E-Stop Circuit (Cat 3 / Cat 4)

An emergency stop (e-stop) circuit must be designed to a defined safety performance level. Category 3 (Cat 3) requires that a single component failure does not lead to loss of safety function — a second failure before the first is repaired may cause loss of safety. Category 4 (Cat 4) requires that even a combination of failures does not lead to loss of safety.

The dual-channel wiring principle achieves this. The e-stop button has two NC contacts wired in separate channels. Both channels must be simultaneously de-energised before the safety relay drops out. A safety relay module (or safety PLC) monitors both channels continuously and will not allow a restart if the two channels are not consistent — for example, if one channel is open and the other is still closed.

Cross-channel monitoring detects short circuits between channels, which is a common failure mode when cable insulation is damaged. The safety relay tests for cross-shorts by momentarily pulsing each channel and verifying the other does not follow.

A manual restart interlock is required for Cat 4: the machine must not restart automatically when the e-stop is released — an operator must consciously press a separate reset button. This prevents an unexpected restart that could injure someone who was inside the danger zone when the e-stop was activated.

Wiring e-stops in series on each channel allows multiple buttons — at pendants, along conveyors, at panel doors — to all stop the machine safely regardless of which one is activated.