SOLVED: voltage on oven inner chamber - Page 1
Forum section: electronics / general

author: mcinque
author_rank: Unknown
timestamp: November 01, 2013, 11:23:35 am
content: A friend of mine noticed a strange thing on his oven for investment casting: with one hand grounded, and with the other touching the plate on the inner chamber (with the long pliers), he experienced a little electric shock (just enough to trow the pliers away while cursing).This was the first time he experienced this behaviour.I've done some measurements with the device turned on, during heating:there is no voltage on the metal chassisthe device is properly groundedwhen the oven is still cold (20-50°C), there is no voltage on the inner chamber (both touching the inner plate or the refractary only)the more the temperature rise, the more voltage I read both on the inner plate or in the refractary only; at 950°C I measured 127V!even without the metal plate inside, there is voltage on the chamberthe current drawing on the ground is 4uAI suspect that there is some thousand of little fractures in the refractary filled by carbon dust that is touching the film resistor inside, what do you think?

author: amyk
author_rank: Super Contributor
timestamp: November 01, 2013, 12:04:58 pm
content: Is 4 uA the maximum current you can draw from it? If so, that's well below the safety standards' limits.

author: mcinque
author_rank: Unknown
timestamp: November 01, 2013, 02:40:03 pm
content: With the instrument I read that value. But with his hand he felt "oooooouuuuuuuchhh!!!" (I've tried too, trust me, you can hear the voltage)Maybe that DMM its crap and say uA instead mA

author: Alex
author_rank: Regular Contributor
timestamp: November 01, 2013, 03:21:49 pm
content: A small neon bulb with or without resistor - Hold one lead with your finger, and the other lead to the plate. Does it flash or stay lit? Is the plate electrically connected to the chassis?Alex.

author: mcinque
author_rank: Unknown
timestamp: November 01, 2013, 03:27:51 pm
content: Quote from: Alex on November 01, 2013, 03:21:49 pmOh, the neon bulb trick! Nice idea, I will try it asap.Quote from: Alex on November 01, 2013, 03:21:49 pmNo, it's removable, it's simply leaning on the chamber.Thank you.

author: Alex
author_rank: Regular Contributor
timestamp: November 01, 2013, 03:30:21 pm
content: Another thing, is this powered from 110 or 230 VAC? Is the voltage you are measuring AC RMS?When you do the neon experiment, if it lights up, what is the plate voltage then?

author: mcinque
author_rank: Unknown
timestamp: November 01, 2013, 03:37:30 pm
content: Quote from: Alex on November 01, 2013, 03:30:21 pm230VACQuote from: Alex on November 01, 2013, 03:30:21 pmAC, but not true RMS (crappy DMM)Tried also with DC, no/low reading.Quote from: Alex on November 01, 2013, 03:30:21 pmI guess you mean what's the voltage I will measure on the plate if/when the neon lights up, right?Thank you.

author: Alex
author_rank: Regular Contributor
timestamp: November 01, 2013, 09:17:57 pm
content: Quote from: mcinque on November 01, 2013, 03:37:30 pmThat's fine, I can't think of something that will distort the sinewave.QuoteYes. Actually, record the plate voltage when the neon is connected, no matter what it does. We are getting closer.

author: G7PSK
author_rank: Super Contributor
timestamp: November 01, 2013, 10:46:42 pm
content: It could well be that it's just an induced voltage on the plate, the element's are most probably coiled and run around the furnace so any conductive material will have an induced voltage on it, it's not likely due to carbon in cracks as at the temperature achieved by such a furnace carbon would burn off.

author: mcinque
author_rank: Unknown
timestamp: November 02, 2013, 09:11:49 am
content: This don't explain to me why when the oven is warming up but it's still cold (20-50°C), there is no voltage (2-3V) on the inner chamber (both touching the inner plate or the refractary only)As I said, the more the temperature rise, the more voltage I read both on the inner plate or in the refractary: If it's induction, since the coil is active, I should be able to read the same voltage value both when the chamber is (relatively) cold or hot, don't you?

author: SeanB
author_rank: Super Contributor
timestamp: November 02, 2013, 07:19:49 pm
content: Ceramics are insulators when cold, but as they heat up they become ionic liquids ( still solid but they behave like an ionic liquid somewhat ) and thus become conductive. This allows current to flow that will float the metal plate inside to something close to mains voltage. Try to swap the power leads to the heater element around so the hot end is now the cold end ( possible as it is likely the element ends are next to each other on metal bolts with easily removeable nuts holding them to the wiring) to reduce the leakage voltage.With a high enough voltage you can actually have the element break in the middle when the ceramic is bright cherry and the ceramic will conduct enough to keep it at bright cherry.Only solution is to either ground the plate with a wire or place some quartz glass blobs on the places where it rests on the ceramic lining to provide extra insulation.

author: mcinque
author_rank: Unknown
timestamp: November 02, 2013, 07:28:00 pm
content: Quote from: SeanB on November 02, 2013, 07:19:49 pmI've learned another thing, thank you!Quote from: SeanB on November 02, 2013, 07:19:49 pmI will try!Quote from: SeanB on November 02, 2013, 07:19:49 pmThis was my first idea before writing on the forum, but I thought was a compromise.Thank you for your suggestions!

author: Alex
author_rank: Regular Contributor
timestamp: November 02, 2013, 07:52:29 pm
content: Quote from: SeanB on November 02, 2013, 07:19:49 pmIsn't the temperature quite low for this? What ceramic is used for the lining?Also. this mechanism can't explain why the plate hasn't always been at high potential when the oven is hot.QuoteThat should only be done (in any case) as a safety measure. If you do it now you are only masking the underlying issue. However if there is an RCD and you earthing the plate trips it, you know the leakage path is strong. So a good experiment.We still don't know how good the path is between the plate and the voltage source.

author: Kryoclasm
author_rank: Regular Contributor
timestamp: November 02, 2013, 09:55:58 pm
content: Quote from: mcinque on November 02, 2013, 09:11:49 amWhen cool, the heating element should conduct easy, but as it heats up, its normal resistance will increase possibly allowing some of the voltage to find a lower resistive path?Maybe inspect the element and its socket/connector, whatever, for build up of debris, carbon or anything.

author: SeanB
author_rank: Super Contributor
timestamp: November 03, 2013, 05:24:07 am
content: Hot there will be a thermostat to control temperature by switching power off. The ceramic composition could degrade with cycles of heating and this could sinter the low melting point glasses into a film that conducts through the ceramic.

author: mcinque
author_rank: Unknown
timestamp: February 28, 2014, 10:03:17 pm
content: SOLVED! (I've tested a brand new furnace with the same results), I'm posting my consideration in case someone could find it useful: the ceramic insulation inside the chamber starts to conduct electricity when temperature rises (at 950C° it's few ohms).That old furnace doesn't have a sensor that cuts the heater when the door is opened, so if you touch the chamber with the metal pliers, is like we're touching live wire, and if we're grounded, you feel the shock (even if minimal current, since the most energy is discharging into the neutral thru the heater).The new furnace has a switch on the door that turns off the heater and prevents this behaviour.Thank you for partecipating to this thread!

author: Tinkerer
author_rank: Frequent Contributor
timestamp: March 01, 2014, 01:00:14 am
content: Quote from: mcinque on February 28, 2014, 10:03:17 pmAh! That explains the behavior of a stove that was replaced a couple months ago.(thing was decades old) When it was on, if you touched the frame around the stove edge and a metal part of a pan while it was on, you would get a shock. Could very well be the same reason. Or I assume it is the same reason, never checked the wiring inside the frame itself as taking it apart wasnt very appealing.