>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 13 May 2005 11:21
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE] PIN PROTECTION
>
>
>        BCC ken as relevant to new property.
>
>> I want to test a 2km (1.6mm dia) steel wire that runs around a
>> residential home for continuity.
>There are several wires parralel to each other, but aren't connected 
>together. All the wires are tightened to wooden poles - no 
>isolators. />
>
>
>Quite apart from the protection issue you are liable to have problems 
>with leakage to ground causing apparent continuity even if the wire is 
>broken.
>
>I think the wire resistance will be in the order of 100 ohms (tell us 
>your figures). 

Using a resistivity of mild steel 1E-7 Ohm/m I make it ~170 Ohms.  I'm
not sure what alloy steel fending wire is, but that should be reasonably
close.


If posts are every 3 metres there will be about 700 of 
>them. To produce a resistance to ground equal to the wire resistance 
>each post would need to have a leakage resistance of 100 x 700 = 700K. 
>In fine weather posts will have resistances far far far higher than 
>this. In rainy conditions the resistance to ground will vary markedly 
>depending on pollution, post condition etc.
>

This is going to be a major snag, the resistance of wet wood can be
surprisingly low.  I used to work for a company making railway
signalling products, and the track circuits used to detect occupancy of
a section had to work down to something like 2 Ohms/km IIRC which
represented worst case wet track and ballast.  Addmitedly, the situation
should be somewhat better in the case of fencing wire.

I would say that, as with the above track circuits, using AC signalling
rather than DC would be a far more reliable method of detecting a cut.
You inject a tone (which isn't a 50/60Hz harmonic) at one end, via a
power amp and use a tuned filter and rectifier at the other end.  Just
FYI The track circuits use an FSK signal and the receiver looks at the
modulation rate as well as the levels and frequencies of the two tones
to provide good protection against activating the receiver from EMI.
Adjacent track use differing frequencies to prevent crosstalk.

You also avoid any electrolytic corrosion problems with AC.

Regards

Mike


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