Safety circuits are the most important circuits on a machine. When a safety circuit faults, the machine will not run — and the temptation under time pressure is to look for a way around it. This guide is deliberately high-level, and the overriding rule is simple:

Safety devices must never be bypassed, defeated or overridden as part of fault finding.

These resources are for general engineering information and do not replace site-specific risk assessments, safe isolation procedures, manufacturer instructions, competent engineering judgement or applicable legislation.

The components of a safety circuit

  • Emergency stops — maintained (latched) buttons that break the safety chain when pressed; must be reset (twist/pull/key) to restore.
  • Guard switches / interlocks — confirm guards and doors are closed; some are safety-rated with coded actuators.
  • Safety relays / safety controllers — monitor the e-stops, guards and field devices and only allow a reset when conditions are healthy.
  • Permissives — conditions the safety controller (or PLC safety task) requires before allowing motion.

Diagnostic logic (high level)

  1. Is a device actually triggered? An e-stop latched in, or a guard open, is the most common cause — not a fault at all.
  2. Reset conditions — safety relays typically require a healthy inputs state and a deliberate reset action (a button, not automatic). Is the reset action present?
  3. Which device? Most safety controllers indicate which input has dropped. Use the diagnostics rather than guessing.
  4. Wiring and terminations — loose connections on a safety chain cause nuisance trips; check M12/M8 connectors, terminal blocks and cable flex points.
  5. Failed device — if one device is genuinely faulty, replace it with an equivalent safety-rated part; never short across it.

What is never acceptable

  • Bridging out an e-stop, guard or safety relay to "get running".
  • Repeatedly resetting without finding the cause.
  • Substituting a safety-rated device with a non-safety-rated one.

Key points

  • Treat every safety trip as real until proven otherwise; the proof is a genuine cause, not a workaround.
  • Use the safety controller's diagnostics to localise — modern units tell you which input dropped.
  • Document the cause so a recurring nuisance trip is fixed at source, not worked around.