Fault finding with a multimeter

[Tom011]

Hi, I’m just wondering what sort of readings I can expect for faults on a multimeter? I’ve only ever used a multifunction tester. What readings can I expect for a short cct fault or a polarity fault? I know that open cct will be 0L/1, cheers

 

[DPG]

Hi, I’m just wondering what sort of readings I can expect for faults on a multimeter? I’ve only ever used a multifunction tester. What readings can I expect for a short cct fault or a polarity fault? I know that open cct will be 0L/1, cheers

Short circuits will typically be fractions of an Ohm, but it depends how far along the circuit the short is.

Remember a multimeter is high impedance, and will not subject the circuit under test to a load (several mA) like an MFT resistance test will. Try measuring with your probes held together, then try with a few metres of cable between them. Remember that contact resistance between probes and the cable will have an effect.

Polarity can be determined by either continuity measurements or measuring using the VAC range.

Ensure your multimeter is decent quality and has a suitable CAT rating for mains voltage measurements.

And yes, open circuits will be infinite resistance - different meters have different methods of showing this.

 

[Tom011]

Short circuits will typically be fractions of an Ohm, but it depends how far along the circuit the short is.

Remember a multimeter is high impedance, and will not subject the circuit under test to a load (several mA) like an MFT resistance test will. Try measuring with your probes held together, then try with a few metres of cable between them. Remember that contact resistance between probes and the cable will have an effect.

Polarity can be determined by either continuity measurements or measuring using the VAC range.

Ensure your multimeter is decent quality and has a suitable CAT rating for mains voltage measurements.

And yes, open circuits will be infinite resistance - different meters have different methods of showing this.

Thank you

 

[Lucien Nunes]

Multimeters tell it like it is... the actual readings of resistance shouldn't be any different to those you would get with an MFT. Obviously the megohm ranges can read resistances in the same order as the lower end of an insulation tester scale but do not subject the insulation to a high test voltage.

What might differ are readings of AC ghost voltages on cables that are not actually energised but are picking up a whiff of capacitive leakage from live cables nearby. Ghost voltage readings are not useful or informative, but can be surprising, e.g. if you find 100V on a cable that's known to be disconnected. Because the multimeter input circuit tends to have a high input resistance, they can read voltages (which are correct) in situations where the lower input resistance of an electrical tester would bleed off the leakage and not register very much. Both instruments are correct, but the multimeter reveals the ghost voltage better.

I would mainly use the multimeter to trace faults, other than to test insulation which needs the MFT.

 

[DPG]

Multimeters tell it like it is... the actual readings of resistance shouldn't be any different to those you would get with an MFT. Obviously the megohm ranges can read resistances in the same order as the lower end of an insulation tester scale but do not subject the insulation to a high test voltage.

What might differ are readings of AC ghost voltages on cables that are not actually energised but are picking up a whiff of capacitive leakage from live cables nearby. Ghost voltage readings are not useful or informative, but can be surprising, e.g. if you find 100V on a cable that's known to be disconnected. Because the multimeter input circuit tends to have a high input resistance, they can read voltages (which are correct) in situations where the lower input resistance of an electrical tester would bleed off the leakage and not register very much. Both instruments are correct, but the multimeter reveals the ghost voltage better.

I would mainly use the multimeter to trace faults, other than to test insulation which needs the MFT.

Thanks Lucien for correcting my error. Of course I should have said that it is voltage readings that will be different with a high impedance meter.

 

[Lucien Nunes]

Ah, hadn't noticed that, was only replying to the OP. I won't make a charge on this occasion.