Mechanical interlocking provides simple solution

Mechanical techniques proven over many years still have much to offer modern industry in safeguarding personnel says EJA Marketing Manager, Ian Williams.

Where access is required to machinery or an installation by an operator, suitable precautions must always be taken to eliminate risk.

It is essential that an operator cannot access the equipment whilst it is dangerous.

The consequences of doing so are obvious and often horrific.

Unfortunately, even armed with this knowledge, accidents are relatively common.

One general reason identified for this is human error, the result of many factors including carelessness, ignorance and bad communication.

Training and safe working practices can improve the situation, but to eliminate the risk altogether it must be physically impossible for the operator to perform those actions that can lead to a dangerous situation.

One of the most simple and effective ways of ensuring this is with a mechanical trapped key interlock system.

The use of mechanical coding or key interlock systems is not new.

The original concept of using keys for safety and security predates the industrial revolution, but in more recent times trapped key interlocks have become synonymous with safeguarding heavy plant and equipment and processes.

Trapped key interlocks were first developed in the UK over 50 years ago and the principle behind such systems remains sound.

Based upon the premise that no one key can be in two places at once, key interlock systems can be configured to ensure a predetermined sequence of events takes place or that all hazards have been eliminated before operators can become exposed to them.

The key interlock is a mechanical system and therefore is widely used in many applications including those where the location of plant, environment or explosive atmospheres make the use of electrical interlock systems unsuitable or expensive to install.

In addition, as unique coding can be provided, a higher degree of security and resistance to tampering may be possible.

Guardmaster's Prosafe range is an example of trapped key interlocking system.

The range is fully inclusive, satisfying the bulk of mechanical interlocking requirements with a series of complementary products, including isolators, solenoid locking devices, time delay units, key exchange units, integral valve interlocks and plus lock out/tag out devices bolt and access interlocks, 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).

Inbuilt reliability of the Prosafe range is ensured by the use of s/s direct acting and linked (positive mode) mechanical components.

The turning of the key is transmitted to the locking action of the bolt via rigid members.

It does not rely on springs or ball bearings, which could fail and lead to a dangerous condition.

The device can only fail if its components break or become distorted.

That situation is prevented by the "over-engineering" and high specification of the components and materials so that they can stand up to all the foreseeable forces with a considerable safety margin.

This approach is adopted by most manufacturers of trapped key interlocks and results in a device with an extremely low probability of failure.

Some devices such as Prosafe take the measures one step further however.

The key features a specially designed section which will shear at a force much higher than that required for normal (or even abnormal) operation but much lower than that required to damage the lock.

In addition, unlike any other interlock on the market the Prosafe has the facility for individual code barrels to be replaced, rather than the complete interlock.

This means that new keys can be introduced into the system quickly should the original ones be lost, and there is no need to keep a complete set of spare keys.

Interlocks in the Guardmaster Prosafe range are offered in single key and dual key access configurations.

Single key access units are designed for the interlocking of hatches, guards and doors on machines where full body access is not required.

They are extremely reliable and have the advantage of not requiring electrical wiring to the guard.

With single key access units the key, after it has been turned to release the guard- locking unit, remains trapped and cannot be removed until the guard is closed and locked again.

Therefore it is impossible to open the guard without first isolating the power source and it is also impossible to switch on power to the machine without closing and locking the guard.

Dual key interlocks in the Guardmaster Prosafe are designed for applications where full body access to an enclosure is required.

They can be used in one of two modes: sequential key exchange, with one key in and one out, or dual key insertion with both keys trapped to operate.

In the former mode, one key, the primary, remains trapped after being inserted and rotated.

The secondary key is then inserted and turned, allowing the guard to be opened.

The secondary key can now be removed and used as the next stage of an interlocking sequence or be retained as a personnel key to ensure that machinery cannot be restarted whilst the keyholder is inside the hazard area.

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