How to measure resistance between a point and earth?

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kieennx

Joined: 08 Jul 2005
Posts: 30
Helped: 1

30 Sep 2005 11:50 how to measure resistance of earth

Hi all,
I have a power battery charger and i want to measure the resistance between one of DC outputs and earth point (this point is connected to device's cover too).
Can anybody show me some ways to do that?
Thanks a lot.
Kieennx.

IanP

Joined: 05 Oct 2004
Posts: 6490
Helped: 1542
Location: West Coast

30 Sep 2005 12:29 measure hv with standart voltmeter

One would expect that this resistance will be pretty low ..
There are several methods of measuring this resistance ..
I will just name a couple of them ..
You can use a low-resistance ohm-meter, and there are several brands on the market, which will measure resistances down to 0.01Ω ..
Other option is to build a low voltage current source of, say 1A (see LM317 data sheet, page 14: http://www.angelfire.com/electronic/hayles/downloads/lm317.pdf or anything you think will do the job), and just measure voltage between these two point using standard voltmeter set up to read mV, or better, if you need ..
Regards,
IanP

kieennx

Joined: 08 Jul 2005
Posts: 30
Helped: 1

01 Oct 2005 4:46 how to measure earth ohm

Hi IanP,
thank you very much gor your help. But i mean the resistance between a point of circuit (exactly the DC power output of battery charger) and earth, not GND signal. The DC power output can be up to 230VDC(at)50ADC, so the resistance between that output and cover of device is expected to be so high for safety. If low, it's very dangerous for operator.
I want to measure the resistance by MCU in device and display out for earth fault alarm, so no ohm-meter or volt-meter outside.
Is there any method that i can do with MCU?
Best regards.
Kieennx

IanP

Joined: 05 Oct 2004
Posts: 6490
Helped: 1542
Location: West Coast

01 Oct 2005 5:10 how to measure isolation on earth

In this case you will need to use a resistor of,say 100kΩ, to provide a reference between 0Vdc of the HV output and the housing (I assume that the MCU GND is identical with the potential of the housing).
Using resistors as a voltage divider divide the output voltage of 230Vdc to something that can be measured, say 5Vdc.
If the equipment is safe (full isolation) you will measure high voltage difference between the housing ond output voltage. If the voltage drops that may indicate that you have higher (danger) voltage on the enclosure ..
Regards,
IanP

smxx

Joined: 18 Aug 2005
Posts: 188
Helped: 8
Location: iran(ahwaz)

01 Oct 2005 6:25 measure resistance to earth

hi
i think u want build positive&negetive GND protection.
for this u can connect mid point of output transformer to gorund and check voltage between positive (negative ) and GND.

zenisle

Joined: 11 Sep 2005
Posts: 131
Helped: 2

01 Oct 2005 15:51 simulations measure resistance

do a simulation in hspice or spectre.

kieennx

Joined: 08 Jul 2005
Posts: 30
Helped: 1

03 Oct 2005 3:14 measure neutral point and earthpoint

Thanks all of you.
Actually, i have to design a three phase rectifier producing DC power of 230VDC(at)50ADC, load independent. The DC power output has 2 line: possitive(L+) and negative(L-). GND signal of MCU is connected with negative line (L-), not machine cover. One of device feature is ability of earth fault alarm for both 2 power lines. That means i have to measure resistance between one of two lines and Earth point.
As all of you suggested, i should measure the voltage of one of two lines compare to Earth. Is that right? And how to do it?
Thanks and best regards,
Kieennx.

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smxx

Joined: 18 Aug 2005
Posts: 188
Helped: 8
Location: iran(ahwaz)

03 Oct 2005 5:56 why star point of transformer need earth

ok
connect star point of 3ph transformer (secoundary side) to earth ,ther for (+) have +25vdc and (-) have -25vdc refer to earth point .
if any of this point connected to earth its have 0 vdc.

kieennx

Joined: 08 Jul 2005
Posts: 30
Helped: 1

03 Oct 2005 7:45 Re: How to measure resistance between a point and earth?

Quote:

connect star point of 3ph transformer (secoundary side) to earth ,ther for (+) have +25vdc and (-) have -25vdc refer to earth point .

Thanks but why 25VDC? Maybe you mean value about 115VDC, am i right?
I thought about this method but it's so dangerous because the cover of machine is connected to earth. So anyone that touch the cover will get a power voltage of 115VDC? Should i take a fuse between neutral (star) point and cover of machine (earth)?
I appreciate all advices from you.
Best regards,
Kieennx.

IanP

Joined: 05 Oct 2004
Posts: 6490
Helped: 1542
Location: West Coast

03 Oct 2005 9:15 Re: How to measure resistance between a point and earth?

Attached is a concept of measuring leakage between DC voltage and Earth ..
As I mentioned before, you need to reference 0Vdc to Earth (enclosure) by adding a 100kΩ resistor.
If you measure (A1-A2) and it is very close to (A1-0V)=A1 then there is no leakage.
If voltage (A1-A2) is different from what you measure (A1-0V) then there is a leakage and MCU should shut down power ..
Regards,
IanP

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kieennx

Joined: 08 Jul 2005
Posts: 30
Helped: 1

03 Oct 2005 11:21 How to measure resistance between a point and earth?

Hi IanP,
Thank you. But i have to detect leakage from both (+) and (-) output of retifier. Your suggested method only does the job with positive line, doesn't it?
Best regards,
Kieennx.

IanP

Joined: 05 Oct 2004
Posts: 6490
Helped: 1542
Location: West Coast

03 Oct 2005 12:24 Re: How to measure resistance between a point and earth?

Unfortunatelly on DC systems this is mission impossible ..
It would be a gold mine if someone designed 100% reliable protection system for both rails: positive and negative ..
Regards,
IanP

kieennx

Joined: 08 Jul 2005
Posts: 30
Helped: 1

04 Oct 2005 6:52 How to measure resistance between a point and earth?

Hi IanP,
finally i decide to connect star point of three-phase transformer to enclosure (and earth) through a resistor. Then i measure the voltage of that point compare to GND signal (negative line). Normally i'll get a half of voltage between two power lines. If not so, i can find leakage error in one of 2 line.
Thanks for your help.
Best regards,
Kieennx.