crwdns2915892:0crwdne2915892:0
A 3 year old Breville waffle machine part no 1923362 type VST072 tripped the rcd/rccb (residual current device/residual current circuit breaker) switch after about some minutes from turning on. It may not have been used for at least 3months.
The waffle maker tear-down showed no obvious distress. The resistance between live and earth pins with a cheap multimeter exhibited rising values typically 200 kohm to 2Mohm over about 20 sec, as did the individual heating elements when isolated. The elements also had a capacitance of 4 to 6 uF which may have caused problems for my cheap multimeter.
After heating the extracted heating elements in an oven for an hour at 150C, the insulation resistance had significantly increased ie no resistance below 2Mohm was detected and it no longer tripped the RDC switch.
crwdns2942213:0crwdne2942213:0
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Images show the waffle maker with lid closed, with lid opened (waffle plates removed) and one of the two identical waffle plates. Waffle plates are removed (normally for cleaning) by pressing the grey buttons on the side.
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To extract the bottom heating element, first undo the 5 black screws in the handle to remove some of the black plastic plastic surround. (Note 2 of the screws are shorter, see green arrows).
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Undo the 4 corner screws in the metal inner casing (eg red arrow)
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Undo 2 screws and brackets holding the heating element to the inner casings eg blue arrow. Now, by pressing in the grey button on the side, you can wiggle the inner casing with the heating element from the outer casing. Care not to lose the sprung loaded grey button and sprung loaded thermostat assembly in the middle.
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Lift up the inner casing of the bottom half to access electrical connections to the element. Pull away insulation covers and undo screws eg orange arrow.
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To make access easier, you can also remove the earth connectors and the thermal fuse (purple arrow) from the inner casing. If you have a multimeter you can confirm that the thermal fuse and thermostat (blue arrow) are closed circuit ie near 0 ohms.
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The lower heating element can now be extracted.
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The upper heating element is removed in the same way, although you don't need to remove any black plastic surround.
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The disassembled items are shown in this image. There is no need to remove anything else for this repair unless you want to do a deeper clean and check wiring in hinge etc.
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With a multimeter you can confirm that both elements have a resistance across the terminals of about 33 ohms. (Note the elements identification is for 115v because both are in series ie 230V across the two. The date is also stamped there.)
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Check the resistance between a terminal and its metal sheathing (use abrasive paper to get a good connection if necessary). It should ideally be above about 2Mohms. My cheap multimeter did not give a steady value but rose from about 100kohms to 2Mohms over about 18 seconds (probably due to inherent capacity of 4-6uF).
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The relatively low resistance to the earth sheathing was probably the cause of the RCD tripping. To solve this I heat-treated the elements in an oven at 150C for 1hr. After this the resistance to the earthed sheathing was satisfactorily off-scale ie above 2Mohm. (The capacitance had also halved).
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Image shows successful test where elements heat to red hot (~700C) without the waffle plates in place in about 2 or 3 mins without tripping the RCD as they did before the oven treatment. The item was consuming 900W, close to the stated 850W on the label.
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Perhaps do not do this test for long as the elements may get hotter than they were designed for, as without the waffle plates in position to transfer heat to the thermostat, there is no working temperature control.
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Comments below are an optional read.
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Note: It is believed that the relatively poor electrical insulation is caused by moisture ingress changing the hygroscopic magnesium oxide to the less insulating magnesium hydroxide. The chemical reaction is reversible by applying heat.
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No weight change however >0.5g was observed on oven heating. This was probably due to the amount being so small as the moisture ingress probably only occurred locally at the terminal end. (No obvious fault was observed to the ceramic sealant)
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The reason why the RCD did not trip immediately, but after amount 3 mins, may be because the electrical conductivity increases with temperature as it does with many ceramics. The reason why the red hot element did not drive off the moisture was probably because it was much cooler at the terminal (see image) and there was not enough time (3 mins).
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The second image shows a typical mains power consumer unit (UK). Note there are different types of trip switches. Here there are two types (i) RCD (green arrow), which is relevant to this repair and measure earth leakage and (ii) MCR (miniature circuit breaker- red arrow and are the modern equivalent of 6/16/32amp fuse wire) and not relevant here.
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Heat treating the extracted heating elements increased the electrical insulation resistance between the element and its earthed sheathing, so it no long tripped a residual current device (RCD) switch. I have used this method before for repairing a bread-maker with RCD tripping issues.
Heat treating the extracted heating elements increased the electrical insulation resistance between the element and its earthed sheathing, so it no long tripped a residual current device (RCD) switch. I have used this method before for repairing a bread-maker with RCD tripping issues.
