Energy Efficiency and

Renewable Energy Bulletin

November, 2010 UPDATED

Earthing PV module frame … AS/NZS 3000:2007 It is especially important that a PV module frame earth (equipotential bond) is provided when connected to NON-ISOLATED INVERTERS [ non galvanically isolated or ‘transformer-less’ ]

There can be an a.c. component on the d.c. supply. This is capacitively coupled to the module frames producing an a.c. voltage. While not likely to be lethal, if not depleted by earthing, an electric shock will be enough to cause a person to fall off the roof e.g. the system owner, while cleaning modules.

If a fault develops in the PV array, the grid voltage will be present – this is likely to be lethal. Earth wires must be run so that the removal of one component (e.g. module) does not interrupt the earthing of other parts of a system. In other words daisy chaining earth connections is not permitted. Minimum recommended CSA - 4mm² The PV earth connection is direct to the earth link not via the inverter earth connection.

SECTION 5 EARTHING ARRANGEMENTS AND EARTHING CONDUCTORS 5.1.2 Selection and installation requires that earthing arrangements shall be selected and installed to ... (c) Mitigate voltage differences appearing between exposed conductive parts of equipment and extraneous conductive parts through equipotential bonding arrangements. 5.6 EQUIPOTENTIAL BONDING 5.6.1 General Equipotential bonding is intended to minimize the risks associated with the occurrence of voltage differences between exposed conductive parts of electrical equipment and extraneous conductive parts. NOTE: Additional equipotential bonding requirements apply for ... (e) photovoltaic arrays, in accordance with AS/NZS 5033 5.6.3.2 Size (a) ...the equipotential bonding conductor required ... shall have a cross-sectional area not less than 4mm².

Refer to the CEC GC Installation guideline.

PV String

GC Inverter

AC Switchboard A N E

Issue 2 June, 2011

EARTH LINK

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Energy Efficiency and

Renewable Energy Bulletin

November, 2010 UPDATED

Earthing and Electrical Safety

Fuse may not operate under fault

To start to address the issues it is first very important to consider why earthing of exposed metal of electrical equipment is mandated by standards. Figures 1-3 show the basic principles on which protection of personnel from shock is provided by earthing exposed metal of electrical equipment as required by standards. C onducting frame

A

Conducting frame

A N AC

Figure 3 Consequences if there is a fault in equipment where the frame is not earthed

Z

N AC

Earth r eturn

Figure 1 Typical metal framed electrical equipment connections

Z

shows a piece of equipment correctly connected under normal operating conditions with no faults.

shows a piece of equipment where the earth connection to the frame as been left disconnected. Under fault conditions the equipment frame may be raised to a lethal voltage and the current flow will be through the person. This is a very dangerous and potentially lethal arrangement. What happens if the neutral connection to an installation becomes disconnected or broken at the point of attachment (POA) to a building ? This is a serious situation and can lead to serious safety issues in an installation which are not specific to PV.

Conducting frame

Fuse will operate under fault A N AC

Earth return

Figure 2 Typical metal framed electrical equipment connections with fault in equipment to earth

Z

shows the same piece of equipment if a fault to frame occurs showing the path of the current flow. Under these conditions no current flows through the person because the frame is connected to earth and the person is standing on earth so there is no potential difference that would produce current flow in the person’s body.

Issue 2 June, 2011

If the neutral becomes interrupted at the POA and the earth of the installation is not a very good return conductor to adjacent earths and neutral connections it can lead to a significant voltage present on the earth/neutral link of the installation as that point is connected to metal cases of electrical equipment including toasters, washing machines and other appliances. Under these conditions the metal cases of those pieces of equipment can be raised to a dangerous voltage with respect to the actual earth. If a person comes into contact with, for example a toaster, and also some earth point not connected to the installation’s MEN point then they may receive a shock. To reduce the likelihood of this type of occurrence the appliance standards require exposed metal of electrical equipment to be earthed.

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Energy Efficiency and

Renewable Energy Bulletin

November, 2010 UPDATED

PV Module Frame Earthing It has been suggested that PV modules should not be earthed because if the neutral becomes detached at the POA then the PV array frame may reach a dangerous potential. If you were to take this advice and not earth an array because of the chance the frame could become live, then you need to carry the thinking through to the conclusion that earthing any metal case is dangerous. So we need to cut all earth wires to toasters, washing machines, etc. which would be a very DANGEROUS situation if any fault developed in the appliance. Currently PV systems with NON-ISOLATED inverters are required by CEC to have module frames earthed. Refer to the CEC GC Installation guideline.

The module frames are no different to a toaster body because the voltages on the module frames are the grid voltage with a superimposed dc component due to the solar. The fault current available can be the full grid fault current. The main issue is fundamental electrical safety principles. There are also leakage currents to earth but this is a secondary issue. If you do not earth the frames and a fault develops in the PV modules or wiring, then the frames can become live with the grid voltage plus some superimposed d.c. voltage depending on the type and location of the fault. This is the fundamental reason why frames are required to be earthed and it is no different to the safety earthing of a toaster.

The EL-042 “Renewable Energy Systems” and EL-001 “Wiring Rules” committees support the earthing of PV module frames for safety.

Issue 2 June, 2011

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