Chapter 3.8
Safe enabling functions
Guards may in some applications hinder operation unduly and monitoring is not always possible. In such cases, giving the operator a push-button, foot switch or two-hand control to start and stop hazardous situations is the safe way to go.
Safe enabling functions
Two-hand control
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 s) to start a hazardous machine part’s movement.
However, “two-hand start” ignores an important requirement. The person must keep pressing the buttons until the hazard is gone! Otherwise, if the operator would let go of one or both of the buttons they could reach the hazard zone and get hurt.
Therefore, the two-hand control buttons must be located at a calculated safety distance. It must not be possible to reach the hazard zone before the moving parts have stopped. The correct standard to base calculations of the safety distance on is (EN) ISO 13855.
Consider the following additional factors when designing two-hand control:
- Be aware that two-hand control can “protect” only one person (or one person per panel).
- Since only one person is protected, access to the hazard zone must be limited to the extent possible (e. g. by guards).
- The operator starts a dangerous movement that might present a hazard to others. So he needs full view of the hazard zone.
The international standard for two-hand control systems is (EN) ISO 13851. It is also referenced in ANSI B11.19 for the USA and CSA Z432 for Canada.
Safe enabling functions
Hold-to-run, enabling, and manual control
Another frequently used way of giving people control of the hazard is the so called “hold-to-run” device.
Typically, it features an enabling button (hardware or on-screen) that triggers a movement or function as long as it is pressed or touched. Thereby the operator can control when the hazard occurs and when it is to stop.
Hold-to-run safety functions can come in two different forms:
- With direct control by one push-button only.
- By indirect control involving an on-screen button and a so-called “enabling” button. The onscreen button is active only when the enabling button is also pressed. This is different from two-hand control, because pressing the two buttons is not monitored for simultaneousness.
The two buttons simply need to be pressed in series. When the enabling button is let go, the hazardous movement stops.
Four major requirements for a hold-to-run function are sometimes overlooked:
- Hold-to-run may only be used in a separate selectable control mode. It is not allowed to use it as part of an automatic operating cycle.
- Letting go of the hold-to-run button or the enabling button must result in stop of the hazardous movement as fast as possible.
- The motion speed, force or stroke must be limited so the operator can realistically let go of the button early enough to be safe.
This is often not the case with manual control, making hand-held machinery like drills and chain saws particularly hazardous.
- A means of escape from the hazardous situation must be provided for. This can be a mobile panel, limited access (person stays away from actual hazard source) or - in most cases - an emergency stop button close by. Most enabling switches have a built-in emergency stop button.
