Why HMIs Are Not Enough for Safety

While an HMI (Human Machine Interface) may come loaded with indicators, data, displays and buttons, it isn’t enough on its own. Industrial machines also need additional safety controls like an estop (emergency stop) push button, power-off button, power-on button, and reset button. These buttons may be conveniently placed next to most HMIs and can be further enhanced with alarms and LED stack lights.

Often operators, line supervisors and engineers will push the estop button for a myriad of reasons, but use should be reserved for its given function, which is only if someone is in imminent danger.

In an emergency, estop buttons tend to be hit with a force reflective of the situation – a force most HMIs cannot withstand. Regardless, safety standards typically require an estop to be hardwired into the system. The best practice is to use an estop with a red, mushroom-shaped head with a cautionary yellow background for visibility. LED illuminated models are also available for a quick visual reference as to its activation.

Estop buttons should not be used to power down machinery. Instead, use a power-off push button for the purposes of turning equipment off over lunch or at night. Power-on buttons are popular in green LED that illuminate when the power is on.

Also recommended is usage of a fault-reset button. While this function may be available on the HMI, operators have a tendency to use more force than necessary when hitting a reset button and tend to hit it many times as well, which can damage the HMI screen. Installing a reset button can save your HMI from such abuse and can be equipped with LED lights to indicate a fault.

While the HMI may be able to technically perform many button functions, it might not be in the HMIs best interest to use it that way. In many cases, use of stack lights, push buttons and indicator lights are better options where someone may not always be within visual distance to read an HMI screen. Other visual and audible tools may allow for a faster response time to any issues and may be read and heard from various locations around the machine. The faster an operator is made aware of an issue, the faster it may be resolved.