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Old 19th Mar 2018, 10:01 am   #1
'LIVEWIRE?'
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Question Using my TMK TEM1555B Insulation Tester on an 80 year old radio

For those who don't know this instrument, it is an 'electronic megger' for want of a better way to describe it, which is used to check insulation at high voltages (0-200Meg @ 500vDC). Is it wise to use this to test the insulation, particularly between the Mains Transformer Primary and Chassis, of an 80 years old Radio?
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Old 19th Mar 2018, 10:17 am   #2
TonyDuell
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Default Re: Using my TMK TEM1555B Insulation Tester on an 80 year old radio

I would use it. It might cause the insulation to break down, but if so, then it is likely that it would have failed with normal mains voltage, at an unknown time. That would be dangerous.

I normally test everything (old and new) that is going on the mains with a 1000V insulation tester. If it won't stand 1000V from the current-carrying parts to earth then I am not happy using it on the mains.
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Old 19th Mar 2018, 12:11 pm   #3
'LIVEWIRE?'
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Default Re: Using my TMK TEM1555B Insulation Tester on an 80 year old radio

Thanks, Tony. I'll certainly do that, and power the radio from a lamp limiter, before applying full mains. Incidentally, the radio in question, which has a double wound mains transformer, according to the circuit diagram, never had a mains earth, but someone in the past has fitted a length of 3-core mains lead to the removable 2-pin mains connector, and presumably connected the earth lead to a bolt or screw in the chassis, Whilst I would prefer a metal chassis to be mains earthed, I wonder if this would add to the stress in an almost 9 decades old mains transformer, so a 2-core lead might be best. The radio, of course, needs an external aerial and earth, to perform properly, so maybe the mains earth was used as a signal ground.
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Old 23rd Mar 2018, 10:09 am   #4
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Default Re: Using my TMK TEM1555B Insulation Tester on an 80 year old radio

Having checked the CPC-supplied probes which I use with the TEM1555B, they are 1000volt Category III. Each incorporates a 500mA FF fuse (though at some time I had replaced the fuse in the red probe with a RS 1A one(must get some 500mAFF fuses!!) The internal fuse in the TMK Meter is a 2Amp type (and marked as such inside the instrument, despite the booklet stating 'Use only a fuse with the 0.5Amp rating-this is important!') The foregoing tells me that the leads & probes are adequately rated for use with an insulation tester such as the TEM1555B, which is no doubt why I bought them.
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