Protected Extra Low Voltage

Often the 0VDC terminal of the 24VDC power supply is connected to earth. The below diagram shows a circuit with a PELV setup.

However PELV implementations should not be used for devices with CPU loads such as HMIs and PLCs. This is well illustrated in the below diagram, where the connection between the noise earth terminal and the 0VDC line allows electrical energy from devices which are rejecting energy to ground (such as inverters, servos and a myriad of other common devices) to reach the power feed of sensitive devices. 

It is for this reason, that devices with CPUs should be fed from supplies whcih are SELV (separated Extra Low Voltage). SELV does not have an earth connection to the 0VDC wire, as shown below.

Making small mistakes like the above design errors can result in control systems which are subjected to unnecessary amounts of electrical noise, and issues such as communications errors, dropped packets, as well as strange behavior, can occur.