Interlocking principles: some of the less well known requirements as set out by EN1088When selecting an interlock, one of the key standards for consideration is EN1088:1995+A2:2008, Safety of Machinery - Interlocking devices associated with Guards - Principles for design and selection. Within the market place it can now be more or less taken for granted that the key characteristics for an interlock, as set out by EN1088, are covered by almost all products.
Guidance is given in EN1088 in the selection of either an 'interlocking guard' or an 'interlocking guard with guard locking'. Essentially if the hazardous area to be protected has a run down time (stopping time) greater than the time it could take an operator to get from the guard access point to the point of danger (access time), then the interlock requires a locking function. This function is almost always provided by a solenoid. However the way in which the solenoid operates does vary, and it is here that units are often incorrectly selected.
EN1088 clearly puts forward that a power to unlock mechanism (with a compression spring to provide locking) is the best solution. In the event of a power failure to the unit the guard will remained locked and therefore the hazardous area remains protected. The locking function when controlling access as a safety function must also be monitored to the same level and principles as the door. Thus dual positive mode actuation contacts are a well tried and proven solution, a function that is often missing from some high tech solutions. This level of control and monitoring however is not required in situations where the solenoid locks the guard for reasons of operational efficiency rather than safety. EN1088 puts forward that due consideration is given to the environment when the interlock is selected. Whilst this appears an obvious point, how often are weak plastic interlocks observed in heavy-industry? A unit may have force break contacts and an undefeatable tongue, but if in practice the unit is damaged or jammed within months of installation, how long is it until the unit has been temporarily 'jumped out' to keep production running?
A final point of interest in EN1088 is that of power interlocking. As opposed to control interlocking, where the hazardous area remains 'live' but under control, EN1088 reminds the reader of the simplest form of machine guarding, that of power interlocking. As the title suggests, power interlocking uses the isolation of all power into the hazardous area before releasing an access key to allow entry through the interlocking guard. With the guard open the power cannot be re-applied. This simple and cost-effective solution, whilst not for everyone (for example those seeking continued but limited functionality within the area or those needing quick and frequent access), provides an increasingly popular choice in an environment where control interlocking is becoming ever more expensive and complicated.
Looking to the future, ISO 14119 (the update to EN1088) is currently being created and in draft format. Whilst much is a carry-over in this new standard other elements have been updated for greater clarity. One area of great interest within the draft proposal is the need to provide an 'Escape release of Guard Locking' (IR) if 'it is reasonably foreseeable that a person can be trapped'. Whilst clearly the new range of Fortress products are available with IR systems it brings back to the fore a more important issue: how to prevent personnel becoming locked within an area in the first place. It is here that the Fortress Safety Key system yet again proves its worth.
Once a Safety Key has been taken by the operator they are assured that the system cannot restart until they exit the hazardous area and return the key. In this way a safety key system, as with the whole Fortress range, is not only fully compliant with the standards but industry leading in providing prevention rather than just a cure to hazardous situations.
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