Last edit: 22/08/2023
The electrical output of an interlocking device has two types of technologies
- It can be a Voltage Free Contact (VFC), or
- It can be an OSSD type (Output Signal Switching Device)
Type 2 interlocking devices often have VFC, but of a “special type”: they are with a Direct Opening Action. The concept is defined in Annex K of IEC 60947-5-1 [32]. Please refer to figure 4.12.1.1; the opening of the door causes the following steps:
- The actuator moves out of the actuating head
- That causes the movement of the Interlocking plunger
- That opens the interlocking monitoring contact
Something may go wrong and, for example, the actuator movement does not open the electrical contact.
In general, that is valid for many sensors. A low pressure switch may have the same issue: the contact is closed because the pressure is normal. If something happens in the process and the pressure drops, the sensor detects it but the contact does not open. Can a fault exclusion be done? Unless some considerations are made by the pressure switch manufacturer, a fault exclusion cannot be done by the user!
In case of the interlocking device, or other sensors using the same technology, a fault exclusion can be done if the Electrical contact has a Direct Opening Action. How does it work? As shown in Figure 4.21 {4.12.1.1}, left hand side, the opening of the contact is through non elastic elements. In the Non-direct opening example, the opening of the contact happens thanks to the spring force. If the spring has a failure, the contact will not open, despite the door opened.
[IEC 60947-5-1] Annex K
K.2.2 direct opening action (of a contact element). Achievement of contact separation as the direct result of a specified movement of the switch actuator through non-resilient members (for example not dependent upon springs).
Direct Opening Action contacts are identified with the following symbol:
Recent interlocking devices have OSSD outputs: those are solid state outputs. The signal goes to an off state when the door is open or when the component detects an internal failure. It has to comply with the standard IEC 60947-5-3 [33].
Direct and non-direct opening action
The use of direct and non-direct opening action of the position switches interlocking devices is commonly done to prevent common cause failures. Figure 3 shows the combination of direct and non-direct mechanical action of the position switches of a Type 1 interlocking devices. That is a way to avoid common cause failures of two mechanically actuated position switches by using associated direct and non-direct mechanical action.
In general, when using Voltage Free Contacts with Direct Opening action, both contacts are closed
when the door is closed. However, it is correct to use one NO (Normally Open) and one NC (Normally Closed) contact. It is incorrect, though, to use only one or even two Normally Open contacts (they are open when the gate is closed).