correct way for measuring resistance?

[namdocmaimun]

when i measure phase to earth i get 2kohm. when i swap the red and black leads, it shows 12mohm

 

why is the reading different so much and which is the correct value? thanks

[stud858]

When measuring resistance there should not be any voltage on the circuit.

Switch breaker off and try again.

 

 

 

[maxpower]

Clue - Your test meter leads are using DC and theres stuff on the circuit that passes current in one direction only.

[WaveHunter]

If she slaps your face, you've gone too far.

[stud858]

1 hour ago, namdocmaimun said:

why is the reading different so much and which is the correct value? thanks 

The resistance to earth from a live wire should be open circuit.

 

Only when there is a fault  such as a live wire that touches the earthed metal case will the resistance go low causing the breaker to trip.

 

Possible all sorts of readings depending on what equipment and what fault.

Different readings in different polarity would suggest diodes in the circuit,

 

But I'm guessing as my first post, make sure power is off when checking resistances.

 

 

[namdocmaimun]

thanks maxpower and stud8588

[bankruatsteve]

Measuring resistance of a circuit that has voltage applied is not possible.  The meter will put out a small voltage to make the resistance measurement.  Any other voltage on the wire will either damage the meter or make the reading meaningless.

[Crossy]

Using a regular multimeter to try measure insulation resistance is very prone to error and weird readings particularly if there are other live circuits nearby. Only a dead-short would read correctly (zero).

 

You really need an insulation tester.

 

By the way 12mohm is pretty darn close to a dead short, or did you mean 12Mohm? There's only a factor of 1,000,000,000 between them.

[namdocmaimun]

yes it's Mohm. i turned off both power and disconnect neutral.

 

only the 2kohm light circuit is suspect. the others are grouped together in the 100kohm range

 

[Crossy]

It seems you have some device in circuit which is confusing the meter.

 

You do have an RCD/RCBO/Safe-T-Cut don't you (2k would be 100mA @220V easily enough to operate an earth leakage device). If you do and it's not tripping I wouldn't worry, if you don't you should install one.

 

Do you have those illuminated light switches?

 

If you can beg / borrow / buy an insulation tester you should get more consistent results.

 

[namdocmaimun]

yes i have illuminated light switch.

 

ok thanks

 

[Crossy]

21 minutes ago, namdocmaimun said:

yes i have illuminated light switch.

2

I suspect that's what is causing your asymmetrical readings although there could be something bigger going on with your flickering lights etc (other thread).