MEN System - Everything You Need To Know
AS/NZS3000 (also known as Australian/New Zealand wiring rules) has defined the MEN system as follows:
“A system of earthing in which the parts of an electrical installation required to be earthed in accordance with this Standard are connected together to form an equipotential bonded network and this network is connected to both the neutral conductor of the supply system and the general mass of earth. (AS/NZS3000:2007 Clause 1.4.66)”
Most of the power authorities in Australia have accepted two types of Earthing Systems:
- The MEN (Multiple Earthed Neutral)
- The CMEN (Common Multiple Earth System)
The low-voltage and high-voltage earth systems are kept separate in the MEN system.
A low-resistance path is created for the earth fault currents by the distribution of neutral conductor. Its ‘potential rise’ is kept in control by connecting it to earth at a few locations all through its length.
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The Origins of the MEN System
Before 1980, there were three accepted wiring systems in Australia.
Earth’s mass cancels the effect of leaking current in an installation by earthing out. A correctly earthed system lets one isolate any defective electrical equipment by the means of protective devices.
Since 1980, the voltage-operated ELCB protection and the direct earthing system have been discontinued here and the MEN system has been used exclusively because of a few reasons:
- In the MEN system, a low-voltage distribution transformer is connected to the soil at both the transformer and base of fourth transmission fault. A timber batten acts as ‘touch potentials’ and provides mechanical protection to the pole.
- The MEN system is designed such that it ensures that when a fault occurs in a way that the active conductor contacted the exposed metal, then the fault current takes the earth conductor as a return path on its way to switchboard. This allows the protective device to open circuit and thus it is made safe.
- It helps to reduce neutral to earth voltage -- which rises because of an increase in the neutral current -- especially for the long low voltage runs.
- When active current passes current to earth, the MEN system uses the distribution of neutral conductor for carrying the resulting fault currents.
Why We Need the MEN System in Australia
MEN System is considered to be the most important safety component of any electrical system in Australia. Because there is no moisture or conductivity in the sandy soil of Australia, you have got to make sure that all the installations you do have the MEN system, with good earthing. It is also required by law and by science.
The Science Behind the MEN System
Talking about Science, I would briefly like to discuss the Ohms Law -- When the resistance goes up, the current goes down. The voltage remains constant. V = I x R.
The higher the resistance, the lower the current.
In a loose connection, the resistance increases.
Why Is the MEN System So Important
It makes the circuit safe.
In an electrical fault where the neutral conductor is disconnected and the active conductor is connected to the exposed metal network, the current flows through the earth conductor to the switchboard, which allows the protective device (such as fuses or circuit breakers) to open circuit. In this way, the MEN (Multiple Earthed Neutral) system makes the circuit safe.
It helps during Earth Fault Currents.
When there are earth fault currents, the MEN system uses the network neutral conductor as the conductive path for installation. It is called the “earth fault current” because the fault current of an electrical appliance or an equipment goes to the earthed frame.
It reduces the fault current.
These faults currents should be large so that the protective devices (such as fuses or circuit breakers) can operate. When an Active to Earth fault occurs, the fault path is through the low resistance circuit to the MEN link to the source of power (such as a generator or a transformer). The neutral conductor provides the low resistance path so that the power can return. In absence of the MEN link, the fault current would return via ground.
It prevents fatal voltage on electrical appliances.
Earth has a higher resistance than the MEN/neutral conductor path. Thus the fault current’s capacity would be limited to a degree that it might not be able to operate the protective devices. As a result of this, a fatal voltage can remain on the metallic enclosure of equipment or electrical appliance.
Thus it is important to install MEN link during electrical installations.
Characteristics of the MEN System:
The main earthing electrode is installed in a soil that is conductive.
The transformer’s (source of supply) neutral should be earthed.
In the main distribution panel, a link connects neutral and earth.
All the exposed metallic parts of the electrical system are earthed for your customers' safety.
Components of the MEN System
How effective the MEN system is depends on:
- The main neutral conductor
- The MEN link
- The connection of the main earthing conductor to the earth electrode. All the earthing conductors should be run separately to a single earth bar.
Let's take a detailed look at various other elements of the MEN System.
Main earthing terminal/bar
It enables the connection of the main earth, protective conductors with equipotential bonding or the ones with functional earthing, and the MEN connection to the general mass of earth.
- They are usually high-conductivity copper or aluminium conductors.
- It is also acceptable to use stranded steel catenary wire, metal conduit, and conductive sheaths.
- When used in cables, all the earthing conductors must meet the CSA requirements.
- The earthing conductors can be connected as tunnel-type connections or soldered connections.
- It is the link between the main earth terminal bar and the main neutral conductor bar.
- Its current carrying capacity must not be less than that of the main neutral conductor.
- An insulated MEN link should be green or green/yellow.
- In case the switchboard is installed outdoors, it needs special attention while establishing earthing and the MEN connection. After testing the installation, you need to recheck the MEN connection.
Main Earth Conductor
- It connects the main earthing terminal/connection or bar to the earth electrode.
- The CSA of any copper main earthing conductor should not be less than 4 mm².
- The smallest aluminium conductor which can be used as a main earthing conductor is 11 mm².
Main earth resistance
The resistance of the main earthing conductor is measured at these two points:
- The main earthing terminal/connection and bar
- The earth electrode, including the connection to the earth electrode.
The measure resistance between these two points must not be greater than 0.5 ohms.
- It connects the neutral of the electrical installation to the general mass of earth.
- Earth electrodes can be copper-clad earth rods, strip electrodes, and rod electrodes. Earth stake, copper strap, and bare copper cable are a few additional examples.
- Its connection to the earth must be located in moist soil such that there is protection from mechanical damage.
- Earth electrodes should come with mechanical protection.
- Its location should be identified at the main switchboard.
LABELING of the main earthing connection
The main earth connection should have a permanent label attached to the connection to the earth electrode with a warning that the main earth conductor could be disconnected.
Earthing resistance of an electrode
- It is made up of three essential elements: resistance of the electrode, contact resistance between the electrode and the soil, and the resistivity of the soil.
- Soil resistivity determines the resistance of an earth electrode as well as the depth it should be driven to for low ground resistance.
Protective earthing conductor
- A bonding conductor that is not the main conductor is called a protective earthing conductor.
- It should be connected to the main earth bar or to the main earth conductor via various methods.
- It provides a safe path for earth fault current so that the protective devices can remove dangerous potential differences, which are inevitable under the fault conditions.
It is a way by which various types of earthing conductors cause various exposed metallic conductive parts and extraneous conductive parts to be at the same or almost same potential.
I hope you have a clear idea of the MEN system now.
You can watch “The MEN System Explained” video on the Electricians Success Academy for further reference.
I would like to welcome your questions, if any.
Thank you so much!
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