Prevention of Unexpected Power-Up

Prevention of Unexpected Power-Up

Prevention of unexpected power-up is covered by many standards. Examples include: ISO14118, EN1037, ISO12100, OSHA 1910.147, ANSI Z244-1, CSA Z460-05, AS 4024.1603, NFPA70[NEC] 430.109(a)(7). These standards have a common theme: the primary method of preventing unexpected power up is to remove the energy from the system and lock the system in the off state. The purpose is to safely allow people to enter a danger zone of a machine.

Lockout/Tagout

New machines must be built with lockable energy isolating devices. The devices apply to all types of energy, including electrical, hydraulic, pneumatic, gravity, and lasers. Lockout refers to applying a lock to an energy isolating device. The lock must only be removed by its owner or by a supervisor under controlled conditions. When multiple individuals must work on the machine, each individual must apply their lock to the energy isolating devices. Each lock must be identifiable to its owner.

In the US, tagout is an alternative to lockout for older machines where a lockable device has never been installed. In this case, the machine is turned off and a tag is applied to warn all personnel to not start the machine while the tag holder is working on the machine. Beginning in 1990, machines that are modified must be upgraded to include a lockable energy isolating device.

An energy isolating device is a mechanical device that physically prevents the transmission or release of energy. These devices can take the form of a circuit breaker, a disconnect switch, a manually operated switch, a plug/socket combination or a manually operated valve. Electrical isolating devices must switch all ungrounded supply conductors and no pole can operate independently.

The purpose of lockout and tagout is to prevent the unexpected startup of the machine. Unexpected startup may be the result of various causes: a failure of the control system; an inappropriate action on a start control, sensor, contactor, or valve; a restoration of power after an interruption; or some other internal or external influences. After completion of the lockout or tagout process, the dissipation of the energy must be verified.

Safety Isolation Systems

Safety isolation systems execute an orderly shutdown of a machine and also provide an easy method of locking off the power to a machine. This approach works well for larger machines and manufacturing systems, especially when multiple energy sources are located on a mezzanine level or at distant locations.

Figure 116 shows an overview of the system layout. Lockable stations are remotely located at convenient access points throughout the machine. When necessary, an operator uses the remote station to turn off the machine and lock the machine in the off state. The control box disconnects electrical and pneumatic power and provides a signal back to the operator that the energy has been disconnected.

Figure 116: Layout of Safety Isolation System

Figure 117 shows the safety isolation system not only removes power from the machine, but also grounds the load side. The operator receives a monitored, visible signal at the remote station indicating the machine is in a safe state and the energy has been dissipated

Figure 117: Machine side is grounded with signal to operator.

Load Disconnects

For local isolation of electrical devices, switches can be placed just prior to the device that needs to be isolated and locked out. The Bulletin 194E Load Switches are an example of a product capable of both isolation and lockout. Figure 118 shows an example of Bulletin 194E.

Figure 118: Load switch with isolation and locking capability

Trapped Key Systems

Trapped key systems are another method for implementing a lockout system. Many trapped key systems start with an energy isolating device. When the switch is turned off by the primary key, the electrical energy to the machine is removed from all the ungrounded supply conductors simultaneously. The primary key can then be removed and taken to a location where machine access is needed. Figure 119 shows an example of the most basic system, an isolating switch and a gate access lock. Various components can be added to accommodate more complex lockout arrangements.

Figure 119: Trapped key isolation and lockable devices

Alternative Measures to Lockout

Lockout and tagout must be used during servicing or maintenance of the machines. Machine interventions during normal production operations are covered by safeguarding. The difference between servicing/maintenance and normal production operations is not always clear.

Some minor adjustments and servicing tasks, which take place during normal production operations, do not necessarily require the machine to be locked out. Examples include loading and unloading materials, minor tool changes and adjustments, servicing lubrication levels, and removing waste material.

These tasks must be routine, repetitive, and integral to the use of the equipment for production. The work is performed using alternative measures, like safeguarding, which provide effective protection. Safeguarding includes devices such as interlocked guards, light curtains, and safety mats. Used with appropriate safety rated logic and output devices, operators can safely access machine danger zones during normal production tasks and minor servicing.