Accurate earth loop impedance testers or MF tester recommendations for industrial type installations/supplies

Are you taking the resistance of your test leads into account?

On the Megger MFT1741+ this has to be set manually in the setup menu under 'Loop' and the factory default setting is 0.07 Ω.

Note that although the Megger MFT1741+ will display loop impedance values down to 0.01 Ω the manufactuer's specification only applies down to 0.3 Ω on the high current range and 1.0 Ω on the low current range. This can be seen on the table on the back of the instrument case, or in the manual for the instrument.

I would recommend getting hold of the booklet by Megger 'Earth Electrode & Earth Loop Impedance Testing - Theory & Applications' which explains the perils and pitfalls of trying to measuring low loop impedance values close-up to the distribution transformer.

I have no connection with Megger other than as a user of their products and services.

There is a general problem with measuring low resistances, as you have to use large test currents to generate a decent voltage drop. Ideally this has to be done with a 4 point (Kelvin) connection, so that the test current does not flow down the wires making the voltage measurement. Multi function testers in particular are a design compromise, and therefore make 2 wire measurements, with modest test currents, and the penalty is that the low end of the range is prone to larger errors. 

A reading like this only verifies the earth is not disconnected, but a knowledge of the transformer regulation and the dimensions of the main cables will allow a more accurate calculated estimate.

Mike

EDIT

Very low loop impedance (or very high PSSC readings..)   are a common source of distress when it comes to worrying about things, for example 6000A rated breakers in a CU. In the days of a moving needle instruments, where the high PSSC numbers and infinity were all bunched down at one end of the scale, much like high resistances on the ohms range,




you sort of knew when you were in deep water as it were because a slight wobble of the head got you from 1000 to several thousand,  while the hundreds were well separated -  the problem of 1 divided by something approaching zero.
The problem with the digital instrument is it happily calculates a single value based on a really small voltage drop , and displays lots of significant figures, but it still does so from a measurement where the uncertainty is rising rapidly at one end of the scale, and does not really convey the natural uncertainty.
It is not really a reliable way to make a measurement. cleaning meter lead connectors may reduce the divergence but it is inherently an increasingly imprecise measurement.
(and conventional  meter leads will add a few tens of milliohms per metre ,even when perfectly clean.)

Mike.

There is a general problem with measuring low resistances, as you have to use large test currents to generate a decent voltage drop. Ideally this has to be done with a 4 point (Kelvin) connection, so that the test current does not flow down the wires making the voltage measurement. Multi function testers in particular are a design compromise, and therefore make 2 wire measurements, with modest test currents, and the penalty is that the low end of the range is prone to larger errors. 

A reading like this only verifies the earth is not disconnected, but a knowledge of the transformer regulation and the dimensions of the main cables will allow a more accurate calculated estimate.

Mike

EDIT

Very low loop impedance (or very high PSSC readings..)   are a common source of distress when it comes to worrying about things, for example 6000A rated breakers in a CU. In the days of a moving needle instruments, where the high PSSC numbers and infinity were all bunched down at one end of the scale, much like high resistances on the ohms range,




you sort of knew when you were in deep water as it were because a slight wobble of the head got you from 1000 to several thousand,  while the hundreds were well separated -  the problem of 1 divided by something approaching zero.
The problem with the digital instrument is it happily calculates a single value based on a really small voltage drop , and displays lots of significant figures, but it still does so from a measurement where the uncertainty is rising rapidly at one end of the scale, and does not really convey the natural uncertainty.
It is not really a reliable way to make a measurement. cleaning meter lead connectors may reduce the divergence but it is inherently an increasingly imprecise measurement.
(and conventional  meter leads will add a few tens of milliohms per metre ,even when perfectly clean.)

Mike.