Requirements on electrical safety

ID: 184995
Requirements on electrical safety 
03.Mar.09 19:55

Rolf Nickel (D)
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Rolf Nickel

Electrical safety is a precondition for operational safety. In order to reach or to maintain a required safety level we must observe two different safety related aspects: safety against environmental interferences and (personal) safety against electrical shock including preventive measures.

1. Safety against environmental interferences
Environmental means in our case especially the electromagnetic environment. Safety against environmental interferences includes safety against electrical overload by transients on the line ("Low-frequency phenomenons").
Additionally we have to take into account "High-frequency phenomenons", the "Radio Frequency" interferences (RFI) from the AC line which disturbe the reception of radio stations. These interferences can be as strong that a weak RF signal will be "covered". A detailed and, according to my opinion, good explanation of these facts we can find here, it is worth a download or print.
Studying the Evox/Rifa-document we will come to the conclusion that we need the PE connection in order to operate such RFI-filters successfully. The advice to use such a filter today is good advice because the power quality of the AC line decreases more and more due to the fact that the number of interfering equipment like switch mode power supplies or electronic energy saving lamps has increased during the last 20 years.

However, for the use of a RFI-filter it is not essential to connect the radio chassis with PE. This can be concluded from the schematic printed on the Siemens filter.

2. Safety against electrical shock
The danger of an electrical shock comes into existence
1.    if an insulation fault exists between primary circuit side and chassis-ground and, additionally,
2.    if there is the possibility of directly touching live parts, e. g. the chassis or the surface of metal parts which are connected with the chassis (switches or potentiometer shafts/metal knobs).
Normally these two possibilities are not effective at the same time, because our radio chassis is usually intended to be operated inside an insulating enclosure (e. g. wooden casing) and the knobs are made of an insulating material (plastic). But let us have a look into the standard IEC 60950-1 (EN 60950-1) as referenced in the Evox/Rifa-document.

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