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Continuing our series on D-I-Y house wiring – NZ style Looking at Light Switches A light switch is a light switch, right? You just flick it and the light comes on . . . Of course – but it is very obvious that many people think what goes on behind the architrave or wall is a black art. Magic, even! A siliconchip.com.au P LOO First of all, let’s have a look at a For a start, a light switch is a “double throw” device – this simply means that you can have it switch between two different circuits if you wish (see Fig.2). It has a “common” terminal (labelled C) and two switched terminals, usually labelled 1 and 2, either of which can be connected to the common terminal depending on which way the switch is positioned. You don’t need to connect to both switched terminals – in fact, in the 1 The standard light switch standard mains light switch. “What’s to look at,” you ask? Well, quite a lot when you look into it (pardon the pun!). We have shown both a photo and a drawing of the back of a light switch. As you can see, there are actually four screw terminals on the back – why, when for a standard switch function you need only two terminals. That’s true, but those four terminals give you quite a bit more flexibility than a simple off/on function. 2 C nd then there are those twoway or even three-way light switches which let you turn the light on and off from two or three locations. If single light switches are magic, two-way switches must be the stuff of sorcery to some! There are many areas of a home where two-way light switches make a lot of sense – any room or hallway, for example, where you can enter and leave by different doorways. But there is a right way and a wrong way to do it! More on this shortly. But before we go on, a warning (again!): even though the following is completely legal in New Zealand, it’s not legal in Australia unless you have an electrician’s or electrical contractor’s licence. As we mentioned last month, you might have a PhD in electrical engineering, or a lifetime of experience in electrical repair or assembly but that counts for nought without that ticket! OK, so we’ll assume you’re in NZ and want to replace a light switch. By Ross Tester The back of a standard light switch mechanism (in this case an HPM brand). The four terminals are C (common), switched terminals 1 & 2 and the non-connected “loop” terminal. In a normal light, terminal 2 is seldom used. And here’s a photo of the same thing. Terminal 2 is covered by a thin shield – if needed the shield is easy to remove. July 2008  11 vast majority of lighting installations, only one switched terminal (and the common) is used, effectively making it a “single throw” switch. Incidentally, it is impossible to use the switch to connect together the terminals labelled 1 and 2 – they can only be individually connected to the C (common terminal). You might notice from the photos that terminal 2 is normally covered by a thin PVC shield. To use the switch in its double throw mode you need to remove this (it’s quite easy with a small screwdriver). The standard switch is also a “single pole” device – again, this means that it will only switch one circuit (eg, switching power to a light). There are “double pole” switches available which can switch two circuits at the same time but these are neither required nor used in light switch applications. An example of a double pole switch is (usually) in the main switch in your powerboard. It will switch both Active and Neutral at the same time. Now, what about that fourth screw terminal? Look at the photo/drawing again. There is almost always one screw terminal off by itself (in fact, usually directly opposite the common and two switched terminals). It almost always has the word “loop” moulded into the plastic alongside. This terminal is simply used as an “anchor point” for other wires – for example, it may connect Active wires or Neutral wires together if required. You will almost certainly find the same terminal moulded into the light fitting itself. Only two terminals connect to the base of the bulb, supplying By contrast, the back of a modern Clipsal 530 BC (bayonet cap) light fitting. It’s quite similar in layout to the light switch, the main difference being the earth terminal (here clearly identified with an “E” as well as the green around it) instead of the “C” terminal of the light switch. Note also the looping terminal (identified with an “L” moulded into the plastic), as well as a more coppery colour. Like the light switch, the looping terminal plays no part except as a potential anchor point. The terminals which actually connect with the base of the lamp (here seen as more of a brass colour) are not polarised and can be connected either way around. 12  Silicon Chip SINGLE POLE SINGLE THROW (SPST) SINGLE POLE DOUBLE THROW (SPDT) DOUBLE POLE SINGLE THROW (DPST) DOUBLE POLE DOUBLE THROW (DPDT) Fig.1: for those not familiar with switch types, here are the four most common. The simplest, which simply makes or breaks a connection in one wire, is an SPST type – this is the switch you’ll most usually find inside appliances. Light switches are almost always SPDT types, even though the majority of the time they are used in SPST mode. SPDT can switch one wire to two different circuits.Where both active and neutral need to be switched at the same time, a DPST switch is used. Double pole switches are certainly not the largest available – you can easily get four pole (and more) switches. But where large numbers of circuits need to be switched simultaneously, a relay or contactor would normally be used. power to it. The third, again usually labelled “loop”, is used the same way as the loop terminal on the switch; that is, as a connection point for other wiring. Switch wiring colours Unlike flexible mains cables (or so-called “extension leads”), building cables haven’t largely switched over to the IEC colours of brown, blue and green/yellow for Active, Neutral and Earth respectively. Building cable, incidentally, invariably means cable with each of the conductors individually insulated and then sheathed with another layer of (usually white) PVC insulation. The vast majority of homes have their switches wired with standard red and black building cable. The red wire is always Active and the black the switched Active back to the light fitting. However, as you know black normally means “Neutral”. Isn’t that a recipe for disaster? It could easily be, which is why you will find more modern homes with the wiring to their light switches in red and white building cable. Red still means Active while white means switched Active. In either case, it means “bitey – beware!” AS/NZS3000 (and the domestic installation standard AS/NZ3018) in fact specifies the colours of cables: Active can be red, white or blue, Neutral black (only) and Earth a green/yellow combination. Earths on lighting fixtures Many older homes (eg, 30 years +) have only two conductors (switched Active and Neutral) connected to the light fitting. But the rules have changed – now most light fittings (referred to as “luminaires”) must also have an Earth conductor connected to the building Earth. The exceptions are double-insulated luminaires or extra-low-voltage types supplied through a transformer (such as halogen downlights). So if you are adding any new fittings or replacing wiring, it makes sense to run an Earth conductor (in green/gold insulation) from the light fitting back to, and solidly, mechanically connected to, a suitable Earth line. All metal light fittings sold today have provision for connection of an Earth conductor. How to replace a light switch Switch mechanisms do fail – especially in moist or salty environments – and therefore need replacement. Perhaps you have decided to replace a good switch with a modern colourschemed one. Or you might be wanting to install a light in a previously unlit siliconchip.com.au At the risk of getting boring, we’re going to say it again! Check that the circuit is dead before doing anything with a non-contact tester. This circuit is obviously still very much alive and therefore dangerous. Incidentally, you should always check the non-contact tester with a known live circuit before use – just in case the battery is flat or the tester has failed. You want to know that it’s working! NEUTRAL 230/240V (WITH BUILDING CIRCUIT BREAKER EARTH OR FUSE IN ACTIVE LINE) LAMP CONNECTION TERMINAL ACTIVE P C siliconchip.com.au wood screws) or onto a plaster wall (larger screws which fasten it to a mounting plate inside the wall). Some have a faceplate, as we discussed last month, which you’ll need to pry off the subplate underneath, to gain access to the two mounting screws. If the switch is mounted on a painted wall, before undoing the screws score around the very edge of the switch plate with a sharp knife so you don’t pull away paint (as we discussed last month). Now remove the screws and gently pull the switch plate away from the architrave/wall. We said gently because old wiring might be brittle and you don’t want to break it off. If the wiring is brittle and/or you do manage to break it off back in the wall, it really is time to call in an electrician and have the wiring replaced. You’ll LOO place. We’ll look at this a little later but in the meantime we’ll assume you’re simply replacing a switch. You might think this is like teaching people to tie shoelaces –but how often do you see loose or untied laces! To many people, what we are talking about here is like a foreign language. Even if you think you know what you’re doing, read on. You might learn something! Before you start, you have the choice of replacing the whole switch or just the switch mechanism (see the side panel overleaf). There’s not a great deal of price difference in them (in fact, the whole switch is often cheaper than the mech!) so if your switch plate is at all marked, dirty or (shudder!) painted on, we’d always replace the whole thing. First of all, as always, turn off the power at the main switch in the fusebox. If you can positively identify the circuit breaker or fuse which controls the light, this can be tripped or removed but it is nice to know that all power is off when the main switch is off. Second, use your non-contact voltage detector to absolutely confirm that the power is removed. What, you haven’t bought one yet? Shame! Don’t proceed any further until you do . . . Incidentally, before use you should always check the non-contact voltage detector on a known live circuit (eg, a power point) to ensure that it is working. The batteries might be flat or it could have developed a fault. Most light switches have two screws which hold them in place, either directly to an architrave (with small probably find there’s a lot more wiring in your home in a similar state. Remove the wires one at a time by unscrewing the tiny grub screws that hold them in place and simply swap the wire to the new switch, placing it in the same-positioned terminal that it came from (be careful not to mix them up!) and tighten its grub screw. Those grub screws have to be done up as tight as possible to make sure the wires are making the best contact they can. This will help eliminate the possibility of “hot spots” developing caused by high-resistance joins. (You should find the wires which come out of the switch are bent over at the end, forming a double wire and if there are two wires going into the one terminal, they should be twisted tightly together – for exactly the same reason as above). If the switch has wires going into the “loop” terminal, swap these over to the loop terminal on the new switch – and the job is almost done. It’s just a matter of replacing the switch plate in the hole the old one came from – as long as you get it the right way up. The switch mechanism itself usually has some tiny writing or logo moulded into the front – this goes down. If you can’t see the writing or logo, activate the switch. Re-insert the screws and if the switch has a cover plate, simply snap that in place. Finally, turn the power back on and check that the switch works and that the off and on positions are in the conventional places. If the off and on are upside down, turn the power back off before remov- REAR OF LIGHT SWITCH 1 NOTE EARTH TERMINAL -MARKED WITH GREEN AND/OR LABELLED “E” LOOPING TERMINAL L E WHITE GENERALLY MEANS A SWITCHED ACTIVE LAMP CONNECTION TERMINAL REAR OF LIGHT FITTING Fig.2: wiring a single light switch isn’t at all difficult but that doesn’t mean you shouldn’t take proper care. Here’s the way it should look – except that you’d generally have some distance between switch and light socket! Older light sockets may not have the earthing terminal. July 2008  13 A C N Fig.3 SWITCH 1 C LAMP A Fig.4 N A SWITCH 2 N C SWITCH 1 tiny internal arc, which occurs every time any mains switch is turned off, will eventually become a major arc between Active and Neutral. If you’re lucky, this will simply blow a fuse or cut a circuit breaker. If you’re not so lucky . . . You might get away with this in a 110V system (does that give the DIY magazine source away?) because the lower voltage would not create such an arc. But it is still wrong and should never be used. Fig.4 shows the right way to do it. When the position of both switches is “up” or both “down”, the light will be on. When either switch is different to its mate, the light will be off. The difference here is that it is (a) legal because both switches are in the Active conductor and (b) it is safe because Active and Neutral are not applied to the two “switched” terminals, so the arc is minimal; too small to cause any problems. In fact, this method of C SWITCH 2 LAMP Here are two ways to wire a two-way switch. We show the first one (Fig.3) because we have seen it in a D-I-Y magazine available in Australia. Apart from being illegal, it’s definitely NOT the way to do it. The version on the right (Fig.4) is the right way to do it and we’ve shown in more graphical format below for those who have difficulty with circuits. When both switches or up or both down, the light will be on. ing the screws and turning the plate through 180°. Switch mechanism orientation Switch mechanisms almost always operate in the vertical plane (ie, you push down on the switch to turn it on) but the switch plate can be oriented either horizontally (eg on a wall) or vertically (eg, on an architrave). Fortunately (by design!) the switch mechanism can be removed from the plate and turned to the required orientation. If you look at the photo opposite, you’ll see some tiny lugs at the base of the mechanism. These “lock” the mechanism in place in appropriatelyplaced notches moulded into the “well” within the switch plate. As always, turn the power off and/ or flick the circuit breaker/remove the fuse before doing this. Check that the circuit is dead with your non-contact mains voltage tester. The simplest way to remove the mechanism from the plate is to push down on one side at the back, while applying gentle pressure to the switch itself from the front. Turn the switch mechanism to the direction you require and push it back in quite firmly. It will eventually snap into place . . . and that’s that. Wiring a two-way switch LAMP CONNECTION TERMINAL LOOPING TERMINAL E LAMP CONNECTION TERMINAL NEUTRAL 230/240V VIA CIRCUIT BREAKER OR FUSE IN ACTIVE LINE AT FUSEBOX EARTH WHITE GENERALLY MEANS A SWITCHED ACTIVE ACTIVE P C P LOO Fig.5: obviously not to scale but here is the way to wire up a two-way light switch with the circuit of Fig.4. The cables between the REAR OF two light switches REAR OF LIGHT SWITCH 1 LIGHT SWITCH 2 (which we’ve shown CONNECTING CABLES CAN BE in blue for clarity) RED, WHITE OR DARK BLUE would probably be red/white BUT MUST BE DOUBLE INSULATED twin building (lighting) cable in most installations. The last 4-5mm of the bared ends of each cable should be bent back on themselves and the grub screws done up as tight as possible to ensure as good an electrical connection as possible. LOO 14  Silicon Chip REAR OF LIGHT FITTING NOTE EARTH TERMINAL -MARKED WITH GREEN AND/OR LABELLED C We mentioned earlier that there is a wrong way and a right way to wire a two-way switch. Both work, although one will only work for a while before the switch is destroyed (it may even catch fire). Refer to Fig.3 – it shows a popular – but very wrong – way to wire a two-way switch. We’ve actually seen this shown (admittedly some years ago) in a do-it-yourself handyman magazine from overseas that was sold in Australia. Apart from being illegal under AS/ NZ3000 because it can switch the Neutral conductor (rule 2.20.1.2), if you have a switch wired like this, it’s only a matter of time before the mechanism fails. Here if either switch is opposite to its mate, the light is on. If both are the same, either up or down, the light is off. However, switches were never intended (nor designed) to work this way. In this, Active and Neutral are only separated by a very small distance inside the switch mechanism and the siliconchip.com.au NEW! CIRCUIT WIZARD A revolutionary new system that combines circuit design, PCB design, simulation & CAD/ CAM in one complete package for your pc. rom: f o m e d e a fre com Downloeawd-wave-concepts. www.n To see the The switch mechanism removed from the switch plate, so that it can be turned through 90° for horizontal mounting (eg, on a wall). The mechanism simply clicks back into place with those lugs aligning with recesses in the plate. wiring can actually be a little easier because you don’t have to provide Active and Neutral at both switches. You do have to provide a two-wire connection between the two switches. Here it can be a little confusing because the standard says that you can use red, white or blue (but never green, yellow, green/yellow or black). You must NEVER use a green/gold wire for anything but Earth. NEVER. Sure, you might know that it isn’t an Earth wire. But what about the contractor who comes in to do some other work? Or the people who buy your house when you move? It is a perfectly reasonable assumption that green/gold is the Earth wire – and if it happens to be a live wire, disaster is not too far away. Of course, you must not use the red wire for anything but Active and the black for Neutral – but that’s just common sense, isn’t it? Remember: twist and tight! And one final point: at the risk of repeating ourselves, when fitting wires to light switches, lights fittings or even power outlets, it is very important to have as much copper wire in contact with the terminals as possible. We mentioned before so-called “hot spots” which usually develop due to the join between wires, or between wires and terminals, becoming a high resistance. And as everyone knows, pushing current through a resistance causes heat. In fact, this is the most common reason that wires and fittings deteriorate (and become a potential fire risk). So always twist any wires that you are joining tightly together; always bend the end of a wire 180° back on itself to create a larger cross-sectional area . . . and always make sure those grub screws (either in the fittings or in connectors) are done up as tightly as you can get them. SC siliconchip.com.au s between difference essional dard & Prof tures’. ick on ‘fea versions cl Stan IDEAL FOR Schools, TAFEs, Hobbyists & Business Circuit Wizard Standard – $202* & Circuit Wizard Pro – $390*post*incin GST Aust. 555Electronics Australia and New Zealand – for orders or more information, please contact 19 Kensington St, Clovelly Park, SA 5042 Tel (08) 8277 8936 email: bwigley<at>senet.com.au www.555electronics.com.au Radio, Television & Hobbies: ONLY the COMPLETE 00 $ 62 archive on DVD &P +$7 P • Every issue individually archived, by month and year • Complete with index for each year • A must-have for everyone interested in electronics This remarkable collection of PDFs covers every issue of R & H, as it was known from the beginning (April 1939 – price sixpence!) right through to the final edition of R, TV & H in March 1965, before it disappeared forever with the change of name to Electronics Australia. For the first time ever, complete and in one handy DVD, every article and every issue is covered. If you're an old timer (or even young timer!) into vintage radio, it doesn't get much more vintage than this. If you're a student of history, this archive gives an extraordinary insight into the amazing breakthroughs made in radio and electronics technology following the war years. And speaking of the war years, R & H had some of the best propaganda imaginable! Even if you're just an electronics dabbler, there's something here to interest you. NB: Requires a computer with DVD reader to view – will not work on a standard audio/video DVD player Use the the handy handy order order form Use on page 23ofofthis thisissue. issue on page 81 July 2008  15