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Your house Virtually every household in Australia uses M.E.N. and it is assumed to be a very safe system. But it is not foolproof. Unless checked out from time to time, it can develop a very dangerous fault. By LEO SIMPSON M.E.N. stands for "multiple earth neutral" and is the system used for wiring the majority of domestic electrical installations in Australia. As specified in the SAA Wiring Rules (AS 3000-1986), the mains neutral wire is connected to earth at the user's switchboard. In most houses this means that the 240V AC mains supply is connected from the power pole in the street via via two cables, active and neutral, with the neutral wire connected to earth via the consumer's water pipe. The earth connection point on the water pipe is usually just at the point where the pipe enters your house. This is important because you need to examine it occasionally to make sure that the connection is sound. As we will show, that connection is vital to the safety of everyone in your household. Typically, the active and neutral 25A SWITCH WATT-HOUR METER oiriN~RE~r FUSE l--,U.-'0---TO OFF PEAK HOT WATER SYSTEM 6OA SWITCH NEUTRAL LINK SWITCH BOARD Fig. 1 76 SILICON CHIP EARTH TO CONSUMER 'S -:WATER PIPE • • g wires from the power pole to your house have a capacity of 70 or 100 amps. This means that the maximum electricity consumption (volts multiplied by amps) for the household is nominally 18.9 kilowatts in the case of 70 amp mains and 24 kilowatts in the case of 100 amp mains. Happily, most homes normally use only a small fraction of their installed wiring capacity although there would be times during winter evenings when it could be easily approached in large households when several radiators, a stove and perhaps one or two other cooking appliances are in use. Why M.E.N? "Multiple earth neutral" is a system which offers improved safety and lower losses in energy transmission. Let's see why. In a normal single-phase 240V AC wiring system (which is what most households have), the nominal voltage of the neutral wire coming into your switchboard is the same as earth, ie, zero volts. But in practice, the voltage is higher because all the current from your house and your neighbours' homes inevitably causes a voltage drop in the neutral ea ble between your power pole and the nearest substation or pole transformer. This is because the resistance of the neutral cable is not zero and so the large currents inevitably mean that there is some voltage present on the neutral. This voltage is a loss in the energy transmission system and is part of the reason why, when At left is a diagram depicting the electrical switchboard of a typical household using the "multiple earth neutral" system. could kill you! The mass of connections to a typical power pole. The neutral return to the substation is the cable to which all consumers are connected (ie, top of photo). Other earthing systems There are two other systems for earthing of consumer electrical installations in Australia. One is known as Direct Earthing while the other ·is referred to as ELCB or Earth Leakage Circuit Breaker. In the Direct Earthing system, the earth wires of the installation are connected to earth via a water pipe or electrode but do not connect to the neutral conductor at the switchboard. In the ELCB system, there are two connections to earth with a sensistive relay coil connected between the two . If a fault current flows to earth, the energy demands are heavy, the mains voltage available at your switchboard is lower than it should be, or would be if it were not for the multiple earth neutral system. Because the neutral wire is connected to earth (ie, the consumer's water pipe), there are actually two return paths for the current pass- potential difference between the two earths will trip the circuit breaker and disconnect the mains supply. Again, in the ELCB system there is no connection between the consumer's earth and the neutral conductor at the switchboard . Throughout most of Australia the ELCB system has been superseded by the MEN system described in the article. The ELCB system was prone to false tripping and improved safety can now be provided by using the MEN system combined with core balance relays. ing through the appliances in your home. Some of the current passes back along the neutral, back out to the power pole and thence via the power lines back to your local substation, pole transformer or whatever. The rest of the current passes back via the earth wire to the water pipe and then via good old Terra Firma itself to the substation. (Editor's note: we have simplified matters here by ignoring the inherent current balance of threephase power energy distribution systems. For a brief explanation, see the panel headed, "Current flow, three-phase and all that".) If you are not familiar with the mains distribution system this may all sound like heresy but it is true. Have a look at Fig.1 which depicts the mains wiring of a typical household installation. This diagram shows the active and neutral connections to the switchboard. The active wire is connected via two watt-hour meters one feeding the off-peak hot water service and the other feeding the rest of the power circuits in the household. Following each watt-hour meter is a large rotary or toggle switch which is typically rated at around 60 amps and is connected via a short length of cable to the Active link. This is a heavy brass bar with quite a few screw connections to take the wires feeding the fuses or individual circuit breakers. The whole Active Link assembly is housed under a plastic cover to avoid accidental contact by anyone who has access to the switchboard. A typical modern houshold may have one 25-amp circuit to feed the stove and oven, three 15-amp circuits to feed all the power points and two or three 8-amp circuits for all the lights. By the way, in the wiring standards, power points are referred to as GPOs which stands for General Purpose Outlet. All the neutral return wires from the household power and lighting circuits are connected together at the Neutral link which is a similar assembly to the Active Link. The difference is that the Neutral link NOVEMBER 1987 77 What's available in clamp-on current meters The Escort ECT-620 has two DC and two AC voltage ranges, two resistance ranges and three current ranges up to 1000 amps AC. It is priced at $155 (tax incl) from Emona Instruments. Phone (02) 519 3933. The Emtek DCT-300 has three DC and two AC voltage ranges, four resistance ranges and three current ranges up to 400 amps AC. It is priced at $141.92 (tax incl) from Geoff Wood Electronics. Phone (02) 427-1676. What's in a clamp-on meter The typical clamp on current meter is a current transformer with a turns ratio of 1 : 1000 with the laminated steel clamp providing the magnetic core of the transformer. With the clamp placed around a cable carrying 1 0 amps AC, the secondary will generate 1 0 milliamps. In analog clamp meters the secondary current is rectified and read on a moving coil meter. In digital clamp meters the secondary current is applied to a shunt resistor and the resulting voltage is read by the digital metering circuitry. Some clamp-on adaptors for digital multimeters will measure DC as well as AC by using Hall effect sensors. 78 SILICON CHIP The Escort ECT-620 has one AC voltage range, one resistance range and two current ranges to 300 amps AC. Priced at $118 (tax incl) from Emona Instruments. Phone (02) 519-3933. also carries all the earth wires from the power and lighting circuits as well as the main earth wire which connects to the water pipe. It also connects to the neutral wire which comes directly from the power pole. So there is a straightforward circuit and it would be natural to assume that all the current which comes in via the active feed wire goes out via the neutral wire. In other words, the current in the active wire is equal to the current in neutral wire. Well, it ain't necessarily so. It is possible that half or more of the return current goes via the Earth wire and the remainder goes via the neutral back to the power pole. To take a particular example, if you were using a 2400 watt radiator which draws 10 amps from the active line, 8 amps might go via the earth wire to the water pipe and the remainder, 2 amps, would go via the neutral return. The simple explanation for this is that the resistance of the neutral cable back to the substation or pole transformer is slightly higher than the very low resistance via the earth path to the same point. Ergo, some current goes via Earth and some goes via neutral. It can't all go via neutral. Having come to terms with this previously unconsidered fact, what does it mean? It means that the supply authorities are able to obtain lower supply losses in the return path than they otherwise would if the MEN system was not used. They also experience less voltage flashovers from neutral to Earth during thunderstorms in appliances which are permanently connected. A common example of this used to occur in stove heating elements. Even though the stove or oven might have been turned off, a lightning strike on a power line would cause many stove elements to break down to chassis. With the MEN system this is not a problem. Where lie the problems? Corrosion is the first problem. Since many small appliances these days use thyristor power controls there is inevitably a DC current imposed on the mains supply. Some of This shot shows Tandy's Micronta clamp-on meter measuring the current in an earth wire to a water pipe. The present model has two AC voltage ranges from 6 to 300 amps AC, and is available from all Tandy stores for $64.95 (Left hand by courtesy of Greg Swain - the rest of him has been edited out.) this direct current will flow via the connection between the earth wire and the water pipe. And the very presence of DC will accelerate corrosion which naturally occurs when dissimilar metals are used, as they normally are. Ultimately, corrosion of the connection between the Earth wire and the water pipe will mean that the connection will be broken. Or if it does not physically break, its resistance will be so high as to be useless. OK, so that means that if you have lost the Earth connection and an appliance such as your washing machine breaks down from active to chassis, the chassis could be lethal. That is bad enough but consider another possibility which is even more likely and which has occurred a number of times quite recently in Sydney. If corrosion has occurred in the connection between your main Earth wire and the water pipe, is it not just as likely that corrosion has progressed in the neutral connections between your house and the power pole? Of course it has. What that means is that the neutral return could now have an appreciable resistance and could easily be around 20 ohms or more. If that is the case, all of the return current will go via the Earth and water pipe. But what if the earth connection has broken? What that means is that if you now have an appliance drawing 5 amps, the voltage impressed across the neutral return path from the switchboard to the power pole will be 100 volts AC. Hmm. So the Neutral link will be floating at 100 volts AC above earth. And all the earths from the various points in the house are connected to the Neutral link. So every appliance plugged in will have have its chassis floating at 100 volts AC above earth - a lethal situation! So your fridge, washing machine, toaster and microwave oven could all be sitting there silently with their metal cabinets at a substantial voltage above earth. Touch one of those and a properly earthed metal object, such as your kitchen sink, at the same time, and it could be curtains! It does happen Don't think that the this scenario is a remote possibility. It happened very recently to a close acquaintance of yours truly. We'll call him Jacko, to protect the innocent. He discovered just such a fault condition had occurred in his home but the reason why no-one had been electrocuted was that his washing machine was separately earthed via a water-pipe just outside his laundry. If he hadn't been the naturally super-cautious person he is, and had not earthed his washing machine separately, years ago, someone could be dead today. But there is more to the story. Having become aware of the dangerous situation with the Earth NOVEMBER 1987 79 parently thinks that current flows into an appliance but not out of it. Eventually, Jacko was able to explain the situation in words of less than two syllables and persuaded said inspector to measure the current in Active, Neutral and Earth, using his tong tester (we'll explain that in a moment). Result: 10 amps in Active, 10 amps in Earth, zero amps in Neutral; inspector perplexed but agrees that something is wrong with the Neutral. Enter council linesman. He renews neutral connection at eaves of house, pronounces situation fixed, leaves. Enter another inspector, tests for current in neutral return, finds none. Problem at power pole. Several days later another pair of linesmen return to renew the active and neutral connections at both the house and power pole. The saga goes on. At the time of writing, an inspector is due to return and pronounce the installation safe . So far, six people from the supply authority have been involved. If all houses in Sydney had to be checked and rectified, it could take a hundred years at this rate. What you should do These two photos show a quick and dirty approach to making a clamp-on meter. The top shows an old relay with the active lead inserted, while below is a transformer doing the same job. and neutral return, our friend Jacko contacted the local electricity supply authority to have the situation rectified. Their reaction does not inspire confidence. On being told that the neutral return was high resistance, the first inspector tested the line with a test-lamp which tests for the presence of voltage only. On detecting the presence of voltage across the active and neutral, he pronounced the situation normal. No problem. On being told that all the current from a 10-amp load was going via the Earth instead of neutral he apparently had difficulty comprehending the situation. (It should be noted here that a test lamp is a very crude test instrument which is essentially a neon 80 SILICON CHIP lamp. It tells the user if there is more than about 90 volts peak present. It does not give any indication of just how much voltage is present. Inspectors seem to place undue reliance on such crude instruments. Enter another inspector, more senior than the first. On having the problem explained to him, and on being asked to measure the currents in active and neutral his response was, "I can measure the current in the Active but I can't measure current in the Neutral because it is all used in the appliance"! Hmm. Here we have a big problem. Here is a senior electrical installation inspector who apparently does not know how current flows! Heap- Do you know where your main earth point is? Take a walk around your house. Where does the water pipe enter the house? That is probably the point where you will find the earth wire connected via a screw clamp. If you can't find it, have a look at your switchboard. There should be a note (in white paint) saying where the earth is. For example, in the author's switchboard, the note says, "Main earth under hall" and lo and behold, if you went under the house (it is partly two-storey) there you would find the earth wire clamped to the water pipe. In some areas where the water service is run in plastic pipe, the authorities specify that the earth connection is made to a long copper-clad steel spike or rod driven at least 1.2 metres into the ground. Alternatively, the earth connection may be made to a strip electrode at least three metres in length and buried at least 45cm underground. When you find the connection, inspect it carefully to see that it is sound and not corroded. If the connection is badly corroded you should have it attended to by a licensed electrician. Alternatively, the earth wire should be cleaned up and reconnected. But before Current flow, 3-phase and all that touching it you should switch off the power with the main switch on your switchboard. If you have another switch, such as for an offpeak hot water service, switch that off too. The reason for switching everything off is to avoid the possibility of having any current flowing in the earth path when you disconnect it. If current was flowing, and the neutral path was poor, you would produce exactly the hazardous situation we have described above. Alright, having determined that your earth connection is satisfactory, how do you check the neutral The second simplification involves the concept of neutral current flowing back to the substation, pole transformer or whatever. This ignores the fact that domestic mains power distribution from the power pole has three phases, each of 24OVAC (with 120 degrees phase difference between each) . In an ideal system, the currents should be balanced so that there is no current flowing in the neutral line. Domestic systems are never ideal though, so appreciable current does flow in the neutral. We have made a number of simplifications in the writing of this article. The first is that we have said that current flows from active to neutral or from active to Earth, as the case may be. In fact, since we are talking about 5OHz alternating current, the current changes its direction 1 00 times a second. It is convenient to think of current flowing from the active line though, because in the words of an electrician we know, "It's the activ_e line that gets ya. It's the one with the juice!" regard themselves as familiar with mains wiring. But there is a way of checking it. There is only one safe and legal way and that is to use a clip-on meter (also known as a tong tester.) return path? At this point we must warn that you must not even think of tampering with the wiring of your switchboard. Not only is it illegal but it could be extremely dangerous, even to those who Special "First Issue" Offer to 'Silicon Chip' readers only! 150Amp Clamp-On Current Probe for just $97.75?1}:S': To coincide with the first issue of 'Silicon Chip' and this special feature on electrical safety, Elmeasco have obtained a limited quantity of clamp on current probes. The Model 4000P is a current transformer capable of measuring ac current up to 150A without breaking the circuit under test. You simply clip the probe to the conductor - just like a clothes peg! The probe connects to any multimeter (lead and plugs supplied) capable of measuring 200mA. In fact the reading on the multimeter will directly correspond. Thus a SA current in the conductor will give a meter reading · of 5mA. You can use the probe with conductors up tq 11 mm in diameter. If you've read the article, you'll know what a great safety accessory this probe can be. Contact your local Elmeasco Office. Model 4000P Specification Current range : 1A to 150A Accuracy : ± 2.5% + 0.15A Division Ratio : 1000:1 Working Voltage : 300V ac rms max Maximum Conductor Size : 11 .1mm (7/16in) ELMEASCO Instruments Pt,,. Ltd. Cheques & Bankcard Welcome NEW SOUTH WALES VICTORIA QUEENSLAND SOUTH AUSTRALIA WESTERN AUSTRALIA 15 McDonald Street, MORTLAKE P.O.Box 30, CONCORD NSW2137 Tel : (02) 736 2888 Telex : AA25887 Fax : (02) 73 3663 12 Marooncah Highway, RINGWOOD P.O.Box 623, RINGWOOD VIC 3134 Tel: (03) 879 2322 Telex : AA30418 Fax : (03) 879 8688 192 Evans Road, SALISBURY P.O.Box 274 SALISBURY OLD4107 Tel : (07) 875 1444 Telex : AA44062 Fax: (07) 277 375:i 241 Churchill Road, PROSPECT P.O.Box 154 PROSPECT SA5082 TEL : (08) 344 9000 TELEX : AA87519 FAX : (08) 269 6411 Scott House, 46-48 Kings Park Road, WEST PERTH P.O.Box 901, WEST PERTH WA0005 Tel : (09) 481 1500 Telex: AA94765 Fax : (09) 321 3026 NO VE MBER 1987 81 The Fluke 80i-410 clamp-on current probe is intended for use with digital multimeters and will measure up to 400 amps AC and DC. It can measure alternating current through a cable without having to break the connection. Now many readers probably have not heard of a clip-on meter let alone have access to one but we'll address that problem a little later. In fact there is no need to measure the current in the neutral return. What needs to be done, and which is a lot easier, is to measure the current in the earth wire to the water pipe. As illustrated in one of the accompanying photos, a clip-on meter does not have meter prods but has a pair of clamp jaws which close around a cable to measure that current. The clamp jaws actually constitute the core of a current transformer. The ea ble to be measured forms the primary winding of the transformer while the meter monitors the secondary winding in the instrument. One of the cheapest available clip-on meters is the Tandy model (Cat No 22-161) which currently sells for $64.95. Checking the earth current Measuring the earth current is simply a matter of fitting the clip-on meter over the earth wire to the water pipe, or over the water pipe itself (between the earth wire connection and the point where the pipe enters the ground), if that is possible. But before you do that you must be sure that your load current is known. The way to do that is switch off all appliances and light fittings in your house, plus the hot water system. Then connect a large heating appliance of known rating. 82 SILICON CHIP If possible, use a 2400W radiator, on its highest setting. This will draw a current of close to 10 amps, depending on the value of the mains voltage and the tolerance on the heating element. So with the 2400W radiator switched on, you know you have a current from the Active of 10 amps. The question is, what is the neutral current? Measure the current in the earth wire, subtract it from 10 amps, and you have the answer. If the earth current is more than say, 5 amps, you have cause for concern. In that case, you should contact your local electricity authority and have them check out your neutral connection. Don't do this test during or just after wet weather. When the ground is saturated, more current will tend to flow via earth than via the neutral path. Most readers will not have access to a clip-on meter but as we have found, it is possible to cobble up a meter circuit to do the job. For one example, we used the core and winding of an old relay and for another, we used a small transformer with part of its windings removed. The object of the exercise is to modify (or butcher) the relay or transformer so that you can fit an earth wire within the magnetic circuit of the core. In the case of the relay, we loosened the screw holding the coil to the armature so that a wire could be slipped into place and then retightened the screw. The voltage developed by the relay coil is then measured across the coil terminals. Similarly, for the transformer, the clamp(s) holding the laminations in place is first removed, then the coil can have turns removed to allow space for the abovementioned wire to be slipped into place. The transformer is then reassembled to allow the measurement to be performed. The only problem with this method, crude and simple as it is, is that you need to calibrate it. Well, this is fairly straightforward and can use the same 2400W radiator, or whatever, as a reference load. You will need a short three-core extension cord fitted with plug and socket. The idea is to separate out the active cable from the power cord and insert it into your cobbled up version of the clip-on meter. Then plug in the radiator and take a note of the reading generated by the coil. This can then be used as a rough guide to the current being measured in the earth wire. Note that it is a rough guide only as such a measuring set-up is unlikely to be linear. Warning Many people will not be qualified or feel confident to make any measurements as described in this article. If they suspect that their electrical wiring is unsafe, they should contact a licensed electrician or the electricity supply authorities. A number of serious questions remain to be answered. Do the electricity supply authorities have any program for periodic checking of customers' Active, Neutral and Earth connections? Or does the first indication come from the customer, complaining that they "got a tingle from that appliance"? And are plumbers and employees of the various Water Boards instructed to take any special electric al safety measures when disconnecting a customer's water supply? In the light of this article there is a previously unsuspected hazard to plumbers and Water Board authorities as well as to the public in general. If readers have any further information on this subject which should be published, please write to us at Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097.