC24V SFN4D-DC12V SFS6-L-DC24V SF3-DC12V SF2D-DC48V SFY2-DC18V SFY2-DC21V SFY3-DC18V SFY3-DC21V SFY2-DC5V SFY3-DC12V SF2D-DC60V SF3-DC48V SFY4-DC18V SFY4-DC21V SFY5-DC18V SFY5-DC21V SFY5-DC12V SFY5-DC24V SFY5-DC5V SF4D-DC5V SF4D-DC48V SF4D-DC60V SFS4-L-DC24V SF2-DC12V SF2-DC24V SF3-DC60V