Author Topic: Normal to read 0 ohms between mains earth and self made earth?  (Read 1004 times)

Offline Egg

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I sticked a long nail in my garden just under the window and got the cable up to my room because i need it for tesla coils. I measured the resistance between the mains earth on the plug and the self made one just out of curiousity and it reads a short circuit.
Is this normal? I did not want to use mains earth because i heard sgtc could be dangerous for the electronics that are plugged in to earth in the house will making a new earth made no sense now?

Offline TizianoBll

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Re: Normal to read 0 ohms between mains earth and self made earth?
« Reply #1 on: June 26, 2023, 04:32:10 PM »
Yes, a multimeter uses 1mA DC to measure resistance, it's a bit too low.
A better way is to use higher AC current (not DC because it could cause electrochemical reactions...).
You can use two multimeters, an insulated transformer and some current limiting (or variac). You send at least 50mA and use the second multimeter to measure the voltage drop.

Anyhow, the bigger issue is not low frequency resistance but the inductance! All the wiring in your house may have good ground resistance but will act as an antenna.

Offline Egg

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Re: Normal to read 0 ohms between mains earth and self made earth?
« Reply #2 on: June 26, 2023, 08:55:19 PM »
I see, thanks for the reply. I will probably try something similar to what you suggested.

Offline Mads Barnkob

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Re: Normal to read 0 ohms between mains earth and self made earth?
« Reply #3 on: June 26, 2023, 10:53:07 PM »
Anyhow, the bigger issue is not low frequency resistance but the inductance! All the wiring in your house may have good ground resistance but will act as an antenna.

I wrote about Tesla coil grounding, EMI and common grounding systems in this article: https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/grounding-circuit-protection-and-emi/

There is 6 cases of high frequency current evaluations on regular grounding schemes from industrial settings. This might answer some of your questions :)
https://kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics
https://www.youtube.com/KaizerPowerElectronicsDk60/join - Please consider supporting the forum, websites and youtube channel!

Offline davekni

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Re: Normal to read 0 ohms between mains earth and self made earth?
« Reply #4 on: June 27, 2023, 03:31:19 AM »
Quote
I measured the resistance between the mains earth on the plug and the self made one just out of curiousity and it reads a
Just for curiosity, try measuring resistance using the same meter, but reverse the two meter leads.  Will likely produce a different reading.  Also try measuring DC voltage with your meter.   Chances are that the nail vs. line ground is creating a battery generating some small DC voltage.  Normal meter resistance measurement is by applying a small DC current and measuring voltage.  Battery may be making voltage read negative.  My meters display negative resistance for these situations.  But your meter may read 0 whenever voltage is negative.
David Knierim

Offline Egg

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Re: Normal to read 0 ohms between mains earth and self made earth?
« Reply #5 on: June 28, 2023, 11:29:17 PM »
Quote
I measured the resistance between the mains earth on the plug and the self made one just out of curiousity and it reads a
Just for curiosity, try measuring resistance using the same meter, but reverse the two meter leads.  Will likely produce a different reading.
You were right. I reversed the leads and now it reads an understandable resistance, although the multimeter still cannot really measure it correctly. It reads a way different resistance in different ranges. I should probably try measuring by applying voltage.
I measured the voltage between the two earths and it reads 700mv which i find really cool. I have also noticed some interesting vibration sound coming from the outlet while measuring the resistance.
Thanks for the article link. I find it really helpful.

Offline davekni

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Re: Normal to read 0 ohms between mains earth and self made earth?
« Reply #6 on: June 29, 2023, 03:46:48 AM »
Quote
I should probably try measuring by applying voltage.
Yes, good idea, of course through a current-limiting resistor.  For TC use, AC resistance matters, not DC.  So apply an AC voltage/current.  Although most accurate if AC frequency is closer to TC operating frequency, line frequency is likely to give reasonable results.  Use a line-frequency transformer.  Or, with appropriate shock precautions and no ground-fault interrupter in your line circuit (if available), an incandescent light bulb from line hot to external ground would work.  If GFI is present, it will trip immediately.
David Knierim

Offline Egg

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Re: Normal to read 0 ohms between mains earth and self made earth?
« Reply #7 on: June 29, 2023, 12:08:32 PM »
Yeah the whole house has ground fault protection from the entry of the circuit box. But as you said if i step down the mains than put the both transformer leads between the two earths with a resistor it shouldnt trip .
« Last Edit: June 29, 2023, 12:13:54 PM by Egg »

Offline Egg

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Re: Normal to read 0 ohms between mains earth and self made earth?
« Reply #8 on: July 04, 2023, 03:13:44 PM »
I have tried measuring the resistance today. I applied 26v from a step down transformer through a 10ohm resistor. voltage across the resistor was 0.85ohm. So the resistance between two earths is around 26:(0.85/10)=300~ohm. I am not sure how accurate my meter is on low ac voltages as it fluctuates a lot, however the result seems reasonable.

Offline davekni

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Re: Normal to read 0 ohms between mains earth and self made earth?
« Reply #9 on: July 04, 2023, 10:24:07 PM »
Seems like a reasonable result, but so would a wide range of values.  Depends greatly on nature of soil and moisture content and depth and surface area of ground rod (and perhaps on how well your line power is connected to earth).
David Knierim

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Re: Normal to read 0 ohms between mains earth and self made earth?
« Reply #9 on: July 04, 2023, 10:24:07 PM »

 


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