Insulation resistance testing on the DC side of PV strings

The insulation resistance measurement guide covers the standard procedure for a de-energised installation: the installation is made dead, and a measurement is taken between live conductors and earth. That premise does not hold on the DC side of a PV installation, because as long as daylight falls on the panels, the PV strings generate voltage — there is no "making dead" in the usual sense. This article covers how the insulation resistance test on the DC side is adapted for that under IEC 62446-1.

Why the standard approach does not work

A PV panel is effectively a voltage source the moment light falls on it, even in overcast weather. There is no switch that de-energises the cells themselves: only the connection from the string to the inverter, or to the rest of the installation, can be interrupted. An ordinary insulation resistance tester, applying its own test voltage (typically 500 V DC) to a conductor while the string itself is also delivering voltage, produces unreliable readings and, in the worst case, can damage the tester through the combination of test voltage and panel voltage.

Method 1 — shorted string

Under Method 1 in IEC 62446-1, the positive and negative poles of the string are short-circuited together at the start of the test, for example with a suitable short-circuiting clamp on the DC connectors. This means there is no voltage difference between positive and negative during the test, and the insulation resistance tester can apply its test voltage between the shorted poles together and earth (PE, or the mounting framework earthing) without interference from the panel voltage. This is the simplest method, but it does require that the string is not damaged during the short: the short-circuit current of a string is not typically dangerously high, but the short-circuiting clamp must be rated for that current and for the full system voltage.

Method 2 — separate R+ and R-

Under Method 2, the string is not shorted; instead, R+ (insulation resistance between the positive conductor and earth) and R- (insulation resistance between the negative conductor and earth) are measured separately, with the string not shorted. This is useful when shorting is not desired or not practical, but it requires a test instrument explicitly designed for this measurement on live DC strings — an ordinary insulation resistance tester is not always suitable, because the present panel voltage is not shorted out during the test and must be handled by the instrument without a faulty reading or damage.

Test voltage and threshold

For the DC side of a PV installation, IEC 62446-1 typically specifies a test voltage of 500 V DC (as is usual for low-voltage installations), with a minimum acceptable insulation resistance of ≥ 1 MΩ for the string. This is a practical minimum: a significantly lower value points to a possible insulation fault, moisture ingress in the junction boxes, or cable-sheath damage, and must be investigated before the string is put into, or kept in, service.

Note: DC connectors from different manufacturers (MC4 and similar) are not always mutually compatible. Always use a short-circuiting clamp and test leads that match the connector system used on the installation, to avoid damaging the connector itself.

Practical relevance

At commissioning of a new PV installation, at periodic inspection under IEC 62446-1, or when investigating a suspected earth fault on the DC side, this adapted insulation resistance test is the way to assess the quality of the DC wiring and the panels without having to force the DC side dead — which is not possible in daylight anyway.

Common mistakes

Using an ordinary insulation resistance tester on a non-shorted, live string without an instrument or method suited to that, resulting in an unreliable or damaged measurement.
Testing at very low irradiance and treating the result as representative — at very low irradiance the panel voltage can be so low that results are not comparable to a test under normal conditions.
Choosing a short-circuiting clamp not rated for the system voltage and current of the string, risking damage to the clamp or the connectors.
Confusing this with the AC-side insulation resistance test of the installation after the inverter, which can indeed be performed de-energised under the standard procedure.