Loop impedance measurement

[Joh-20219182]

Hi, All

I have a megger LTW325 and fluke 1664FC to measure loop impedance by using high current mode. But both results are quite different. megger LTW325 is given a value 0.04ohms and fluke 1664fc is given a value 0.12 ohms. Means megger can give a current at 230V/0.04=5750A but fluke is only 230V/1.12=1916A.

Do you have any advices or expereince on this megger and fluke meter?

 

[Darkwood]

I assume you have zeroed your leads first or accounted for lead resistance when taking the reading?

 

[Vortigern]

Something to discuss with the tech team of the manufacuturers as they would know details like that. They have been most helpful to me in the past when something of that nature has arisen.

 

[pc1966]

Check you are using the probes supplied with the MFTs. The one I have (DiLOG) is "zeroed" at the factory for the supplied lead resistance and there is no provision for nulling that out (I asked them). It only takes 0.08 ohm to explain what you are seeing.

The other obvious things are clean connections on the probes and plugs, and if both meters are in-calibration (and have not be abused too much since they were checked).

 

[nicebutdim]

Check you are using the probes supplied with the MFTs. The one I have (DiLOG) is "zeroed" at the factory for the supplied lead resistance and there is no provision for nulling that out (I asked them). It only takes 0.08 ohm to explain what you are seeing.

You sure about that? My 9110 uses one of the function keys for nulling - seems a bit odd that they'd make a meter on which replacement leads would cause errors. Conveniently, it holds that measurement until batteries are replaced.

Here's Rev2 manual for the 9118, which details lead zeroing around page 16.

 

[pc1966]

I think I found it did not change the Zs value and asked them, but I can check tomorrow by deliberately using higher R leads and nulling them for low-ohm measurement.

 

[nicebutdim]

I believe that once leads are nulled, they're taken into account for any resistance measurement until batteries are replaced. Lead nulling process is separate from any test, but IIRC it has to be carried out on continuity setting.

 

[pc1966]

A test is worth a thousand words!

 

[nicebutdim]

A test is worth a thousand words!

Doesn't make any sense that they'd be factory nulled as this would be a nightmare if leads need to be replaced. Also makes little sense that the meter would hold details for leads, but only apply them to one particular test.

 

[pc1966]

Doesn't make any sense that they'd be factory nulled as this would be a nightmare if leads need to be replaced. Also makes little sense that the meter would hold details for leads, but only apply them to one particular test.

You are 100% right, but that implies a sensible design where the low-R null value is incorporated in to the Zs computation.

The jury is still out...

 

[nicebutdim]

Looking forward to learning the results. Don't have another set of leads here or I'd be trying this now.

 

[pc1966]

I think I have some 4mm multimeter leads that can be stacked, will do for a 'careful' test with the pins covered up.

 

[timhoward]

I don't think anyone has asked if either device has been recently calibrated?

 

[ipf]

I don't think anyone has asked if either device has been recently calibrated?

#4 mentions it.

 

[pc1966]

I couldn't find the leads, and discovered it won't null out the ~1.1 ohms of my 50m extension lead, but I did find that enabling/disabling the lead zero for low-ohms made no observable difference to the L-N PFC measured (was seeing about 0.21 - 0.22 ohms in all cases)