On 05/01/2018 21:31, Roger Hayter wrote:
newshound wrote:
On 05/01/2018 08:52, Roger Hayter wrote:
newshound wrote:
Got a heating element in a wallpaper stripper which is blowing 30mA RCD.
Testing live to earth on an old (but "electronic" rather than wind-up)
Megger (which seems to provide 320 volts DC) gives ~ 5 MegOhm.
Testing on a DVM gives ~ 200 kOhm, corresponding to a little over 1 mA
leakage current on mains.
So why the trip, especially given the Megger reading? Or is the megger
f**ed?
Whatever, I guess I need a new megger, any views on the £30 Chinese
models from eBay, or should I be looking at a second hand Avo / Robin /
Fluke etc? This is not for "professional" use, it probably only comes
out a couple of times a year.
Can you test the leakage to the casing of the element on its own removed
from the stripper? If it is still 5 megohms then it is defective. The
discrepancy in the readings is surprising but the cause is uncertain.
Can you test both instruments on a one megohm high voltage resistor?
That test was with the element removed. I agree that 5 meg is defective,
but wouldn't necessarily trip an RCD. Didn't have any suitable resistors
to hand when checking, will have to drag out a deeply buried box of
"bits too useful to throw away".
Real purpose of post was to get views on cheap Chinese instruments.
The trouble with cheap Chinese imports is that they may be good,
mediocre or plain fraudulent, and it is hard to tell in advance.
The market for insulation testers is mainly to electricians and I would
suggest buying one from a UK based wholesaler who has to take some
responsibility for what is being sold. e.g.
http://cpc.farnell.com/c/test-equipm...earchlookahead
Good point, I suppose this one
http://cpc.farnell.com/tenma/72-9400...ohm/dp/IN07406
is just about affordable (for my very limited use)
BTW, your element might leak a lot more when it is hot. I don't know
the full failure mechanism, but they tend to fail abruptly with an arc
to earth internally at some point.
Realise that, but all this testing was done "cold". Took the element out
when it failed open circuit, just did the insulation resistance test out
of interest.