IS THE POLARITY OF TEST LEADS IMPORTANT WHEN MEASURING THE INSULATION RESISTANCE OF A CABLE?
The technical department of SONEL S.A receives many customer questions: “Is the polarity of connected test leads
important when measuring the insulation resistance e.g of a power cable?”
A professional electronic technician, who measures the resistance of a resistor would answer that it is not significantly important, as the results of measurements made in both directions, would not differ from each other. However, the situation is quite different in case of performing measurements on power facilities. The manner of connecting test leads, in case of the cable mentioned above, may significantly affect the obtained results.
At the beginning, we have to know what types of current we are dealing with during measurements and why the measurements of insulation resistance can not be identical to those made on a resistor. Try to imagine a current that flows in a normal, unconnected alkaline battery, causing its gradual self-discharge. Sources indicate that self-discharge rate is 0.3% per month. Their capacity is approx. 3000 mAh. It means that the current flow in an unconnected battery is approx. 12.5 μA, which corresponds to the resistance of 80 MΩ, measured with the voltage of 1000 V.
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
MIC-10k1 Insulation Resistance Meter
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 1
Sources of interference and protection against them:
- leakage currents related to reduced insulation resistance flowing through RUPŁ1 and RUPŁ2, are eliminated:
– RUPŁ1 by GUARD lead (blue) using a band on the insulation between red (+) and black (-) leads,
– RUPŁ2 by shielding on black (-) lead, marked with thin, blue line, - possible leakage currents between the black cable shield and its core. Represented by RIZOL:
– 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 IZAKŁ1 and RSKR1 in combination with RSKR3:
– design of the meter eliminates the influence of interference currents by the internal shielding of the measuring system, - possible leakage currents caused by IZAKŁ1 and RSKR1 in combination with RSKR2:
– 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 RX. 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 IZAKŁ2 and RSKR1 or RSKR3 in combination with RSKR2:
– 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 RX. 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.
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Authors:
Grzegorz Chrzanowski, SONEL S.A.
Wojciech Siergiej, SONEL S.A.
Oct 19, 2023