Fullscreen HTML5Med Res (??MB)HTML4

Appliance Earth Leakage Tester By JOHN CLARKE Used in conjunction with a digital multimeter, this Appliance Earth Leakage Tester can be used to check the safety of earthed and double-insulated equipment. Most importantly, it tests equipment when it is powered from the 230VAC mains supply and operating normally. Features & Specifications Features •  Monitors earth leakage current via imbalance between Active & Neutral current flow •  Measurement displayed on multimeter in voltage mode •  AC output and true RMS (DC) output •  Easy measurement conversion (100mV on the DMM = 1mA leakage current) •  Powered from a 9V battery; power LED indicates battery state Specifications Frequency response: <10Hz to >6kHz (-3dB) Linearity: <1% deviation for measurements from 1-5mA True RMS: crest factor up to 5 Power supply: 9V battery Current drain: typically 2.5mA Battery voltage: operates down to 7.4V; indicator LED indicates battery state 26  Silicon Chip I N LAST MONTH’S issue, we presented the Appliance Insulation Tester which tests at 500V or 250V DC. However, that tester cannot do a proper test of any appliance which is switched on by remote control or does not use a mechanical on/off switch. Such appliances are very common these days, starting from large-screen TV sets and working down from there. Chances are that you have a dozen or more appliances with remote-controls or pushbutton switches. In the latter class will be washing machines, microwave ovens and vacuum cleaners. So this Appliance Earth Leakage Tester is an essential item to have on hand if you want to be sure that none of your appliances presents a safety hazard. As we noted in last month’s article siliconchip.com.au PLUG (PIN SIDE) N A MAGNETIC FLUX DUE TO CURRENT IN ACTIVE WIRE iA E iN N A CURRENT TRANSFORMER E 1000-TURN CURRENT SENSING WINDING A life-size view of the Talema AC1015 15A current trans­ former used in this design. MAGNETIC FLUX DUE TO CURRENT IN NEUTRAL WIRE SOCKET (INLET SIDE) Fig.1: how earth leakage current is measured. The Active & Neutral leads to the appliance are fed through a current transformer and if the currents in them are unequal, the transformer produces an output from its secondary. on the Appliance Insulation Tester, you should not rely your home’s safety switches (RCDs) to fully protect you. If one of your appliances does become faulty and you are unlucky enough to be in the fault current path, the RCD may well save your life but you could still get a very severe shock in the process. And you if you have a weak ticker, the RCD may not save your life – there is no absolute guarantee! Measuring earth leakage Our Appliance Earth Leakage Tester is based on a low-cost current transformer. It comprises a ferrite toroid through which are wound 1000 turns of enamelled wire connected to two output pins. The transformer is encapsulated in resin with a hole in the centre to allow the primary windings to be fed through. Isolation between the centre hole and secondary winding is 4kV. Further isolation is provided because the wires that pass through the core will also be insulated. The particular transformer we are using is rated for up to 15A primary siliconchip.com.au current and up to 60A before core saturation. To measure the earth leakage current of an appliance, the Active and Neutral wires are passed through the centre hole of the current transformer, as shown in Fig.1. If there is no earth leakage current, the magnetic flux due to the Active and Neutral currents will cancel and there will be no output voltage generated by the 1000-turn secondary winding of the transformer. On the other hand, if the Active and Neutral currents are not exactly the same, then the difference between those currents will be due to a leakage path to earth. As a result, there will be a differential magnetic flux and there will be a resulting output voltage from the 1000-turn secondary winding. For Class 1 appliances where the exposed metal parts are connected to mains earth, the leakage current can be directly measured. Alternatively, for double insulated equipment where the mains earth is not connected to the appliance, an earth probe must used to connect any exposed metal to the mains earth so that the leakage current can be measured. Amplifying the voltage Now even though the transformer has a 1000-turn secondary winding, its output is quite tiny at about 1µA per mA (or 100µV/mA across a 100Ω load) and this is far too low to be useful. We would need to amplify this by a factor of 1000 to produce a useful signal of 100mV per mA of differential current. This amplified signal can be measured directly with a digital multimeter, using the low AC voltage ranges. Mind you, if you do use a digital multimeter, it should be a “true RMS reading” meter. Multimeters that do not have true RMS readings are prone to severe reading errors if the leakage current waveform is non-sinusoidal, as is common with switchmode supplies and rectified supplies in mains equipment. Since most DMMs are not “true RMS reading”, the circuit described here includes a true RMS AC-to-DC converter to allow the multimeter to accurately measure the leakage current on its DC voltage ranges. As an aside, when an appliance contains a switchmode power supply, any earth leakage current will contain components at 50Hz, 100Hz plus many higher frequency components exceeding several kHz. Electromagnetic interference (EMI) suppression filtering in the appliance will suppress but not eliminate frequencies higher than this. Circuit details As mentioned above, the signal output from the current transformer’s winding is very low with a 100Ω resistive load. A low resistance load is necessary to ensure the output is linear with respect to the appliance earth leakage current. However, an alternative method that does not require a low value loading resistor but still results in a linear response is to convert the current in the transformer secondary winding to a voltage using a transimpedance amplifier. This is shown in the main circuit of Fig.2 which has one side of the current transformer secondary tied to half the supply voltage (Vcc/2) and fed to the non-inverting input (pin 3) of op amp IC1, a TLE2071CP. IC1’s inverting inMay 2015  27 K A A 4.7k 20 1 5 EARTH PROBE TERMINAL SC  68k 220pF CALIBRATE VR1 50k N SOCKET EARTH LEAKAGE CURRENT TESTER 1M A 100pF D1 1N4148 K A PLUG A 1N4148 100 µF 47 µF 100k VR2 OFFSET ADJUST 4 2 A 150Ω E E N A A N K –Vs 4 C AV 5 1 5 6 IC1 7 3 100k CT1 AC1015 10 µF 100 µF D2 K 1N4148 Vcc/2 100k Vcc 28  Silicon Chip Fig.2: the complete circuit diagram of the Earth Leakage Current Tester. The output from the current transformer is fed to IC1 which acts as a current-to-voltage converter. Its AC output at pin 6 is in turn fed to IC2, a true RMS AC-to-DC converter. ZD1 6 OUTPUT IC2 AD736 2 1 CC VIN +Vs IC1: TLE2071CP Vcc Vcc/2 10 µF 100nF 8 C OM 7 CF 3 100nF 10 µF Vcc/2 K AC OUT COMMON 100 µF DC OUT A 2.2k ZD1 5.6V Vcc K K A 1N5819 9V BATTERY S1 λ POWER LED1 A K D3 1N5819 The Appliance Earth Leakage Tester is housed in a standard UB1 plastic utility box and is fitted with a PCB front panel. The panel comes predrilled with screened lettering, to minimise case preparation. put (pin 2) monitors the other side of the transformer via a 10µF capacitor and 150Ω resistor. IC1 acts as the current-to-voltage converter. Its transimpedance value is 100mV/µA which, when combined with the transformer’s input:output ratio, results in the required 100mV/mA of differential current. But due to the way it works, no voltage appears across the transformer, so the load impedance “seen” by the transformer is very low. The 150Ω resistor between the transformer and op amp input pin 2 is there to limit current flow in diodes D1 and D2 should the output from the transformer exceed the supply rails. Diode D2 limits the input to pin 2 at just over the Vcc supply, while D1 limits the input to just below the ground. Note that the earth leakage would need to be around 45A before the diodes begin to conduct but that could happen with a major short to earth in an appliance under test. siliconchip.com.au Parts List The aforementioned transimpedance value is determined by the 68kΩ resistor and series-connected 50kΩ trimpot VR1 between pin 6 of IC1 and the transformer secondary. The 100pF and 220pF capacitors across the feedback resistances provide a ~6.5kHz high-frequency roll-off, preventing RF pick-up in the amplified waveform. The DC offset at pin 6 due to Vcc/2 is zeroed out using VR2. Any DC offset will typically be within 0.34mV of the Vcc/2 rail but in the worst case could be up to 4mV and this is fixed by adjusting this trimpot. IC1’s output connects to the AC output terminal (for measurement with a DMM) and is also fed to pin 1 of IC2, an AD736 true RMS AC-toDC converter. As shown in Fig.3, the AD736 comprises an input amplifier, a full-wave rectifier, an RMS core, an output amplifier and a bias section. The input amplifier has two inputs: a high impedance buffered input at pin siliconchip.com.au 1 double-sided PCB, code 04203151, 86 x 130mm 1 PCB, coded 04203152, 88 x 26mm 1 blue PCB, code 04203153, 90 x 151mm (front panel) 1 UB1 jiffy box, 158 x 95 x 53mm 1 15A current transformer, Talema AC1015 (RS Components 5374508) (CT1) 1 1.5m mains extension lead 1 150mm length of 10A Earth wire (green/yellow) 1 double-screw BP connector to join Earth wires in mains cable 2 BP connectors to join Active & Neutral wires 1 9V PCB-mount battery holder (Altronics S-5048, Jaycar PH9235) 1 9V battery 2 8-pin DIL IC sockets (optional) 2 cordgrip grommets to suit 10A 3-core mains cable (7.4-8.2mm diameter) and 3mm-thick case (Altronics Type C, Cat. H-4280) (Do NOT use cable glands) 1 black shrouded safety multimeter test lead (Altronics P0404A, Jaycar WT-5325) 1 set of shrouded banana to banana test leads (Altronics P-0414) 1 SPDT toggle switch, PCBmount (Altronics S-1315) (S1) 1 red safety banana socket (Jaycar PS-0420) 1 black safety banana socket (Jaycar PS-0421) 1 green safety banana socket (Jaycar PS-0422) 1 yellow safety banana socket (Jaycar PS-0423) 3 No.4 x 6mm self-tapping screws for 9V battery holder 4 M3 tapped x 15mm spacers 8 M3 x 6mm machine screws 6 100mm cable ties 3 PC stakes 1 50mm length of 0.7mm diameter tinned copper wire 1 50kΩ multi-turn trimpot (code 503) (VR1) 1 100kΩ multi-turn trimpot (code 104) (VR2) Semiconductors 1 TLE2071CP PDIP low-noise high-speed JFET op amp (RS Components Cat. 834-140, element14 Cat. 2387529) (IC1) 1 AD736JNZ PDIP True RMS AC-to-DC Converter (RS Components Cat. 522-9133, element14 Cat. 9605061) (IC2) 2 1N4148 diodes (D1,D2) 1 1N5819 1A Schottky diode (D3) 1 5.6V 1W zener diode (ZD1) 1 3mm high brightness red LED (LED1) Capacitors 3 100µF 16V PC electrolytic 1 47µF 16V PC electrolytic 3 10µF 16V PC electrolytic 1 100nF MKT 1 220pF ceramic 1 100pF ceramic Resistors (0.25W, 1%) 1 1MΩ 1 4.7kΩ 2 100kΩ 1 2.2kΩ 1 68kΩ 1 1kΩ* 1 10kΩ* 1 150Ω * for calibration Fig.3: inside the AD736 True RMS AC-to-DC Converter. It comprises an input amplifier, a full-wave rectifier, an RMS core, an output amplifier & a bias section. May 2015  29 TO DOUBLE-SCREW EARTH BP CONNECTOR CT1 AC1015 EARTH LEAKAGE TESTER C 2015 04203151 EARTH WIRE LOOPED THROUGH STRESS RELIEF HOLES 15130240 9V BATTERY HOLDER 5819 IC1 TLE2071 150Ω 68k D3 LED1 & S1 MOUNTED UNDER S1 100 µF 10 µF LED1 100 µF 2.2k D1 5.6V ZD1 100k 4148 COM 100nF K 100k IC2 AD736 D2 100nF 100pF POWER A 1M AC V ~ OFFSET 10 µF VR2 100k 4.7k CAL. 220pF 4148 DC V EARTH PROBE VR1 50k 100 µF 47 µF 10 µF Fig.4: follow this PCB layout diagram and the photo at left to build the unit. Note that LED1 & power switch S1 are mounted on the underside of the board. 2 and a low impedance, wide dynamic range input at pin 1. We use pin 1 input as it produces a wider frequency response. The output of the input amplifier is full-wave precision-rectified before the signal is applied to the RMS core. RMS conversion essentially squares, averages and then takes the square root of the value. Averaging is done using capacitor CAV at pin 5 (ie, the 47µF & 100µF capacitors connected in parallel on the circuit). The output amplifier buffers the output from the RMS core and allows for optional low-pass filtering to be performed via external capacitor CF (10µF in our circuit), which is con- nected across the feedback path of the amplifier. This additional filtering stage helps reduce any output ripple   Table 2: Capacitor Codes Table 1: Resistor Colour Codes   o o o o o o o o No.   1   2   1   1   1   1   1 30  Silicon Chip Value 1MΩ 100kΩ 68kΩ 4.7kΩ 2.2kΩ 1kΩ 150Ω 4-Band Code (1%) brown black green brown brown black yellow brown blue grey orange brown yellow violet red brown red red red brown brown black red brown brown green brown brown Value 100nF 220pF 100pF µF Value 0.1µF   NA  NA IEC Code EIA Code   100n   104   220p   221  100p  101 5-Band Code (1%) brown black black yellow brown brown black black orange brown blue grey black red brown yellow violet black brown brown red red black brown brown brown black black brown brown brown green black black brown siliconchip.com.au AUDIO SIGNAL GENERATOR 50Hz that is not removed by averaging capacitor CAV. Power supply WOW WOW WOW WOW WOW WOW WOW WOW WOW WOW WOW WOW WOW WOW 10kΩ 1% RESISTOR WOW WOW WOW WOW WOW WOW WOW WOW WOW OUT AMPLITUDE GND AUDIO SIGNAL GENERATOR OR AC PLUGPACK (SEE TEXT) siliconchip.com.au EARTH LEAKAGE TESTER AC1015 C 2015 04203151 DIGITAL MULTIMETER 15130240 9V BATTERY HOLDER EARTH PROBE 4.7k LED1 & S1 MOUNTED UNDER S1 10 µF POWER K LED1 100 µF 2.2k 5.6V D1 100k ZD1 100k 100nF 100 µF D2 IC2 AD736 100nF 1M + 100pF 4148 – D3 A AC OUT ~ VR2 100k 5819 10 µF 68k 220pF OFFSET IC1 TLE2071 CAL. 150Ω VR1 50k 4148 DC OUT DC mV Construction The assembly is straightforward, with most of the parts mounted on a PCB coded 04203151 and measuring 86 x 130mm. This is housed in a UB1 plastic case (see photos) and a second PCB coded 04203152 (88 x 26mm) is slid into the side pillars of this box to provide the necessary isolation between the mains wiring and the low-voltage measurement circuitry. A third PCB coded 04203153 (90 x 151mm) is used as the front panel. It takes the place of the original plastic lid and is screen printed and predrilled. Fig.4 shows the parts layout on the PCB. Most of the components are mounted on the top side, the exceptions being LED1 and power switch S1 which are mounted on the underside. Begin construction by installing the resistors. Table 1 shows the resistor colour codes but we also recommend checking each one with a multimeter before installing it on the PCB, as some colours can be difficult to read. Diodes D1-D3 and zener diode ZD1 can go in next. Be careful not to get these mixed up and make sure they are installed with the correct orientation. Follow with the two ICs, again CT1 ADJUST VR1 ON PCB FOR CORRECT READING ON DIGITAL MULTIMETER COM Power for the circuit is provided by a 9V battery, fed via reverse polarity protection diode D3 and switch S1. A 100µF capacitor bypasses the resulting nominal 8.7V supply. In addition, the supply rails to IC2 are decoupled using 100nF capacitors, one across the Vcc supply and another across Vcc/2. Battery voltage indication is provided by LED1 connected in series with 5.6V zener diode ZD1 and a 2.2kΩ resistor. When the battery is fresh there will be an 8.7V supply. With a nominal 1.8V voltage drop across the LED and 5.6V across ZD1, that leaves 1.3V across the 2.2kΩ resistor and so there is a 590µA LED current which gives a relatively bright LED (a high brightness LED is specified). As the battery goes flat, the battery voltage decreases and so the current through the LED diminishes. The LED current drops to near zero with a 7.4V supply which is about the end point for the battery as far as this circuit is concerned. WOW WOW WOW SET AUDIO SIGNAL GENERATOR’S OUTPUT LEVEL TO 10VAC 50Hz SINEWAVE ACROSS 10kΩ 1% RESISTOR (OR SET OUTPUT LEVEL TO 1VAC & USE A 1kΩ 1% RESISTOR – SEE TEXT) 100 µF 47 µF 10 µF Fig.5: this diagram shows the set-up used for the calibration procedure. It involves passing a 1mA current through the current transformer and then adjusting VR1 for a 100mV reading on the multimeter (see text overleaf for further details). taking care to ensure that they are orientated correctly (they go in with their notched ends towards the battery holder). Note that IC1 is the TLE2071 while IC2 is the AD736. You can either solder them directly to the PCB or install them using IC sockets. The next step is to fit PC stakes at the Common (COM), AC and DC output connection pads (these stakes are later wired to the three output terminals). Once they’re in, install the capacitors. The MKT and ceramic types can be installed either way around but the electrolytic types are polarised and must be orientated as shown on Fig.4. Note that the positive leads are longer. VR1 & VR2 are next and must be fitted with their adjustment screws positioned as shown. VR1, a 50kΩ trimpot, could be marked as 503, while VR2, a 100kΩ trimpot, could be marked as 104. Don’t get them transposed. The battery holder and current transformer can now be mounted in place. The battery holder is held in place using three No.4 x 6mm selftapping screws. Underside components All that remains now to complete the PCB assembly is to install LED1 and switch S1. These both go on the underside of the PCB. Install the switch first, then fit a single nut to its mounting thread and wind it all the way up to the switch body. Don’t solder the LED in place though. For the time being, simply push it down onto the underside of the PCB, making sure that its anode lead is orientated as shown. Its leads May 2015  31 This is the view inside the completed Appliance Earth Leakage Tester. Arrange the wiring so that the Earth BP connector will be on one side of the current transformer and the Active & Neutral connectors on the other side and don’t leave out the barrier PCB. will be soldered later, when the front panel is fitted to the PCB. Adjustment & calibration Now for the test and calibration procedure. First, insert a 9V battery into the holder and switch on power. Check that there is power (approximately 8.7V) between pins 7 & 4 of both IC1 and IC2. Pin 3 of IC1 and pins 2 & 8 of IC2 should be at half the supply. This voltage can be measured with the multimeter’s negative probe connected to the 0V rail. The next step is to adjust the DC output offset at pin 6 of IC1. That’s done by connecting your multimeter (set to measure DC mV) between the COM and AC V terminals on the PCB and adjusting VR2 so that the reading is as close to 0mV DC as you can set it. For example, we were able to adjust our prototype to obtain a reading which flickered around 0.05mV. Important note: even though you are measuring between the AC V and COM terminals on the PCB, you are adjusting for a minimum DC voltage and you should get a reading which is a fraction of a millivolt DC. If you accidentally switch to the AC millivolt 32  Silicon Chip range on the DMM, you are likely to get a much higher reading because the circuit will be reacting to stray hum fields. The next step involves passing a current of 1mA (or thereabouts) through the transformer and you can do this with a sinewave signal generator that can deliver a 10VAC signal at 50Hz. The set-up is shown in Fig.5 and uses a series 10kΩ resistor to provide the 1mA current via a single wire loop through the current transformer. First, connect the signal generator probes as shown and adjust the level for 10VAC RMS across the 10kΩ resistor, as measured with a multimeter. That done, connect your multimeter (set to measure DC mV) between the DC V and COM PC stake terminals on the PCB, apply the 1mA signal through the toroid and adjust VR1 for a reading of 100mV DC. If your signal generator cannot deliver 10VAC across the 10kΩ resistor, just set it to the maximum available and note the signal level reading. Then adjust VR1 for a reading that corresponds to the current flowing through the 10kΩ resistor. So if, for example, your signal generator can develop a 3VAC signal across the 10kΩ resistor, adjust trimpot VR1 so that the multimeter reads 30mV when connected to DC V and COM. If your signal generator only delivers 1VAC or thereabouts, a 1kΩ 1% resistor should be used instead of the 10kΩ resistor to provide the required 1mA calibration current. The calibration accuracy needs to be within ±5%. If you don’t have an audio signal generator, you can do the calibration with an AC plugpack. For example, we found a 9VAC plugpack in the junkbox and measured its output across a 10kΩ resistor. It was 10.45V. In that case, 10.45V across the 10kΩ resistor would result in a current of 1.045mA through the toroid and you would adjust VR1 for a reading 104.5mV DC. Final assembly With the calibration now complete, you can finish the PCB and front panel assembly and install it in the case. Begin by fitting the red, black, yellow and green shrouded banana sockets to the front panel PCB and secure them with the supplied nuts. Do not over-tighten these nuts; if you do, the plastic thread will be stripped. The red socket is for the DC output, the yellow for the AC output, the black for Comsiliconchip.com.au CABLE FROM 3-PIN PLUG CABLE TIES BARRIER PCB UB1 BOX INSULATED SCREW (BP) CONNECTORS KEEP THIS AREA CLEAR FOR CURRENT TRANSFORMER CORD GRIP GROMMETS CABLE TIES DOUBLE INSULATED SCREW (BP) CONNECTOR FOR EARTH WIRES CABLE FROM MAINS SOCKET S1 EARTH PROBE POWER EARTH WIRE LOOPED THROUGH STRESS RELIEF HOLES K A 5819 ~ AC V 15130240 C 2015 04203151 CAL. COM TESTER 4148 EARTH LEAKAGE 4148 9V BATTERY HOLDER IC1 TLE2071 IC2 AD736 OFFSET AC1015 CT1 FRONT PANEL PCB - LED1 5.6V 9V BATTERY DC V Fig.6: follow this wiring diagram to complete the Appliance Earth Leakage Tester. Make sure that the BP connectors are all securely attached to their respective wires and be sure to use a double-screw BP connector for the Earth leads. Once the wiring is completed, secure the leads with cable ties as shown. mon and the green for the Earth probe connection. Now attach four M3 x 15mm tapped spacers to the PCB’s mounting holes using M3 x 6mm screws, then fit the front panel in position over switch S1 and secure it in place using four M3 x 6mm screws into the spacers. Once siliconchip.com.au it’s secure, push LED1 into its hole in the front panel, then solder it in place. Next, wind the switch nut up so that it contacts the underside of the front panel, then fit a nut onto the top of the switch and tighten it down. Finally, complete the assembly by soldering wires between the three PC stakes and their adjacent banana sockets, as shown on Figs.4&6. Preparing the case The first job with the case preparation is to trim the internal ribs on the ends of the UB1 case, as they prevent the front panel from sitting down May 2015  33 TOP EDGE OF BOX 15.9mm 15.9mm 14mm 14mm BASE Fig.7: the holes for the two cord-grip grommets must be profiled exactly as shown, to ensure they grip they mains cords securely. onto the four corner pillars. These ribs can be cut down using sharp side-cutters or a hobby knife. You then need to drill and shape holes for two cord-grip grommets in the top end of box. As shown in the photos and Fig.6, these grommets are used to secure a mains plug lead and a mains socket lead. It’s important that these two holes be shaped so the grommets (and the cords) are securely captured in the panel. Fig.7 shows the hole template and a photocopy of this can be sticky-taped to the box and the hole outlines scribed out with a sharp hobby knife. The two holes can then be drilled, reamed and carefully filed to shape (don’t just drill round holes; they will not secure the grommets correctly). Note: do not use cable glands; the plastic nuts come undone too easily to ensure secure clamping. Next, cut a 1.5-metre (or longer) mains extension cable in half and strip about 150mm of outer insulation from each end, then feed them through their case holes and clamp them in place using the cord-grip grommets. Check to make sure that they are securely clamped – it must not be possible to pull the lead out from the grommet. Note also that there are different types of cord grip grommet. The most common is only suitable for use with a thin panel (typically aluminium or steel). The grommets specified for the Appliance Earth Leakage Tester are for thicker panel material, in this case 3mm – see parts list for specified type. Its now just a matter of trimming and stripping the various mains wires, twisting them together and terminating them in BP (blue point) connectors – see Fig.6. Use one-screw BP connectors for the Active and Neutral leads and a double-screw BP connector for Why Not Use A Current Clamp Meter? An obvious question when making leakage current measurements is why not just use an extension cord that has its Active and Neutral leads separated from the Earth lead, so that a clamp meter can simply measure the differential Active and Neutral current? 34  Silicon Chip Apart from the legalities involved in using a “doctored” extension cord, the problem is that you would need a specialised clamp meter that can measure current down in the mA range with at least 5% accuracy. However, most clamp meters are unsuitable as they are designed for high currents, with typical ranges of 40A and 400A, and have insufficient resolution or accuracy for a 1mA reading (let alone 5% accuracy). Clamp meters with a 40A range and a 4-digit display have only 10mA resolution, for example. siliconchip.com.au MAINS APPLIANCE TO BE TESTED – MUST BE SWITCHED ON ON NOTE: NO EARTH PIN ON DOUBLE INSULATED EQUIPMENT PLUG PROBE TO METAL PARTS FOR DOUBLE INSULATED APPLIANCES PLUG IN X GPO (POWER SWITCHED ON) X www.siliconchip.com.au PLUG IN LEGAL LEAKAGE LIMITS CLASS 1 (MAINS EARTHED) EXAMPLE READING SHOWS 100mV =1mA OF LEAKAGE CURRENT Fig.8: here’s how to use the unit to test an appliance for excessive mains current leakage. The DC voltage reading on the DMM is used to calculate the leakage current, with 100mV DC equivalent to a 1mA leakage current (eg, 100mV equates to 1mA leakage, while 245mV reading equates to 2.45mA leakage). Note that if the appliance is earthed via the mains, then you do not need to connect the earth probe to exposed metal. the Earth wires and make sure that all connections are secure. As shown in Fig.6, keep the Active & Neutral leads from the plug fairly short and make sure that the three Earth wires are secured by both screws in the double-screw BP connector. As shown in Fig.6, the Active and Neutral wires from the socket lead are looped through the current transformer (CT1) before going to their respective BP connectors. Note the area that needs to be kept free from any BP connectors, to leave room for the current transformer when the PCB/front panel assembly is fitted in position. Note also that an Earth wire is run from the double-screw BP connector and is looped through strain relief holes in the main PCB and connected to the earth banana socket. Once the wiring has been comsiliconchip.com.au DIGITAL MULTIMETER PORTABLE RCDs WITH FUNCTIONAL EARTH 5mA MAX 2.5mA MAX CLASS 2 (DOUBLE INSULATED) USE EARTH PROBE TO EXPOSED METAL 1mA MAX CORD EXTENSION SETS PORTABLE OUTLETS AND RCDs 1mA MAX ENSURE APPLIANCE IS POWERED AND SWITCHED ON FOR TEST APPLIANCE EARTH LEAKAGE TESTER DC mV EARTH PROBE DC OUT AC OUT TO DMM – + POWER X pleted, slide the 88 x 26mm barrier PCB into the side pillars in the box, as shown in Fig.6. This barrier isolates the mains wiring from the rest of the (low-voltage) circuitry. Do not leave the barrier PCB out – it’s an important safety measure. Finally, fit cable ties where indicated to hold the mains wiring together. These will prevent individual wires from moving and possibly coming adrift. The PCB/front panel assembly can then be fitted in place and secured using four corner mount screws. Be sure to position the Earth BP connector to one side of the current transformer and the Active & Neutral connectors to the other side. Testing appliances When testing appliances, the condition of the mains plug, lead and earth (mV DC RANGE) 100mV DC = 1mA LEAKAGE SCOPE OUTPUT (AC) COM X connection should first be checked. Make sure that mains wires are not frayed, repaired with insulation tape, broken or exposed. Appliances that have metal parts earthed via the mains plug should also initially be checked using a digital multimeter (DMM). The DMM is used to check the resistance between the earth pin on the mains plug and any exposed metal on the appliance and the measured resistance should be 1Ω or less. Note that before taking any readings, the DMM should be checked for a 0Ω reading with its probes shorted together. If it’s not close to 0Ω, then the probe tips, the banana plugs at the ends of the probe leads and the DMM’s input sockets may require cleaning. Inserting and removing the banana plugs in the sockets a few times is a good way of May 2015  35 Appliance Insulation Tester Or Appliance Earth Leakage Tester: Which One Should Be Used? There are two types of testers described in the Australian Standards AS/NZS3760 – In-service Safety Inspection And Testing Of Electrical Equipment. These are an appliance insulation tester and an appliance earth leakage tester. We published a suitable insulation tester design last month and this applies a DC voltage (either 250V or 500V) between the Active/Neutral pins and the appliance earth and measures any leakage current flow between them. The problem is that if the device being tested contains relays or solidstate mains switching, the applied voltage may not reach some of the internal circuitry which could possibly have significant earth leakage and thus this test could miss a potentially hazardous fault. By contrast, this earth leakage tester measures the current flow when 230VAC mains is applied to the unit. Since it is operating normally, any internal switching can be activated and thus mains voltage can reach all of its circuitry and its earth leakage can be checked more thoroughly. However, the AC waveform peak of around 325V DC is lower than 500V and thus this test may not pick up leakage due to marginal insulation which could cause problems during power surges (eg, in a storm). The peak voltage is also relatively brief so any leakage which occurs only at the highest voltages could be underestimated. Ideally, you should use both tests to check an appliance and you certainly should do an earth leakage test on any equipment with a remote control or standby mode. Note that in either case, when testing earthed equipment it’s necessary to first verify that its earth connection is good, as explained in the text. a 1mA leakage current. So, for example, a 245mV reading equates to a leakage current of 2.45mA. If the appliance is earthed, then you do not need to connect the earth probe to exposed metal but you must do so for correct readings on double-insulated appliances. Note that some metal parts may be painted or anodised and you may need to scrape away some of the coating so that a proper connection can be made. A case screw is often a good place to make a connection. Using a scope Fig.9: using the Appliance Earth Leakage Tester with a scope. In this case, the yellow scope waveform shows the earth current leakage from a doubleinsulated set-top box. cleaning the contacts. Fig.8 shows how the unit is used to test an appliance. The appliance is plugged into the tester’s socket lead, while the tester’s mains plug is plugged into a GPO wall socket. The GPO and the appliance itself are then switched on and a DMM used to take the reading. Note that switching the appliance on may be a multi-step process; if the 36  Silicon Chip appliance is in a stand-by mode, the measurement will not be valid as some of the circuitry may not be powered. In many cases, it will be necessary to apply power and then press the on/off pushbutton, either on the unit itself or on its remote control. The DC voltage reading on the DMM is then used to calculate the leakage current, with 100mV DC equivalent to The oscilloscope waveform at left shows the earth leakage from a doubleinsulated set-top box, as measured at the tester’s AC output. This set-top box has a switchmode power supply that includes electromagnetic interference (EMI) bypass capacitors that are grounded back to its metal case. The earth leakage waveform shows the higher-frequency components within the 50Hz envelope and these extend far beyond 20kHz. Note that the leakage is not a sinewave but one that reflects the high crest current flow typical of switchmode power supplies. We measured the RMS amplitude o this waveform on the scope along with the DC voltage reading (green trace) SC and they were almost identical. siliconchip.com.au