Options to Improve the MEN System into the 21 st Century

Transcription

1 Options to Improve the MEN System into the 21 st Century Chris Halliday Electrical Consulting and Training Pty Ltd, Gladstone NSW, Australia. Web: Abstract Network and customer installation neutral integrity cannot be guaranteed and earthing system impedances to the general mass of ground are increasing as conductive water pipes are replaced with non-conductive types. Those inside premises are placed at risk, where there is poor neutral integrity and the installation has conductive water pipes and/or earthed metal appliances, which is the majority of electrical installations within Australia. This paper identifies deficiencies in the Multiple Earth Neutral (MEN) system and analyses options aimed at minimising the risks to customers from poor neutral integrity. 1. Introduction Before the objectives of this paper can be explored it is important to understand the role of the MEN connection at the customer s installation. Prior to 1980, Australia utilised the direct earthing system, voltage operated Earth Leakages Circuit Breaker (ELCB) protection and the MEN system of earthing. From 1980, the direct earthing system and voltage operated ELCB protection were discontinued in Australia and the MEN system was to be used exclusively from then on. The installation MEN connection helps to minimise the voltage on the neutral link to the general mass of ground (i.e. the neutral to earth voltage), and anything connected to the neutral link such as the equipotential bond to the waterpipes, should the supplying neutral develop a higher than normal impedance or even if the neutral completely open circuits. The neutral to earth voltage at an installation also rises as neutral current increases under normal conditions, particularly for long Low Voltage (LV) runs, and is reduced by the MEN connection. The MEN also helps to stabilize the neutral point and phase voltages at the installation for three phase systems, particularly for long Low Voltage (LV) runs where the neutral point may be offset slightly because of voltage drop in the neutral. The MEN does not generally play a significant role in earth faults within the installation as most of the return current will flow back via the neutral (if it is intact). This paper utilises statistics and requirements in NSW, however similar issues and requirements are expected for the rest of Australia. 2. Why Improvements Are Necessary? The integrity of the supply neutral within installations and the distribution company s neutral cannot be guaranteed, even if high quality products are used. Faults can develop, vandalism can occur (e.g. copper thief), accidents can occur (e.g. car hitting a pole and dislodging the neutral), and connections can be unintentionally left loose or can deteriorate over time, particularly in coastal areas and where copper to aluminium connectors are used. Electrical Consulting and Training Pty Ltd Page 1

2 In years gone by and since the beginning of the MEN system in Australia, the return path, when the neutral became faulty, was generally maintained by the conductive water supply pipes. Not only did the water pipes provide a good connection to the general mass of ground but the neighbour s installation was often bonded to their waterpipes and so the majority of the return current would travel back via the waterpipes to the neighbour, back through their waterpipes and neutral. This ensured safety within the customer installation but put plumbers at risk if they cut the waterpipe without a bridging lead connected across the cut or where repairs had to be made. Many water authorities and councils are unintentionally removing the risk to their plumbing staff by installing non-conductive water mains for new installations or when replacing existing metallic pipes. Water meter replacements particularly put water authority plumbers at risk and so many water authorities are deliberately installing a section of non-conductive pipe in the water service to remove the risk to their staff on a permanent basis. Where this occurs, the electrical earthing system impedance of the customer installation to the general mass of ground is often significantly increased, even if an earth electrode is in place. The 1.2 metre copper clad steel electrode is the most commonly used earthing electrode. Depending on the soil type, the moisture content of the soil, and the age of the connection, the impedance of this to the general mass of ground can vary up to around 1000 ohms, with ohms more typical. Preliminary testing has shown that reinforcing in concrete foundations provides a superior earth electrode than a 1.2 metre electrode. The loss of the return neutral path from an installation that is drawing load, will see a significant rise in voltage on all earth conductors connected to the neutral link where electrodes are poorly connected to the general mass of ground e.g ohms. Lethal voltages are likely on all metalwork bonded to the main earth such as the metallic switchboard, metallic taps, and non double insulated metallic Class 1 appliances. NSW distributors are required to provide a Network Performance Report annually to the NSW Department of Water and Energy. Energy Australia, Country Energy and Integral Energy reports for 2007/08 show the numbers of electric shock incidents caused by a poor or open circuit neutral return path (refer Figure 1). The risk of an electric shock within a property from a poor or faulty neutral is calculated as approximately 1 in 6000 for NSW based on the 2007/08 annual reports for the three NSW distributors. Using the risk matrix provided at Appendix A, a consequence of catastrophic and a likelihood of unlikely, the resultant risk is assessed as Extreme and therefore control measures should be implemented. The risk in other states is likely to be similar. Figure /08 Electric Shocks Incidents for NSW for Poor or Faulty Neutrals Distributor Area No. of Shock Incidents Caused by Poor or Faulty Neutrals Customer Numbers Chance of an incident per year based on customers numbers Energy Australia 48 1,580,933 1:32,937 Integral Energy ,322 1:16,100 Country Energy ,000 1:1853 Total 522 3,214,255 1:6158 Electrical Consulting and Training Pty Ltd Page 2

3 3. MEN System Improvement Options 3.1 Low Impedance Earth Path A low impedance earth path should be able to limit the neutral to earth voltage to safe levels at the installation should the neutral fail. To determine what the maximum value of impedance of the return path should be to ensure safety, the maximum allowable neutral to earth voltage needs to be determined first. IEC provides guidelines for the automatic disconnection of supply to protect against indirect contact. Whilst this standard primarily provides guidance for the disconnection of circuits within an installation, the same touch voltage guidelines could be used to ensure an installation is safe, should the neutral conductor fail. Figure 2 is from this standard. The Lp curve in Figure 2 is primarily for wet locations, wet skin and low floor resistance (200 ohms) and therefore provides guidance for some of the worst touch voltage conditions that can occur e.g. a person showering, standing on a concrete floor and touching a tap bonded to the neutral, with the neutral open circuited (refer Figure 3). A sustained touch voltage limit of 25 volts therefore applies using the guidance of Figure 2. This voltage is likely to reduce slightly due to a reduction in the overall impedance caused by the parallel path of the human body when it makes contact with the touch voltage source. This reduction will be ignored in this paper as it helps build in a small safety margin. Now that the maximum touch voltage for the loss of a neutral return path has been determined, the value of impedance of the earth return path must be determined. But to do this, the value of load impedance turned on at the time needs to be determined. Figure 2 IEC : Maximum Duration of Prospective Touch Voltage Electrical Consulting and Training Pty Ltd Page 3

4 Figure 3 Diagram used to Calculate Installation Earthing System Impedance A 230 Volts Supply Installation load normally 100 A 2.3 ohm N Neutral Link Distribution Substation LV Earth 1 ohm Burnt off service neutral Installation earthing system 25 volt max The maximum single phase load that should occur is 100 amps as Clause of the NSW Service and Installation Rules requires the use of additional phase(s) once the installation goes over 100 amps. Clause requires for installations supplied by more than one phase that the maximum demand in an active service conductor is not more than 25A above the current in any other active service conductor. Therefore the maximum neutral load at an installation should be 100 amps (though it could be greater where overloads occur and before a 100 amp service fuse is likely to operate). The load impedance of an installation at maximum neutral load i.e. 100 amps, is: Z = 230/100 = 2.3 ohms The maximum return current through the ground will flow when the impedance from the general mass of ground back to the neutral system is at its lowest, which in an urban area could be less than one ohm. The following calculations show: how much current is able to flow and restrict the rise in potential to 25 volts on the installation earth; the earth electrode impedance at the premises using the maximum load of 2.3 ohms. Using Kirchhoff s Voltage Law to calculate the current: 230 V = 2.3I I 205 = 3.3I I = amps Z earth = V/I = 25/62.12 = 0.4 ohm The calculated value of installation earthing system impedance of 0.4 ohm is an extremely low value of impedance and difficult to achieve in practice. Figure 4 shows the values of earth electrode impedance Electrical Consulting and Training Pty Ltd Page 4

5 calculated for both a 25 V and 50 V touch voltage with various return path earth values and using 2.3 ohms for the installation load. Even with a 50 V touch voltage limit, let alone the more stringent touch voltage requirements of IEC/TR (refer Figure 5), the calculated value of installation earthing impedance to the general mass of ground is unrealistic, particularly if the return path impedance is <1 ohm as is often the case in urban areas. The only practical way to achieve this is to have another conductor as a protective earth conductor as occurs with the TN- S system. This option would be cost prohibitive and impractical in Australia due to the size of the existing low voltage network. Figure 4 Resistance of Earth Electrode versus Earth Return Path Figure 5 Strong Muscular Reaction for Alternating Current 50/60 Hz Electrical Consulting and Training Pty Ltd Page 5

6 3.2 Fault Loop Impedance Monitoring/Protection Fault loop impedance is tested by applying a load for a very short time, even as short as one cycle (20 ms), and measuring the voltage drop of the system. The impedance is given by: Fault loop impedance = V/ I V - change in voltage in volts I change in current in amperes An appreciable increase in fault loop impedance over time could indicate a faulty neutral connection or conductor. And so, fault loop impedance monitoring could be used to protect against a faulty or open circuited neutral. Aurora Energy is providing the first generation of warning devices to customers to help prevent electrocutions from a failed or poor neutral. [1] Figure 6 provides the details of a standalone fault loop monitoring protection system but this could easily be incorporated into a smart meter. Warnings might be given at >0.5 ohm and the supply could be isolated if 1 ohm is exceeded. Figure 6 Monitoring Fault Loop Impedance at an Installation A 230 Volts Supply N Fault Loop Testing Device Installation load Neutral Link Distribution Substation LV Earth Installation earthing system The advantages of this type of scheme include the system impedance and hence neutral integrity is constantly monitored, significantly reducing the risk to those within the installation. To trip the supply off within 0.4 second, then fault loop testing could be carried out for 1 cycle and then again after another 9 cycles. This continual testing is required as a neutral fault could develop in the cycle after the test and not be picked up until the next test. An allowance for the mechanical trip would also need to be included and so testing would need to occur slightly more often than quoted. The amount of power used for the testing would depend on the size of the current used and the period of the test. This would equate to 5256 kwh/year and a cost of approximately $ /year based on 20c/kWH and a 25 amp test every 10 cycles. This Electrical Consulting and Training Pty Ltd Page 6

7 is a significant cost/year and increase in greenhouse gases. A 15 ma test would significantly reduce the energy used to 3.2 kwh and $0.63/year but may not provide reliable testing/protection. The disadvantages of this system are that the supply would be continually tripping off due to variations in the supply voltage that would be included into the fault loop calculation. It also has the other inherent disadvantages of fault loop testing such as the phase angle of the load in the neutral may cause underestimation of the system impedance and inaccuracies due to harmonics and DC offsets. Testing less often would reduce energy costs and greenhouse gas emissions but would increase the risk to those within a premise, as the touch voltage limits of Figure 2 and 3 and the time constraints of Figure 3 are not likely to be achieved. The electronics used for this testing would need to be protected from impulsive transients from load switching and lightning to ensure continued operation of the system and protection of persons. 3.3 Neutral-Earth Voltage Monitoring The neutral link voltage of an installation could be monitored to the general mass of ground as shown by Figure 7. The sensing relay would need to be set to 25 volt and to ensure isolation of the load within 0.4 second for the same reasons as those provided in Section 3.1. This approach is similar to the old voltage operated ELCB protection used in Australia prior to Figure 7 Monitoring N-E Voltage at an Installation A 230 Volts Supply Installation load N Neutral Link Distribution Substation LV Earth Burnt off service neutral Installation earthing system >2 metres Voltage Sensing Relay Remote earth electrode A section of plastic pipe would need to be included in the water main to prevent intermittent tripping from neighbouring premise faults that would have occurred if the waterpipe between these premises were electrically connected. This was one of the main problems with the old voltage operated ELCB system of earthing. Protection, in the form of a Metal Oxide Varisters (MOV), would also be required to protect the sensing relay electronics from impulsive transients from lightning and load switching and ground potential rise from lightning strikes. The later would require the MOV to be connected across the two earth electrode inputs to the relay. The Electrical Consulting and Training Pty Ltd Page 7

8 earth electrodes would need to be installed so the remote electrode is not influenced by the main electrode i.e. at least 2 metres away from the electrode and also from other conductive items. This form of protection has been trialed successfully at one premise in NSW for the last 2 years. The sensing relay could incorporate into and become the installation main switch. For domestic installations it could also include ma, Type S, RCD protection, as recommended by Clause of the Wiring Rules for protection against the initiation of fire, and truly become a life saving switch. 3.4 Ratio of Neutral to Earth Current The ratio of neutral to earth current could be used to monitor and protect for a loss of neutral. This could be done in a smart meter or in a separate device. Current transformers (CT s) or whole current monitoring could be used to monitor the ratio. The disadvantage of this system includes false tripping if ratios changed quickly e.g. where equipotentially bonded water piping is altered or earthed electrical equipment is added to or changed within the installation. Less speedy ratio changes would occur as soil moisture alters but this is not seen as overly critical as protection algorithms should be able to be adaptive to cope with such changes. The loss of the incoming supply may disrupt monitoring, especially if the ratio was changed due to rain while the power was off. 4. Other Options 4.1 TT System The TT system is another option to consider (refer Figure 8). It uses direct earthing, which was used prior to 1980, and is still in use in parts of Australia, where metallic waterpipes provided the low impedance return path for faults. The loss of neutral in a single phase system would simply cause loss of supply and not an excessive neutral to earth voltage rise at the installation. However, we must ensure that earth faults are removed as quickly as possible. This would necessitate Residual Current Device (RCD) protection on all circuits to ensure protection from inadvertent direct contact and also indirect contact. The value of the installation and substation earth values become less critical using this system of earthing and RCD protection. A 30 ma RCD could be used to clear touch voltages in less than 0.4 second. The fault loop impedance is calculated as follows: Z earth = V/I = 230/0.03 = 7666 ohms Therefore the maximum allowable earth electrode impedance is approximately 7636 ohms when allowing a maximum of 0.5 ohms impedance in the active mains conductor and a maximum of 30 ohms at the substation earth. This value should be easily achieved at most LV installations in most soil types without much effort. Electrical Consulting and Training Pty Ltd Page 8

9 Figure 8 TT system A 230 Volts Supply RCD protection Class 1 appliance N Distribution Substation LV Earth Installation earthing system Backup protection could be provided in case of the failure of an individual RCD, as RCD s do fail. This could take the form of a mA RCD for domestic installations and 1 amp earth fault protection of commercial and industrial applications. This would require a reduction in the total loop impedance circuit as follows: Domestic - Z earth = V/I = 230/0.3 = 766 ohms Commercial - Z earth = V/I = 230/1 = 230 ohms The above values are still relatively easily obtained for each load type. Disadvantages of the TT systems include that the overall impedance to the general mass of ground will not be as low as that for a TN system of earthing. Therefore lightning safety will not be as good as that provided by the MEN system. Also, the loss of a neutral in a three phase system will cause the neutral and phase voltages to float to levels dependent on the load impedance of each phase, which is likely to damage customer equipment. Under and over voltage protection would therefore seem to be a requirement as well. 4.2 Other Options Other options have not been explored at this stage. Electrical Consulting and Training Pty Ltd Page 9

10 5. Conclusions and Recommendations The risks posed by poor or faulty neutrals in NSW have been assessed as Extreme and therefore requires mitigation. Various improvement options for the MEN system and the IT system have been explored as options for the Australian electricity system for the future. A low impedance earth return path has proven to be impractical but cannot be totally dismissed if any form of MEN system is used. The use of the metallic reinforcing in footings should be encouraged as the primary means of an earth electrode where this form of earthing is available. Fault loop impedance monitoring and the TT system with RCD protection are other alternatives that show promise but do not provide the same degree of protection as neutral to earth voltage monitoring. Neutral to earth voltage monitoring provides the best option. The sensing relay could incorporate into and become the installation main switch. For domestic installations it could also include ma, Type S, RCD protection and truly become a life saving switch. MOV protection will be required to protect the device from impulsive transients. Electrical Consulting and Training Pty Ltd Page 10

11 Appendix A Risk Matrix Level Insignificant Minoir Moderate Major Catastrophic Description of Consequence No injuries, low financial loss First aid treatment, on-site release immediately contained, medium financial loss Medical treatment required, on-site release contained without outside assistance, high financial loss Extensive injuries, loss of production capability, off-site release contained with no detrimental effects, major financial loss Death, toxic release off-site with detrimental effect, huge financial loss Step 2: Using the following table, the likelihood the risk will occur is determined. Level Likelihood Almost Certain Is expected to occur in most circumstances Likely Will probably occur in most circumstances Possible Might occur at some time Unlikely Could occur at some time Rare May occur only in exceptional circumstances. Step 3: Using the risk matrix below, the risk class/ranking is determined. Consequence Likelihood Insignificant Minor Moderate Major Catastrophic Almost Certain H H E E E Likely M H H E E Possible L M H E E Unlikely L L M H E Rare L L M H H Class/Ranking Description / Requirements E extreme risk H high risk Additional controls required and risk reassessed M moderate risk L low risk Proceed with existing controls in place Electrical Consulting and Training Pty Ltd Page 11

12 Appendix B References [1] AS/NZS 3000 Wiring Rules AS/NZS Effects of current on human beings and livestock Country Energy, Energy Australia and Integral Energy Network Performance Reports 2007/08 IEC Electrical installation guide Part 413: Protection against indirect contact Automatic disconnection of supply IEC/TR Effects of current on human beings and livestock Part 5: Touch voltage threshold values for physiological effects. NSW Service and Installation Rules Electrical Consulting and Training Pty Ltd Page 12