Solar power - A guide to PV system electrical testing


To make the most of potential new business opportunities in the growing solar panel sector‚ contractors need to understand the technology and the electrical testing associated with PV system installation‚ says Jim Wallace of Seaward Solar

Energy regulator OFGEM recently reported the 2010 year end total of solar power installations reached 45MW. Month by month numbers of domestic PV installations ended the year with over 2‚000 per month during November and December.

This solar microgeneration ‘boom’ reflects the impact of Feed-in Tariffs (FITs) introduced from 1 April 2010 and also the 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.

Given the obvious synergies‚ it is widely expected grid connected PV systems will quickly become a mainstream electrical contracting industry activity. 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 be working with 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.

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.

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 - including mitigating potential hazards present during and after the installation phase. In particular, it is vital the long term performance and electrical 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 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. 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 will allow 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 standards
The general requirement is that grid connected PV solar systems are tested according to 17th edition electrical wiring regulations but there also additional requirements for PV systems.

Engineering Recommendation G83/1 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. Installers are required to complete G83/1 with information on various tests‚ system details and a range of supporting information to satisfy the requirements of the Distribution Network Operator.

Installation of domestic grid connected PV systems falls with the scope of Part P of the Building Regulations and it the responsibility of installer to ensure 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.

‘IEC 62446: 2009 Grid connected PV systems – minimum requirements for system documentation, commissioning tests, and inspection’ specifies the minimum requirements for PV system documentation‚ commissioning tests and inspections.

Building or electrical works in the vicinity of the PV array are also likely and the ownership of a system may also change. 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.

In short the standard sets out measures to ensure:
t The PV panels and electrical supply connections have been wired up correctly
t That the electrical insulation is good
t The protective earth connection is as it should be
t There has been no damage to cables during installation

Under electrical tests the standard sets out specific requirements for:
t Earth continuity of array frame to earth and connection to main earthing terminal
t Polarity of all DC cables
t PV string open circuit voltage test
t PV string short circuit current test
t PV array insulation test  
t Operational test – PV string current
t Functional test
t Irradiance
IEC62446 also requires inverter details to be recorded and MCS requires that installation records are kept.

Testing times
Between them‚ the various installation requirements for PV systems are designed to ensure the electrical safety of the installation and installation personnel - and the verification of performance/power output of the system.

However‚ in what is still a relatively new area for many contractors‚ there remains a lack of understanding of some issues.

For example, many inverters have transformer isolation between the AC and DC side, preventing DC fault currents from being fed into the electrical installation. Transformerless inverters are increasing in popularity due their increased efficiency and reduced cost and physical size.

IEE Guidance Note 7 advises where an electrical installation includes a PV power supply without at least simple separation between the Ac and DC sides, an RCD installed to provide fault protection shall be type B.

However, the guidance also states where the inverter is, by construction, not able to feed DC fault currents into the electrical installation, an RCD of type B is not required. Manufacturers of transformerless inverters commonly provide declarations to this effect - avoiding the need for a type B RCD.

One area of PV installations where there is there is also much debate is that of protective earthing and equipotential zones.  Where the PV array is not Class II, exposed metal parts must be connected to protective earth. When the electrical installation is a Protective Multiple Earthing (PME) system, the recommendation is that the PME is not taken outside the equipotential zone.

In such cases, the recommendation given in the DTI Guide to the installation of PV systems is connection to earth is via an earth spike. However, there is much debate as to whether a PV is inside or outside the equipotential zone. The array itself is mounted on the exterior of the building - however, conductive parts such as fixings or brackets may be accessible from inside the building as they pass through the roof. Also‚ in terms of test instrumentation‚ different PV electrical tests 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 – considerations which have led to the introduction of a new generation of integrated testers capable of performing all of the tests required by IEC 62446.

Ongoing verification of performance
The installation of PV system by householders is clearly only undertaken after careful consideration of the costs involved and potential return on investment provided by lower energy bills and FIT payments. 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. This is particularly important for installers working on ‘roof rental’ schemes were installation has been provided free of charge in return for receipt of the FIT payments. In such circumstances proper metering will always give an indication of system performance but effective electrical testing is vital not only to prove the safe installation of a new system but also to verify ongoing functional performance over extended periods.