Two-hand control devices as a protective measure (ISO 13851)

The machine safety guide and the emergency stop categories guide describe how a machine is safely stopped. ISO 13851 covers a different aspect of machine safety: how to prevent an operator's hands from reaching into the hazard zone during a hazardous movement (for example, a press or punching tool).

What a two-hand control device is

A two-hand control device (Two-Hand Control Device, THCD) is a protective measure that protects the operator against reaching into a hazard zone during a hazardous situation, by placing the control elements at a specific position and distance from the hazard zone. The operator must hold both hands simultaneously on two buttons separated from each other to start the hazardous movement — this prevents one-handed operation and rules out an operator jamming one button with an object to keep one hand free.

Safety category

A two-hand control device is generally classified as a category 3 or 4 safety system under ISO 13849-1 — the generic standard for safety-related parts of control systems referenced by ISO 13851 (itself a so-called type-B2 standard).

What the standard does not cover

ISO 13851 does not specify on which machines a two-hand control device must be applied, which type of two-hand control device is required for a specific application, or what distance must be maintained between the control device and the hazard zone. Those choices follow from the risk assessment of the specific machine and, where applicable, a machine-specific type-C standard.

Note: a two-hand control device protects only the operator themselves — not any bystanders who may also be within the hazard zone. Additional measures (guarding, light curtains) are needed to protect bystanders.

Practical relevance

When converting or maintaining a press, punch, or similar machine in a workshop, it is important to know that a two-hand control device is not on its own a complete machine-safety solution: it covers the risk for the operator, but not for any second person near the machine. This consideration belongs in the risk assessment that also determines the choice between stop categories (see the emergency stop categories guide).

Common mistakes

Assuming a two-hand control device protects everyone present — the standard explicitly protects only the operator who operates the buttons.
Applying a two-hand control device without determining the required distance to the hazard zone — this distance follows from the machine's stopping time and the reach-access analysis, not from the standard itself.
Allowing the two-hand control device to be bypassed by bridging or jamming the buttons — this undermines the entire protection principle.