Trapped key system protects multi-door 90m oven

Trapped key interlocks simplify safety provision and provide safer access into the largest of machine installations as shown on a continuous bakery oven having 32 separate doors.

The ability of Allen-Bradley Guardmaster's Prosafe range of trapped key interlocks to simplify safety provision, while, at the same time, providing safer access into the largest of machine installations has been demonstrated at Spooner Vicars on Merseyside.

The stainless steel Prosafe units are being employed to ensure that 32 separate doors on a 90 metre long triple hybrid continuous bakery oven are securely interlocked and isolated before personnel access is allowed.

The triple hybrid oven is unique among a line of such hybrid equipment produced by Spooner Vicars, a leading company in the design, manufacture and supply of high quality food handling and food process plant.

Over the past few years Spooner Vicars has been at the forefront of the supply of hybrid ovens to the baking industry.

These combine facilities for radiant and convection bakes to produce a more flexible controllable oven.

The triple hybrid goes further, adding radio frequency operation on the last three zones of the oven to help balance moisture control and produce an even better product.

Hybrid ovens are typically long pieces of equipment - the triple hybrid extends to almost 90 metres.

They integrate continuous conveyors and have many doors along their length to allow full body operator access for general maintenance and product inspection.

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.

Colin Oakes, an Electrical Projects Engineer at Spooner Vicars, decided that the best system to achieve this level of safety was one comprising an Allen- Bradley Guardmaster rotary key switch isolator with electronic time delay, and 32 dual access trapped key interlocks.

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.

Upon expiry of the time delay 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 because, 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.).

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