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Your House Water Pipes could ELECTROCUTE You! Do you have RCDs (so-called “safety switches”) fitted in your home’s switchboard? Do you think that eliminates the chance of electrocution? Well that isn’t the case because the copper water pipes in your home could easily become live and dangerous! That is why some people have been electrocuted in the shower . . . U ndoubtedly modern Australian homes have safe wiring when they are first built. They would not be “signed off” as such before occupancy certificates were issued if they weren’t. But corrosion and wiring or plumbing modifications over the years can easily produce a dangerous situation which could cause a fatal electric shock. It is partly because of the M.E.N. Electrical Wiring System used in Australian homes. 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 3000-2007), the mains Neutral wire is connected to Earth at the user’s switchboard. In most houses this means that the 230VAC mains supply is connected from the power pole in the street via two cables, Active and Neutral, with the Neutral wire typically connected 14  Silicon Chip to Earth via the consumer’s water pipe (or a separate Earth stake). 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 Earth connection is vital to the safety of everyone in your household. Typically, the Active and Neutral 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 16kW (kilowatts) in the case of 70A mains and 23kW in the case of 100A 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 By LEO SIMPSON households when air-conditioning, a microwave oven, a stove and perhaps one or two other cooking or heating 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 230VAC 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 a little higher because all the current from your house and your neighbours’ homes inevitably causes a voltage drop in the Neutral cable between your power pole and the nearest substation or pole transformer. This is because the resistance of the Neutral cable is not zero and the large currents passing through it inevitably means that there is some voltage present on the Neutral line. This voltage siliconchip.com.au CIRCUIT BREAKERS RCD WATT-HOUR METER 80A GPOs OVEN COOKTOP AIR CON ETC RCD CIRCUIT BREAKER SERVICE FUSE LIGHTS RCD ACTIVE WATT-HOUR METER RIPPLE TONE CONTROL (OR TIME SWITCH) 20A CIRCUIT BREAKER OFF-PEAK HOT WATER HEATER FROM STREET NEUTRAL NEUTRAL LINK This simplified diagram depicts the electrical switchboard of a typical home using the “Multiple Earth Neutral” (M.E.N.) system. It does not take into account the fact that more Neutral connections are required than shown here (eg, to the RCDs and off-peak hot water tone control), nor the fact that some appliances (stoves, cooktops and air conditioners, for example) may be supplied from a circuit not protected by an RCD to minimise nuisance tripping. The Service Fuse may be mounted on the switchboard or it may be mounted on the barge board, etc, where the wires from the street are attached. is a loss in the energy transmission system and is part of the reason why, when energy demands are heavy, the mains voltage available at your switchboard is lower than it should be; the Multiple Earth Neutral system mitigates this to some extent. Because the Neutral wire is connected to Earth in your switchboard, there are actually two return paths for the current passing through the appliances in your home. Some of the current passes back along the Neutral line, 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 inhersiliconchip.com.au ent current balance of three-phase 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 simplified 230VAC mains wiring of a typical modern household installation with electric (off-peak) hot water heating. 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 circuit breaker which is typically rated at around 80A (amps) and is connected NEUTRAL WIRES EARTH WIRES FROM GPOs ETC BUILDING EARTH TO WATER PIPE OR EARTH STAKE via the RCDs (residual current devices) which protect the power and lighting circuits in the household. From there, a number of circuit breakers feed the individual circuits for the stove, oven and GPOs. By the way, in the Australian wiring standards (AS/NZS3000), 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. This also connects to all the earth wires from the power and lighting circuits as well as the main earth wire, which in turn connects to the water pipe or in some cases to a mains earth stake driven well into the ground. It also connects to the Neutral wire which comes directly from the power pole. August 2014  15 they otherwise would if the M.E.N. system was not used. They also experience less voltage flash-overs 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 M.E.N. system this is much less of a problem. Wherein lie the problems? On this one pole are the Neutral and three Active phase wires (on the crossbar at top) with the Neutral having the most connections; below that are two cable TV/broadband services and telephone lines. Normally, the phone and at least one cable TV line (Telstra) would be underground but this pole sits in a hole drilled into solid rock. Note that we haven’t mentioned an Earth wire – under the M.E.N. system, the Earth connections are virtually always made at the consumer’s premises. (We are ignoring the separate Neutral links which are used for each RCD. These additional links are used because the RCDs monitor the balance between the Active and Neutral currents in their respective circuits). So it is a fairly 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. It ain’t necessarily so! It is possible that as much as half (or even 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, 6 amps might go via the Earth wire to the water pipe and the remainder, 4 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 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 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 become physically opencircuit, its resistance may 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 happens quite frequently. 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 is. What that means Current flow, 3-phase and all that 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 50Hz alternating current, the current changes its direction 100 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 Active line that gets ya. It’s the one with the juice!” 16  Silicon Chip 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 230VAC (with 120° 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 typically single-phase, so appreciable current does flow in the Neutral return. siliconchip.com.au is that the Neutral return could now have an appreciable resistance and could easily be around 5 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 25 volts AC. Hmm. So the Neutral link will be floating at 25 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 its chassis floating at 25 volts AC above earth. In time, this could get much worse. 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. Of course, another corrosion scenario is also possible and it also does happen. What if corrosion in the Neutral connection has resulted in high resistance or an open-circuit? In that case, all the current will flow in the Earth. All your appliances will continue to work and will be perfectly safe. Your RCDs will continue to monitor for any imbalance between Active & Neutral currents into your electrical system. No problem. But all that current is now flowing in your Earth connection. And in many homes, that Earth connection is via your water pipes. If you have an AC clamp meter, your can check this yourself. Just wrap the clamp around the water pipe just where it goes via your water meter, as you can see in one of the pictures in this article. You can see the clamp meter is reading a substantial AC current! Does this ring alarm bells? Well, it should. That water meter is a potential death trap! If a plumber needs to disconnect the water meter, he may be interrupting a circuit in which a substantial mains current is flowing. If the water meter is removed, there could be a very high voltage between the two ends of the pipe and there is a danger that the poor unsuspecting plumber could be electrocuted. Fortunately most plumbers know about this hazard and before removing a water meter, their standard practice is to bridge around the water meter with a set of car battery jumper leads, before it is removed. Mind you, plumbers have been known to create a hazard further downstream from the water meter by replacing a length of copper pipe with If this isn’t enough to scare you, perhaps it should be. The load (inside the house) was a nominal 2400W heater, so the total current drawn is just shy of 10A. That’s what we’d expect to see on the main Neutral line (the left photo). But it’s only 4.32A – the other 5.06A is actually flowing through the (green/yellow) Earth wire in the photo at right. If your Earth connection is in any way faulty . . . siliconchip.com.au August 2014  17 plastic, thereby breaking the Earth connection and creating a shock hazard. So what might be the likely signs that you have a hazardous condition in your wiring or water pipes? Perhaps the most dangerous is where people receive an electric shock or tingle from the water taps over the kitchen sink, or worse, while in the shower. If this ever happens to you or one of your acquaintances, you should immediately contact your electricity supply retailer. They should have an inspector out to check the situation in very short order. And often they will find in a defect in the Neutral wiring back to the supply in your street. But you don’t have to wait for this dangerous situation to develop. You can make a few checks yourself. Switch on and place the clamp meter over the Earth wire to your water pipe or Earth stake. Some AC current is bound to flow and it could be 6 amps or more. So what if you measure a current of close to 10 amps? That’s when the alarm bells should be ringing because that means that you have no Neutral current and no Neutral connection out to the wires in your street. That means that your water pipes and taps could become live and lethal, as outlined above. 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 What you should do 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 may be a note (in white paint) saying where the Earth is. In some areas where the water service is run in plastic pipe, or the copper pipes do not make intimate contact with the ground (eg, when the pipes are run above ground or on rock) 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. Use an AC clamp meter But there is a better way to check the integrity of your Earth connection and that is to use an AC clamp meter. First, you need to have a substantial AC current flowing in your home’s wiring and the best way to ensure that is switch on an electric radiator, preferably one rated at 2400 watts because that means that it will pull a current of close to 10 amps. 18  Silicon Chip This reading of 5.39A in the water pipe between the house and the water meter is not at all atypical – it can often be more. But just imagine if a plumber (or anyone else) disconnected the water meter or severed the pipe – it could be very easily be lethal! siliconchip.com.au via earth than via the Neutral path. 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 electrical safety measures when disconnecting a customer’s water supply? The simplified diagram of a typical switchboard on page 15 does not show any details of all the Neutral connections which are necessary for the RCDs. This photo shows typical RCD Neutral link connections. Currents can flow even when the power is turned off! As part of the preparation for this article, I used a Digitech AC clamp meter (Jaycar Cat QM-1561) to measure Neutral and Earth Currents at my switchboard. I also measured the current flowing via the water meter and I noticed that much greater currents were flowing in the water meter than could possibly be attributed to my home consumption when the load was only standby power from cordless phones and other appliances. So while the total consumption was much less than 100W, the current in the Earth was as much as 4A, or more! I turned off the power at the main circuit breaker and the large Earth current was still there. Checking the Neutral, I found that virtually the same current was flowing in the Neutral. How could this be? This was quite independent of anything in my home. I also measured the current flowing in my next door neighbour’s water meter and found that it also had substantial current flowing, even though no-one was home. At this point I contacted Energy Australia, my electricity retailer, and they sent an electrician out within hours. He first confirmed that about 55% of the load current was flowing in the Earth and the rest via the Neutral. But he used a much larger load current. Instead of simply switching on a siliconchip.com.au 2400W radiator as I had, he over-rode water meter can lead to burnt O-rings, the Zellweger ripple tone control switch leading to a complaint of tainted water to allow the hot-water tank elements to from consumers. SC cut in. This gave a total load current of 31A. He also confirmed that current flows in the Neutral and Earth when the mains supply fuse is disconnected. He said that this is not uncommon and does not necessarily indicate a fault in the household system. He then proceeded to do a system integrity check with a Fluke 1654B Multifunction Tester and came up with figures of about 0.35 ohms for the Neutral path and 0.3 ohms for the Earth which he then pronounced to be quite safe. The test procedure is according to Australian Standard AS/NZS3017. (An alternative instrument for this testing would be the Metrel Instaltest 3017 from Emona Instruments [www.emona.com.au] which was reviewed in the September 2008 issue of SILICON CHIP). He was very thorough during the testing, wearing three pairs of gloves, rubber boots and protective goggles which is a mandated procedure. He also confirmed that anyone finding high currents flowing in the The Metrel Instalset AS3017 Test Set from water meter should contact their Emona Instruments. It’s not cheap (in both electricity retailer. Apparently they senses of the word!) but if you’re involved get most calls of this nature from in installation or checking of electrical plumbers or from Sydney Water. installations, it makes short work of what Interestingly, high currents in the could otherwise be laborious tests. August 2014  19