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Q&A Circuit for earth conductor presence detection

The section you quoted says: "by injecting a small amount of current into the ground". With a solid ground connection, the voltage on their ground pin shouldn't change no matter what current is in...

posted 4mo ago by Olin Lathrop‭ · edited 4mo ago by Olin Lathrop‭

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Olin Lathrop‭ · 2024-07-19T15:39:14Z (4 months ago)

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The section you quoted says: <i>"by injecting a small amount of current into the ground"</i>.
With a solid ground connection, the voltage on their ground pin shouldn't change no matter what current is injected onto is. If they inject a current and see a resulting voltage change, then they know the ground connection is not solid.
The question then becomes voltage change relative to what? I have no special knowledge about their circuit, but if I was faced with this, I'd probably generated an "average" voltage from the two hot phases. I'd filter that to get rid of the power line frequency and give it time to settle. Now you have an internal voltage that shouldn't be changing relative to the ground connection. You can inject short pulses of a few mA and a few 10s or 100s of µs and verify that you don't get corresponding changes in the difference between your internal reference voltage and the ground connection.
I expect a lot of noise, so I'd probably do these pulses regularly and synchronously measure the voltage difference. I'd then get a filtered value of difference with pulse on and difference with pulse off, and compare the two. If you continually do pulses, like 10 µs on, 10 µs off, then after a few line cycles I'd expect you get a good enough signal.
~~Again, this is all speculation. I don't know how they are actually doing it, and I haven't tried this. There are probably some gotchas you discover as you try to implement something like this.~~

The section you quoted says: <i>"by injecting a small amount of current into the ground"</i>.
With a solid ground connection, the voltage on their ground pin shouldn't change no matter what current is injected onto it. If they inject a current and see a resulting voltage change, then they know the ground connection is not solid.
The question then becomes voltage change relative to what? I have no special knowledge about their circuit, but if I was faced with this, I'd probably generated an "average" voltage from the two hot phases. I'd filter that to get rid of the power line frequency and give it time to settle. Now you have an internal voltage that shouldn't be changing relative to the ground connection. You can inject short pulses of a few mA and a few 10s or 100s of µs and verify that you don't get corresponding changes in the difference between your internal reference voltage and the ground connection.
I expect a lot of noise, so I'd probably do these pulses regularly and synchronously measure the voltage difference. I'd then get a filtered value of difference with pulse on and difference with pulse off, and compare the two. If you continually do pulses, like 10 µs on, 10 µs off, then after a few line cycles I'd expect you get a good enough signal.
Again, this is all speculation. I don't know how they are actually doing it, and I haven't tried this. There are probably some gotchas you discover as you try to implement something like this.
<blockquote>"You can inject short pulses of a few mA and a few 10s or 100s of µs and verify that you don't get corresponding changes in the difference between your internal reference voltage and the ground connection."
From the "recreated neutral" point?</blockquote>
No, specifically NOT that. That generated neutral is a signal that has very limited current capability. And, in the ideal case, it has the same potential as ground, so you can't easily cause it to dump current onto the ground.
Surely there is already rectified power from the two line phases available in the system somewhere. I'd use that to source the few mA to inject onto the ground.

#1: Initial revision by

Olin Lathrop‭ · 2024-07-19T12:26:31Z (4 months ago)

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The section you quoted says: <i>"by injecting a small amount of current into the ground"</i>.

With a solid ground connection, the voltage on their ground pin shouldn't change no matter what current is injected onto is.  If they inject a current and see a resulting voltage change, then they know the ground connection is not solid.

The question then becomes voltage change relative to what?  I have no special knowledge about their circuit, but if I was faced with this, I'd probably generated an "average" voltage from the two hot phases.  I'd filter that to get rid of the power line frequency and give it time to settle.  Now you have an internal voltage that shouldn't be changing relative to the ground connection.  You can inject short pulses of a few mA and a few 10s or 100s of &micro;s and verify that you don't get corresponding changes in the difference between your internal reference voltage and the ground connection.

I expect a lot of noise, so I'd probably do these pulses regularly  and synchronously measure the voltage difference.  I'd then get a filtered value of difference with pulse on and difference with pulse off, and compare the two.  If you continually do pulses, like 10 &micro;s on, 10 &micro;s off, then after a few line cycles I'd expect you get a good enough signal.

Again, this is all speculation.  I don't know how they are actually doing it, and I haven't tried this.  There are probably some gotchas you discover as you try to implement something like this.

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