Oven with all elements leaking to earth.  - Page 1
Forum section: electronics / repair

author: Stuart Coyle
author_rank: Contributor
timestamp: May 21, 2022, 12:32:08 am
content: I have an oven that works but trips the RCD as every element except one has a low resistance to earth in the order of 100 to 300k.The elements all measure ~50ohms across the element which is fine.The oven was stored idle for quite some time and as you fellow Queenslanders know we have had exceptionally wet weather. Do people heremore experienced in these sort of things agree that moisture in the elements is the likely problem, or are some ovens notdesigned to work on circuits with RCD protection and leak stupid currents to earth?I guess the fix is easy, replace the offending elements, but I'd like to know why they are so low resistance to earth.

author: ozcar
author_rank: Frequent Contributor
timestamp: May 21, 2022, 12:53:23 am
content: I had a brand new element that tripped the RCD. I was able to heat the oven up using another one of the elements (so maybe not an option for you), and left it like that for a while. It never tripped again (it got used a couple of times a week at least).

author: themadhippy
author_rank: Super Contributor
timestamp: May 21, 2022, 01:18:31 am
content: seen this many times in the past,if your confident in what your doing you could try running it up without the earth connected to dry out the moisture.Remember DO NOT touch any part of the oven and make sure nothing or nobody can come into contact with it whilst the earth is disconnected.

author: Stuart Coyle
author_rank: Contributor
timestamp: May 21, 2022, 01:29:22 am
content: Not going to run it with the earth disconnected. I might just take the elements out and try drying them in another oven. I also have a pottery kiln downstairs, in which I could dry them or if I get fed up, melt them.

author: bdunham7
author_rank: Super Contributor
timestamp: May 21, 2022, 01:40:19 am
content: Quote from: themadhippy on May 21, 2022, 01:18:31 amIsn't it easier--and safer--to bypass the RCD?

author: themadhippy
author_rank: Super Contributor
timestamp: May 21, 2022, 02:21:06 am
content: Quoteeasier-no,you'd need to move both live and neutral to,if available, a non protected way in the consumer unit,in the uk at least, a single rcd protecting the whole installation is pretty common so unless you want to bypass the rcd for whole installation theres no easy waysafer-all you gain is overload protection from an earth-live fault

author: bdunham7
author_rank: Super Contributor
timestamp: May 21, 2022, 02:32:18 am
content: Quote from: themadhippy on May 21, 2022, 02:21:06 amI was thinking that any leakage would be conducted to ground and small leakage would not lead to a shocking potential. So no live oven chassis during the bakeout. I've not seen many Australian service panels (Dave's was an eye opener) but here we just plug in a non-GFI breaker for stuff like that, if there's even a GFI on an oven circuit.

author: themadhippy
author_rank: Super Contributor
timestamp: May 21, 2022, 02:42:18 am
content: Quotedepending on the insulation resistance the leakage current could be high enough to give a fatal shock ,but below the operating point of the fuse and anything connected to the installations electrical earth,water pipes gas pipes other circuits etc, would also have that shock risk present

author: Stuart Coyle
author_rank: Contributor
timestamp: May 24, 2022, 12:25:48 am
content: So I heated the elements up in another oven to drive off the moisture. They all measured >30M ohms after that which should be fine. I don't have a megger to do a real insulation test. I put them back in the oven and after a while RCD trips again and one element now has 250k to ground. I think running them causes a breakdown of insulation. I guess they are cactus, new elements to be ordered.

author: floobydust
author_rank: Super Contributor
timestamp: May 24, 2022, 12:39:49 am
content: I think moisture in the heating element is an old wives' tale. As if it's raining inside the oven or something. The heating element metal tube has silicone end seals as well.The heater's wire does expand and contract greatly with use, pushing around the magnesium oxide powder insulator inside, and can touch the outer tube.If there is a ground-fault I think the arc leaves some conductive residue and perhaps shaking things up moves the powder around and causes that resistance to change.You'd really have to do an autopsy on the heating element, I would say it's shorting to the tube somewhere once orange hot.edit:https://www.watlow.com/blog/posts/what-is-heater-bakeout-and-why-is-it-importantWatlow says this:"A common issue with heaters is the likelihood of moisture being drawn into the element from the atmosphere. This phenomenon typically occurs when heaters are stored for an extended period prior to use. Metal sheath heaters are hygroscopic, meaning that they will absorb moisture from the surrounding atmosphere. When enough moisture has accumulated in the heater, a condition known as a “wet heater” will occur. Wet heater problems are more common in locations with relative high humidity, but they can also occur if heaters are stored and unused for extended periods of time (about 30 days).""In some instances, heaters can become so “wet” that it is possible to observe water exiting the heater during a conditioning cycle. In addition to location, higher line voltage heaters and large process heater bundles are more prone to absorbing moisture."How moisture is absorbed by a heater"Water forms in a heater through natural chemical and atmospheric processes. Take a tubular mineral insulated heater, for example. Magnesium oxide is used as insulation in a tubular mineral insulated heater because it has high thermal conductivity and low electrical conductivity. The chemical compound is used to transfer heat from the element to the sheath.""Magnesium oxide readily absorbs moisture to form magnesium hydroxide. As the unit cools after use, the pressure inside the heater falls and air is driven into the heater. Once inside, the moisture in the air can exist as dihydrogen monoxide – commonly known as water – in liquid or vapor form, or as magnesium hydroxide."How to check for a wet heater"If a user suspects a wet heater is present or is unsure, it is best to run a meg-ohm precheck on the heater to avoid potential damage to the heater. A meg-ohm precheck is conducted with a Megger test instrument, which provides a measure of insulation resistance in ohms or meg-ohms. A lower Megger reading indicates that the user should condition the heater prior to use. Please refer to the installation and maintenance manual provided with your heater for the recommended meg-ohm level prior to applying full power to your heater.""Other indications that moisture may be present in the heater include tripping the ground fault circuit interrupter (GFCI), the heater element experiencing a “burn through” fault, high speed fuses open in the power controller and branch circuit fuses or circuit breakers open."How to remove water from a heater"If moisture is in the heater, the concentration of magnesium hydroxide will increase as the temperature rises. At 100 degrees Celsius, the chemically bonded water is maximized. As the temperature increases to more than 330 degrees Celsius, the magnesium hydroxide starts to decompose. At 430 degrees Celsius, the chemical compound is converted back to magnesium oxide and water.""A “bakeout” process is used to remove this moisture. A bakeout is the use of high temperature and sometimes a vacuum to remove moisture or water vapor from the heater. A bakeout artificially accelerates a process known as outgassing, which is the release of water vapor trapped or absorbed in the heater.Heater conditioning can be an expensive, complicated process. In its simplest form, a conditioning process is performed by placing the heater in a specialized oven running at 120 degrees Celsius for up to 40 hours. Some heaters require only a cycle of six to eight hours. After the cycle is complete, the heater should be retested with a Meggers instrument to determine if the required meg-ohm level has been attained.""It is important to note, the moisture released from within the magnesium oxide does not appear to contribute to the heater failure. As such, 120 degrees Celsius is sufficient to remove water from the heater and is a safe temperature for the other components of the heater, including terminations, wires and seals."