A guide to electrical testing
For those involved in the growing solar panel sector‚ awareness of installation and safety standards is crucial for system performance‚ says Jim Wallace of Seaward Solar.
The recent solar microgeneration boom reflects the impact of Feed-in Tariffs (FITs) that came and the associated arrival of free installation schemes that enable the property owner to benefit from solar electricity while the installer receives the FIT payments.
Any PV installation seeking funding from the FITs initiative must use Microgeneration Certification Scheme (MCS) approved contracting companies. MCS is a quality assurance scheme and approval is therefore a pre-requisite for any company seeking to take advantage of the growing demand for solar panel installation.
Alongside MCS accreditation‚ the installation process itself is unlikely to be too difficult for a qualified electrician‚ although there are significant differences from the usual installation wiring technology that they are likely to encounter on a day to day basis.
PV installation overview
The installation of PV systems presents a unique combination of hazards linking the risk of electric shock with the implications of working at height. It follows therefore that stringent health and safety practice must apply to the installation, commissioning and testing of a PV system.
PV arrays produce a DC voltage when exposed to sunlight. In the wiring system associated with PV panel installation‚ the DC current generated by the solar array is converted to AC by means of an inverter which then feeds into the AC mains supply of a building. It follows that electrical work on PV panel installation involves working on both the DC and AC sides of the circuit.
From the outset, therefore, the designer and installer of a PV system must consider the potential hazards carefully and systematically devise methods to minimise the risks. This includes mitigating potential hazards present during and after the installation phase.
The consideration of panel operation under both normal and fault conditions is essential at the design stage to ensure the required level of safety. It is then important to ensure that the long term safety of the system is not compromised by a poor installation or subsequent poor maintenance.
Much of this comes down to the quality of the installation and the system inspection and testing regime.
PV systems are unusual in that the energy source cannot be switched off. If there is daylight falling on a PV panel it will produce electricity and it is possible for a relatively small array of only a few panels to deliver a lethal shock.
Another important point is that PV panels generate DC voltage, which is not always commonly used by electricians in their normal work. In addition‚ because of the current limiting properties of PV cells‚ they are incapable of producing sufficient fault currents to operate over-current protection devices such as fuses. Once established a fault may remain undetected, not only posing a hazard for an extended period but also wasting energy generated by the PV system.
Undetected faults may also develop into a fire hazard over time. Without fuse protection against such faults, elimination of a fire risk can only be achieved by both good system design‚ and careful installation with appropriate inspection and testing.
Special measures must therefore be taken during installation of PV systems to eliminate the risks of dangerous working and latent electrical problems.
These include completing the DC wiring before connection is made to the panels and avoiding working with both positive and negative conductors simultaneously. This allows the effective isolation of the DC system (via a DC isolating switch and module cable connectors) while the array is installed and the effective isolation of the PV array while the inverter is installed.
Installation and testing
Between them‚ the various installation requirements for PV systems are designed to ensure the electrical safety of the installation‚ the electrical safety of installation personnel and the verification of performance and power output of the system.
In general terms the installation of domestic grid connected PV systems also falls with the scope of Part P of the Building Regulations and it is the responsibility of the installer to ensure that systems are installed according to the existing BS7671 electrical installation standard – the 17th Edition IEE Wiring Regulations.
However‚ the inspection and testing of DC circuits associated with PV arrays requires special considerations. The IEE Guidance Note 7 Special Locations provides guidance on solar photovoltaic (PV) power systems.
Also required is completion of Engineering Recommendation G83/1 which is the installation commissioning confirmation form for the connection of Small Scale Embedded Generators‚ such as PV arrays‚ of up to 16A per phase with public low voltage distribution networks.
IEC 62446: 2009 Grid connected PV systems – minimum requirements for system documentation, commissioning tests, and inspection specifies the minimum requirements for system documentation‚ commissioning tests and inspections.
Building or electrical works in the vicinity of the PV array is also likely and the ownership of a system may also change. As a result‚ the standard recognises that only by the provision of adequate documentation at the outset can the long term performance and safety of the PV system be ensured.
The standard therefore sets out the information and documentation that should be provided to the customer following the installation of a solar panel system and also the initial (and periodic) electrical inspection and testing required.
In short the standard sets out measures to ensure that the PV panels and electrical supply connections have been wired up correctly, that the electrical insulation is good, that the protective earth connection is as it should be and that there has been no damage to the cables during installation.
Under electrical tests the standard sets out specific requirements for:
• Earth continuity of array frame to earth and connection to main earthing terminal
• Polarity of all DC cables
• PV string open circuit voltage test
• PV string short circuit current test
• PV array insulation test
• Operational test – PV string current
• Functional test
IEC62446 also requires inverter details to be recorded and ported and MCS requires that records are kept.
The installation of a PV system by property owners is clearly only undertaken after careful consideration of the costs involved and potential return on investment provided by lower energy bills and FIT payments.
As a result, the verification of system performance and energy output from the panels is therefore particularly important and a major reason why periodic verification and testing of the system can also be very important – as well as being essential to comply with warranty and PV system guarantees.
In many cases simple electrical faults or wiring failures can cause a serious inefficiency in the ability of the panel to produce power.
In such circumstances proper metering will give an indication of system performance but effective periodic electrical testing is vital not only to prove the safe installation of a new system but also to determine and verify ongoing functional performance over extended periods.
In terms of test instrumentation‚ different PV system electrical tests currently require the use of different testers – typically including an earth continuity and insulation resistance tester‚ a multimeter and DC clampmeter.
Using such an array of instruments can be cumbersome and time consuming – factors that have now led to the introduction of a new generation of integrated testers capable of performing all of the tests required by IEC 62446.