Case study: baking safely behind closed doors
sector over the last few years, and plants are now much safer
places than they were even just a decade ago.
But that is not to say that they can not be made safer still. Spooner Vicars, one of the UK's leading suppliers of bakery machinery, is constantly looking to improve the safety of its products, and recently turned to compatriot EJA for help.
EJA's Allen-Bradley Guardmaster Prosafe trapped key interlocks have been installed in a 90 metre-long triple hybrid continuous bakery oven from Spooner Vicars in a plant operated by one of Europe's leading baked goods suppliers.
The stainless steel Prosafe units are being employed to ensure that the 32 separate doors on the oven are securely interlocked and isolated before personnel access is allowed.
Spooner Vicars' triple hybrid oven combines facilities for radiant and convection bakes - as do most hybrid ovens - but has the added functionality of radio frequency operation on the last three zones of the oven to help balance moisture control and produce an even better product.
Like all hybrid ovens, it has an integrated continuous conveyor system, with many doors along its length to allow full body operator access for general maintenance and product inspection.
But this provision creates problems for design engineers when deciding which safety system can provide the optimum protection for bakery personnel. Any system must ensure that the whole oven is isolated both electrically and mechanically should a maintenance operative need to enter the oven.
Spooner Vicars opted for the Allen-Bradley Guardmaster rotary key switch isolator with electronic time delay and 32 dual access trapped key interlocks to help meet these rigorous demands.
According to EJA, operation of the rotary key switch (RKS) isolates the electrical supply to the oven, initiating a time delay to ensure that radio waves, and the high levels of static they generate, are fully dissipated before operator entry is attempted.
Once this time delay is over, the isolator key is released and carried some distance by the operator, finally being inserted into a special key exchange unit with 38 removable keys, all with the same code to fit any door.
Once any of these keys are removed, the isolator key is trapped and will remain so until all keys from the key exchange are returned to the unit. Only then can power be restored to the machine.
The keys from the key exchange unit are used in conjunction with dual access interlocks on the oven doors. These allow maintenance personnel and operators to enter the oven whilst in possession of a secondary key; as a result, it is impossible for them to be locked into the oven and for power to be restored while they are in the oven.
Apart from the obvious safety advantages, the adoption of the Allen-Bradley Guardmaster trapped key system means that safety provision on the machine is much simplified: once power is isolated by the RKS, only mechanical operations have to be considered. This also offers the added benefit of removing problems of installation and maintenance of cabling on the 90 metre long oven.
Allen-Bradley Guardmaster's Prosafe range is fully inclusive, satisfying the bulk of mechanical interlocking requirements with a series of complementary products, including isolators, key exchange units, integral valve interlocks and lock out/tag out devices, all of which employ the same dependable key principle and are tested to 100,000 operations (equivalent to 27 years service at 10 operations per day).
