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Topic Title: Recommendations for a new Earth fault loop impedance tester
Topic Summary: Accuracy of ELFI testers
Created On: 31 August 2012 03:26 PM
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 31 August 2012 03:26 PM
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Parsley

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Hi

Recommendations please for a new ELFI/PSCC tester that can test accurately at <0.2 ohms.

Fluke recommend their 1654 as it has 3 decimal points but I'm sceptical.

Thanks
 31 August 2012 03:48 PM
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perspicacious

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Have a read of Geoff B's reply and search for the other topics where Geoff has contributed on this topic

Regards

BOD


http://www.theiet.org/forums/f...y%20near%20transformer
 31 August 2012 03:54 PM
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perspicacious

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This is the old topic I was looking for

Regards

BOD

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 31 August 2012 04:35 PM
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Parsley

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Thanks Bod

Why would 3 points make the reading more accurate? The fluke tech dept said the tester was able to go below 0.2 accurately, but like the other posts state whats the point?

I'm aware of the issues when being close to a TX with large supply conductors.

I got a test sheet back from one of my guys this week for a 5th floor flat in central London (400amp incoming supply to cutout in switchroom and 25mm 3C SWA submain via ryefield DB). The Zdb recorded was 0.04 ohms and pscc 4KA, which concerned me with T&E wiring.
I checked the the PD's manufacturers data and the final circuits conductors K2S2 was > A2S.
I estimated a realistic measurement would have been around zdb of 0.15-0.20 ohms and pscc of around1.5-1.2KA.

Regards
 31 August 2012 06:05 PM
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John Peckham

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No way 0.04ohms on the end of the 25mm SWA on the 5th floor with the head in the basement.

These wizo meters may give a resolution to 3 decimal places but resolution is not accuracy. Getting an accurate reading to 2 decimal places (1/100 ohm) is a big ask so getting a reading with a resolution of 1/1000 ohm to be accurate is pie in the sky.

I would say my old Robin 4120DLs give the most consistent readings and send a decent shot (25A) around the loop. My calibration man Mike Olding of On site Calibration Services rates the Robin 4120DL to be the best loop tester and he should know as he calibrates all types in the thousands each year.

I love my Megger LTWs as they do both a high and low current 2 wire loop test. As a back up I have my LRCD220 RCD tester and loop tester combined in my test kit bag. Good loop tester does a 2 wire high current test but not a 2 wire low current test.

I also have a Megger LT300 high current tester that will do a wide range of frequencies up to 400Hz that I have for testing some kit that went in to a container that was going to somewhere hot and dusty.

If I could only have one lop tester it would be an LTW. If I am getting daft readings on site or very low impedances then I break out the old Robin.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 31 August 2012 06:58 PM
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dickllewellyn

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I use an mft1730 day to day, but I regularly use an old socket and see loop tester (can't remember model number) which was cheap as chips, and really reliable. I believe the early socket and see testers were made by an ex robin staff, so that could well be the reason!

-------------------------
Regards
Richard (Dick)

"Insert words of wisdom and/or witty pun here"
 31 August 2012 08:52 PM
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Parsley

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Thanks guys

John,I agree 0.04 is unrealistic. Do you believe your old Robin is accurate below below 0.2?

Regards
 31 August 2012 10:07 PM
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dbullard

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Originally posted by: John Peckham

I love my Megger LTWs as they do both a high and low current 2 wire loop test. As a back up I have my LRCD220 RCD tester and loop tester combined in my test kit bag. Good loop tester does a 2 wire high current test but not a 2 wire low current test.



If I could only have one lop tester it would be an LTW. If I am getting daft readings on site or very low impedances then I break out the old Robin.




Hi John,

Interesting, I have a mft 1720, LT7, and the afore mentioned Robin and a LTRCD210, If I get any odd or daft readings I tend to dig out my LT7 as this old Loop tester has always given me the most consitent readings of all, I have allways had seperates and dont fully trust the readings that the MFT chucks up from time time.

Regards

Daren

-------------------------
..... Dont pee in my pocket and tell me its raining ......


www.quest-electrical-sw.co.uk
 02 September 2012 04:43 PM
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sparkyaj

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I Have a new Megger meter which measures much closer, but will have to look it up for model number. Purchased it last year after the multi finction testers which Megger recomended where MILES out! When I challenged the readings there meters could not read low enough to provide a good reading when testing near TX's,

Think its a Megger LTW425 - measures down to 0.001 :-)
 02 September 2012 05:19 PM
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alancapon

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Originally posted by: sparkyaj
. . . there meters could not read low enough to provide a good reading when testing near TX's,

Think its a Megger LTW425 - measures down to 0.001

As has already been mentioned, a resolution of 0.001, but I doubt it will be an accuracy of 0.001. Close to a transformer, the reactive impedance will dominate the resistance. As far as I know, all loop testers give a result which is resistance not impedance, and therefore no good in this scenario.

Regards,

Alan.
 02 September 2012 05:42 PM
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sparkyaj

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The only way you get a true reading will be to obtain all the winding details of the TX and loss's then calculate the output and compare it from your calc to your meter reading. I have to say this meter is about as close as you will get. No meter will be 100% due to the amount of variations on site. I will dig out the spec sheet in the week and post it back here.

This meter blast both the Flue and Megger multifunction testers out of the water for accuracy mind.... I have had some detailed discussions with the reps about this over the past year or so. Hope that helps a bit.
 02 September 2012 08:31 PM
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perspicacious

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"I will dig out the spec sheet in the week and post it back here."

How about posting the figures off your most recent calibration certificate?

Regards

BOD
 02 September 2012 08:34 PM
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John Peckham

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The LTW425 has an accuracy of +-5% +- 0.01Ohms in the range of 0.3 to 1000 ohms.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 02 September 2012 10:44 PM
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sparkyaj

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John - Hope this helps. Kindly provided by Megger to me last year.

Loop testing close to the source of supply (Transformers)

End users often complain of problems experienced when measuring loop impedance close to the source of supply, these recurring problems appear irrespective of the make of instrument used. These typically include instruments that are unable to take measurements due to the high noise environment, instruments that are unable to provide a high PSC value or give erratic readings.

Many of these issues have been largely over come with the LTW425 loop tester which gives resolution to 0.001Ω, however users' need to understand the application, limitations and issues when testing close to the source of supply. To illustrate this, the following example considers transformer characteristics and then compares and contrasts the calculated fault current and measured fault current for a 315 kVA three phase transformer. After which sources of inaccuracies and errors will be considered followed by health and safety issues.

Transformer characteristics

Transformer windings are highly inductive, looking to a three phase 315 kVA transformer the typical specification for reactance per phase is 0.0268 Ω, as opposed to resistance per phase of only 0.00901 Ω. (Ref. Table 16a, Cook, 2002, p.328).

For the transformer above the impedance can be calculated at 0.0282 ohms.

This aspect is significant in comparing calculated as opposed to measured values, as "Cook, (2002)", demonstrates the significance of reactance in the impedance value as opposed to resistive elements.

Accessing the specification for the transformer (on the manufacturers name plate), we can calculate fault current at the transformer as shown in the example below, prior to comparison to the measured value.


Transformer (name plate) loop impedance calculated value

From a transformer in the field, using name plate values the following calculations can be made. Transformer type, 3 phase 11kV Primary/ 415V secondary (315 kVA rating)
Percentage impedance voltage (from name plate) Vz = 4.8 %

Vz is the percentage impedance voltage - the voltage applied to the primary which results in full load current when the secondary is short circuited.

Full load current, IFL = 315000 VA / (3 x 240 V) = 438 A per phase

Where loop impedance = (240V x (4.8/100))/IFL = 0.026 Ω

Isc = supply voltage/ loop impedance

= (240 V x 100 x IFL) / (240 x 4.8)

Therefore: Isc= 438 x 100/4.8 = 9.11 kA

This calculated value can be compared to the measured value below for a 315kVA transformer, with the measurement taken two metres from the transformer through the protective devices.

Transformer measured value

Isc = Working voltage/measured impedance
Measured loop impedance = 0.015 Ω
Isc = 240V/ 0.015 = 16 kA

We can see that the measured fault current value is substantially higher than calculated. This is due to the impedance value being measured is actually based largely upon the resistive element only, which is therefore lower than the calculated value, leading to a higher Isc reading.

The resistive measurement is higher than that suggested by "(Cook, 2002)", however do bear in mind the measured value in the field includes the resistance of 2 metres of cable, switchgear and probe contact resistance.

Conclusions

When testing close to the source of supply such as a distribution transformer the overall impedance is largely driven by the reactance and not the resistive elements. As one moves away from the close proximity to the distribution transformer the impedance becomes largely resistive, significantly reducing the proportion of reactance in the measurement, hence in turn reducing the error. Any measurements made close to the distribution transformer are largely inductive, therefore resulting in a low resistance reading and higher short circuit current.
A practical example is to conduct a loop test a substantial known distance from the transformer, typically >90 metres, then deduct the R1 + R2 of the known distance to give the required Ze value. Simply an Ohm's Law calculation will then give a vastly more accurate PFC value. Obviously parallel paths may affect readings.
Therefore measurements require knowledge of the application specifics and interpretation of results.

This application issue emphasises the importance of the calculated value, where both the transformer manufacturer and cable data is paramount in achieving a calculated value as opposed to a measured value.

Sources of measured error

Test lead connection resistance

Loop measurements include the resistance of the test leads and any contact resistance that is present. The resistance of the test leads is usually taken into account with the instrument calibration. Connection resistance can significantly add to a low resistance loop measurement. Any exposed metal will tarnish over time so it is important to clean the contact area to make an accurate measurement.

Instrument specifications

Instrument specifications are defined within EN61557, this is recognised within the IEE Guidance note 3, (2008), where earth loop impedance instruments are recognised as being suitable for values down to 0.2 Ω. Beyond this the reading is dependent upon an understanding of the application specifics and interpretation, which is the intention of this article.

Health and safety

Prior to the commencement of testing Megger recommend that risk assessment of an electrical installation should also consider the instrument category rating required and the need of fused test leads.

When testing close to the source of supply it is recommended that instruments with CAT IV protection are used. The Megger LTW range met the requirements of CAT IV 300V, which enables them to be used on 415V three phase systems. These instruments are protected internally by high integrity fuses and clearance distances appropriate to Category IV, 300V to earth systems. However a significant risk may arise during testing where the electrician short circuits unprotected high energy supplies, which results in explosion, burnout of test apparatus and arcing etc without rupturing the instruments internal fuse. Examples of the way that the supply might be shorted out are: -
. Human error on the part of the electrician
. A test lead is trapped and shorts together or down to earthed metalwork
. The integrity of the test apparatus has been damaged by suspect repair, overload, abuse etc.

Fused test leads will rupture, protecting the operator. However the disadvantages are:

. An open circuit fuse in a test lead will give a dead circuit indication on the instrument.
. GS38 compliant probes use 500mA fuses, hence they are unsuitable for making high current loop measurements. Thus a significantly larger fuse rating is required in order to carry out a test.
. The fused lead itself will add a small variability into a test result, which may or may not be significant depending upon the particular application.

Test leads fused at 10Amps will provide protection from short circuit faults, but will not fail under normal testing conditions or add any significant loop resistance under the majority of instances. To assist users the LTW can store a test lead offset to take into account the use of fused leads. In summary, 10Amp fused test leads can provide an additional level of safety and could significantly reduce the risk under short circuit fault conditions, use of these is dependent upon your risk assessment and company safety procedures.


Bibliography


Cook Paul (2002) Commentary on IEE Wiring regulations, 16th Edition BS7671: 2001.The Institution of Electrical Engineers, table 16a, (p.328).

IEE (2002) Guidance Note 3 Inspection and testing, The Institution of Electrical Engineers, London, 2008.
 03 September 2012 12:13 AM
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alancapon

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Looking at the spec sheet from Megger's website for the LTW425, it tells us that the display resolution is 0.001Ω, and the accuracy is ±5% ±0.01Ω over an operational range of 0.30Ω to 1000Ω.

It is also worth noting that these accuracy figures would seem to be valid for an ambient temperature of 23°C ±2° and a source voltage of 230v ±1%.

Regards,

Alan.
 03 September 2012 06:49 AM
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GeoffBlackwell

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Just as a little aside here - and to throw the cat among the pigeons:
1) how do you measure resistance, or for that matter, impedance or reactance?;

2) how do you think a loop impedance tester works.

Enjoy.

Regards

Geoff Blackwell

Edit: Spelling!

Edited: 03 September 2012 at 08:54 AM by GeoffBlackwell
 03 September 2012 08:48 AM
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perspicacious

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"the accuracy is ±5% ±0.01Ω over an operational range of 0.30Ω to 1000Ω."

5% of full scale value is the usual quote so our reading could be ±50.01 Ω Or are they trying to suggest it is ±5% of the reading displayed (which is unconventional in instrument usage)?

Miaow

BOD
 03 September 2012 10:00 AM
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Parsley

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Thanks guys

As per Geoff's email on this post and the ECA book review, we all appear to be wasting our time and money.

The problem I find is generally we don't have access to TX's and design information is usually non existent on site. We need to record a value and we want confidence in what the tester indicates.

Regards
 03 September 2012 11:54 AM
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John Peckham

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Parsley

"We need to record a value and we want confidence in what the tester indicates."

Let me know when you have found and instrument that does that?

I will stick my neck out and answer GBs questions. I think that loop testers measure the volt drop across a known resistor inserted in to the earth loop. Others charge and discharge a capacitor and measure the residual voltage. Do I win the prize? The newer loop testers do this a number of times and display the most common result as a resistance (impedance).

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 03 September 2012 12:33 PM
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Parsley

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John

What prize would you like?

I'm afraid this was a bit of a stupid post and I apologise, I wanted other peoples views. I might get the chance to see the Megger in action, on a big supply in the near future.

Regards
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