A low-cost cure for e-stop flaw

Most emergency stop contact blocks have a potentially disastrous flaw. The normally-closed blocks can become separated from their actuator so that they will not open, no matter how hard or how often the actuator is hit.

This can happen if the block is incorrectly installed, or is not secured correctly following maintenance or a machine upgrade. In some cases, high levels of vibration can cause the block to become dislodged. Damage can also occur during routine maintenance.

Whatever the cause, there is usually no warning of the condition because the block cannot be seen by operators. The first sign may be in an emergency, when the e-stop button fails to work.

To avoid this risk, Rockwell Automation has come up with a simple device which adds a set of self-monitoring, early-make, normally-open contacts wired in series with the standard normally-closed contact. These extra contacts close automatically when the block is correctly installed, but if it becomes separated from the actuator, they interrupt the circuit, and thus alert the operator to the fault.

There is a patent pending on the self-monitoring contact block (SMCB) idea, which came out of Rockwell`s acquisition of the safety specialist EJA. The UK list price of the enhanced contact blocks is £3.80.