They were only on this run and not any of the lines in the area. And what is the function of the flat u-shaped guards in front of each?

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  • \$\begingroup\$ Being the lightning capital of USA , I suspect it has something to do with current limiting perhaps adding line inductance, in order to limit the half cycle follow current limit for the over voltage absorbers to the right which can bring down the trunk without current limiting or at least slow it down for the breakers upstream. \$\endgroup\$ Commented Dec 21, 2016 at 21:23
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    \$\begingroup\$ hm, might also simply be current clamps for monitoring – the boxes look large enough to house a couple of batteries and a data logger \$\endgroup\$ Commented Dec 21, 2016 at 21:36
  • \$\begingroup\$ While not the answer to this question, there are also devices similar to this that are a lot less expensive. They have a flag that deploys when the current exceeds some maximum value. They require someone to notice the flag, though. \$\endgroup\$
    – Steve
    Commented Dec 22, 2016 at 17:10

1 Answer 1


It's a fault circuit indicator. Specifically, it's the Sentient MM3 model which monitors the current through the conductor it's attached to. It also draws its power from the magnetic field resulting from the current passing through the line. It logs and can wirelessly transmit the data it gathers which helps the local electric utility in quickly pinpointing problems -- ideally in advance of the problem becoming an actual outage. For example, an intermittent fault could indicate a sagging tree branch hitting the line when the wind blows.

The little metal circle thing is an arc shield from the same company. It's intended to save the sensor (which is not inexpensive) from a traveling arc, as from a lightning strike down the line.

And no, I have nothing to do with that company but have some experience in the utility industry.

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    \$\begingroup\$ This is the perfect example of what SE is for. This question is absolutely non-googleable, but someone with the right knowledge can answer it nicely right away. \$\endgroup\$
    – Eugene Sh.
    Commented Dec 21, 2016 at 21:46
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    \$\begingroup\$ good find @Edward , I wonder how many power interruptions they get in Hurricane lightning season with a few hits per sqkm per second. Toronto has a very solid grid. with very few brief interruptions except the year of ice damage in some parts. \$\endgroup\$ Commented Dec 21, 2016 at 21:52

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