6 min read
A common mistake in machine safety is to treat the emergency stop button as "the safety system". A safety function is a three-link chain: detection (button, door switch, light curtain), evaluation (safety relay or safety PLC) and safe stopping (contactor, or the drive's safe torque off function).
Performance level first
Before choosing components, a risk assessment defines the required performance level for each safety function. Severity of injury, frequency of exposure and possibility of avoidance determine that level. A safety relay bought before the level is known may be the right device in the wrong architecture.
When is a safety relay enough?
- With few functions (one or two emergency stops, a single guard door) a relay is cheap and quick to commission.
- If the logic is fixed and unlikely to change, a hard-wired solution is easier to maintain.
- On small machines the documentation burden is noticeably lighter.
When does a safety PLC become necessary?
- When there are multiple zones, doors and light curtains, or zone-based stopping is required.
- When conditional functions such as muting, partial stop or speed monitoring are needed.
- When safety status must be reported to SCADA.
- When the machine family will grow and the logic is expected to change.
The job does not end at commissioning
Once installation is complete, every safety function is tested individually and recorded. A periodic inspection plan is then handed over: emergency stops, door switches and light curtains must be physically exercised at set intervals. These records are the first thing an auditor asks for.
Frequently asked questions
Is wiring the emergency stop straight to the contactor enough?
On simple machines it provides a basic stop, but it is single-channel: a welded contact or a broken wire goes unnoticed. As the required performance level rises, dual-channel monitoring and a feedback loop become mandatory, and that is what a safety relay or safety PLC provides.
How is safe stopping done on a machine with a drive?
The safe torque off (STO) input available on modern drives is used. This safely prevents the motor from producing torque without opening the main contactor every time, so the machine restarts faster and contactor life improves.
Should the emergency stop be category 0 or 1?
Category 0 removes power immediately and suits machines where coasting is not hazardous. Category 1 stops under control before removing power and is preferred on high-inertia machines where abrupt stopping is dangerous.
How many emergency stops can one safety relay monitor?
Because the buttons are wired in series there is no technical limit, but a series chain cannot tell you which button was pressed. On multi-station machines needing that information, a safety PLC monitors each one separately.