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Low Voltage in Australia – The Peril of its Amenity

Access to reliable low voltage electricity is essential to developing societies across the world and the human consumption of electricity is on the rise. Electricity however is a fundamentally hazardous substance that is responsible for great harm when handled incorrectly or unexpectedly. SafeWork NSW cites electrocution as the second leading cause of death in the construction industry. Electric shock can also cause significant harm and contributes to many secondary effects such as falling off a ladder; with falls from height being the major cause of fatality in construction. Other effects include lost time, ongoing medical problems and nuisance shocks and tingles which require significant investigation to solve, and often great difficulty eliminating.

The RCD

The introduction of residual current devices (RCD) in 1991 led to a steep decline in the number and severity of electrical shocks. However, RCDs alone cannot detect and isolate faults where the shock victim becomes in circuit with the active and neutral, or insufficient out of balance current flow to earth means the RCD cannot operate. The RCBO which includes an overcurrent element is also unlikely to ensure safety of the shock victim due to relatively high pickup current required to isolate the circuit under fault. For this reason and because there are many older residential installations still without RCDs installed, there has been a spate of workers crawling through roof spaces and under floors receiving electric shocks which unfortunately in too many instances result in fatality.

This being the case, RCDs while having a significant impact on electrical safety are not perfect and certainly not the ‘silver bullet’ for all LV electrical problems. This next electrical problem may well be as significant an issue that prompted the 1991 mandate for RCD installation.

The High Resistance Neutral

In Australia and particularly at the consumer level, precisely where AS/NZS 3000 ‘Wiring Rules’ is aimed, we prefer the electrical LV system colloquially termed the MEN system. It’s an odd term because it refers to Multiple Earthed Neutral which is very rare and problematic at the consumer level. The MEN system is employed by the Distribution Network Service Provider (DNSP) where they earth the neutral conductor in multiple locations. What we employ in Australia is best described by the IEC 60364 series of standards using the two letter codes TN, TT and IT where the MEN system is most closely defined by the TN earthing system. To be abundantly clear, the system we have in Australia at consumer level is the TN-C system, however, we create a TN-C-S system by installing a Neutral-Earth link within the consumer switchboard.

Figure 1: TN-C system (left) and TN-C-S system (right) (Source: Wikipedia)

The TN-C system is typically how the DNSP reticulates LV through our streets and residential developments. It’s quite rare for the DNSP to reticulate a Protective Earth (PE) with the active and neutral conductors. It is only once the service mains enter a residence, and once the Neutral-Earth link is installed that the system truly represents the TN-C-S system.

So, what’s the problem? The main issue with our Australian MEN system is the increasing incidence of the high resistance neutral problem. This coupled with the trend away from metallic (water and gas) to non-conductive services means the redundancy of the system is degrading and fast. Almost certainly in this day any new residential development would be established with non-conductive water or gas services to the home. Therefore, a high resistance neutral supply side of the Neutral-Earth link will, under normal load conditions, create a voltage rise on the neutral as a larger proportion of the load current flows through earthing system of the consumer’s installation. While the high resistance neutral problem is often associated with a broken neutral connection, this need not be the case as even the physical or geographic nature of the installation can lead to a voltage rise on the earthing system. So in other words the neutral does not need to break for this problem to create the conditions for an electric shock to occur, even a very long service connection may cause a neutral voltage rise due to the higher return circuit impedance.

The Conditions for Alarm

The high impedance neutral has led to an increase in reported incidents. The problem is that our typical protective devices, including the RCD, is not designed to detect the problem and so it remains a persistent hazard while the electrical service is used. The neutral-earth voltage rise need not be significant to cause harm, in fact one could argue that lower voltages produced by a high resistance neutral are the most concerning because they persist undetected because the electrical system continues to operate without any real perceived problem. The AS/NZS 60479 document contains useful data on the physiological effects of electric currents on the human body. This data can be interrogated to understand the full range of electrical shocks victims both report and succumb to.

Figure 2 (left) and Figure 3 (right) (Source: AS/NZS 60479)

Consider a person, wet from swimming in their salt-water clorinated pool, is exposed to a 25 Vac neutral-earth voltage rise. According to the data presented in AS/NZS 60479 their body impedance for hands to feet contact is 650 Ω if they represent 50% of the population. This situation may cause as much as 38 mA of current through their body including the heart which places their circumstance in the so referred AC4.1 region of figure 20 meaning they are at increasing risk of ventricular fibrillation or VF. It follows that even voltages as low as 1-2 Vac are sufficient to cause perception of an electrical shock including involuntary muscular contraction, enough to cause a fall from a ladder or height. In between the current sufficient for perception, up to those that cause VF, are currents that prevent ‘let-go’ and really cause long term damage such as burns, impeded breathing, and organ damage, including the nervous system. Clearly the high resistance neutral is a significant problem and one which arguably requires industry intervention to reduce the risks of this severe hazard.

The Solution

Over the years work has been done to develop technologies based around detection of the high resistance neutral. This article won’t cover those devices, but just like the RCD it is expected that there is some level of neutral-earth voltage rise caused by the high resistance neutral that may go undetected.

There is a solution that eliminates the problem, however, it is an unpopular one. The overwhelming prevalence of the MEN or TN system of earthing here in Australia has meant that the rather useful alternative system, the TT system goes largely unused. Apart from some specialist applications, too few DNSPs and LV designers would get excited about the application of the TT system. In fact, we’ve had experience where LV designers have incorrectly stated that the TT system is not supported by AS/NZS 3000 or that TT systems are not allowed, which is certainly not true.

Figure 4: The TT system
Figure 4: The TT system

Figure 4: The TT system (Source: Wikipedia)

The TT differs from the TN in that the neutral is only connected to earth at the supply transformer (or generator). At the consumer, there is no Neutral-Earth link, and the only earthing system at the consumer is what is intentionally installed for the correct operation of the TT system. The key fact – the TT system is completely immune from the high resistance neutral problem as there is physical separation of the neutral conductor and earth at the consumer. The TT system does require consideration and actions to ensure it can operate safely, and IEC 60364 requires circuits operating under TT have residual current device protection. Therefore the earthing system at the consumer must meet a target resistance, for instance <8000 Ω with a 30mA sensitivity.

There would need to be greater understanding by the industry and greater coverage of the TT system in AS/NZS 3000 for it to be truly viable to defeat high resistance neutral problems. Electricians would need to be educated so TT systems weren’t inadvertantly modified to TN by mistake. And the MEN system as we know it, would be degraded with less overall interconnectivity which assists the DNSP particularly at HV distribution voltages.

In conclusion, TT systems may not be the silver bullet either to the high resistance neutral problem but they certainly would be a useful alternative in some circumstances. In Australia we require more education and understanding of the system because we are well behind other developed countries that do employ the TT system with great effect such as Japan and Italy where it is actually mandated.

Safearth consultant: Bill Tocher – Engineering Director | Consulting Manager, Safearth

For more information: enquiries@safearth.com or 1800 327 844