Chapter 3.8
In cases where guards hinder operation or monitoring is impractical, providing operators with push-buttons, foot switches, or two-hand controls ensures safe starting and stopping of hazardous situations. With the following information we will explain and discuss the different options and their advantages.
Alternative 1
Two-hand control is sometimes referred to as “two-hand start,” meaning that two buttons must be pressed simultaneously or within a short time span (0.5 seconds) to initiate the movement of a hazardous machine part.
The term "two-hand start" may cause designers to overlook a crucial requirement: the operator must continue pressing the buttons until the hazard is eliminated. If the operator releases one or both buttons prematurely, the operator could potentially reach the hazard zone and be injured.
To prevent this, the two-hand control buttons must be positioned at a calculated safety distance. This ensures that it is impossible to reach the hazard zone before the moving parts have completely stopped. The correct standard for these calculations is (EN) ISO 13855.
Consider the following additional factors when designing two-hand control:
The international standard for two-hand control systems is (EN) ISO 13851. This standard is also referenced in ANSI B11.19 for the USA and CSA Z432 for Canada.
Alternative 2
Another frequently used way of giving people control of the hazard is the so called “hold-to-run” device.
This typically involves a button (hardware or on-screen) that triggers a movement or function as long as it is pressed or touched. This way, the operator can determine when the hazard occurs and when it is to stop.
The same principle applies to manual control. However, it is not usually considered a safety function. For instance, a hand drill features manual control, but one would hardly consider the possibility to let go of its trigger as a safety function.
Hold-to-run safety functions can come in two different forms:
Four major requirements for a hold-to-run function are sometimes overlooked:
