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). 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.

Probably measuring the wire resistance and then looking for a change 
of more than a certain percentage, rather than a complete open 
circuit, would be in order.

If you want lightning protection you are going to have to "condition" 
the measuring signal and then couple it to the PIC by a  suitable 
isolator. You can make a custom isolator with a LED and a suitably 
long tube and sensor that will be "lightning rated".

If you expect or can accept that not all wires will be cut at once 
then you can make a very effective circuit by using 2 or more wires in 
a bridge or similar circuit. That way if the state of one wire is 
different from the rest you can trip the alarm. There would be 
extremely few circumstances where something could cut 2 or more wires 
at once in such a way that a bridge circuit would not detect it.

For interest, what is the application? What are you expecting to cut 
the wires?




        RM 

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