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SAFETY WATCH Safety Watch is an occasional feature in SILICON CHIP drawing attention to issues of electrical safety in the workshop and home. Making your electric radiator safe Does the cord or plug of your electric radiator get hot? If so, you should think seriously about installing a cord and plug more suited to the job. These days many electric radiators have plugs and cords which are just not suitable for the task. We're talking particularly about appliances rated at between 2000 and 2400 watts and which draw between B and 10 amps from the mains. If they're used on the lower power settings, they're OK but when you switch up to full power on those cold winter nights, the cord and plug starts to go a bit limp literally. Two of the photos accompanying this article show a particular type of plastic 3-pin plug which is commonly fitted to the cords of electric radiators. It is nominally rated at 10 amps [as indicated by the lOA marking between the pins) but when the appliance is drawing this current, the plug gets very hot - so much so, that the plastic starts to melt. This shows up where the cord enters the rear of the plug [Fig.1) and around the brass Active and Neutral pins (Fig.2). Part of the problem is that the pins are fabricated out of thin brass and thus have a higher resistance to the current flow than plugs with solid brass pins. When you are drawing 10 amps, even quite a small resistance will result in a lot of power dissipation. Say, for example, that the total resistance associated with the pin of a power plug is only 50 milliohms or 0.050 [this figure includes the contact resistance plus the resis16 SILICON CHIP tance of the pin itself). With 10 amps flowing, there will be 5 watts dissipated. With two pins having the same resistance, the total power dissipated in the plug will be 10 watts. Now 10 watts of heat dissipated in a plug is enough to make it stinking hot. It certainly is enough to melt it. So you can see that resistance must be avoided as much as possible. Often combined with these puny plugs is a puny power cord which also has a nominal rating of 10 amps. The operative word here is "nominal". As far as we're concerned, it doesn't mean 10 fairdinkum, full-on continuous amps; it means pony sized amps which probably come out of the same labs that used to do the exaggerated horsepower tests for cars. Such cords are OK if used for a short period but are not adequate for radiators that are used at maximum setting for hours on end. These puny 10-amp power cords have a plastic sheath with an overall diameter of about 8mm. When they are running at 10 amps they get quite warm and limp. If they are running over a shag pile carpet or are in any situation where air circulation around the cord is poor, the cord will get surprisingly hot. Some blower type radiators are of all plastic construction and so are double insulated. This means that they have a 2-core flex which is even thinner in overall diameter at only 6mm. In some situations, we believe these cords and plugs get so hot that they could eventually lead to a fire. In our opinion, they should not be approved by the electrical authorities for 10 amp applications. We reckon they're OK for any appliance drawing up to 5 amps on a continuous basis but for anything more than that they are not suitable. Replace the cord and plug Fig.1: carefully inspect the rear of the plug. If the insulation is starting to melt, the plug should he replaced. There is only one type of 3-pin plug which is suitable for an appliance which draws 10 amps and this is the HPM Cat. 99. This is not made of soft plastic but hard white phenolic. It has solid brass pins and a large cord entry at the rear. Heat will not distort these plugs. So even if you have poor connections inside the plug, the plug will not melt. And there is only one type of Problems? ... and you don't have our .120page catalogue . .. Fig.2: the pins on plastic plugs used at high current can loosen as the plastic melts. Here, the active pin (at the end of the screwdriver blade) has worked quite loose, creating a hazardous situation. 3-core flex which we would recommend. It is rated at 15 amps continuous and has an overall diameter of 10.5mm. It is labelled 301.25mm and 1.5 sq mm. What this means is that the three cores are each made up of 30 strands of 0.25mm copper wire which gives an overall conductor cross section of 1. 5 sq mm. Why go for a 3-core flex which is rated at 15 amps'? There are two reasons but they add up to the same result - the cord won't get hot. The first reason is that this heavy duty cord has thicker insulation and is therefore less likely to kink and strain the internal conductors. Second, the voltage drop along the cable for a given current is a lot less than in the so-called 10-amp cables. This means more voltage in the radiator and less heat in the cable. cable. That way, if you decide that it looks too hard to mess about with, you can let an electrician do the job. By the way, many radiators don't have a long enough power cord - if you decide to replace the cord you can make it longer. Let's assume now that you have . purchased two or three metres of the recommended cable and the HPM plug. Remove the existing power cord from the radiator and note the length of the Active, Neutral and Earth wires. Depending on how the cord is installed, these will probably have different lengths. You must duplicate these. The longest wire determines how much of the outer insulation must be removed. Stripping the cable When you buy your HPM Cat. 99 plug it will come with instructions on how to wire it and how much to strip the cable. That's OK for the plug end but what about where it terminates inside your radiator'? What you have to do is to duplicate the connections inside your radiator. In fact, it would be a good idea to have a look at these connections before you go to purchase any Fig.3: the HPM Cat. 99 plug is the one that we recommend for 2400W radiators. It has solid brass pins and will not distort due to heat build-up. At last . .. a TRADE catalogue for the consumer ARISTA ... your one-stop problem solver. Video plugs and sockets ... Video extension speakers .. . Video flyleads . .. Video RF interference filters ... Video splitters ... Indoor antennas ... Video switching units . . . Down converters . . . Video speaker controllers . . . Video camera lights . . . Video tape rewinders .. . Video cine adaptors ... Video head cleaners . . . Video splicing kits . . . Video storage cases . . . Video dust covers . . . Video leads . . . Scart plug leads ... Video dubbing kits . .. Video headphones .. . Video shotgun and wireless microphone systems .. . Pre-amplifiers with video inputs .. . Video camera stands ... Just about anything you want. ... Try us ... NOW! Get your catalogue FREE from your local ARISTA dealer or send $2.50 P&H and your return address to: ARISTI\. ELECTRONICS PTY LTD PO BOX 191, LIDCOMBE, NSW 2141 OCT0BER1989 17 Fig.4: a Stanley utility knife with a sharp blade is the recommended tool for stripping mains cable. Fig.6: use the Stanley knife to strip about 12mm from the ends of the inner cores but be careful not to nick the individual strands of wire. The inner cores should duplicate the lengths of those on the original cable. Fig.5: be careful not to nick the insulation of the inner wires. If you do, cut off the damaged section and start again. Naturally, you have to strip both ends of the cable, one end to suit the radiator and the other to suit the plug. But many people are not sure how to go about stripping cable. The recommended tool is a Stanley utility knife with a sharp blade. The idea when stripping the outer sheath from the flex is to make sure that you don't nick or cut the insulation of the three individual insulated wires. If you cut the insulation of these, you can't safely wire the plug. A cut in the insulation can eventually lead to a flashover in the cord and perhaps even a fire. 18 SILICON CHIP So if you do nick one of the three cores, you must cut off the damaged portion and start again. Fig.4 shows how to start stripping the cord. You bend it over and carefully cut around half the circumference but don't go deeper than 1mm. As you bend it, the cable covering will then split open as shown in Fig.5. Having done that, bend the cable over in the other direction and cut it in the same way. The outer covering will then slip off. Note that if you have nicked one of the inner cores it will be immediately visible as you bend it over. Now strip about 12mm from the inner cores as shown in Fig.6. Again, go easy with the knife otherwise you will nick the individual strands of wire. As you can see in Fig.6, you just roll the wire along the blade edge so that it penetrates but doesn't cut through. Then by bending the core back and forth, the insulation will break and can be slid off. Terminate the cable inside the radiator first. Make sure that all connections are secure and that the cable is anchored exactly as before. Incidentally, if the radiator is a double-insulated appliance, do not connect the Earth wire - just clip it off. You can now terminate each of the three wires on the mains plug. Don't forget to slip the plug back over the cable before you terminate the wires. There is nothing more annoying than wiring up a mains plug and finding you have left the plug cover on the table, or on the floor! Now before you apply power, get out your multimeter and switch it to a low Ohms range. Check that you have continuity between the Earth pin of the mains plug and exposed metalwork on the radiator. Check that you have continuity between the Active and Neutral pins. If the radiator has a switch you will have to move this to the on position to obtain a reading. If the radiator has a rating of 2400W, its resistance in the highest heat setting will be about 240. Finally, switch your multimeter to the highest available Ohms setting and check that there is an open circuit (ie, no reading or no pointer deflection) between the Earth pin on the plug and either the Active or Neutral pins. That done, your radiator can be put back into use. One further point: if the power plug still gets hot when the radiator is at the maximum heat setting, then the mains power point itself may be a little tired and may need replacing. ~