Important new solar requirements for the sizing of earthing and bonding conductors are crucial to adhering to standard AS/NZS 5033:2021, writes Michael Shaughnessy, technical program specialist at the Clean Energy Council.
On 19 November, 2021, the new AS/NZS 5033:2021 Australian standard was published. It became mandatory in NSW immediately, with other states and territories following on 19 May, 2022.
Among many changes detailed in the new standard, there are updated requirements to the way solar installers and designers size earthing or bonding conductors. The purpose of this article is to provide a consistent approach in the interpretation of Clause 4.6.5 of AS/NZS 5033:2021.
Introducing the powered neutral
The sizing of earthing and bonding conductors now depends on whether the inverter has a “powered neutral”, whether there is DC overcurrent protection, and where that fault might occur. The standard has introduced the term “powered neutral”, which is broadly defined as some electrical connection between the DC and AC sides of the inverter.
The manufacturer can inform whether a particular inverter has a powered neutral. AS/NZS 5033:2021 suggests that inverters with powered neutrals are typically non-separated single-phase inverters, three-phase transformerless inverters with voltage balancing capability, or all multiple mode inverters (Figure 4.13 Note 4 in the standard).
The reason for the introduction of powered neutrals is there are a number of different failure modes when it comes to non-separated inverter installations. It is important to understand these failure modes and the fault currents that can occur (either AC or DC). This will give a better understanding of the current-carrying capacity required for the earth cable. Therefore, read Appendix C in the standard before trying to determine the earth cable size.
Determining earthing requirements of a system
Standards do not usually set out requirements based on more than one fault occurring. However, often when an array earth fault occurs, another fault occurs in a non-separated inverter as a result. This completes a fault path, such as those that can be seen in Appendix C of the standard.
Once you have this understanding, the best way designers and installers can determine the earthing requirements of a system is to go to AS/NZS 5033 Section 4.6. The “Decision Tree” in Figure 4.13 leads us to which column of Table 4.6 you should be focusing on for a particular solar installation.
The combination of “Decision Tree” Figure 4.13 and Table 4.6 provides good initial guidance in determining the earthing conductor sizing of the system that is to be installed (assuming they are copper conductors).
The following explains how the columns in Table 4.6 can be interpreted to determine the earthing conductor size of a system.
Table 4.6 (a) (i) and (ii)
The phrase “At least the same size as the string, sub-array, or array cable, which is associated with the equipment to be earthed, as defined in Clause 4.4.2” is saying the earth conductor shall be the size of:
- The string cable where a string fault could occur.
- The sub-array cable where a sub-array fault could occur.
- The array cable where an array fault could occur.
This may result in earth conductors reducing in size as you work towards the extremities of the system.
Table 4.6 (a) (i) and (b) (i)
The phrase “As per the earthing requirements of AS/NZS 3000 taking into account the size of the active conductor of the inverter AC cable” is saying the earth conductor shall be the size of the earth cable that would be run with the AC active conductor as per AS/NZS 3000 cl. 5.3.3.
Table 4.6 (b) (ii)
The phrase “As per the earthing requirements of AS/NZS 3000 taking into account the size of the active DC conductor of the array or sections of an array with DC overcurrent protection” is saying the earth conductor shall be the size of the earth that would be run with the DC active conductor as per AS/NZS3000 cl. 5.3.3.
Determining the largest calculated earth size
It should be noted that the two paragraphs under Table 4.6(a)(i) are headed by “The larger of the following” so whichever paragraph results in the largest earth cable size will be the minimum.
The largest calculated earth size shall be maintained through to the earthing system (main earth bar) because the fault path can flow through the MEN of the electrical installation.
In areas not subject to lightning, inverter AC cable carried earths may be used as long as it is of sufficient size according to AS/NZS 5033:2021 Table 4.6 and meets the minimum earth size of 4mm squared (Clause 4.4.2.3, 4.6.4, 4.6.5).
Clause 4.6.5 also clarifies that module mounting framework may be regarded as a protective earthing conductor when PV modules are mounted to it as per the manufacturer’s instructions and they are earthed by connection of a protective earthing conductor (cable) to that framework.
After all of this has been analysed, calculated and installed, the array earthing system test shall result in resistance not more than 0.5 Ω between all exposed conductive parts and the installation earthing system (main earth bar), as per the first paragraph of Clause 4.6.5.
The Clean Energy Council will continue to provide additional information to Clean Energy Council accredited designers and installers and members through additional technical information that can be found in the new Technical Resources area of the Clean Energy Council Installer Portal.
