A Megger DCM340, a
versatile clampmetre that’s ideal for solar PV applications
Correct inspection and testing ensures a top quality installation and ensures that a customer’s investment yields a healthy return on investment
Shock tactics Within a solar PV wiring system, 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. Simon Woods, Megger’s UK wholesale and distribution sales manager, explains: ‘Because testing is on a DC circuit as opposed to AC, there are additional risks due to the difficulty in isolating the circuits.’ Tim Painter, operations manager with ECA member and
MCS qualified installer, The Green Electrician, adds: ‘Most electricians do not come into contact with DC so there are risks involved for anyone who is not fully conversant with it. It’s just one of the reasons why specialist training in this area is so important.’ The inspection and testing of DC circuits associated with
solar PV arrays requires special consideration and the IEE Guidance Note 7 Special Locations provides guidance on solar PV. It is advisable to complete the DC wiring before connection is made to the panels and avoid working with both positive and negative conductors simultaneously. Also, the energy within a solar PV system cannot be
switched off, so in daylight conditions electricity will be produced and it is possible for a only a few panels to deliver a lethal shock. Jim Wallace, product and technology
Di-Log DL6402 digital clamp meter
Five alive
Seaward Solar has produced a five-point guide to solar PV system electrical commissioning tests. IEC 62446 and national requirements such as the MCS recommend a sequence of electrical tests to demonstrate the safe and correct installation of a solar PV system. n Earth continuity. Where protective earthing/grounding and/or equipotential bonding conductors are fitted, such as bonding of the array frame, continuity should be tested.
n Polarity test. For reasons of safety and prevention of damage to other equipment in the system, the polarity of all DC cables should be verified before other tests are performed.
n PV string open circuit voltage. This is measured to ensure correct installation and operation of each PV string. Measured values should be compared with expected values. For systems with multiple identical strings, values should be within five per cent of other PV strings in array.
n PV string current. Short circuit current is measured to ensure correct installation and operation of each PV string. Operational current is measured with the system in normal operation mode and compared with the expected value. For systems with multiple strings, values should be within five per cent of other strings in array.
n Array insulation resistance. Measured from the array positive and negative to earth and compared to maximum acceptable value specified by national requirements and IEC 62446. It is also important to note that test voltage is selected
according to the PV system voltage – 250V for systems less than 120V, 500V for systems between 120V–500V and 1000V for systems over 500V.
46
ECA Today November 2011
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