Induction hob tripping RCD

Sparks · 2005-04-21T22:38:23+00:00

Just checking it's not just me, but I think my new induction hob is faulty!!It is connected to the RCD consumer unit (Because the cable that feeds it was feeding the oven, and was already connected there before you ask!)When turning on the local isolator near the hob, the RCD trips.If I reset the RCD it will trip either immediately or after a few secondsIf I isolate the hob (Via the double pole isolator, OR by the single pole MCB in the CU) the tripping stops.I tried disconnecting it's earth (and keeping away from it!), and the tripping stoppedIf I measure the current from it's earth wire to earth, the RCD trips (no surprise there then!)If I measure the current between its earth wire and Neutral, I am getting about 2.9AIf I measure the voltage, it is about 120vNow when it was first turned on (this afternoon), it tripped the RCD immediately, then after a few resets, it seemed to settle down - I even boiled some water on it to test it.I thought it must have been a bit damp or something (Although it has been sitting in a warm house for about two weeks!)We went out and set the alarm as we would usually do, only to be called by the alarm about 10 minutes after leaving the house telling me my power had gone off (The hob had been powered up, but "off" for a good hour)Now the RCD refuses to stay on with the hob powered up - This is when I tested it.Do I send it back, or connect it to the non RCD CU!(I personally would think 120v @ 2.9A leakage to earth is not too clever, so would think it is faulty, but would just like to see what others think)...If I hadn't have connected it to an RCD, then this fault (assuming it is) would never have been noticed (Unless it got worse, and tried to pull more than 32A, tripping the MCB!)If the earth had then become disconnected somewhere between it and the CU, then someone touching it may have got a packet from it.Is not RCD protecting things like this really as wise as people make out?(The same goes for an old dishwasher we used to own,...

Apr 21, 2005 23 Replies

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Andy Wade 21 years ago

Yep, you've cleared up that possible misunderstanding.

I don't think anyone has asked what sort of meter you're using. At the risk of raising a cry of "grandmother, eggs, sucking," are you sure that the reading was really 2.9 A and not perhaps 2.9 mA? I'm thinking that some of the Fluke DMMs just display a number on current ranges, and whether it means amps, milliamps or microamps depends on which socket you've plugged the red test lead into.

Even 2.9 mA's a bit on the high side, but much more feasible than having nearly 3 A flowing in the earth wire.

What happens if you use a 12V 50W bulb in place of the meter? At 2.9 A it should light up nicely.

Perhaps it's too late and we'll never know...

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Michael Chare 21 years ago

Yes, in this case the appliance was correctly earthed. The problem for the repair man was that the bottom plate formed part of the earth circuit. A better design would have ensured that the internal compnents were earthed even with the bottom plate removed. (Atleast that is how it appears, I have not studied it in detail)

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Peter Parry 21 years ago

2.9mA is about right for a noise filter (usually 2-3.5mA)

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Andrew Gabriel 21 years ago

For Class I IT appliances (ie. computer), max permitted is 0.75 mA. They are normally about half this though. I think this limit applies to most types of Class I appliances which plug in to a standard socket outlet, but they are all defined in different EN documents, so it would take some research (and considerable expense) to get a full list of figures. Wired-in appliances are probably allowed higher leakage.

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