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Build a “Safety Switch” Checker By JOHN CLARKE Many homes now have “Safety Switches” installed on their mains switchboards and these give a high level of protection against electrocution from faulty appliances. But how do check that the safety switch is protecting every power point? By using our RCD Checker which is simple to build and even simpler to use. S INCE THE INTRODUCTION of mandatory installation of RCDs in all new homes, there has been a huge increase in the availability of these products. There is no doubt that RCDs increase overall safety when using mains power and are a vital piece of equipment when using power tools. Indeed, under WorkCover regulations, all tradesmen on building sites are required to use RCDs when running power tools. Before we go too far in this article, we should explain what an RCD is. Features * Suitable for home use, tradesmen, electricians, musicians, etc * Creates an Earth leakage current to test RCD * Push to test operation * Neon indicator shows presence of power and checks internal components * Neon indicator to check Earth connection and Active/Neutral terminals * Suitable for type II RCDs (10-30mA rated residual current) * Applies residual test current from 32.5mA at 192VAC to 45.5mA at 264VAC. * Complies with Australian Standard AS 3190-1994 for RCD to trip at or above rated residual current of 100% +5% -0% between 80% of rated supply to 110% of rated supply (240VAC). FEBRUARY 2000  53 Our RCD or “Safety Switch” checker is built into a plugpack supply case. It simply plugs into the outlet to be checked. “RCD” is an acronym for “Residual Current Device”. While sometimes called a “Current-operated Earth leakage device” most people refer to them as a “safety switch”. They protect the user from electrocution by disconnecting the 240VAC supply if there is current flow to Earth. The RCD does this by monitoring the current flow between the Active and Neutral lines for the 240VAC mains supply. In a normally operating appliance there is current flow from the Active lead through the appliance and then back through the Neutral lead. This is shown in Fig.1a. Both currents i1 and i2 should be exactly the same value. If there is a fault in the appliance, as shown in Fig.1b, some of the current flowing from the Active lead may flow to Earth (i3) instead of to Neutral. If this is the case it may be that either there is a breakdown in the appliance between Active and Earth or someone is conducting current away from Neutral via their body. In other words they Fig.1a shows an appliance in normal condition – earthed but with no leakage. Fig.1b shows the same device with a fault. Fig.1c shows the “self test” in an RCD and Fig.1d shows how our checker can tell whether the RCD is operating correctly. All of these diagrams are fully explained in the text. 54  Silicon Chip are suffering electrocution. The RCD detects this difference between the Active and Neutral current and disconnects the power should the difference reach a predetermined level. The difference between the Active and Neutral current is called the residual. An RCD cannot protect against electrocution if the current flow through the person is between Active and Neu- tral. This is because the RCD cannot differentiate between appliance current and current through your body. (The moral of this is never to work on an appliance which is connected to the 240VAC mains supply!) Note that any appliance with exposed metal parts can become an electrocution hazard and this includes many appliances which are labelled as “double insulated”. All that has to happen is a for a leakage path to develop between the Active mains terminal in the equipment and the exposed metal parts; ie, usually the case. Then, if the case is not earthed, it will be live and a potential cause of electrocution if a person touches it. There are several types of RCDs. Type I RCDs have a rated residual current of 10mA and a 40ms tripping time. Type II have a rated residual current of between 10 and 30mA and 40ms tripping time at 500% residual over-current. It takes some 300ms to trip at the rated residual current. Type III RCDs have a rated residual current of between 30 and 300mA and a 50ms tripping time at the 500% residual current. Finally, type IV has type III current characteristics with selectable tripping times. Type II RCDs are the most frequently encountered. You should test your RCDs periodically with their own self-test switches. But this does not tell the whole story. Firstly, the self-test switch on the RCD merely tests its own operation. It does Fig. 2: the circuit of the RCD Checker could hardly be simpler: three resistors, two neons and a switch! Just remember that all this is at mains potential and no RCD Checker can protect you against an active/neutral path. Never work on a plugged-in RCD Checker. not tell you which power points are protected. If you have an RCD installed in your switchboard, you can use the checker to test each power point for its operation; not all power points will necessarily be connected. Secondly, power points which include an RCD do protect other power points connected to it but only those that are “down line” from it – that is, further along the mains circuit from the switchboard than the power point concerned. You can use the RCD Checker again to find out those which are protected and those which are not. Third, if you are using power in a premises where you are uncertain about the presence of an RCD, you can test for this on each power point. Finally, electricians can use the RCD Checker to verify that their installation is effective on each power point. Using the RCD Checker to trip out an RCD is a very convincing test of its effectiveness. there is no access to the Neutral input side of the RCD at the power point. The full circuit is shown in Fig.2. It comprises two Neon indicators, some resistors and a pushbutton switch. The “Valid Earth” Neon connected between Active and Earth lights to show there is an Active supply and a connection to Earth. This indicator is useful to verify that the Earth circuit exists on the power point. If there is no Earth connection, the RCD Checker cannot trip the RCD under test and the power point should be checked before using it. The “Power/Test” Neon monitors the Active and Neutral supply via the test current resistors, R1, R2 & R3. The Parts List 1 3-pin plugpack case (Jaycar HB-5900 or equivalent) 1 RCD Checker label, 40 x 32mm 1 250VAC plastic pushbutton momentary SPST switch (S1, Jaycar SP-0716, DSE P-7568, Altronics S-1080) 2 plastic Neon bezels with resistor (Jaycar SL-2630, DSE P-8116, P-8117, Altronics S-4016) 1 small cordgrip grommet to fill hole in case 1 100mm length of blue 250VAC 7.5A 250VAC wire 1 200mm length of brown 250VAC 7.5A 250VAC wire 1 100mm length of green/yellow 250VAC 7.5A 250VAC wire 1 20mm length of 1mm insulating tubing 1 20mm length of 3mm heatshrink tubing 1 100mm length of 12mm ID heatshrink tubing 1 small cable tie Resistors 2 2.2kΩ 5W wirewound resistors 1 1.5kΩ 5W wirewound resistor Miscellaneous Neutral-cure silicone sealant Test method So how does the test switch for an RCD work? Fig.1c shows the circuit for the internal self-test method used on an RCD. When the test switch is closed, the Active current passes through resistor R1 to the Neutral on the input side of the RCD. Thus all the current through R1 is assumed by the RCD to be the residual. R1 is chosen to trip the RCD between 80% to 110% of rated voltage (eg, from 192VAC to 264VAC for a unit rated at 240VAC). Fig.1d shows the circuit for the SILICON CHIP RCD Checker. In this case the test current flows from the Active to Earth. We have used this method for two reasons. First, because it simulates a true Earth fault and also because Compare this “opened out” photo of the RCD Checker with the wiring diagram (Fig.3,) while building your checker. While it is very simple, take extra care – it is a mains device, after all! FEBRUARY 2000  55 Neon will only light if the three resistors are in circuit and there is power from the Active to Neutral. When the test switch is pressed, the Power/Test Neon will go out and residual current will flow from Active to Earth. The Valid Earth Neon will stay alight until power is disconnected. So if you are using the RCD Checker and you press the button, both Neons will go out for an RCD that is working properly. Note that the switch must be held down for at least 300ms (1/3rd of a second) to ensure that the RCD is given sufficient time to trigger. Construction The RCD Checker is constructed in a plugpack case with the switch and two Neons mounted on the case lid. The three resistors mount around the mains pins inside the case. Fig.3 shows the wiring details. Begin construction by drilling the holes for the switch and two Neons. These are drilled as shown in Fig.4. Insert and secure the two Neons and pushbutton in place. The front panel label (Fig.5) fits into the rectangular moulding above the Neons and switch. Use the wiring diagram of Fig.4 when connecting up the components. Assembled and ready to go, this side-on shot shows how neat a package the plugpack case makes. The cord entry grommet (on the bottom) is not used and for safety, the hole should be sealed with silicone sealant. The resistors are placed around the plug pins with a short length of insulating sleeving on the resistor lead connecting to the Active pin. Solder the resistor leads together and secure a brown mains wire to the free end of the 1.5kΩ resistor and insulate it with a short length of heatshrink tubing. The wiring to the Neons and switch must be followed carefully and use 250VAC-rated wire. Slip a short length of heatshrink tubing over the Neon and switch body before connecting the wires. Then pull the tubing up over the terminals and shrink it with a heat gun. Secure the resistors with some silicone sealant (neutral-cure acid free, eg, roof and gutter sealant) and cover the resistor wires with a dob as well. Leave to cure overnight. Secure the wires together with a cable tie. The case has a cord entry point at the lower righthand side which must be filled to prevent accidental contact with any internal wiring. We used a small cord grip grommet in the hole which effectively sealed off the opening. Alternatively, the hole could be “plugged” with some more silicone sealant. Secure the case together using the supplied screws, with the two longer ones at the top. Checking operation Fig.3: wiring the checker should take no more than about half an hour – ten minutes to assemble, then twenty minutes to check your work before it is plugged in. 56  Silicon Chip Plug the RCD Checker into the power point and switch on the power point switch. Both Neons should light. The power/test Neon will light to indicate supply between the Active and Neutral and that the test resistors are in circuit. If it does not light, check SMART FASTCHARGERS® 2 NEW MODELS WITH OPTIONS TO SUIT YOUR NEEDS & BUDGET Now with 240V AC + 12V DC operation PLUS fully automatic voltage detection Fig. 4 (left): drilling details for the front half of the Jaycar HB-5900 case. Use these REFLEX® chargers for all your Nicads and NIMH batteries: Power tools  Torches  Radio equip.  Mobile phones  Video cameras  Field test instruments  RC models incl. indoor flight  Laptops  Photographic equip.  Toys  Others  Fig. 5 (above): the samesize label which fits the cut-out in the Jaycar HB-5900 plugpack case. the resistors for continuity and correct value and that there is power available. The Earth Neon will light to show there is an Earth connection on the power point. If this does not light, you either have the Active and Neutral connections in the power point transposed or there is a faulty earth connection. Either way, you should engage a licensed electrician to correct the problem. Pressing the Test switch for half a second (ie, more than 300ms) should trip the RCD. Both Neons should extinguish and the RCD should show that it has tripped. If it has not tripped, check that the RCD can be tripped with its own self-test switch. Any faulty RCD should be immediately replaced. If the RCD can be tripped with its own test switch but not with the RCD Checker, check the rated trip current. The SILICON CHIP RCD Checker is for use with 10-30mA RCDs only and will not test type III RCDs which will take a higher residual current before SC tripping. Rugged, compact and very portable. Designed for maximum battery capacity and longest battery life. AVOIDS THE WELL KNOWN MEMORY EFFECT. SAVES MONEY & TIME: Restore most Nicads with memory effect to capacity. Recover batteries with very low remaining voltage. CHARGES VERY FAST plus ELIMINATES THE NEED TO DISCHARGE: charge standard batteries in minimum 3 min., max. 1 to 4 hrs, depending on mA/h rating. Partially empty batteries are just topped up. Batteries always remain cool; this increases the total battery life and also the battery’s reliability. DESIGNED AND MADE IN AUSTRALIA For a FREE, detailed technical description please Ph (03) 6492 1368; Fax (03) 6492 1329; or email smartfastchargers<at>bigpond.com 2567 Wilmot Rd., Devonport, TAS 7310 Silicon Chip Binders REAL VALUE AT $12.95 PLUS ACTIVE/NEUTRAL TRANSPOSITION Until fairly recently, the connection of the Active and Neutral lines to a power outlet was not given the careful attention that it is today. In many older houses (say, pre 1950) you often find the Active and Neutral lines transposed, or swapped at the power point. (In a correctly wired outlet, when you look at the three “holes” from the front, the active is on the left side and the neutral on the right. Earth, of course, is ALWAYS at the bottom (vertical). Indeed, until the late 1960s "double adaptors” were sold which themselves transposed Active and Neutral! Of course, devices still work when plugged into transposed outlets and RCDs also work. It’s normally only a combination of appliance and earth faults which brings out the horrors of an Active/Neutral transposition. This device will not check RCDs where the Active and Neutral are transposed. However, it is very useful for determining whether you have an Active/Neutral transposition. If you have a power point which is known to work but the “valid earth” neon does not light, treat it with suspicion. It could be a broken or high resistance earth – which of course must be fixed immediately – or it could be an Active/Neutral transposition. It’s worth having the outlet checked out by a licenced electrician – for your piece of mind and your safety. P&P  Heavy board covers with 2-tone green vinyl covering  Each binder holds up to 14 issues  SILICON CHIP logo printed on spine & cover Price: $A12.95 plus $A5 p&p each (Australia only) Just fill in & mail the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. FEBRUARY 2000  57