Earthing systems explained
Earthing systems explained
Was the MEN system ever good? and what’s changed?
- Earthing systems
Rethinking the Earth Electrode: A Critical Look at MEN System Safety
This is my attempt to clearly lay out a basic understanding of earthing systems with a particular focus on why we don’t use isolated systems, and questioning the MEN system. I take a critical thinking approach here, which means what the regs require, or what is commonly believed will be put to one side in an effort to understand the electrical principles at play here. I will not explain the ABCs, I’ll just endeavour to keep it clear and technical. This article is designed to stand alone, but it is inspired by and is a response to some recent articles by Athol Gibson and Warren Harris.
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Why Not a Fully Isolated System?
The first question is: why not have a fully isolated system? Prior to RCDs, construction workers used isolation transformers to increase safety when working with hand tools. If isolation from earth creates more safety here, then why not isolate an entire installation?
- Earthing systems
There are two reasons why isolating an entire installation is different from isolating a single appliance
The kind of isolation we are talking about here involves having no fault current through a person, and also having no fault current through protective earthing conductors. For a single appliance this is safe because if one active conductor contacts the metal frame, there is no shock risk. It’s the same principle that means birds can safely sit on power lines. But if two appliances have faults each to a different active conductor, these then pose a serious electrocution risk. If someone touches both appliances at the same time they will get a serious electric shock.
In medical situations fully isolated systems are used. They do this primarily to reduce the need for protective devices to immediately isolate the power in the event of a fault. Instead, operators and technicians are alerted to the fault. RCDs can offer similar safety but they do this by tripping extremely fast at the smallest fault. In medical situations suddenly losing power can create other problems. An isolated system is safe if only one fault occurs. Whichever conductor is part of the first fault becomes like a grounded neutral posing no shock hazard, but if a different conductor comes in contact with a metal frame then an electric shock hazard is created between these two points. This system relies on complex and expensive monitoring systems, making it not suitable for widespread use in regular installations.
The second reason fully isolated systems don’t work is[a] because they lack robust paths for managing transients, stray voltages, and surge currents. Even in an “isolated” system, conductors exhibit distributed capacitance to earth and nearby structures, allowing small leakage currents and voltage drift. Without a proper earth reference, voltage transients (lightning, switching surges, arcing re-strikes) have no low-impedance path to dissipate, which causes insulation stress. Over time, this can cause partial discharges, insulation degradation, or failure under peaks.
So fully isolated systems are not an option. We at least need a transformer earth to protect equipment and people. But do we need earth electrodes at each installation? The current MEN system in NZ has been in use for 100 years, it has the backing of industry experts, and is used in many other countries. This proves it’s a fairly safe system, but it doesn’t explain the why behind this system and it doesn’t tell us if it’s the best system or if elements of it could be improved.
Athol Gibson’s Challenge and Warren Harris’s Reply
Athol Gibson recently put out an article questioning the value of the installation earth electrode. For the longest time the importance of the installation earth electrode has been dogma. This was the first time I’d seen a trusted industry expert questioning this. His concern with the MEN system centred on the risk of main neutral faults. This is where the supply neutral fails before the MEN link and before any RCDs. The electrical safety regime in NZ follows a two-fault logic. The goal is to make it so two faults must be present simultaneously for harm to be possible.
Warren Harris wrote a response to Gibson where he made a case that the installation electrode is still important. He acknowledged the weakness of the MEN system by explaining that there are two faults that by themselves pose serious risk: a transposed mains, and a high-impedance main neutral. A transposed mainscan only occur if a technician transposes the conductors and fails to test[b]. That means two separate errors must occur for the fault to arise, but the two fault safety regime is not really satisfied here because once the fault is present this single fault is dangerous.[c][d] However, it’s worth pointing out that a TT system is immune to this kind of fault. That’s because a TT system has no connection between the earthing conductors and neutral. It also has whole-installation RCDs that trip if any kind of shock hazard is present.
The following diagrams illustrate the safety protection afforded by MEN vs TT systems. The first one shows how MEN systems are vulnerable to far more faults. The 2nd one illustrates how TT systems are designed. The 3rd image shows how there are very few faults in which fault protection will fail to work in TT systems.
Warren Harris also points out that these faults are not hypothetical but inevitable. Property owners typically run electrical installations until failure, rather than conducting preventative maintenance that could catch issues early. He further notes that the main neutral conductor fails more often than the phase conductor. This is because live connections are generally better shielded from the elements, while neutrals are often more exposed.
So, Warren Harris acknowledges the risk of neutral faults in MEN systems, but he still thinks it’s a good system. We’ll look at his reasons later, but for now let’s get a good understanding of main neutral faults.
Neutral Faults in Practice
When the main neutral connection is partially or fully lost, the installation’s load current seeks return via the MEN link through the earth mass and back to the distribution network neutral. Because this is high-impedance compared with the neutral, the installation’s bonded earth system floats up towards phase potential. The higher the neutral impedance, and the heavier the loading, the higher the earth potential rise. If just one low-current light is connected the voltage won’t be high. Turn on the oven and the earth voltage goes up. If a customer calls complaining of ‘tingles’ and ‘shocks’ and you suspect this fault, they need to turn the power off until an electrician gets there — removing the supply takes the danger away.
The Perth Tragedy (2018)
A failed main neutral conductor caused a home’s earthing system to become live. A young girl suffered severe brain damage when she touched an energised garden tap. The home’s RCDs didn’t trip. The system, with its functional earth electrode, performed exactly as designed — and the result was catastrophic.
An Isolated Installation
I once attended a lost neutral fault at a house built on tall timber piles. I measured 180 V between independent earth and the installation earthing system. This meant the meterbox and all earthed metal appliances were energized at 180 V. And yes, I did a low impedance voltage test, this was not ghost voltage. Interestingly, not one person inside the house had felt a shock. This apparent safety was due to two factors: isolation and distance. Being raised off the ground and far from neighbouring earths, the occupants inside were not part of an effective circuit.
However, this did not mean the installation was remotely close to being safe (>50Vac is electrically unsafe). Had someone stood barefoot on damp ground while touching an energised garden tap, they could have bridged that 180 V potential directly to earth. The hazard was still present, but its consequences were mitigated by high resistance and luck.
Metal cladding and EV chargers are becoming more commonplace. Earthed metal cladding or a charging EV at this property could have led to a fatal electric shock. This fault would have livened the metal frame of a charging EV, and the EV would likely be parked close to soil, increasing the chances of a person bridging the 180 V to earth and suffering a severe electric shock. EV chargers also draw a lot of current and this added load stresses supply systems leading to more failed mains connections.
The Traditional Rationale for MEN
The Fault Clearance Myth
It was once taught that the earth electrode helped trip fuses or MCBs. This was true once upon a time. In the past metal pipes not only created an excellent connection to the earth mass, but they also acted as a shared fault conductor joining installations. If a neutral was lost then resulting voltage on the earthing system would travel through these metal pipes to the earthing system of neighbouring installations, through the MEN link, and back down the neighbour’s main neutral. The result is that voltages could not build up and supply fuses could trip if too much voltage was getting dumped onto the earthing system.
But this is no longer the case. Houses are now connected with plastic pipes and electricians instead install an earth rod. A typical New Zealand electrode has an impedance of 10–100 Ω or more. This is far too high to allow the hundreds of amps needed to operate an overcurrent device fast enough to achieve safety.
Even in ideal conditions — say 20 Ω — only ~11 A would flow at 230 V. That stands no chance of clearing a 60 – 120A supply fuse to the installation. Fault clearance relies on the MEN link and neutral conductor, not on current through soil. NZS 3000 (the ‘wiring rules’ states unambiguously that the earth electrode does not usefully contribute to lowering the earth loop fault impedance (ELFI).
RCDs Are Blind to the two nasty MEN Faults
Against high-impedance neutrals and polarity reversals, the sub-circuit RCD are simply ineffective as shown in fig. 1. An RCD detects imbalance between the active and neutral conductors passing through its sensor. In a neutral fault, the diversion of current into the earth happens after the current has already passed back through the RCD’s neutral coil. To the RCD, the circuit looks balanced, so it doesn’t trip.
The only way to protect against a mains neutral fault with RCDs is to place an RCD upstream so it protects the whole installation — as is standard in TT systems in Europe. This is not done in New Zealand. Another way would be to monitor phase-to-neutral voltage. If it gets too low or too high then a fault is detected and power can be disconnected or an alert activated. This is called O-PEN or OPDD detection.
Modern 30 mA RCDs are extremely sensitive and will trip reliably even if the electrode resistance is high — to several kOhms. In practice, the difference between a pristine electrode and a poor one makes no practical difference to their operation.
The One Weakness of TT Systems
TT systems are not flawless. They carry a lesser but real vulnerability: when a neighbouring MEN installation develops a neutral fault, fault current can be driven into the soil, raising the local earth potential. A nearby TT installation within that zone can experience that same rise on its own earth electrode, even though it has no direct connection to the faulted system. This can, in theory, cause a voltage to appear between exposed conductive parts in the TT installation and true earth. Note the neighbouring MEN installation is already unsafe too, so this isn’t really a ‘TT’ issue.
It’s important to understand that this is not a flaw of the TT system itself but a by-product of MEN systems injecting current into the ground. The TT site is a victim, not the source. In practice, the resulting voltage gradients in the soil are usually small and localised, and the likelihood of harmful current through a person is far lower than the risks posed by lost-neutral or transposition faults inherent in MEN systems. Still, it’s a reminder that every earthing system exists within a wider electrical ecosystem — and that the dangers of MEN don’t stay neatly confined to the installation where they begin.
Warren Harris’s Defence of Earth Electrodes
Warren Harris has argued that earth electrodes remain critical because they:
- Provide a return path for load and fault current if the neutral is broken.
- Help limit voltage rise in open-neutral situations.
- Assist RCDs in detecting earth leakage.
- Are required by AS/NZS 3000, which confirms their importance.
- Must be properly maintained to prevent corrosion or loss of integrity.
These arguments deserve careful scrutiny.
Replying to Warren Harris’s Points
Return Path for Current and Limiting Voltage Rise
There’s some truth here. A properly functioning earth electrode will allow some current to flow, thus lowering the voltage on the earthing system. But this might also mean the fault lingers much longer before someone notices it. This is because the flowing current allows devices and appliances to continue to function in some conditions. If the neutral and earth mass were isolated, not only would there be no electric shock current path, but the power would stop working altogether under fault conditions, triggering an electrician to be called. The longer a fault like this lingers, the more chance of someone touching a live garden tap in wet conditions, which could be fatal.
Assisting RCDs
Warren argued that electrodes can still help by providing an alternate path that allows RCDs to detect imbalance. Let’s break this down. The RCDs in question are between 10mA and 30mA. If a 230v fault occurs an RCD will still trip even with a 7000 ohms impedance to earth. Even the 500ma RCDs used in TT systems can trip with 450ohm impedance to earth.
But even if we assume the electrode does help RCDs trip. It’s a double-edged sword. The same feature that helps RCDs to trip also creates a possible return path through a human body. This is fine for faults that RCDs can handle, but in NZ we don’t RCD-protect pumps, hot water and ovens — and RCDs cannot help with main neutral faults. In Australia they put everything on RCDs, but they also get a lot of nuisance tripping. For faults that RCDs don’t help with, the electrode just creates more risk. We discussed one case where things were safer precisely because people were more isolated from ground.
Legislation Requires It
Warren Harris mentioned that Standards require it and suggested that the standards would only require it if was indeed important. It’s certainly true that the standards require it. But standards reflect consensus, not immutable truths. Regulation can lag behind. A lot has changed since this system was first implemented and critical analysis in light of these changes raises some questions. Quoting AS/NZS 3000 shows compliance requirements, not proof that the measure is optimum in today’s risk environment.
Integrity of Electrodes
Warren Harris is right that electrodes corrode and fail. But maintenance only matters if the function matters. If the safety contribution of electrodes is marginal at best, ensuring their “integrity” becomes an exercise in maintaining something that does not significantly enhance protection.
There’s another angle that often seems overlooked: the lack of any defined benchmark in our regulations for what constitutes a “good” earth electrode in MEN installations. In nearly every other critical safety domain, we demand precise measurable criteria — conductor cross-sections, volt drop, fault loop impedances, RCD operating times, etc. But for the earth electrode, there is no compulsory limit and no test to show the electrode is performing.
Contrast that with TT systems. In France there is a design guidance threshold of 100 Ω (or less) for the installation earth electrode under certain conditions, tied to the sensitivity of the protective RCD. In those systems, the electrode is truly part of the safety chain — it must allow sufficient current to flow so the RCD can detect and act on faults.
However, in our MEN system, removing the installation electrode would not clearly break any of the fault-clearing loops (which rely on the metallic PE → neutral path). So I struggle to see what safety risk is systematically created — if any — by omitting the electrode entirely.
Conclusion: A 100-Year-Old Habit vs. the Lindy Effect
There’s a concept called the Lindy Effect, which suggests that the longer a technology survives, the longer it’s likely to persist, implying a certain robustness. One could argue the MEN system is “Lindy,” having served us for a century.
But this argument fails when the environment changes so fundamentally that past performance no longer predicts future safety. The MEN system was designed for a world without RCDs, with simple loads and conductive copper pipes. Today’s world of high-current EV chargers, grid-tied inverters, and insulating plastic pipes presents challenges it was never designed for. The MEN system’s 100-year history is not a testament to its timeless perfection but a marker of its age. The continued adherence to its original form isn’t proof of resilience; it’s institutional inertia.
Interestingly, New Zealand is something of an outlier for maintaining MEN as the default system. In many European jurisdictions, TT or TN-S options are allowed or even required in domestic installations — especially to improve safety in neutral fault scenarios. The fact that NZ’s wiring rules remain locked into MEN reflects not technical superiority, but path dependence.
If we continue to insist on installation electrodes without questioning their actual function, we risk treating a century-old assumption as untouchable dogma, rather than testing whether it meaningfully contributes to safety in a modern RCD-protected world. The safest path forward is not to blindly trust the past, but to engineer a system fit for the challenges of the present.
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