Low resistance measuring - High vs. Low current methods 4

[lz5pl]

I have to buy some micro-ohmmeter for field testing of contact resistance of CB's, disconnector switches, busbar joints, etc.
I have a choice between instruments using high DC current and low DC current. Both types have sufficient resolution and accuracy, that with low DC (actually 2A)can be used also for measuring of transformer windings resistance.
Could you advice me whether there are some advantages/disadvantages of both methods, as well as some specific standard requirements? I work in the IEC world, but suggestions based on ANSI requirements are also welcome.

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[Skogsgurra]

First of all, I would make sure that the instrument has four wire capability.

Then, I would look for an 'anti-thermal EMF' feature. Sometimes, the thermo-EMF influences super low ohms readings and it may be necessary to do two measurements with reversed polarities to compensate for that.

Using high current makes the thermo-EMF problem less pronounced, but the equipment is heavier and I do not see any reason to use high current.

So, low current, four wire and (perhaps) a possibility to compensate for thermal EMF is what I would look for. Plus storage and print-out possibilities.

Gunnar Englund

http://www.gke.org

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[ScottyUK]

I like the DO4 and DO5 from Cropico, both very good instruments from the light-current end of the spectrum. The big mains-powered 200A ductor types from the likes of T&R are more suited to field work in terms of their construction but they are heavy, needs mains power, and aren't as accurate. It's no good being able to resolve 1[μ][Ω] when the uncertainty is [±]5[μ][Ω]!

My big concern with using a high-current test set on a transformer is that it needs to be gently ramped up in current and back down otherwise there can be a huge inductive voltage produced when the test leads are disconnected.

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If we learn from our mistakes I'm getting a great education!

 

[lz5pl]

Skogsgurra, I am also inclined to go to low current measurement, even due to the lightweight instrument (and also good price, instrument is produced here in Bulgaria). Compensation of thermo-EMF problem is provided in the algorithm - measurements are displayed as a mean value of several (30 ?) measurements with changed polarity. Four-wire measurement is mandatory, of course. Another useful option is that measurements are independent of power source stability - every field guy knows what happens with voltage when welders are working nearby.
My only concern was if any standard requires using of high currents, as most of traditional instruments on the market are that type. But I suppose it is mostly because technology that allow to produce precise low current microohmeters have not been available before.

ScottyUK before to find that low current instrument I also considered T&R instrument. I am quite happy from their primary injection test sets. But to add one more heavy box for transportation ...




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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[Skogsgurra]

Sounds like you know what to look for already. I am not involved in large transformers and such equipment, so I responded from a kW level, so to speak.

I have seen 'low ohm' tests carried out with ordinary ohm-meters with tens or hundreds of milliohms in test leads.

One recent case was the mapping of bonding resistance over a 100x200 m2 area. Two wire ohm-meter was used and resistance measured never below .4 ohms. It happened in an industrialized country...

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[lz5pl]

One recent case was the mapping of bonding resistance over a 100x200 m2 area. Two wire ohm-meter was used and resistance measured never below .4 ohms. It happened in an industrialized country...
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------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[ScottyUK]

I'd go for the lab-type instrument for the majority of instances, and if you ever have to start measuring down in the 10's of n[Ω] then hire the equipment for the job, or keep an eye on the surplus market and buy it at a fraction of list price.

I had an HP6681A supply with a precision shunt and a couple of good bench multimeters - a Keithley 2000 and an Agilent 34401A - and managed to get results better than 1% accuracy at 10n[Ω]. The thermal emf problems Skogs mentioned are very evident when the volt-drop you're measuring is a few microvolts, and it helps if the meter has an averaging function.


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If we learn from our mistakes I'm getting a great education!

 

[Zogzog]

ANSI/NETA specs require a minimum of 10A output for some tests, some standards are starting to require 100A and will be more common in the future. I have always been happy with our AEMC microhmeters.

 

[electricpete]

Great discussion above.

To summarize what I heard (and agree with):
[*]4-lead measurement – most important
[*]Thermal EMF's tend to become important at low measurement levels.
[*]Manual or built-in-auto reversing of polarity compensates for most interfering noises such as thermal EMF.
[*]High current requires slow rampdown to avoid voltage transient... also sometimes slow rampup due to test equipment limitations.

I would add one item:
High current creates higher voltage drop and tends to create higher signal to noise ratio against for all possible sources of noise. Auto reversing of measurement polarity may not provide perfect cancelation and output stability depending on variability and frequency of the interfering noise. This becomes a more important consideration when interfering quantities become very large compared to measured quantities as might be the case in field measurement of low resistance using low current. I think some of the low current devices will be more sophisticated than others in terms of method used for identifying and cancelling interference (megger mentions a "point / slope method").

Now a question. I have noticed as zogzog said there is a tendency to specify/require the higher current measurement such as 10A or 100A when referrring to contact resistance measurements. I don't know why that is (any suggestions?). I don't think there are typically any non-linear effects even at contact surfaces (do others agree?). I think maybe it is just that contact resistances tend to be very low when measuring directly accross contact (compared to for example winding resistances) and the specifier is making some assumptions about the ability of test equipment to provide required accuracy/stability/repeatability in presence of noisy environment. (?).

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[electricpete]

Correction
[*]High current requires slow rampdown to avoid voltage transient... also sometimes slow rampup due to test equipment limitations.
Should've been:
[*]High current for inductive winding resistance measurements requires slow rampdown to avoid voltage transient... also sometimes slow rampup due to test equipment limitations.

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[Zogzog]

EP, I agree with your points but the OP listed, CB's, switches and bus connections as what he needs to test, so I would recommmend at least a 10A microhmeter.

 

[electricpete]

Yes, I agree with you zogzog. Using 10A or more seems to be standard practice for contacts from what I have seen. (It's what we do at our plant. It's what Gill recommends in his book. You mentioned the NETA standard)

I am still curious why.

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[ScottyUK]

I suspect the reason is that the manufacturers of equipment supplied into the electrical maintenance industry have a fairly strong voice on the standards committees, so they write the standards to favour what they produce. It's easier to use a higher current with a relatively cheap voltage measuring stage than to use a lower current with an expensive and complex precision voltage measuring stage. I suspect the high voltage / high current types are more tolerant of imperfect measuring techniques too.

The only time when higher voltage and higher current is definitely a necessity is when semi-conductive materials like carbon are in the circuit because they exhibit very non-linear behaviour at low voltages.


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If we learn from our mistakes I'm getting a great education!

 

[rhatcher]

We have successfuly used 10A DLRO's (digital low resistance ohm meters) for many years to test circuit breaker/contactor/switch contact resistance, bus bar joint contact resistance, ground circuit joint resistance, and transformer/stator/field coil/armature resistances. The resolution and accuracy of these instruments is in the range of a few micro-ohms for a 'field' type instrument like the OP is looking for. These units are also relatively inexpensive, very portable, and very durable. A 'lab' type instrument can read in the nano-ohm range but they are more expensive, less portable, and more delicate to the rough handling seen in field service. Also, for the type of testing that the OP is looking for, nano-ohm readings are not necessary.

I have not seen a specification requiring a 100A low resistance tester in many years. My idea is that this requirement was based on the fact that in the past the measuring part of the instrument (including the dial type meter) did not have good accuracy at low voltage and that the high current was required to produce the higher voltage necessary for a good reading.

 

[gattie]

I would use the old fashioned Ductor type tester.
I employed it many times very successfully to measure transformer winding resistances & busbar connections.
The make was Megger.

 

[ScottyUK]

Really it depends on the size of breaker and busbar doesn't it?

We can all make guesses, but an instrument which is perfectly adequate for testing distribution bus joints and breakers might be useless on a generator circuit breaker or main bus. For example, the joints at an 18kA generator main bus wouldn't even register on a Ductor, and if it did then the reading would be meaningless. I've tried; why do you think we had the huge 500A power supply and lab meters?


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If we learn from our mistakes I'm getting a great education!

 

[rhatcher]

Good point ScottyUK. I am thinking from the OP's post that he is measuring smaller stuff but who knows for sure. In any case, you are right. If he doesn't understand what he is testing versus the limitations of his equipment then he will be in trouble.

 

[electricpete]

One more thought, in the smaller equipment world (think card connectors, ic's etc), the operating currents are small, the conductors are small, their heat capacity is small, their area for heat dissipation is small, and self-heating effects would be monstrous if you tried to inject something like 10A.

So for those applications there is need for instruments that work on very low measurement current. But that reduces the magnitude of measured voltage which requires more sensitive and precise voltage measurements which drives cost and complexity (and imo noise susceptibility) up.

In the power equipment world, we have large conductors with higher heat capacities and more surface area for heat dissipation and self heating would usually not be a concern at levels like 10A. I think that is why we can often get a cheaper alternative that accomplishes what we want using higher signal levels and less sensitive measurement electronics.

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[lz5pl]

Very interesting and very helpful for me discussion . Thank you all !
Please find attached catalogue sheet for the instrument I have in mind. Texts are in Bulgarian language, but data in the table are clear enough (I have translated texts there). I am considering type M2420A, another type has same ranges, but with higher accuracy, which is not necessary for me.

According to me this instrument should be OK for my needs. My only concern was about standard requirements for measurement current. As I understand from the discussion, most probably there is no such standard defined, so 2 A current is looking OK.

By the way, I asked a colleague from independent calibration laboratory for his experience with the instrument. He stated that they have calibrated several of these in last years without any problems in the accuracy and stability. So I am going to invest in one. But your opinions will be highly appreciated too.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[edison123]

Very good discussion. I am myself in the process of procuring a DLRO from an Indian manufacturer. It costs, including the software, about 2200 USD.

linky

http://www.motwane.com/MWRM4.htm

What do you guys think ?

Muthu

http://www.edison.co.in