(at least, in our panels)
The usual solution for earthing in a control panel is an earth block. Less commonly there may be an earth stud to use, but this can be a nuisance if you have more than one earth (for example, some bonding), because no more than one connection should be made to one earth stud. But there's a better way...
Earth terminals have a metal clip onto the din rail, which is to connect the inner conductor to the din rail itself. This isn't just an 'extra' earthing point though - this can be used as a proper earth connection.
The actual current rating can vary, but for the Wago terminals we use on 35mm din rail it's 70A. That's often enough to provide a valid earth connection for the entire control panel. So the earthing in the panel can look like this:
there's a terminal next to the isolator to terminate the earth with the incoming supply
all of the external equipment has its own earth terminal
the panel door is bonded to the mounting plate
Job done.
Another advantage of this is you're left with plenty of spare earth connections. Each terminal has at least two entry points, if one is being used for the field equipment then the other one is available for something else. These can be useful for things like connecting the 0V on the controls circuit to earth.
Remember to calculate the maximum potential current that the din rail may have to handle in total. There's a couple of things to look out for beyond just the maximum current though.
Structure: consider the earthing hierarchy, for example ground on signal cables should have a common earth point before they're joined onto the mains earth.
Resistance: you must check the resistance on the earth path! As always this is a crucial part of the safety of the system.
Labelling: all grounding points, regardless of the method used, must be clearly labelled for easy identification and maintenance.
Professional guidance: due to the complexities involved, consulting with a qualified electrician is crucial, especially for critical applications. Always prioritize safety and adhere to electrical safety regulations.
This blog post provides a general overview and does not replace professional advice. Consulting a qualified electrician is essential for ensuring proper earthing practices.