Open-load / circuit break detection 2

[benjaminc]

Hi folks. Here’s the problem: I’m working on simplifying the manufacture of some geophysical equipment which involves running an AC signal (typically around 300-500 Hz) through the ground. We typically see around 1A of current in the ground at voltages up to 300V. The circuit is composed of a power supply which generates the signal, two long insulated wires with electrodes at each end, and the ground in between those electrodes, which can be spaced thousands of yards apart.

For safety reasons, we’ve developed a circuit interrupter module which detects (fairly cleverly) when one of the wires has been cut or the circuit has been broken for any other reason, and nearly instantaneously cuts power. A GFCI is obviously useless, since the ground is already part of the circuit, but this device allows us to prevent live-wire accidents due to cut wires. The module works fine. However, it’s a complex proprietary device that we have to manufacture ourselves, and I was wondering if there might be any off-the-shelf products which would do something similar.

Essentially, I’m looking for something which could detect a dramatic fall in the AC current either before (60 Hz) or after (300-500 Hz) the power supply (ideally after, since the power supply will draw some nominal current even with a broken circuit). Is such an open-load or circuit-break detector commercially available? Any suggestions or comments would be much appreciated.

Thanks,
Benjamin

 

[waross]

Check the refrigeration industry. They have some load detectors that work on 60 Hz and may work on 300-500Hz.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

There are a few monitors which switch at some threshold based on the power being delivered to a load, either as a power factor monitor or as a true active power monitor. When a lead breaks you will presumably have capacitive leakage only, or capacitive plus high resistance, so the active power into the ground should drop by a measurable amount. The frequency might be a problem. Check out Siemens, Charter Controls, and Dold as a few manufacturers to get you going. There are a few others too.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[waross]

This is what I am thinking Scotty.

http://www.symcom.com/product.php?pid=777&catid=57

http://www.symcom.com/documentation/spec/pdf/SS_LSRX.pdf

Ten turns through the hole gives you 0.5 amp sensitivity.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

I was intentionally distinguishing between current sensing and power sensing because a long cable can have a significant capacitive leakage, epscially at higher frequency.

I was thinking about the Siemens 3UG4641, but it is listed for 50/60Hz only. The other brands also appear to only have 50 / 60HZ ranges, but maybe worth a call?


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[waross]

Understood. Good point.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Worm shocker?

I saw one of these at work on a fishing expedition once. Two electrodes into the ground about 10 feet apart, HF signal, worms came crawling to the surface.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[benjaminc]

Thanks for the suggestions - I'll look into those options. It's possible that we could use the 50/60Hz option before the power supply; it'll just require more investigation/characterization of the power supply's characteristics.

 

[jghrist]

What happens to the voltage when the circuit is broken? If in increases to more than the normal maximum 300V, it may be better to use an overvoltage relay to trip the output instead of trying to detect a change in the current which is less than 1A and probably significantly less in some situations.