Safeguarding Against Machine Hazards

Interlocking Switches

For applications that require a simple and permanent type of safeguarding used around the hazard as a barrier to prevent access or contain a dangerous hazard, a fixed guard can provide an ideal fit. However, since fixed guards don't move, they may need to be disassembled for maintenance and repair issues, which can result in the guard not being reinstalled. Fixed guards shouldn't be used if personnel require moderate to frequent access to whatever is behind the guard. For these applications, an interlocking guard can be employed.

Interlocking is a safeguarding solution used to monitor the position of a guard or gate. The interlocking device can be used to de-energize circuitry or actuators, control personnel access and prevent the machine from starting when the guard is open or not in place.

The means of interlocking include mechanical, pneumatic and hydraulic systems, but the most frequent is probably an electro-mechanical switch with “positive-opening” operation. “Positive opening” refers to the design of the switch in which operation (i.e., moving or opening the guard) results in a direct physical force opening the normally closed electrical contacts.

Light Screens/Curtains

Optical-based safeguarding devices typically are comprised of two basic components of optical technology: an LED and phototransistor. The most common optical safety system is the safety light screen (also referred to as a safety light curtain), an optoelectronic device used to guard a machine hazard. Safety light screens contain an LED emitter array and corresponding phototransistor receiver that together create a sensing field with a specified detection capability. If all the phototransistors detect light from their corresponding LEDs, the light screen is “complete,” and the safety outputs allow the machine to continue operation. When an opaque object, such as a hand, interrupts a light beam, the light screen sends a stop signal to the safety-related controls of the machine, which react immediately to stop the hazardous motion prior to the hand reaching the hazard.

Safety light screens often are used to guard high-access applications, such as a hand-fed process or an application where regular maintenance is required, since they allow frequent access to an area and can be positioned in relatively close proximity to the hazard. Additionally, safety light screens can be less expensive to install and maintain than hard guards, and they still allow for protection of multiple individuals while also permitting good visibility of the hazard.

The International Electrotechnical Commission IEC 61496-1/-2 is the international standard for design and construction of light curtains, and specifies two types: Type 2 and Type 4. Type 4 light curtains typically are used for high-risk applications and comply with OSHA and ANSI requirements for control reliability because of the redundancy and automatic self-checking circuitry required of this type of device.