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This is about use of earth screw terminals which are often provided in power supplies.

Below is an Embedded Switch Mode Power Supply with a metallic case:

enter image description here

Here I can understand how the earth wiring is useful in case of line is touching to the case. This would save someone being exposed to line voltage.

On the other hand below is a DIN Rail Power Supply with a plastic case:

enter image description here

It also has an earth terminal. In this case how is the earth wiring useful since the case is already insulator itself (plastic)?

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  • \$\begingroup\$ Prevent it from floating, filter interference into it, requirements by standards, for that specific product only the designer knows exactly why. \$\endgroup\$
    – PlasmaHH
    Oct 12, 2017 at 9:50
  • \$\begingroup\$ I see so it could be something else than safety \$\endgroup\$
    – user16307
    Oct 12, 2017 at 9:51
  • \$\begingroup\$ provide a connection to deal with more than one cable. \$\endgroup\$
    – Solar Mike
    Oct 12, 2017 at 9:53

2 Answers 2

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EMC is the first thought - earthing certain line filters inside the plastic case may be a requirement for correctly using this power supply.

However, the earth symbol has a circle around it so it means "protective earth" (or PE) i.e. it is used for reasons of safety and the circled earth symbol is usually implying that the unit is built and CE marked so, it's mandatory to provide a PE wire.

enter image description here

So, that's its main function but, because it is always required to be connected to a PE, the PE may also be used for EMC reasons and the CE marking/EU directive applied might require it for these secondary reasons.

The unit appears to have CE mark so check its documentation to see what that says about the PE. As for what it protects, it's possible that the step-down transformer relies on a PE to protect the output circuits from rising to unsafe levels. Maybe the output is actually connected to PE?

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  • \$\begingroup\$ Data sheet specifically warns "Do not use the power supply without proper grounding (Protective Earth). Use the terminal on the input block for earth connection." \$\endgroup\$
    – Trevor_G
    Oct 12, 2017 at 12:42
  • \$\begingroup\$ That is not exactly a fully insulated case either. It looks like it would be rather too easy to poke the low voltage wires between the slats in the top while fishing cables around. some of their imaging also shows some significant heat-sinks in there which would need to be grounded if the case were deemed to be insufficient. \$\endgroup\$
    – Trevor_G
    Oct 12, 2017 at 13:01
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enter image description here

Figure 1. Functional diagram showing internal earth connection to input filter. Source: PIC 120E.242C page 9.

enter image description here

Figure 2. A typical mains filter. Source TDK-Lambda FAQ.

The earth link of Figure 1 is probably very similar to that required in Figure 2.

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