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Earth Leaking Heating Element
Moving soon to a smaller apartment with very little storage space we went through a lot of things that we had to get rid of. A rule of thumb was: if we didn’t use the item during the past 12 months it had to go… well, not really, and certainly not for my electronic stuff, but this was the general idea! During this exercise we came across a small table grill. It looked like almost new and we didn’t remember using it a single time during the past three years while we were living in this house. Probably we just forgot about it, having an outdoor barbeque. This is what happens when you have too much storage space… Objects get put aside and forgotten about! This one definitely qualified for the “has to go” category.
Before deciding what to do with this appliance I resolved to test it, thus I inserted the power plug into the wall socket. The grill started heating then, after a few seconds, the house circuit breaker tripped off. Ah, this started to be more interesting! The quiet way the circuit breaker when off, without any spark or other kind of typical “short circuit bang”, let me think that the cause was an earth leakage. I took the appliance up to the workbench for investigation.
The first measurement, using a digital meter between Live and Neutral of the power plug, indicated a resistance of 36 Ohms. This could be considered as normal for a powerful appliance (1600 W / 240 V).
Next step, measuring between Live and Earth, the digital meter was showing inconsistent and unstable readings. It was time to switch to the analog meter. This was one of those situations where the analog meter is much better. An insulation leakage is not a linear load and its value will depends on the current passing through. The analog Ohmmeter supplies more current to the device under test than the digital meter, so we have a better chance to get some reading. The meter displayed a stable value of around 50 kΩ, which indicated an insulation leakage and explained why the circuit breaker tripped off… The insulation resistance must show infinite under any scale of the analog meter. I could also have done an insulation test but the present measurement was enough to highlight the problem.
Let’s remind how this works. A typical household circuit breaker has two functions: a) to interrupt the circuit if the line current goes beyond a certain value and b) to interrupt the circuit if there is an Earth leakage current. Those devices are called Residual Current Circuit Breakers (RCCB) or Earth Leakage Circuit Breaker (ELCB).
The figure below shows one of the RCCB installed in our house. It is labeled 40A and 0.030A. It means that it will trip off if the current is higher than 40A (3 phases) or if the leakage to Earth is higher than 30mA!
In a house or apartment the Neutral is usually connected to the Earth in the entrance switchboard. From there, three separated lines are fed to the power points. Each piece of equipment will be connected to the Phase (Live) and Neutral. The frame or cabinet will be connected to the Earth as seen below. When there is no fault the current flowing from the Phase line will be the same as the current returning through the Neutral. There is no current flowing through the Earth wire.
In case of a fault, part of the Phase current will return through the Earth line instead of the Neutral line. The current returning through the Neutral will no longer be equal to the current flowing from the Phase:
The RCCB measures the current difference between the Phase and Neutral wires. If the difference is more than 30 mA (for most countries) the circuit breaker trips off. The following picture shows the operations principle of a RCCB:
The hart of the device is a magnetic core with three windings. The supply current will create a magnetic flux into the core. The return current (through Neutral wire) will also create a magnetic flux but in the opposite direction. If both currents are equals, those two magnetic fluxes cancel each other. In case of a fault, both fluxes do not fully cancel each other. There will be a residual magnetic flux which will induce a current into the third winding. Remember we are talking about AC, it acts like a transformer. This current energizes the relay to trip off the circuit breaker. Pressing the test button also will create a current imbalance, hence trip off the device.
If the equipment cabinet is not connected to the Earth (broken wire or dodgy installation) the leakage current might flow to the Earth through someone touching the cabinet. A current of around 30 mA (0.030 Amps) is considered potentially sufficient to cause serious harm or a cardiac arrest if it stays for more than a fraction of a second. RCCBs are designed to disconnect the conducting wires fast enough to prevent serious injuries from electric shocks.
Back to our grill, we could suspect that the leakage was inside the heating element. To be sure, let’s open the appliance and disconnect the heating element:
As we can see there is not much in it and it is easy to disconnect the heating element and measure it. As expected the results were the same as measured directly from the power plug. So the heating element is definitely faulty. As the grill is like new but have not been used for more than 3 years I suspected that the problem could have been caused by moisture entering the element. Can it be fixed? I wondered if, by heating the element, we might get rid of the moisture but I wanted to be sure so I did a search on the Internet. Sure enough I found a site where someone restored a coffee machine heating element by heating it on a gas flame. And it worked…
From there, I imagined that I could heat the element by running the grill during a few minutes after temporarily disconnecting the Earth wire (green/yellow in the picture) so the RCCB would not trip off. This can be dangerous and one must be very careful not touching any part of the equipment while doing so. But here we go, for two minutes only:
After a couple of minutes unplug and let it cooling down, reconnect the Earth wire, put everything back together and measure again. Bingo, the Live to Earth resistance went up to around 500 kΩ. This was still not good enough but it was a conclusive improvement. So I decided to give it more time. If it took several years for the moisture to get in we cannot expect it to get out within a few minutes only. So I let it rest for a day (switched off of course)! On the following day the resistance had risen to over 1 MΩ. One more resting day and it barely moved the pointer of the multi-meter on the x10K scale. At this point it was possible to run the grill, Earth connected, without having the RCCB tripping off.
Then I performed an insulation test that shows 1340V. Perfect!
Was the problem fixed and will it return? Time will tell us. It probably depends on how often the ones who will inherit the grill will use it and where the appliance will be stored.
Gerald Musy, Penang- Malaysia
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