Robin loop tester

[Jurassicspark]

I got my hands on an old robin (now fluke) KMP4120DL loop tester. I got it calibrated, and it's pretty good and basic. I'm going to hand it over to my apprentice to use, as I can't trust him with my megger. (He's already lost my drill.)


There are three options to select from: 20, 200, and 2000. Surprisingly, when I select 20 and 200, it gives an over limit symbol however, on the last option (2000) it displays 1Ω. Circuit I'm testing is a 32a ring.
Does anyone know what's happening here? (manual avaliable for this item online)

Let me know your opinions,
Cheers.

 

[Deleted member 267285]

I’ve been looked at getting one, so will be interesting to hear professional views.

 

[JohnW2]

I got my hands on an old robin (now fluke) KMP4120DL loop tester. ... There are three options to select from: 20, 200, and 2000.

What are those three options meant to represent? All three are no large to be useful ranges in ohms for loop impedance testing.

Does anyone know what's happening here? (manual avaliable for this item online)

... in which case you have presumably looked at the manual, and therefore ought have have discovered the answer?

Kind Regards, John

 

[plugwash]

What are those three options meant to represent?

Afaict the normal convention on meters is that the name of the range is one count higher than the max reading of the range.

All three are no large to be useful ranges in ohms for loop impedance testing.

Are they?

Looking at the manual ( https://docs.rs-online.com/bb02/0900766b80052a0b.pdf ) it appears to be a 3 and a half digit meter. So a "20 ohm" range would give a resolution of 0.01 ohm. That seems fine for testing domestic/small commerical TN supplies.

The 200 ohm and 2000 ohm ranges are presumably intended for testing TT supplies.

My first thought is it seems the meter is faulty, which leaves the question of how it passed calibration. Were the same leads used? did the person doing the calibration actually do it properly?

 

[JohnW2]

Afaict the normal convention on meters is that the name of the range is one count higher than the max reading of the range.

Indeed so.

Are they? Looking at the manual ( https://docs.rs-online.com/bb02/0900766b80052a0b.pdf ) it appears to be a 3 and a half digit meter. So a "20 ohm" range would give a resolution of 0.01 ohm. That seems fine for testing domestic/small commerical TN supplies.

Yes, that's true. Maybe I wrote in haste In fact, now having looked at the manual, it does indeed say that the ranges are 0 - 19.99Ω, 0 -199.9Ω and 0 - 1999Ω.

The 200 ohm and 2000 ohm ranges are presumably intended for testing TT supplies.

Again, that's true - although the 200Ω range is, in practice, pretty redundant, since once the loop impedance is above 20Ω, a precision of 'to the nearest ohm (as provided by the "2000" range) is really adequate

My first thought is it seems the meter is faulty, which leaves the question of how it passed calibration. Were the same leads used? did the person doing the calibration actually do it properly?

Indeed. If it really is indicating 'over-range' on the 20Ω and 200Ω ranges, despite displaying only 1Ω on the 2000Ω range, then, as you say, it presumably must be faulty - in which case, as you also say, it's hard to see how it got through calibration. However, I see that the manual says this ...

... it says "usually", so I wonder if there are some situations other than over-range impedances which will result it it display that symbol. Might it, for example, perhaps be an olden-day equivalent of the wretched "Err 5" ("Excessive Noise") so often displayed by some more recent Fluke meters when measuring loop impedance?

Kind Regards, John

 

[Jurassicspark]

yeah thought so, meter most likely giving incorrect readings.

 

[JohnW2]

yeah thought so, meter most likely giving incorrect readings.

Sure, if it's giving 'internally inconsistent' readings on the three ranges (with a loop impedance within the range of all three), then something must be wrong with it ...

... if the impedances being measured is around 1Ω, then it ought to display that result (with greater precision) with the "20" and "200" ranges, as well as on the "2000" range.

Kind Regards, John