The concept of SPAN-OF-CONTROL

Last edit: 18/04/2023

The standard ISO 11161 of May 2007 introduces the concept of span of control, which identifies the predetermined portion of a machine under control of a specific device.

The ISO 11161 clarifies which types of devices shall have a span of control:

[ISO 11161: 2007] 8.2.2 Devices having a span of control
Each of the following devices shall have a defined span of control:

  • Emergency stops
  • Enabling devices
  • Gate interlocks
  • Presence-sensing devices
  • Means for disconnecting
  • Local control mode

One of the devices can have a span of control combined from the spans of controls for other devices (e.g. an emergency stop device can have a span of control that includes the span of control for two gate interlocks).

In that edition of the reference standard for integrated-manufacturing-system safety the emphasis was on the Emergency Stops with guidelines on how to implement them correctly.

[ISO 11161: 2007] 8.11 Emergency Stop
All IMS emergency stop devices shall have the same span of control or shall have clearly identified spans of control.
All emergency stop devices for a task zone shall have the same span of control. The span of control may include multiple zones.

Thus, a machine should be covered by a single span of control but it is still possible to define several spans of control associated with different emergency functions. This concept is also mentioned in the EN 13850 regarding emergency stop devices.

[EN 13850: 2015] 4.1.2 Span of control of emergency stop device(s)
The span of control of each emergency stop device shall cover the whole machine. As an exception, a single span of control may not be appropriate when, for example, stopping all linked machinery could create additional hazards or unnecessarily affect production.

With the new edition of the ISO 11161 more attention is paid to the different type of devices and the distinctness of every single span of control. In addition, the identification of the span of control becomes mandatory for manual reset devices.

[ISO 11161: 202x] 7.4.4 Identification of span(s)-of-control

The span-of-control shall be clearly distinguishable or identifiable as to not be confused with other spans-of-control.
Means of identifying span-of-control may include displays, color-coding, signs, shapes, layout and location, markings and more.

The assignment of multiple spans of control must ensure the identifiability of each control circuit, allowing an immediate reaction in case of danger.

As said, what will soon become much more important is the control of the Manual Reset; a guideline-example is shown below.

[ISO 11161: 202x] 7.12.5 Manual reset

When applicable, the span-of-control of the safeguarded space shall require manual reset before a hazardous situation(s) can occur (see also IEC 60204-1 for additional information on manual reset devices).
Manual reset of the SRP/CS shall only be performed after the safeguarded space associated with the span-of-control of the reset device is clear of all persons and extraneous support equipment.
Any accessible location within the entire safeguarded space associated with the span-of-control of the manual reset device shall be visible from the manual reset device location.

Manual Reset shall not itself initiate any hazardous conditions. The reset actuator shall be situated outside the hazard zone and shall only be actuated from outside the safeguard, in complete visibility conditions of the hazard zone. Where visibility is not complete, additional protective measures shall be provided.
The revision of the standard has extended this point by dedicating it entirely to access within the hazardous zone.

In conclusion, the Span(s) of Control isa a key element for a proper implementation of many safety-measures without affecting the functional performance of the machinery: when referring to emergency stops, manual resets or interlocking devices it is essential to reach a full understanding of their spans of control, which must be clearly specified and fully consultable.

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