Insulation is used as a safety mechanism in an electrical installation to resist the flow of electrical current, and therefore insulation resistance is a measure of how well that insulation is performing. Insulation resistance is the term applied to the measured value (in Ohms) of an insulating material that surrounds electrical conductors, which in most cases is electrical cables. Therefore, the insulation resistance test measures the total resistance between two specific points that are separated by electrical insulation, ie. how effective the insulation is in resisting the electric current flow. The insulation resistance test is carried out when an installation is new and being commissioned, but also to determine how the insulation is performing, or how it has degraded, over time.
This is a particularly pertinent test, therefore, when considering a solar PV system, due to the components being installed externally and exposed to harsh weather as a fundamental part of their operation.
What problems are caused by poor insulation?
Electric current can flow directly through an insulating material, or around its surface, and the severity of this current flow depends on the amount of voltage used and the condition of the insulating material. Excessive current flow from a conducting material can cause significant problems in an electrical system, including power loss, short circuits, flash arcing, overheating, fire and electric shocks.
So the insulation resistance test in any electrical installation is essential in ensuring a safe system and compliance with regulatory standards. The test confirms that the components of a solar PV system are not losing their insulation over time, as well confirming the insulation properties are adequate upon a new installation.
Why the insulation resistance test is essential in
commissioning and maintaining a solar PV system
Commissioning a solar PV system verifies that all the component parts are operating correctly before the system is put into service. It identifies any installation issues and verifies that the installation is in line with the design spec of the system. It also establishes baseline figures from which the performance of the solar PV system can be monitored. The insulation resistance test forms an essential part of the commissioning process.
The insulation resistance test verifies the electrical insulation integrity of any circuits in a solar PV system. These are usually found in a combiner box. The combiner takes the wire feed from several different solar panels (also called arrays) and combines them in one main feed. This enables the tester to identify insulation issues during commissioning and later during routine maintenance checks of the system. This is important for three reasons:
- Prevent electrical faults – Having adequate insulation resistance levels reduces the risk of electrical faults during operation, such as arc faults, ground faults and current leakage. These can lead to system downtime and system inefficiencies.
- Compliance – Regulatory standards obligate an installer to carry out insulation resistance testing, to validate that the string modules of the solar PV system meet suitable insulation resistance thresholds.
- Identify insulation degradation – Over time a solar PV system can be impacted by physical damage or moisture ingress, which in turn will degrade the insulation properties of the conducting materials. If this is not detected during routine maintenance checks it could compromise the reliability and safety properties of the system.
Due to Solar PV systems being live when they are exposed to sunlight, it is not possible to perform the insulation resistance test with a traditional instrument or multi-function tester. Instead a specific Solar PV tester such as TIS PV-ISOTEST, TIS PVCHECKS or PVCHECKS- PRO PLUS supplied by Test Instrument Solutions. These test tools allow live insulation resistance testing up to 1,500V DC.