Although Masterspec electrical sections did mention earth bonding, users have recently pointed out that some BCA's were requesting more information. So it became obvious we needed to provide more detail and try to clarify this confusing subject.
This article is intended to provide a background of earthing and equipotential bonding of conductive elements of the building that do not form part of the electrical installation (electrical cables and equipment etc). Although earthing and equipotential bonding are an essential part of electrical safety, they are different methods of preventing electric shock and are often confused. We'll try not to get too technical, as that can get very complicated for the non-specialist. But it may give you a feel as to when you need to consult an Electrical Engineer etc. These requirements relate to virtually all buildings including houses.
What is the difference between earthing and equipotential bonding
In basic terms, the idea of earth and equipotential bonding is to prevent electric shock.
Let’s use a kitchen as an example. If a dishwasher were to develop an electrical fault, the current would be taken away and travel down the electrical installation earthing path. Without that earth connection, the metal outer casing of the dishwasher could become 'live'. The principal of earthing is to prevent this situation occurring, by limiting the duration of touch voltages.
Equipotential bonding is a connection of all at risk metallic (conductive) parts using a protective bonding conductor. Taking our faulty dishwasher above, if you were to touch it and then touch an un-bonded metal tap connected to un-bonded metal pipes in your kitchen, you could become an alternative earth path and could receive a potentially life-threatening electric shock.
The lack of bonding (as well as earthing) would create a chain effect of all metallic parts becoming live in the kitchen. It’s quite a scary scenario which is why earthing and bonding are so essential and why it is important to know the difference between the two. Bonding is intended to limit the magnitude of touch voltages.
AS/NZS 3000 Electrical installations (known as the Australian/New Zealand Wiring Rules)
Both AS/NZS 3000:2007 (cited in NZBC G9/VM1) and AS/NZS 3000:2018 (not cited in NZBC) cover the subject of earthing arrangements and earthing conductors in their respective Section 5. The two versions are very similar and for the most part identical in the way they deal with conductive elements of the building that do not form part of the electrical installation.
MEN earthing system
With few exceptions, New Zealand and Australia use the Multiple Earthed Neutral (MEN) system as the basis of electrical distribution and installations. Basically within an electrical installation, the earthing system is separate from the neutral conductor, and is connected to exposed conductive parts of equipment. Internationally this system is considered safe and reliable when it comes to dealing with short circuits, earth faults etc.
What this means for most installations is that the main switch board will have a main earth bar and a separate main neutral bar. However for extra safety the neutral bar is connected to the earth bar by a MEN connection. The main earth bar has a main earthing conductor directly connected to the earth electrode. The earth electrode is usually that familiar metal rod driven into the ground with a lug and a sizeable green/yellow cable attached.
This system provides a path for a short circuit or fault to flow back through the system back to the main earth bar and then safely to ground. Other interventions like RCDs etc. will cut supply if triggered, which provides extra safety.
Earthing conductive elements
With the MEN system in place, generally most of the electrical installation (electrical cables and equipment) is taken care of. However AS/NZS 3000, 5.4.6 Structural Metalwork And Conductive Building Materials, introduces the requirement to earth at risk conductive building elements. The key term is "at risk", which is the risk of conductive parts coming into contact with live cables/equipment or damaged cables. There are steps the electrician can take to deal with this, so the electrician has to make the risk assessment.
Obviously, nothing electrical in the vicinity of the conductive elements or some form of non-conductive separation, means there should be no "at risk" and therefore no earthing requirement.
There is a clause in AS/NZS 3000:2007 but not in AS/NZS 3000:2018 which makes a difference. AS/NZS 3000:2007, 220.127.116.11 Domestic Electrical Installations, states "Structural metalwork forming the frame of a dwelling shall be earthed". Which means that a design under 2007 (to NZBC G9/VM1) for a dwelling, any structural metal frame must be earthed regardless of risk. It also begs the question what is exactly meant by "domestic" and "dwelling"?
There is nothing in the Standard(s) that talks about exposed or non-exposed conductive elements with regard to earthing. So the assumption has to be that it applies to most exposure situations.
Examples of what might need to be earthed are:
- Structural steel frames or members
- Light steel structural framing
- Exposed conductive materials, like metal sink/tub or vanity benches etc, with electrical units or equipment in contact
- Depending on the situation and risk other conductive items like metal cladding/lining, roofing etc.
AS/NZS 3000, Section 5.6, Equipotential Bonding
Clause 5.6.1 states "Equipotential bonding is intended to minimize the risk associated with the occurrence of voltage differences between exposed conductive parts of electrical equipment and extraneous conductive parts". In other words, without equipotential bonding if you touch electrical equipment (i.e. dishwasher) with a fault and also touch metal which is not part of the electrical installation, the potential voltage will not be equal, so in trying to be equal it may want to go to earth through you.
Equipotential literally translates to "equal potential" (voltage), so bonding together conductive parts allows the potential to be substantially the same and therefore safer. Exposed extraneous conductive parts that are in some way connected to ground (earth) are the main risk because of unequal potential. For instance the metal water pipe coming from under ground to the metal sink tap in the stainless steel sink bench would normally have to be bonded.
Bonding is usually simply a large earth wire in good secure contact with the exposed extraneous conductive part(s) which is usually connected directly back to the main switchboards main earth bar and then of coarse to the earth electrode.
You may be surprised to hear that concrete with reinforcing is considered conductive if water is present. But a metal pipe isolated from ground by a piece of plastic pipe is not.
Unlike earthing, equipotential bonding only deals with exposed extraneous conductive parts.
Examples of what might need to be equipotential bonded are:
- Conductive water piping (including tap etc) and exposed related connected conductive surfaces (like metal sink benches or metal cladding etc). Not required where pipes are isolated by non-conductors (plastic pipe etc) from the mass of earth
- Other conductive piping (not earthed by other means) and exposed related connected conductive surfaces
- Concrete reinforcing - for floor or wall forming part of a room with a shower or bath, or the shell and surround of a swimming/spa pool
- Built-in Swimming pool and spa pool - exposed conductive parts of electrical equipment, as well as exposed conductive, fixtures, fittings and pool structures within 1.25m (horizontally) of pool edge
Note: equipotential bonding can be overdone, non-risk electrically isolated metal parts (i.e. an aluminium window) bonded to other at risk metal parts, could elevate risk due to the unexpected nature of the result (no one expects a window to be even briefly live). Seek expert advice if there is any doubt.
Masterspec has added clauses and guidance, along the lines of this article, to all the relevant Electrical Sections.