Yes you need an insulation (megger) tester.
Depending on the technology, some oven elements insulation absorbs water from the air if left unused. Running them dries them out. Which is a problem if they trip the rcd.
These are he ones insulated with magnesium oxide (white powder). They can be dried with a blowlamp sometimes. (Starting 150mm from a terminal and working towards the terminal to drive the dampness out.) The magnesiun oxide is is sightly hygroscopic.
While heating can help, its only a partial solution, since once the insulator has got water in it, it becomes magnesium hydroxide which is somewhat more conductive than the oxide.
Many years ago, I needed to connect a data logger to the sections of a resistor bank operating at high temperature and voltage (1). I discussed the application with a supplier of what seemed a suitable device (MICC thermocouples), and spent a lot of time getting everything connected.
Tried it in anger, and the data logger input card blew up. Complained to the supplier about the apparent disappearance of insulating properties.They responded that I must have been heating them. As you can imagine, I was excessively reasonable when I shared with them my belief that a thermocouple might well be expected to encounter such a condition.
Mark II involved secondary electrical insulation, more hands, selector switches and squared paper.
We occasionally had problems getting MICC heating elements through insulation test. Running them for a while could improve matters, but sometimes it got worse. I had a vague idea that H2O became free H2O, but the details are obscured by the mists of time.
(1) Brake resistor for 1983 Tube Stock - withdrawn from service before me.
Chris
Guessing is pointless. Get yourself a megger and find out, Meanwhile you can often find the problem by testing with an ordinary multimeter - but not always.
Oven elements deteriorate by absorbing water vapour. When operated they can drive it out again, reverting to their original chemical state.
The level of risk involved in oven earth leakage is about zero, for non-TT installs. I'd be inclined to put an oven on a nonrcd feed when permitted.
NT
Mm, something I've been considering... there are a stack of cheapie Far-East insulation testers available on ebay, most notably the following models from numerous sellers for £35-£40:
I appreciate they aren't Meggers, but does anyone have any experience of any of these and are they likely to cut the mustard for my purposes? Do any look better or worse than others?
Also, are they straightforward to use? (my experience of buying Far-East electronic tat off ebay is that appalling or absent instruction manuals can be the major problem!
All a bit odd though as I've replaced both the main and grill elements in the past few months, in the belief that both were knackered.
FWIW Immersion heater elements are prime RCD "leakage trip" causes...
You can get the real thing for similar money second hand:
(lots of people seem to offload the old single function meters in favour of multi function testers these days)
My main worry about cheap test gear used for mains related work, is that most of it has woefully inadequate input protection stages. Now normally that means the difference between the meter exploding in your hands, rather than just blowing a fuse, but the outcomes can be far less good.
With an insulation resistance tester, these issues are perhaps less serious, in that you should only be using it on de-energised circuits - although you obviously need to take care that you are on the right circuit.
If you understand the basic principles of doing the tests, then the meters themselves are usually pretty straightforward. Can often be as simple as a button marked "test"
Last time I went look for a fault with one of these, it turned out to be the lady of the house had washed the wall behind the cooker in the morning and ended up getting the plug on the cooker all soggy!
(I think I posted the tale of slowly working back from the element, disconnecting ever more bits of cooker until in the end all I had left was a flex coming into the case that was not connected to anything else
- and it was still failing!)
I remember reading that one!
If an element isn't used for months or (in my case) years, the initial heating does drive out the water and that condenses around the (still cold) part where the sheath, insulation and supply meet. Any salts splashed from cooking will make the water more conductive and trip the RCD. I removed the offending elements, scrubbed and rinsed the terminal area and then heated the elements in the oven. No more tripping. Since then I get every element red hot once a year - on a cold morning in the winter - to prevent a repeat (don't turn on all of the elements at once!).
Theyre nothing more than resistance meters running off a 500v generator. Sw itch on, apply probes.
Its trivial to make a mains powered insulation tester if you dont want to s pend the £40. Diode, capacitor gets you 330v. Resistor, multimeter. That' s it. The resistor or resistor chain limits i to a safe value & sets the sc ale.
NT
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.