How to dectect when a (hot) wire has been cut

[edugimeno]

Hi there!
A client needs to find a way to tell when a wire that powers a remote water pump has been cut. There's been some cases of copper robbery in the area.
This wire can have power at any time (380V 3-ph) to start the pump. Or it may be off
The system has to be able to tell when the wire from the frequency regulator to the motor has been cut.
The motor ability to work shouln't be affected by this security system
and the system has to be able to safely work and avoid false positives either when the power os on or off.

I can't think of any solution for this right now, anyone?

Thanks in advance!
Eduardo

[ted]

Apply to one of the phase 380V through resistor ( calculate the value of resistor so curret is abouth 100mA ) and use AC Leakage Current Sensors.

[jackrae]

You'd need a 40watt resistor to give 100ma

Perhaps it's be better to use a capacitor to bypass one of the phase contacts.  The advantage of the capacitor is that no heat is generated.  However, a lengthy line of 3-phase cable will have inter-phase capacitance so the feed capacitor must be at least an order of magnitude greater than this to ensure reliable operation, say 0.22mF or greater.  The capacitor should be a table X type and rated for operation at least 400v

With the current flowing down one phase, this can then be detected as a voltage by measurement on the other two phases, so verifying all three phase lines.

[edugimeno]

The thing is...I need to detect when the cable has been cut, even when the motor is not running.
These are irrigation pump motors, which may be completly off during the whole winter, but even at that time, the cable has to be monitored for theft. And while the motor works it shouldn't "fry" tue monitor system.

These are beefy motors which can get up to 100 KW of power

Thanks

[jackrae]

Both suggestions meet your requirement of detection with the motor not running !

[MarkT]

However, a lengthy line of 3-phase cable will have inter-phase capacitance so the feed capacitor must be at least an order of magnitude greater than this to ensure reliable operation, say 0.22mF or greater.

No no no, 0.22mF is hugely too huge, thats about 12 ohms at 50/60Hz, the pump will have more impedance
than that!
0.47µF is more like it,  5k or so, ie 75mA which is plenty.

The capacitance of the cable is likely to be 100pF/m or so, ie it would take several km of cable to
match 0.47µF

[jackrae]

I used an "m" to signify "micro"  as most of the geeks on here use "m"

My fault for dumbing down the topic

[MarkT]

m stands for milli, always.  Use µ or u for micro.  This is an international standard over 50 years old.
No true geek would get an SI prefix or unit wrong!

[ This seems to be a confusion mainly related to capacitance, people seem to understand
ms v. µs and mm v. µm ]

[edugimeno]

Apply to one of the phase 380V through resistor ( calculate the value of resistor so curret is abouth 100mA ) and use AC Leakage Current Sensors.

Thanks for the suggestion but Im unsure on how this would work. Being a leakage detector, to me it looks like the GF switch that detects difference in current on 2 phases. Is that the case? If so, I can't see how this would work with no power to the motor.
If you could please elaborate on how you'd use this, I would appreciate it
Thanks

[jackrae]

How "remote" is "remote"    100kW is a fair size motor to be running off 380/400 supply with a 'long' cable run.

Why not put the starter box at the pump end, feed permanent power to the starter box, run a signal line to control the starter, then maybe the miscreants will learn not to cut live lines when they try and steal the main cable. 

[edugimeno]

This is not something in my hands. This is intended to be a final product for users to prevent theft of their cables. They sometimes run from the wires from the tool hut to the location of the water pump and this can't be changed.
I've seen runs of several hundered yards
Maybe the largest runs are not the ones in the 100Kw range, the last one I saw with this power was actually running off a local power generator, but others do run remotely, maybe at lower wattages

[INTP]

Just run a parallel trip wire. They won't bother separating wires if you loom or wrap them. They'll cut both.

Keep it simple.

[edugimeno]

Just run a parallel trip wire. They won't bother separating wires if you loom or wrap them. They'll cut both.

Keep it simple.

Like I said before I can't modify their existing cable run or component location, this cable is already running buried, not always inside a pipe, this solution would end up being more expensive in installation than in parts
I need something that can monitor their present wires and sense when they have bee cut, at either end

[INTP]

I'm surprised there are thieves that would bother digging up a buried cable if it is as extensive as you say.

Look into the technology used for cable length meters.

[edugimeno]

Believe me its happening here...High Speed Train cables were stolen once here in Spain. They need to jump over a really high fence, climb to the towers, take to risk to get fried by 25Kv and find a way to roll really heavy wires before the train shows up and runs over them
Let alone getting in a field, cut 3 wires, pulls them up along several hundred yards, and put them into the truck

Thanks, I will take a look at that link