tech info nov 2010 cec iss2 (1)

Energy Efficiencyand

Renewable EnergyBulletin

November, 2010UPDATED

Issue 2 June, 2011 of 3

EarthingPV module frame …

It is especially important that a PV module frameearth (equipotential bond) is providedwhen 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 framesproducing an a.c. voltage. While not likely to belethal, if not depleted by earthing, an electric shockwill 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 gridvoltage will be present – this is likely to be lethal.

Earth wires must be run so that the removalof one component (e.g. module) does not interruptthe earthing of other parts of a system.In other words daisy chaining earthconnections is not permitted.

Minimum recommended CSA - 4mm²

The PV earth connection is direct to the earth linknot via the inverter earth connection.

Refer to the CEC GC Installation guideline.

AS/NZS 3000:2007

SECTION 5 EARTHING ARRANGEMENTSAND EARTHING CONDUCTORS

5.1.2 Selection and installation requires thatearthing arrangements shall be selected andinstalled to ...

(c) Mitigate voltage differences appearingbetween exposed conductive parts ofequipment and extraneous conductive partsthrough equipotential bonding arrangements.

5.6 EQUIPOTENTIAL BONDING5.6.1 GeneralEquipotential bonding is intended to minimize therisks associated with the occurrence of voltagedifferences between exposed conductive parts ofelectrical 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.

PV String

GC Inverter

A

N

EEARTHLINK

AC Switchboard




Issue 2 June, 2011 of 3

Z

Earth return

Conducting frame

AC

N

A

Fuse will operateunder fault

Z

Earth return

Conducting frame

AC

N

A

Z

Conducting frame

AC

N

A

Fuse may notoperate under fault

Earthingand Electrical Safety

To start to address the issues it is first veryimportant to consider why earthing of exposedmetal of electrical equipment is mandated bystandards. Figures 1-3 show the basic principleson which protection of personnel from shock isprovided by earthing exposed metal of electricalequipment as required by standards.

Figure 1Typical metal framedelectrical equipmentconnections

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

Figure 2Typical metal framedelectrical equipmentconnections with fault inequipment to earth

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

Figure 3Consequences if there isa fault in equipment where theframe is not earthed

shows a piece of equipment where the earthconnection to the frame as been left disconnected.Under fault conditions the equipment frame may beraised to a lethal voltage and the current flow will bethrough the person.This is a very dangerous

and potentially lethal arrangement.

What happens if the neutral connection to aninstallation becomes disconnected or broken atthe point of attachment (POA) to a building ?

This is a serious situation and can lead to serioussafety issues in an installationwhich are not specific to PV.

If the neutral becomes interrupted at the POA andthe earth of the installation is not a very good returnconductor to adjacent earths and neutral connectionsit can lead to a significant voltage present on theearth/neutral link of the installation as that point isconnected to metal cases of electrical equipmentincluding toasters, washing machines and otherappliances. Under these conditions the metal casesof those pieces of equipment can be raised to adangerous voltage with respect to the actual earth.If a person comes into contact with, for example atoaster, and also some earth point not connected tothe installation’s MEN point then they may receive ashock.To reduce the likelihood of this type of occurrencethe appliance standards require exposed metal ofelectrical equipment to be earthed.




Issue 2 June, 2011

PV Module Frame Earthing

It has been suggested that PV modules should notbe earthed because if the neutral becomes detachedat the POA then the PV array frame may reach adangerous potential.If you were to take this advice and not earth anarray because of the chance the frame couldbecome live, then you need to carry the thinkingthrough to the conclusion that earthing any metalcase is dangerous.So we need to cut all earth wires to toasters,washing machines, etc.which would be a very DANGEROUS situationif any fault developed in the appliance.

Currently PV systems with NON-ISOLATEDinverters are required by CEC to have moduleframes earthed.

Refer to the CEC GC Installation guideline.

The module frames are no different to a toaster bodybecause the voltages on the module frames are thegrid voltage with a superimposed dc component dueto the solar.The fault current available can be the full grid faultcurrent.The main issue is fundamental electrical safetyprinciples. There are also leakage currents to earthbut this is a secondary issue.

If you do not earth the frames and a fault develops inthe PV modules or wiring, then the frames canbecome live with the grid voltage plus somesuperimposed d.c. voltage depending on the typeand location of the fault.

This is the fundamental reason why frames arerequired to be earthed and it is no different to thesafety earthing of a toaster.

an
The EL-042 “Renewable Energy Systems”d EL-001 “Wiring Rules” committees support
of 3

the earthing of PV module frames for safety.

tech info nov 2010 cec iss2 (1)

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