I am often in a situation where I have to work on an old switchboard that is in a house built in the 60s or 70s, and sometimes earlier.
These old houses were built when the Wiring Rules were different than they are now. The technology of houses, and the equipment that is installed in them has also changed, and this has resulted in subsequent revisions of the wiring rules – stands to reason.
In this article, I specifically want to talk about the earthing system in houses, what it does, and why it requires repair and upgrading.
Throughout this article, I build a case for the replacement of your main earth, and your earth electrode. Why it is necessary, and the clauses that relate to it’s mandatory replacement are listed at the bottom of this article.
The MEN system
In Australia, we use a system called the Multiple Earthed Neutral (MEN) system. This is also known as the TN-C-S system. The Neutral at each home (any premises with an electricity supply), must have an earth electrode and this must be connected to the supply neutral (through a Neutral and MEN link).
I don’t specifically want to go too far into the reasoning, or specifics of this system, but want to point out some changes.
Before 1979, the water pipes supplying homes were allowed to be used as the main earthing electrode. Therefore the Supply Neutral was connected to the water pipes – the water pipes formed the earth electrode.
Back in the 70s, this was perhaps reasonable. Loads inside houses were low, pipes were always copper or galvanised steel.
There was still potential problems and this exposed residents and users of buildings to electric shock risks.
Since 1979 (I believe), the Wiring Rules has specified that each premises has a separate earth electrode, with the water pipes bonded to the earth system separately.
There are a number of very good reasons for this.
1. Pipes degrade
Pipes degrade (particularly galvanised pipes), and continuity decreases. With a decrease in earth continuity, your electricity system becomes more dangerous in the event of a fault.
Plumbers have been killed when repairing pipes within the water system due to pipes being used as a main earth.
2. Conductive Building Materials
We now understand that other building materials can be conductive. Older houses were generally brick and timber. However changes in technology has resulted in different building materials being used.
Newer homes are now generally concrete slab constructions (which are conductive – they have conductive steel structures within them), steel frames, steel beams etc. All of these conduct electricity to some degree, and all can become a hazard in certain situations. This has led to a need to connect them all together (bonding) and to earth them.
3. Plastic water pipes
The new water distribution system is plastic – HDPE (High Density Poly Ethylene – or a variation of). Plastic doesn’t conduct electricity, but water does. This has led to a situation where the floor of a kitchen might have a different voltage to the tap at your sink. This can lead to an electric shock, or a death under certain situations.
4. Gas pipes
Gas pipes within homes are usually copper, but street mains are plastic (HDPE or similar). Without bonding to the earth system, there is an increasing likelihood that different areas of your house will have a different voltage to different surfaces and your gas pipes. You might have a voltage difference between the oven, and you tile floor for instance, or the gas cook top and your sink. Touching both at the same time could result in an an electric shock if there is a difference in voltage.
Potential difference is usually what we might refer to as voltage. However, the correct term in this context is potential difference.
With all of these technology changes, and better understanding of electricity, the Wiring rules have had to evolve to try to better protect occupants.
One of those changes has been the earthing system.
Under certain conditions, it is possible to have a different potential present between different house surfaces.
We might see that the floor of your kitchen has a potential of 20V compared to the soil outside. But if the tap at your sink has a potential of 100V (which is possible), you have a potential difference (or voltage difference) of 80V which could be enough to cause injury or even death.
Why is this?
A variety of reasons could lead to potential differences in different areas of the house. This can be due to earth leakage of appliances (small amounts of electricity passing to the earth conductor through your appliances), bonding to your pipes, plastic pipes, different areas of the house touching the soil in different areas (the soil has differences in potential with distance – and soil is conductive). Your neighbour might have a fault at their premises, causing electricity to pass into their pipes – and hence yours!
This can result potential differences between different surfaces in your house.
I remember as a young person touching the tap while standing in the shower and regularly receiving a tingle. Looking back, as this was an old house, it was highly likely that the main earth was connected to the water pipes, and that there was no earth electrode (or that the earth electrode was in poor condition)
So this leads us on to equipotential bonding – where all conductive building materials are connected together – and all connected to the main earth.
For some time now, the Wiring Rules has specified that all conductive building materials now need to be connected together, and connected to the main earth at the switchboard.
This is to try to limit the potential difference between different building materials – reduce the likelihood that touching two at the same time might result in an electric shock.
Now that we have gone through the preliminary part of this article, explaining why we earth and bond conductive parts of buildings, we can now talk about one of the main parts of that building – the Main Earth, and Earth Electrodes.
What is an earth electrode, and why do we need it?
As mentioned above, our electricity network uses the MEN system. The neutral at every premises is earthed to the soil through this earth electrode.
The earth electrode is usually a big metal rod (earth stake) that is driven in to the soil. It is driven a minimum of 1.2m deep depending on soil conditions.
Sometimes the earth electrode is an interwoven mesh (earth mat), or a 3.5m long thick copper wire (strip earth).
To a degree, this stabilises the voltage at each house by providing a static zero potential (reference zero volts), and provides a low resistance fault path to earth – though that is limited and can have its own consequences!
What happens if I don’t have a separate earth electrode?
Well this depends on the circumstances.
If you have an old house and it is in good condition, your risk could be low. Note I have said “could”.
If your pipes are used as an earth electrode (as many are), your electrical hazard is far higher than someone who has a separate earth electrode.
If there is an electrical fault on a neighbour’s property you are at an increased risk of your pipes carrying electricity into your property. The metal pipes conduct electricity into your premises, and if you don’t have a separate earth electrode, and have your pipes bonded, you are at an increased risk of injury to electric shock.
Water pipes are being changed to plastic in many streets in Australia. This is because street mains are degrading and rupturing. When they rupture, are degraded, or need replacing, often they are changed over to plastic.
When they are changed over to plastic, your water pipes are no longer continuous and no longer provide an effective earth.
If you don’t have an earth electrode the potential difference between different metal surfaces could suddenly become very high, with an increased likelihood of electric shock.
I refer to my previous article where a young girl in Western Australia received a near fatal electric shock and is now permanently brain damaged because of a bad earth and a neutral fault.
What are the rules?
As a matter of interest I clarified this with Ausgrid Inspectors recently, because it is important that I provide clear advice and guidance to my clients.
Their response was:
When making additions or alterations to a premises, it is a requirement that a separate earth electrode be installed where one is not present.
i.e. where pipes currently form your earth electrode, a new independent earth electrode (earth stake, mat, or strip) be installed. Where installing new consumer mains, it is mandatory that a new earth electrode be installed.
Where alterations to a switchboard are being made, it is a requirement that an earth electrode be installed where one is not present.
One of the clauses that define this is presented in The Service and Installation Rules of New South Wales July 2018. (obviously this relates to NSW – my state)
Clause 1.17.7 – Earthing
New electrical installations, and alterations or additions to existing installations must be earthed using a Multiple Earthed Neutral (MEN) system complying with the requirements of AS/NZS 3000.
from AS/NZS 3000:2018
1.9.3 Alterations and repairs
Alterations to electrical installations shall comply with all relevant provisions of this Standard.
The current rules are quite specific and indicate where and how an earth electrode must be installed. For instance, an earth electrode must be installed at least 600mm from the nearest storm water pipe! Not easy to do sometimes, and VERY expensive when an earth has to be remote to the building!
Do you need to replace it?
If you are altering your switchboard, adding load (anything to your circuit), or making any extensions, you don’t have a choice.
Should you replace it?
Definitely – even if you aren’t making any changes to your electrical system.
Bye for now,
#earthing #Costs #Compliance #safety #MainEarth #Electrode
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