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Low Voltage Installation Earthing: Understanding the MEN system, its behaviour and common pitfalls
The MEN (Multiple Earthed Neutral) or TNCS system of earthing required in Australian installations is an important and ubiquitous safeguard against indirect electric shock. However, the wide application of neutral earthing can often create poorly diagnosed issues of current flow in earth paths and persistent voltages, some of which are harmless but of which some can and have been fatally hazardous. This article explains the MEN system, its nuances and common misconceptions. Through examples, the benefits and consequences of neutral earthing can be seen including situations that can result in harmful conditions.
MEN System of Earthing
The MEN or TNCS system as defined in IEC 60364, is a standard electrical earthing configuration. The MEN system describes how the neutral is treated with respect to earth, that is, earthed at the supply source and downstream at multiple installations. A basic illustration is shown below.
Between the supply source and an installation, the neutral is a PEN (Protective Earth Neutral), serving both functions as the name suggests. An important aspect is that PEN conductors are not switched anywhere on the network. They connect through LV open points meaning that the number of installations forming the MEN usually far exceeds the number of customers supplied from a distribution transformer.
At a main switchboard the Neutral is earthed for the last time at the E-N link which is commonly referred to as the MEN link. Beyond this, a 3rd wire (or 5th wire in a 3 phase installation) protective earth is reticulated to provide earthing connectivity to all class 1 equipment and outlets.
The MEN approach to earthing is distinct from other earthing configurations such as T-T, I-T, earth isolated and double insulated where there is no low impedance path between protective earthing, earth and the neutral, or no protective earthing at all.
The MEN system has several benefits such as efficiency in the number of wires used for power distribution; redundancy in the PEN earth path; low hazard levels for faults on a an otherwise healthy system; easy to manage and hard to defeat; and the ability to readily detect insulation failures.
The E-N link is the physical connection between the Neutral and protective earths at an installation. Often there is only one, located at the MSB, but sometime on larger sites with more complex distribution, the neutral can be earthed at multiple locations.
In the event of insulation failure beyond the MSB, the E-N is the connection via which fault current flows from the PE back into the PEN to the supply TFR to complete the fault circuit.
It allows for a low resistance fault path, to enable enough current so that protection should reliably operate and maintain voltages at ‘safe’ levels.
An E-N connection that is broken or has poor integrity presents as a high impedance, causing fault current to flow through the installation earth stake. If the resistance of the earth stake is not sufficiently low, the fault current may be limited to levels similar to load current.
Safe versus Risk
Safety limits for voltage hazard through indirect contact are a function of voltage and contact duration. The higher the time exposure, the lower the tolerable voltage.
It’s important to understand ‘safe’ rarely has a meaning in the context of contact with electricity. Small voltages well below ELV can be unsafe and almost every level of energy in LV distribution carries risk of harm. In the context of electricity, designing for safety objectively means the absence of death. Rarely is consideration given to thresholds for pain, muscle contraction or let-go, breathing difficulty, or the harm that can be caused by these physiological effects.
As designers, if we are permitted to design for voltage rise on protectively earthed parts up to touch voltage safety limits, we should anticipate seeing severe shocks and tingles under certain circumstances. This is vital to understand, because it’s what happens next that can save lives.
Voltages between neutral and ground can occur on a healthy system. Not all voltage hazards that can cause a shock are symptomatic of a problem that will lead to an eventual fatality, but you can’t rule that out unless the mechanisms causing that voltage are understood.
Many DNSPs are cognisant of increased vulnerability to shock in certain situations such as wet areas, however few provide guidelines for design beyond the Wiring Rules.
Healthy MEN under fault
When a fault occurs within an installation in a healthy MEN system, the fault current path is low impedance and a large current flows in the protective earth, through the E-N connection and back to the source via the PEN. This is detected and cleared by overcurrent or residual current protective devices.
By design, the earth path resistance is sufficiently low to keep protective earth voltages below levels that could cause fatality, and protection operation ensures the hazard is short duration.
Concentrated or Sparse MEN
In built up areas with high concentration of installations and a highly interconnected MEN with conductive water piping, it is very hard to lift the neutral voltage, even under fault. In these circumstances, the neutral and PE voltages typically are very low, even with significant load imbalance and embedded generation.
By contrast in a sparse MEN, the assumptions about low impedance return paths may not hold, especially where non-conductive water mains piping is used. There are use cases which lend themselves to neutral voltage rise, and several for which shock investigations have been carried out.
Areas with low installations density supplied by long LV service runs, in high resistivity soil, or with significant load imbalance can suffer from persistent neutral voltage rise due to higher neutral impedances between installation and source. Installations such as caravan parks and campgrounds, mobile home villages, coastal surf clubs and marinas often report persistent nuisance shocks. Elevated neutral (and protective earth) voltage has been identified at such installations to cause routine ‘severe tingles’ at public amenities, kitchens, lighting bollards, handrails and demountable buildings. This can cause nuisance shocks which are painful, but the systems otherwise appear healthy, with no insulation failure or protection operations.
These persistent hazards can be very worrisome as it is often not easily determined whether the system is ‘healthy’ or whether there is a high impedance fault that may degrade and worsen the hazard.
Break at E-N
Several high-profile incidents have occurred due to breaks in the E-N or neutral service mains. In 2014 a fatality occurred at a limestone quarry in Cudal in Central West NSW when a fault occurred on a process motor. A missing E-N connection forced fault current to flow through the soil to complete the fault circuit, lifting the voltage on the neutral at the supply transformer to 230V. The transformer also fed an out-house residence via PEN which raised the protective earthing at the residence to 230V, leading to the electrocution of a resident in the shower while contacting the tapware. In this incident the fault did not clear as the fault path was high impedance, and fault current was significantly reduced below protection settings.
In 2018 at a Perth residence a break in a mains PEN caused load current to return via the installation electrode and created persistent voltage rise on the installation protective earthing as high as line volts. An 11 year old girl was severely shocked at a garden tap suffering burns and a crippling lifelong brain injury.
MEN and circulating currents
Some installations are particularly good sources or sinks of neutral current from the broader street system. For example, an installation near to a supply transformer that is a low resistance return path for neutral current spilled at other installations. In this situation a current is caused to flow in the PE and N even under no-load.
At larger industrial sites, or large substation earth grids, the LV neutral can be earthed at multiple locations with parallel earth paths. This leads to load sharing between neutral and protective earths, process pipes, cable tray and other earth paths which can be exacerbated through induction from cable lay causing ‘circulating currents’. The current in earth conductors and parallel paths can cause alarm however they are not usually hazardous, and can often be tolerated so long as the earth path is adequately rated for the current it carries.
Conclusion
The MEN system used in Australia is an excellent approach in most applications for minimizing electrical hazards under load and fault. There are, however, circumstances where the MEN system of earthing can lead to persistent hazards and current flows on and in earthed parts that are difficult to diagnose. Understanding the mechanisms of causation is very important to being able to distinguish between a healthy system or the presence of a failure that can lead to severe harm of fatality.
By Matthew Bale, CPEng, Engineering Director – Safearth
For more information on Safearth’s products and services, visit their website at www.safearth.com