CT open circuit protection

installation
#1

Does anyone know whether the CTs that Sense supplies have some sort of built-in protection from open circuit operation? A CT with no load attached to its output terminals will have an “infinite” voltage generated if the measured conductor has current flowing. Typically this is a shock hazard and will destroy the CT by damaging the conductor insulation. Do the Sense CTs have protection built in against this?

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#2

I don’t think this apples for this situation as its not an actual “Load”
the inductive clamps are outputting. They are simply reading what
is passing by/through them.
If your talking about the monitor being protected, I’m sure like a high
Quality multimeter it has protection for the designed range. The transformer
outside your home would provide overload protection between the lines and
your meter.
Please correct me if I’ve misunderstood.

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#3

I’ll ask :+1:

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#4

Our CTs have an integrated burden resistor. Thus, the only way they would ever have no load is if they were seriously damaged.

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#5

can you explain how the clamps have a load? I’m probably not
understanding but I’m thinking the clamps are connected to Line
and there is not a load powered by the sense. I’m not seeing how
the output side is doing anything more than sending readings.

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#6

Probably a high R resistor in parallel with the Sense (across the two Sense input pins), built in to the CTs.

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#7

Ryan, thanks for getting that answer. Good to know we don’t have to worry about equipment damage or shocks if we unplug the CT lead from the monitor with the CT’s connected to a loaded service entrance.

Sam, without getting too heavy into electromagnetics, you can think of a CT as a device that forces a current on its secondary windings (in the Sense case, the leads that connect to the monitor) in proportion to the current on the primary winding (in the sense case, the current flowing through your service entrance conductors). So, if you have a CT with a 100:1 ratio, 1 amp would be induced in the secondary leads when 100A of current was flowing through the measured mains conductor. The voltage across the secondary leads is then dependent on the impedance of the load connected to the leads. If a 1 ohm resistor were connected across the secondary leads, 1 volt would be developed across the resistor in our case with 1A of secondary current. If a 1k ohm resistor was connected, 1000v would be developed across the resistor. An open circuit is equivalent to an infinite resistance, so in a perfect world, an infinite voltage would be present at a set of open-circuited CT secondary terminals when any primary current is flowing. In the real world, there are various losses and imperfections in real CTs, so the voltage is high but limited. Often, it’s limited by the breakdown voltage of the CT winding insulation, so an open circuit will mean burned out insulation.

Ryan says there’s a burden resistor integrated into the Sense CTs, so even if the leads are disconnected, the resistor is still connected across the CT output terminals, and nothing will spark or smoke.

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#8

Very good explanation, thank you

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