earth impedance

[bernardgreen]

It is annoying when training is dumbed down to the minimum necessary to get the "qualification" and the new qualified person then has to learn how the real world works. Annoying for the newly trained and annoying for those who have to provide real world education.

 

[slippyr4]

by what measure is your first post on this thread "real world"?

How often in the real world does an electrician have to consider the possibility that a circuit might have at one point in time had some DC applied to it which has caused a "resistive" metal oxide film which is semiconducting and screwing his attempts to measure impedance with a resistance meter?

Never, is how often. It's batsh*t insane is what it is.

 

[bernardgreen]

Never, is how often. It's batsh*t insane is what it is.

Never ? If you believe that then you never worked at a hilltop site ( radio communications equipment ).

Did the OP say he ( or she ) was studying for domestic electrical installation qualification ?

 

[slippyr4]

He didn't say that, you're right. But, given what this forum is about, it's a very reasonable assumption to make.

Your input on this thread has taken the usual pattern - add wrong or irrelevant information then google like mad to find the most tenuous justification for the tripe you write.

 

[JohnD]

The test paper is aimed at people training at quite a basic level.

Which week do you learn the difference between DC resistance and AC impedance?

Hilltop off-grid transmitter sites are not relevant to this test.

 

[securespark]

And I would say to you, "Know when to start."

Leave Bernie alone: he's an institution.

 

[slippyr4]

And I would say to you, "Know when to start."

Indeed. Never argue with idiots.

But I find it hard not to react* when a post is so obviously completely wrong. If the OP put down Bernie's answer in an exam, that question would be scored wrong. In an exam, you don't have the luxury of spending the rest of the day googling arcane literature to attempt to justify your answer.

* i'm quite pleased with that pun

 

[Spark123]

inductance.

Reactance, both capacitive and inductive.

 

[JohnW2]

I'll give the OP a clue to jog his memory .... DC impedance is the same as resistance, but AC impedance is resistance and ???

I don't think that clue will help him much - at 50Hz, there is not going to be enough reactance around for resistance and impedance to be noticeably different (when measured with AC). As Bernard has said, it's the electrolytic effects of DC which can result in resistance increasing, even if (as will be the case) there is near-zero reactance involved.

Kind Regards, John

 

[securespark]

Glad we have sorted that one out.

 

[SparkyTris]

Thanks for some useless responses.
I am well aware of the theory of how an earth impedance might differ from 0hz to 50hz and onwards down the wavelengths. I was simply interested to hear of any real-world anecdotal evidence of such.

 

[SparkyTris]

regarding the DC effects; is it possible for a chronic dc bias to appear across a mains supply? if so, by what mechanism(s)?

 

[JohnW2]

regarding the DC effects; is it possible for a chronic dc bias to appear across a mains supply? if so, by what mechanism(s)?

I haven't thought too deeply, but my first reaction is that that could probably only happen if something pretty dodgy were connected to the supply.

Kind Regards, John

 

[bernardgreen]

if something pretty dodgy were connected to the supply.

Power control by Asymetrical Phase Angle Control comes to mind. Ancient technology
Positive half cycles go through un cut, Negative half cycles are cut by phase angle control. Creates a DC component in the otherwise AC circuit. Possibly used on some pre-emptive controllers charging night storage heaters. In those days one thyristor and one diode was a lot cheaper than two thyristors. This could create significant DC along the supply.

Some old valve radios used half wave rectified mains for the HT supply to the valves. A few milliamps at most.

 

[SparkyTris]

Actually the comments regarding the "copper oxide layer" were something that did not initially cross my mind; so thanks, Bernard, for actually the only practical response so far on this thread.
what I was thinking of here was practical cases when it would prove unsafely misleading to use a dc resistance meter.
developing that case, where the earth conductor is resistive during half of the 50hz cycle, ie a diode, what would its impedance appear as?