Ground Faults, Isolation (ISO) Faults, RISO Low Faults and Insulation Resistance Faults with Solar PV Systems
Published: February 2024
After a number of years exposed to wind, rain, snow, ice and sometimes animals; solar panel systems can start to develop faults. The most common faults we find related to exposure are ground faults, isolation (ISO) faults, RISO low faults and insulation resistance faults. In this article we take a look at what these faults are, the possible causes and what steps are taken to identify and resolve them.
What are Ground, Isolation (ISO), RISO Low and Insulation Faults?
These faults are often identified by solar inverters which will refuse to startup when they find them; sometimes they clear themselves only to reappear again at a later date, this can sometimes be weeks or even months later; other times the solar inverter will flat out refuse to start again until the fault has been cleared.
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Different solar inverters from different manufacturers call these faults by different names but they're all referencing the same thing, what the solar inverter thinks it has found, is a problem with the integrity of the DC cabling. Solar PV inverters are powered from the solar panels (the DC side), they shut themselves down at night and start themselves up again in the morning when the sun comes up. As part of the solar inverter's startup procedure, as well as testing itself and it's own safety components, it will also test the integrity of all the cabling connected to it. This includes all of the cabling to, from and between the panels as well as the mains, solar inverters will only startup and feed power into the electrical system when it's safe to do so. |
One of the tests the solar inverter carries out at startup is insulation resistance testing of the DC circuits. Insulation resistance testing involves measuring the resistance between the bare wires and earth/ground. It's looking for a high reading, essentially it's measuring the integrity of the cable sheathing, the insulation/covering around the bare wires. Resistance is measured in Megohms (1000 Ohms), for example, bare wires on a wet roof would offer zero resistance to ground/earth and likely give a reading of zero Megohms. A well insulated solar circuit, even in soaking wet conditions, we would expect to measure at least 20-30 Megohms of resistance. Solar inverters will begin to raise alarms and stop generating at around 1 Megohm.
What causes Ground, Isolation (ISO), RISO low and Insulation Faults?
Poor Electrical Connections Solar panels are daisy chained together into strings, one panel to the next, each panel with four electrical connections, two in the junction box, two to connect to the next panels in the string. When optimisers are used such as with SolarEdge or Tigo systems, the optimisers are daisy chained together, each panel connecting into an optimiser, optimsers have 8 electrical connections each. In addition, there's electrical connections made at the solar inverter and at the DC isolators.
If any one of these electrical connections has been poorly made or isn't quite true, it could, particularly over time, worsen and be the cause of a DC fault current being able to make it's way to ground/earth, prompting the solar inverter to shutdown or stop generation.
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Moisture A well made electrical connection is to some degree weatherproofed but they're not usually designed to withstand sitting in pools of water, moss or birds nests. Ideally they should be tucked and tied safely away under the panels but this isn't always the case. Being constantly wet or exposed to bird droppings etc will eventually degrade the seals allowing moisture into the connection. Once the wires are exposed to the environment particularly moisture they will corrode and breakdown. Wind Damage Poorly secured cables can get caught in the wind and rub on tiles/masonary which will rub away the insulation and expose wires. Solar panels that have been mounted onto rails that are either too small/not strong enough or with not enough mounting hooks can flex when the wind gets underneath them and result in smashed glass. The top layer of a solar panel is made of tempered/toughened glass so although they are pretty strong, once the glass gets damaged it will shatter exposing the panel's internal circuitry to the environment. |
Physical Damage and Animals As well as falling tree branches, cricket balls, stones and careless workers; birds and rodents can quickly cause problems with solar panel cabling. Birds nesting amongst solar panel cabling in addition to their droppings corroding seals around electrical connections, can stretch cables with the weight of their nests and put pressure on the electrical connections, rodents will happily chew the insulation off cabling.
How are Ground, Isolation (ISO), RISO low and Insulation Faults Fixed?
Testing The first step in finding and repairing these faults is testing. What we need to know before going any further is, is that what the solar inverter is measuring is correct. Insulation resistance testing excluding the solar inverter will confirm if the readings are correct and that the solar inverter is functioning and measuring correctly. At the same time all of the easily accessable electrical connections in and around DC isolators and the solar inverter can be inspected and tested and if faults are found repairs can be done there and then.
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Isolating the Fault The next thing we would like to have is a better idea of where the fault is, particularly if solar panels are installed on multiple roofspaces. If the solar inverter is fed by more than one circuit, at this stage it might be possible to find and isolate the faulty circuit and get the others back working. A DC string is essentially one circuit that starts and ends at the solar inverter, it's not usually so straightforward but if the original installer has left very accurate information and circuit diagrams, measuring voltage to earth at each end of the circuit can sometimes tell us where physically the fault might be located. Locating and Fixing the Fault Ground mounted systems are easier but solar panel cabling on roofs can only be accessed by removing panels which in every case will need a scaffold, it's not possible to do this work (or even access the electrical connections) when working off ladders. We usually (but not always) find that any faults severe enough to trigger the solar inverter to stop generating are relatively easy to find once the panels are off and we have access to all the cabling. |
Testing is carried out in logical steps, rows of panels, sets of panels, per panel etc until the fault is found. Everything involved with repairing the fault uses common and inexpensive parts, cable, connectors etc which we will have with us. If the fault is with a damaged or faulty panel, if it can't be repaired, the faulty panel can either be excluded from the circuit or a well sized and electrically matched panel can be installed in it's place.
Completion We put everything back, securing and weatherproofing connections correctly, making any other repairs to the roof and system whilst we are there. If required we install a guard to prevent animals getting underneath the panels and test again on completion, recording the results and then recommission the solar inverter.
Solar PV Systems & Equipment, including Electrical & Roofing Systems
Grid-tie inverters, Hybrid Inverters, AC Coupled Inverters, Battery Storage Inverters, Off-Grid Inverters, Charge Controllers, Transfer Switches, Hot Water Controllers, Optimisers, Lithium Batteries, Lead Acid Batteries, Solar Panels, Mounting Systems, Test Equipment, Earthing Equipment, Electrical Distribution Equipment, Cable and Accessories
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Deliveries to anywhere in the UK are quick, tracked and accurate, technical support (by phone and on site if needed) and design/product advice is available before, during and after installation. The recording and collating of serial numbers, factory and other test results, manuals any other information required for equipment warranties, add ons and extensions is included. Trade, DIY and Retail Clients Welcome. |
Bespoke system design, equipment testing and integration support as well as on-site technical support and job/equipment specific training is available for off-grid, hybrid, larger and or more complex systems.
Mains/Grid Isolation and Surge Protection
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A whole house surge protector is installed to provide protection from transient overvoltages originating from the mains/grid. A whole house surge protector is installed directly inline and as close as possible to the incoming mains/grid supply meter, this allows for surge protection for all circuits and equipment including solar inverters, routers, stereos and other sensitive electrical equipment within the network. The addition of a 100Amp lockable isolator also allows for safe and convenient isolation of all electrical equipment within the network including consumer units, solar inverters, battery storage units and EV chargers from the mains/grid in one place. Surge protectors are in compliance with the recently updated 18th edition amendment 2 of BS7671. |
Solar PV Surge Protection (DC Surge Protection)
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DC surge protection devices (SPDs) are installed between the solar panels and the solar inverter to protect both the solar inverter and the downstream electrical equipment from transient overvoltages of an atmospheric origin impacting the electrical system via the DC side of the system / the solar panels. DC Surge protectors are in compliance with the recently updated 18th edition amendment 2 of BS7671. |
Articles: Further Information, Calculators & Solar Inverter Fault Codes
Lots of articles, calculators and technical information including links for further reading. Covering a range of topics related to the installation and maintenance of solar photovoltaic and electrical systems in the UK.
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Engineering Recommendation G98Grid Connections for Micro-Generators including Solar PV Systems and Elecricity Storage Systems in the UK. Under 16Amps Per Phase, grid synchronised. BSI - PAS 63100:2024 - Protection Against Fire of Battery Energy Storage Systems for use in DwellingsThis Publically Available Specification (PAS) from the British Standards Institution (BSI) was sponsored by The Department for Energy Security and Net Zero. Although not yet a British Standard, this guidance was developed in response to an urgent demand for clarity as to good practice across the industry including product, service and process standards. How to provide backup power to a house using a portable generatorIn this article we show you how to provide backup power to your home using a portable diesel, petrol or LPG backup generator. We look at changeover switches, the importance of earthing, backup generator loadings, how to isolate non essential loads. We ask where to locate the backup generator when it's in operation, how best to safely isolate the grid/mains power supply and switch to a backup generator supply. Mains Power Supplies & Earthing Systemsmains power supply and Earthing Systems: Location & accessability, supply systems, earthing, bonding, labelling, installing RCDs. |
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