500 volt Megger

[Diabolical Artificer]

" If it ain't broke... Then fiddle with it 'til it is! " That made me laugh. Nice one : )

A.

[russell_w_b]

I know someone whose technique is to fiddle with (or 'improve' as they put it) something that works perfectly well, then, when it stops working, a flurry of panic ensues as a hasty attempt to undo the last 'fiddle' is made. Tempting sometimes, but wrong.

This technique is at the extreme end of the repair scale, with the other extreme being 'leave it untouched, but look at it occasionally, until the fairies fix it'. Also tempting sometimes, but equally wrong.

Many of us swing by these two extremes at some point, but settle down eventually to the middle ground of logical, progressive fault-finding.

Or so I like to think...

[turretslug]

This thread prompted me to dig out my 1945 dated 500V megger- not much used but the mottled maroon chunky Bakelite makes it too nice to pass on....- and check the actual output. Sure enough, never more than a little over 300V (10M input Fluke 83), so off with the lid to investigate. There was a 100nF 1500V metal case Dubiler cap in series with a 5k BTS composition resistor across the generator output, the cap naturally under suspicion. Sure enough, it registered a little over 2M on a 9V supply DMM, when subjected to the output of my 1988 500V Thorn "megger", it came out as virtually short. Replacing it with a 100nF 1600V polypropylene restored an easy 500V, and the cranking is also a whole lot easier.... The resistor measured around 5.8k, so fair enough, it can hardly be critical and stayed in place. The original cap was a special with a single angled foot anchored by the fixing screws of 2 50k bobbin wirewounds, but the replacement is light enough for a dob of glue to hold securely.

Thanks to the ex 2 base for highlighting this problem, and I wonder how many of these venerable devices are measuring OK (as they inherently will) but with much reduced test voltage and un-necessary gearing stress? (We used to use these widely at work, the energy capability was useful for drying'n'frying slightly leaky connections in long multi-jointed control cable runs to restore operation- but that same energy capability was eventually deemed hazardous, and they were confiscated )

[Barnmead]

Redesign and rationalisation of Megger range took place in the 1960's and the DC generators used since the 19th century were replaced with AC generators diodes and capacitor(s) between 1960 and 1963. No need for slipping clutch.