Earthing of Festo components

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We are repeatedly confronted with the question of whether it is necessary to earth Festo components.

Which components must be earthed?

According to IEC/DIN EN 60204-1 (Electrical equipment of machines, General requirements), components which are operated with a voltage of less than 25 V AC or 60 V DC on a PELV power supply unit do not require protective earthing on the components, as no hazard can be generated through electric shocks nor through indirect contact. SELV / PELV (Safety Extra Low Voltage / Protective Extra Low Voltage) Both of these are safe low voltages and have the same voltage levels. The difference lies in the earthing. SELV voltages must not be earthed. PELV may be earthed (typ. ground to earth). For this reason, PELV is always used in mechanical engineering. It is permissible to use an SELV power supply unit and to earth its output voltage (ground to earth); in this case a PELV has automatically been generated. On a PELV power supply unit, the input voltage is galvanic (electrically non-conductive) i.e. reliably separated from the output voltage, and the output voltage is earthed (i.e. either the plus or ground are connected with the earth potential; usually the ground).

Why must they be earthed?

Why do some components which are operated with 24 V have to be earthed? The reason for this is not a protective earthing to prevent electric shocks, but to protect the components against electromagnetic interference(EMC). Another topic often mentioned in connection with this is the connection of our solenoid valves. The coil has 2 pins in order to connect 24 V plus and the ground. However, the plug used has 3 connections for 24 V plus, ground and earth. In this case, does the earth terminal in the plug have to be connected? Yes, it must be connected. Seen from a purely technical point of view, this doesn't make sense, as the valve used must be supplied via PELV (and therefore also without a protective earth conductor) and the earth terminal is not required for protection against electric shocks. The plug itself has a connection for the protective earthing, but this is not required either for this type of valve nor for this plug. However, should the valve have to be replaced once in the machine life cycle, for example with a 24 V valve with protective earth conductor, the protective earth conductor must already be in the plug. For clarity's sake, here is a practical example: does a protective contact socket have to be earthed if we only plug a Euro flat plug into it (a hairdryer, for example)? Yes, because it could also be used with a grouding plug. In practice, earth terminals are often left out. If the valve is operated with a PELV power supply unit, the voltage is already earthed (see above), and the permissible maximum voltage of 25 V AC or 60 V DC is not exceeded, so that no hazards can be generated through electric shocks. However, if we want to be totally accurate, the earth terminal must be connected in the connecting plug. 

Special topic ATEX:

ATEX coils have to be earthed. The reason for this is not EMC or protection against electric shocks, but to prevent sparking. The ATEX standard specifies that all metal parts must be earthed so that they cannot build up electric charges, which could then lead to the hazardous formation of sparks. Through earthing, all metal parts have the same electrical potential so that charging individual components with a voltage which might generate sparks is no longer possible.