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2.1 Electric safety circuits according to EN 60204/VDE 0113 part 1

2.1.1 Emergency-stop devices in gereral

EN 60204/VDE 0113 part 1, section 9.2.5.4 stipulates that with the help of an emergency-stop device a machine or drive must be switched off as quickly as possible in the case of danger. There are two ways of doing this:

an emergency-stop switch
control circuits which are designed in such a way to allow all corresponding main circuits to be switched off by means of a single command.

The emergency-stop function must be effective as a category 0 or 1 stop and must take priority over other functions. The stop category must be determined by means of risk assessment.

Switching off selectively

An emergency-stop device must switch off the energy supply to the machine drives which can lead to hazardous conditions as quickly as possible. General switching off of the energy supply is not necessarily required.

Preventing actuation

Similarly, resetting an emergency-stop device must not cause restarting of the machine (circuit diagrams 1 and 2). Emergency-stop devices must

have a red handle with a yellow background,
be readily accessible,
and easy to reach.

Contact elements which are positively opened are compulsory for manually-operated emergency-stop devices, whereby, according to DIN VDE 0660 part 200, "positive opening operation" means the contact elements are directly mechanically activated.

Permitted actuation

Emergency-stop devices can be activated as follows: Using

the handle of the emergency-stop device
one or more mushroom-head pushbuttons
pull-cords which release the emergency-stop command on actuation or breaking
pedal-operated switches or foot switches without a mechanical guard.

Circuit diagram 1 "Emergency-stop switch" in the power supply of consumers with dangerous movements: with series-connected main switch

Circuit diagram 2 "Emergency-stop switch" and additional control circuit

Important!
Normal red buttons or mushroom-head puchbuttons of a different colour are not permitted for emergency-stop devices!
"Emergency-stop switch"

Only for very simple machines (stop category 0) is the use of an "emergency-stop switch" possible.

The emergency-stop switch must

be fed by the power supply of electrical circuits which can lead to dangerous movements in the system (c.f. also circuit diagrams 1 and 2)
simultaneously be able to switch off the current of the largest motor in locked-rotor condition and

the sum of the currents of all other consumers during normal operation – whereby the electrical circuits to be switched off for an emergency- stop must be taken into account.

The emergency-stop switch can

be activated by hand or remote control
be provided with corresponding tripping devices for protection against overload and short-circuit
simultaneously serve as the main switch if it additionally fulfils the requirements of a main switch.

Circuit diagram 3 "Emergency-stop contactors" for simple drives with only one contactor for one or more motors

Circuit diagram 4 All contactors directly de-energized via emergency-stop control devices

Use of the "emergency-stop switch"

An "emergency-stop switch" may only be used for switching off/on directly for comparably simple machines.
An "emergency-stop switch" may not be used if not all electric circuits may be switched off immediately. Example: drives which are braked electromotively.
An "emergency-stop switch" may/can often not be used simultaneously as a main switch. These are often cases where individual branches may not be switched off (c.f. circuit diagram 1).

Contactors may not b used as emergency-stop devices!

Exception:

If only one contactor must be de-energized to switch off a machine or branch, this contactor can fulfil the emergency-stop function (circuit diagram 3). This is only permissible if consumers are connected directly and there are no further contactors in series.

Restart after an emergency-stop

EN 60204/VDE 0113 part 1 specifies that after an emergency-stop a restart must be prevented. In addition to the emergency-stop device there must therefore also be other control circuits (see also circuit diagram 2).

Switching off with a single command

All main circuits can be switched off with a single command – if the control circuits are correspondingly constructed. This is the case if the control circuits are connected in series with an emergency-stop control device e.g. an emergency-stop mushroom-head pushbutton (circuit diagram 4). As many emergency-stop control devices as desired can be connected in series. The only restriction is the length of the conductors. For direct current operation the voltage drop caused by the conductor and the number of contact elements must be taken into account. For alternating current oper- ation there are restrictions due to the capacity of the conductor. For further details refer to the manual on "Switching, Protection and Distribution in Low-Voltage Networks" section 4.3.2.

Electronic Controls

For reasons of safety emergency-stop commands must not be carried via single-branch electronic controls (PLC) alone. In accordance with the norm, electro-mechanical components must be used. The state of the art is the use of conventional devices e.g. contactor controls.

General requirements according to EN 60204/VDE 0113 part 1:

Only those drives have to be switched off which can cause hazardous condi- tions or if they are not switched off would lead to damage of machines or the work in process. These drives are usually controlled by electromagnetic equipment. Switching off electromagnetic equipment such as contactors or magnetically operated valves takes place directly – via the control device – or indirectly via relays or contactor relays. The individual switching devices must be able to switch off "their" equipment.

2.1.2 Emergency-stop circuits with increased reliability

Faults can occur during the regular control procedure - with grave consequences for man and machine. Additional electric circuits which are only activated in the case of a fault provide greater safety. These special safety circuits are usually made up of contactor relays.
In the literature a distinction is made between two basic circuits:

Circuits made up of two contactor relays with overlapping contact elements
Circuits made up of three contactor relays with positively driven contact elements.

Electric safety circuits should not be used for switching during normal operation.

Protection during malfunctioning

If a contactor relay malfunctions in the first circuit above (see circuit diagram 5) the safety circuit remains fully effective i.e. after activating the emer- gency-stop device the enable circuit is switched off. The circuit does provide redundancy in accordance with EN 60204/VDE 0113 part 1 section 9.4 but in addition restarting of industrial machines should only be possible after removal of the fault.

Prerequisite: positively driven operation

Practical experience with circuits of the type in diagram 5 has, however, shown that restarting the machine despite malfunctioning of a contactor relay is possible after an On command. A circuit made up of two contactor relays with overlapping contacting does not fulfil the conditions of positively driven operation – it operates without internal monitoring of the contactor.

What is positively driven operation?

For "normal" controls positively driven operation is not prescribed but, however, is imperative for safety circuits. Information on positively driven opera- tion can be found in the Safety Regulations ZH 1/457 2nd edition, 1978 for controls of power-operated presses in metal processing: positively driven operation for contactor relays means that the make-contact elements and break-contact elements must not at any time be closed simultaneously.

Circuit diagram 5 Contactor combinations consisting of 2 contactor relays with overlapping contact elements

Circuit diagram 6 Contactor combinations consisting of 3 contactor relays with positively driven contact elements. "Circuit currently used"

Circuit diagram 7 Contactor combinations consisting of 3 contactor relays with positively driven elements. "New" circuit with lower residual risk

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