Which of the following best differentiates physical guards from interlocked safety devices?

• A. They stop a machine when a guard or door is opened.
• B. Guards require electricity; interlocks do not.
• C. Guards are for visibility; interlocks provide noise reduction.
• D. Guards are used on all tools; interlocks are not.

Answer: (A)

The key idea here is how guarding vs. a safety interlock actually behaves to protect you. A physical guard is a barrier that blocks access to moving parts, but by itself it doesn’t automatically stop the machine when you open or remove it. An interlocked safety device adds an active control: when the guard or door is opened, it trips the control and stops the machine, preventing access to hazardous energy while the risk is present. That stopping behavior is what truly differentiates interlocks from simple guards.

So, the statement that the system stops a machine when a guard or door is opened captures the essential safety function of interlocked devices. The other choices are not reliable descriptions: guards aren’t defined by electricity, visibility, or noise changes, and it isn’t accurate to claim guards are used on all tools while interlocks are not.