Detect Short from 12V DC System to Mains Earth

[simoon]

Good Afternoon

I have low voltage system where 12VDC and RS485 type data is passed down a two core cable (0V) and (12V+DATA) around a building. These two cables are powered from an isolated source and I need both cables to 'float' with resepect to Mains electrical earth. What I am looking for is a cunning circuit which will detect if either of these two cables are inadvertantly shorted to mains electrical earth. Is there a standard type circuit for this type of requirement ? the only way I can think of doing it would be to create a circuit between one of my conductors and the Electrical Earth but this would be doing precisely what I am trying to avoid !!

 

[IanP]

Total isolation and in the same time dc leakage detection is not possible ..

Unfortunatelly, you need a common reference to monitor any leakages or shortcircuits ..

One method is to introduce a dc offset (say 10Vdc - now your 0V is 10V and 12V becomes 22V) through a high-ohm resistor (something like 10MΩ) and monitor the voltage accross this resistor ..
Any leakage current, even very small, will cause the offset voltage to drop ..

Regards,
IanP

 

[simoon]

Thanks Paul

Excuse me for being a bit dim..any chance you could knock up a quick schematic of what you are suggesting. !!

Thanks