Back to home pageSafety Relays - Introduction/FAQs

What Are Emergency Stop Relays?
What Is Redundant Circuitry?
What Are Positive Guided Contacts?
What Is Self Checking?
Why Are Safety Relays Desirable?
Which Specifications Apply?

What are Emergency Stop Relays?

An Emergency Stop Relay operates with an Emergency Stop Button to ensure that under emergency fault conditions the electrical supply to a machine/process is swiftly and reliably disconnected.

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What is Redundant Circuitry?

Redundancy consists of two or more relays with parallel coil circuits operated by the same control function. The contacts of these relays are connected in series to ensure that the emergency stop circuit will disconnect the supply even if one of the relays has failed.

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What are Positive Guided Contacts

Positive guidance exists when all the contacts of a relay/contactor are forced to operate in their correct mode during each relay operation. This is achieved by means of a mechanical drive bar which forces the contacts to behave in the desired manner. Each contact in its open position must maintain a minimum clearance between the contact faces.

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What is Self Checking?

Pressing the ON/Reset button to close the main circuit, will apply power to the Emergency Stop relay. A series of internal operations, checks the integrity of each parallel relay circuit with a failure on any circuit inhibiting the start operation.

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Why are Safety Relays desirable?

Existing Harmonised European Standards require that machine mounted switches, relays and sensors incorporated in safety circuits should be used in a manner that "minimises their chance of failure to a dangerous condition." One cost effective solution is to interface the machine emergency stop pushbuttons or safety gate switches to the control system via an emergency stop relay. This relay should incorporate positive guided, cross monitored inputs and outputs. These features will significantly reduce the chance of relay failure, characteristics which are not inherent in standard control relays which often tend to fail in the closed position. Dold safety relays are manufactured to the above requirements and incorporate the following features:

  • Redundant circuitry.
  • Positive guided contacts.
  • A self checking facility.
  • Minimised failure to closed output.
  • Cross monitored inputs (dependent on model).
  • Third party approval as safety products.
NB Fitment of a Dold safety relay will not automatically enable your control circuit to comply with the existing standards. All aspects of the control system must be taken into consideration and a risk assessment made, to verify the safety risk categories required for the machine and associated control systems.

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Which specifications apply?

Listed below are some of the standards pertaining to emergency stopping and Risk Assessment:

  • BSEN 60204-1
  • IEC 204-1
  • BSEN 418
  • EN 954-1
  • BSEN 1050
  • EN 292-1-2
  • EN 1037

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