9 - Control circuits and control functions

Last edit: 05/05/2023

Chapter 9 of the Standard details control circuit and control functions.

In the control function, the Standard summarizes the three categories of stop functions:
–    Stop category 0;
–    Stop category 1;
–    Stop category 2.

Paragraph 9.2.3 details the functions of the machine, where we find:

–    Start: the start function shall operate by energizing the relevant circuit
–    Stop: stop function shall override related start functions
–    Emergency operations (emergency stop, emergency switching off)

  • Emergency stop: it shall function as a stop category 0 or as a stop category 1. The choice of the stop category of the emergency stop depends on the results of a risk assessment of the machine.
  • Emergency switching off: it is accomplished by switching off the relevant supply by electromechanical switching devices, effecting a stop category 0 of machine actuators connected to this incoming supply.

Paragraph describes the operating modes and the protective measures to provide when the machine is design and constructed to allow its use in several operating modes. In particular, the Standard specifies to fit a mode selector which can be locked in each position. Each position of the selector shall be clearly identifiable and shall correspond to a single operating mode. The Manufacturer shall improve protective measures for each operating mode.

It is clearly specified that mode selection by itself shall not initiate machine operation.

The Standard describes in the following paragraphs hold-to-run control [§]; two-hand control []; enabling control [§]; combined start and stop controls [§].

Section 9.3.6 provides the requirements to be followed when suspending safety functions and/or protective measures.
It may be necessary to suspend safety functions and/or protective measures to perform adjustment or maintenance operations. The Manufacturer must provide a selector switch to meet this function. However, the selector switch must simultaneously:

– disable all other operating (control) modes;
– permit operation only by the use of a hold-to-run device or by a similar control device
positioned so as to permit sight of the hazardous elements;
– permit operation of the hazardous elements only in reduced risk conditions (e.g. reduced
speed, reduced power / force, step-by-step operation, e.g. with a limited movement control
– prevent any operation of hazardous functions by voluntary or involuntary action on the
machine’s sensors.

Chapter 9.4 describes the control functions in the event of a fault. In particular, when faults or disturbances in an electrical equipment can lead to a dangerous situation or cause damage to the machine or production, the Manufacturer must consider appropriate measures to reduce the likelihood of such faults or disturbances occurring. The measures prescribed and the extent to which they are implemented, either individually or in combination, depend on the level of risk related to the respective application.

Examples of such measures that may be appropriate include but are not limited to:

– protective interlocking of the electrical circuit;
– use of proven circuit techniques and components;
– provision of partial or complete redundancy or diversity;
– provision for functional tests.

Electrical control systems must have an appropriate level of performance as determined by the machine’s risk assessment.
Measures to reduce risk in the event of failure include, but are not limited to:

– use of proven circuit techniques and components;
– provisions of partial or complete redundancy;
– provision of diversity;
– provision for functional tests.

Section 9.4.3 describes protections against failure of control circuits. The Manufacturer must take measures to reduce the likelihood that insulation faults on any control circuit will cause a malfunction, such as inadvertent startup, potentially dangerous movement, or prevent the machine from stopping.
Measures to meet these requirements include but are not limited to the following methods:

– method a) Earthed control circuits fed by transformers;
– method b) Non-earthed control circuits fed by transformers;
– method c) Control circuits fed by transformer with an earthed centre-tap winding;
– method d) Control circuits not fed by a transformer.

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