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Detect Short from 12V DC System to Mains Earth
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simoon



Joined: 10 Nov 2007
Posts: 3
Post10 Nov 2007 20:06   

Detect Short from 12V DC System to Mains Earth
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 !!
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IanP



Joined: 05 Oct 2004
Posts: 6492
Helped: 1542
Location: West Coast
Post11 Nov 2007 3:19   

Re: Detect Short from 12V DC System to Mains Earth
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
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simoon



Joined: 10 Nov 2007
Posts: 3
Post16 Nov 2007 21:00   

Re: Detect Short from 12V DC System to Mains Earth
Thanks Paul

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

Thanks
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