The problem starts when we measure the resistance of hundreds or thousands of GΩ, instead of MΩ. In such cases, the measured current is not the current that discharges commercially packed batteries. This current is 1000 or even 50 000 times lower (as the test current during 40 TΩ measurement at 10 000 V voltage). In this situation, the measurement is influenced by all kinds of interference, i.e.:

• currents flowing in the circuit resulting from the geometrical arrangement of the wires. The measured current is so small that any undesirable current that flows through the insulation of cables is comparable to the test current and may significantly affect the measurement result. Therefore, avoid arranging cables one on the top of the other. SONEL S.A. provides a shielded test lead, which eliminates the problem, if it is used during the measurements 

• when measuring such low currents and without knowledge of insulation state, in the presence of currents from various sources not generated by the meter, e.g.: current leakage from other sources in the vicinity, electrochemical half-cells, etc. the current input to the meter can receive currents affecting test results (stray currents)

• changes in the measured current flow resulting from the movements in the area of test leads and the measured object. This is a phenomenon similar to that used in capacitive touch screens. A moving person becomes one of the capacitor covers. The role of dielectric is performed mainly by air. Person(s) moving in the vicinity of test leads and measuring object change the capacitance by changing the distance between the covers of this 'parasitic' capacitor and the capacitance change causes a current flow (the meter provides the difference of potentials). A shielded cable eliminates this phenomenon, however the tested object may be unshielded; therefore the tested object should be appropriately connected with the meter. And if the object is still unshielded, it is recommended to avoid movements during the measurement. Below we will discuss the method of connecting test leads that will shield the object by positive electrode of the meter, which also reduces the effect of parasitic capacitance

• external low-frequency electric fields, where the meter and tested object are present. High-frequency fields are filtered. Low-frequency electric fields (below several Hz), in particular those having a period longer than 1 s are very similar to direct current (DC) and may be noticeable as fluctuations of the result

• phenomena related with the polarization of dielectrics

• corona discharge from the sharp ends of wires

Having the above knowledge, we may try to examine two scenarios of connecting the insulation meter to the tested object. We will check the possible sources of additional current that may enter the input of the test lead (black) but without flowing through Rx resistor, that represents the measured insulation. This means the resistance between the cable core and the shield. During the measurements, a shielded cable is used - provided by SONEL S.A. as standard.

CASE ONE

Fig. 1. High voltage in the cable core, return (test) lead on the cable shield

 Sources of interference and protection against them:

•  leakage currents related to reduced insulation resistance flowing through R_UPŁ1 and R_UPŁ2, are eliminated:
  – R_UPŁ1 by GUARD lead (blue) using a band on the insulation between red (+) and black (-) leads,
  – R_UPŁ2 by shielding on black (-) lead, marked with thin, blue line,

•  possible leakage currents between the black cable shield and its core. Represented by R_IZOL:
  – internal design of the meter ensures that voltage on the test lead (-) and its shield is effectively the same (inaccuracy is at the level of a few mV). Pursuant to Ohm's law, the current depends on the difference of potentials and the resistance, which is very high in this case. In the worst case, hundreds of GΩ. Therefore it can be assumed that this influence is not significant, 

• possible leakage currents caused by I_ZAKŁ1 and R_SKR1 in combination with R_SKR3:
  – design of the meter eliminates the influence of interference currents by the internal shielding of the measuring system,

• possible leakage currents caused by I_ZAKŁ1 and R_SKR1 in combination with R_SKR2:
  – the influence is not eliminated,
  – in this case, there is a risk that the test current gets into the measuring input, partially omitting the tested resistance R_X. This is the case when the leakage current (or other interfering current), which flows through the insulation of red lead (+) and through the external insulation (sheath) of the tested cable and through the ground will enter the measurement input,

• possible leakage currents caused by I_ZAKŁ2 and R_SKR1 or R_SKR3 in combination with R_SKR2:
  – the influence is not eliminated,
  – in this case, there is a risk that the test current gets into the measuring input, partially omitting the tested resistance R_X. This is the case of the interference current (originating in other source), which flows through the insulation of red lead (+) or through the insulation of the meter and external insulation (sheath) of the tested cable as well as through the ground to enter the measurement input.

SECOND CASE

Fig. 2. High voltage on the cable shield, return (test) lead on the working core of the cable

• leakage currents related to reduced insulation resistance flowing through R_UPŁ1 and R_UPŁ2, are eliminated:
  – R_UPŁ1 by GUARD lead (blue) using a band on the insulation between red (+) and black (-) leads,
  – R_UPŁ2 by shielding on black (-) lead, marked with thin, blue line,

• possible leakage currents between the black cable shield and its core. Represented by R_IZOL:
  – internal design of the meter ensures that voltage on the test lead (-) and its shield is effectively the same (inaccuracy is at the level of a few mV). Pursuant to Ohm's law, the current depends on the difference of potentials and the resistance, which is very high in this case. In the worst case, hundreds of GΩ. Therefore it can be assumed that this influence is not significant,

• possible leakage currents caused by I_ZAKŁ1 and R_SKR1 in combination with R_SKR3:
  – design of the meter eliminates the effect of interference currents by the internal shielding of the measuring system,

• possible leakage currents caused by I_ZAKŁ1 and R_SKR1 in combination with R_SKR2:
  – current I_ZAKŁ1 does not influence the measurement, because it cannot enter the measurement input via R_SKR1 and R_SKR2. For example, in the absence of I_ZAKŁ1 current flow, induced by external sources, then the leakage current related to the test voltage, flowing through R_SKR1 and R_SKR2 would also not flow, due to the absence of required difference of potentials. If I_ZAKŁ1 current flow is present (induced by external sources) then it would be possible to close it only by the meter grounding via R_SKR3 and/or R_UPL1 or R_UPL2 - this would cause only the change of test voltage value, which is measured and this change will be taken into account during the measurements,
  – in this connection – the influence is eliminated,

• possible leakage currents, caused by I_ZAKŁ2 and R_SKR1 or R_SKR3 in combination with R_SKR2:
  – in each case, no current flows into the measurement input, it may only enter the meter grounding input, or have a rather insignificant effect on the measuring voltage, which is measured by the meter and its possible change is taken into account on the displayed for test results for insulation resistance,
  – in this connection – the influence is eliminated.

 Therefore, SONEL S.A. recommends connecting the tested object in accordance with case two, as shown in Figure 3 and 4.