Some problems and limitations with Earth Loop Impedance Testers

This document was produced in response to a question from an engineer working on large retail and commercial properties who noticed variations in earth loop readings across different multifunction testers. This document is not intended to be a definitive explanation of all aspects of earth loop impedance tests and testers.

Customer data:

Ze at source was 0.16 ohms on a TNCS supply.

High current testing results:

Megger MFT1730 measured 0.22 ohms at the socket

Fluke measured 0.04 ohms

Kewtech tester no1 measured 0.02 ohms

and Kewtech tester no2 measured 0.8 ohms

On a larger installation with a 400amp incomer fed via a 45 meter 185 4 core swa cable, using the armouring as a cpc from a panel board fed from a local transformer Zdb was measured as 0.28 ohms

Loop Test measurement accuracies.

Instrument accuracy statements are often overlooked but will have a significant effect on loop test readings, particularly at the bottom end of the measurement range.

The Megger MFT1730 used for the tests has a published accuracy of:-

L-N/L-L tests ±5% ±5 digits

L-E tests (? Reference conditions apply)

0.1 Ω ~ 39.9 Ω ±5% ±5 digits ± noise margin

40.0 Ω ~ 1000 Ω ±10% ±5 digits

Display range 0.01 Ω ~ 1000 Ω

Looking at the L-N/L-L tests the accuracy statement says 5% ±5 digits.

The 5% part of the statement would mean the 0.16 ohms measured value could be displayed between 0.152 and 0.168 ohms – but with the product only having 2 digits of resolution this would round to a reading between 0.15 and 0.17 ohms.

For a value of 0.16 ohms the 5 digits part of the specification has an even larger effect and describes the variability that could occur with the least significant digit of the displayed value.

So 0.16 ohms ±5 digits could be displayed between 0.11 and 0.21 ohms and be within the accuracy statement.

Combine the 2 parts of the accuracy statement and a value of 0.16 ohms could be displayed between 0.10 and 0.22 ohms and still be within the product accuracy spec.

When you look at the LE tests there are some additional factors to be considered.

Firstly it states that “reference conditions apply” and also quotes a “± noise margin” which I was unable to find a definition for.

So in addition to the ±5% ±5 digits accuracy statement there are additional unspecified inaccuracies that will occur if you are not operating under reference conditions, which is essentially a controlled lab type environment, and additional errors will be incurred as a result of electrical noise.

In the real world the electrical noise could represent earth leakage currents, harmonics or other power quality issues.

Furthermore, if you look closely you will see the lowest measurement range for LE tests is stated as 0.1 to 39.9 ohms. So in actual fact the least significant digit is now the 0.1 instead of 0.01 which means the ±5 digits equates to ±0.5 ohms on this range.

I can't comment on the Fluke and Kewtech readings obtained by the customer as I am not aware which product models were used and so can't quote their accuracies – however the accuracy statements, and issues of electrical noise, basically exist across all manufacturers.

So in summary for a Ze value of 0.16 ohms, on a high current (LN/LL) test, the Megger product could display between 0.10 and 0.22 ohms and be within spec. and so a reading of 0.22 ohms was OK from that point of view.

If the product is used on the LE range then the accuracy of reading can be even worse yet still be within the product specification, particularly if that specification has criteria like ± noise margin in it.

How did loop testing get so complicated and what are the related issues?

35 years ago a loop tester would basically perform what people referred to as a high current, or 25 amp test. In reality it wasn't actually 25 A but put quite simply the loop tester would place a 10 ohm resistance across the supply being tested and measure the voltage drop across the resistor. Using ohms law it could then calculate the “resistance” of the loop and display it. These tests were generally performed for 2 half cycles of the mains supply.

With such a high test current loop testers were able to produce pretty accurate results.

Since then installation equipment and products have evolved, and loop testers today are very different instruments.

Instead of big “beefy” test currents the introduction of RCD's and a desire to keep products small, lightweight and able to perform large numbers of tests without going into thermal overload, pushed manufacturers to try many different testing methods.

Without going through all of them, the various tricks to avoid RCD tripping include test currents below 15mA and/or test durations that are so fast the RCD does not have time to react.

Additionally, in the name of reducing product size, cost and heat dissipation, many of the “high current” tests today only operate at a “couple of amps” or some even lower than that.

Its fairly easy to understand that with lower and lower test currents it becomes more and more difficult to obtain accurate measurements as the external influences become more significant. Hence a “non-tripping” LE loop test with a 15mA test current, which is over 1500 times smaller that a “traditional” test current, having poorer capability/accuracy.

Test current duration, or test frequency as it may be referred, also has some other interesting implications.

When a 10 ohm resistor is placed across the supply the load sees the full 50Hz of the mains supply, and accordingly the results are fairly relatable to a real life condition.

But what if a modern loop tester is using a higher frequency test signal, or applying its load to the supply for very very short periods of time, which is essentially the same thing?

Since the supply impedance is made up of resistive and reactive components it stands that the reactive parts, which are frequency dependent, will appear different at higher frequencies than at the much lower power frequency. This then requires the product to perform some kind of internal compensation which introduces further inaccuracies.

Also items in the installation may respond to higher frequencies in less predictable ways and effect the readings. Ever heard of RCD uplift on loop test readings?

Summary

Todays loop tests have had to incorporate fairly sophisticated measurement techniques in order to work on modern installations and meet customer demands for size, weight and performance.

Test methodologies are different across all manufacturers as they strive to produce the best one, and invariably this will lead to different readings on different products in different situations.

The variability in readings should be (and I assume is) taken into consideration in the published specifications from all the manufacturers.

The question for the user, that I can't answer, is does the variability in readings mean the loop test range capability is or is not good enough for the job in hand.