Electronics > Projects, Designs, and Technical Stuff

Designing a Burn Down Transformer

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MagicSmoker:
Yeah, I definitely agree with Marco that a hi-pot tester that can't be used for more than 3 seconds is a fail. For a quick-n-dirty solution I would use a Variac to supply either an isolation transformer or an old-school neon sign transformer (not the new style with built-in protection against ground faults and shorts). Use the neon sign transformer to encourage a partial insulation breakdown to finish the job, so to speak, and the isolation transformer to energize the heating cable at a reduced power level. Old-school NSTs have magnetic shunts in the transformer that turn them into current-limited power sources so are almost ideal for hi-pot testing as-is, while supplying an isolation transformer at less than its rated primary voltage will also limit the maximum current it can deliver, though additional resistance or reactance might be warranted for extra safety.

But as Marco already pointed out, there's nothing safe about the equipment needed to do this kind of testing, and while there are other means of finding faults in cables that are intrinsically safe - like time-domain reflectometry and RF injection/detection - they aren't accurate enough to pinpoint where to dig/rip up the floor/etc. like a hot spot on a thermal camera.

digsys:
Thinking about it - a long time ago a friend had that problem with his heated concrete slab. The only way we could find the break, and luckily it was a ring loop, was to inject a good quality RF signal into the cable and trace out the circuit - then looked for obvious "dips" , "inconsistencies" - unfortunately there were none - then drilled "breaks" in the cable run - re-checked with the RF, and when that didn't help, injected 24VAC and tested a load at the break. A slow process, but worked out ok.
We found the crack

james_s:
It's a valid technique. I once used a 9kV neon sign transformer to find a break in the winding of a HID ballast that had taken a whack. The high voltage instantly arced across the fault, clearly visible in subdued lighting and I was able to locate and repair it with minimal invasiveness.

IanB:
First of all: this is an underfloor heating element designed for direct connection to the mains. I do not see the point of an isolation transformer here?

About how such things may apparently work:

Feeding a constant current source into a wire loop will produce local hot spots at points of high resistance.

If the buried heating element has a break and has gone open circuit, then a constant current source with a high compliance voltage can arc across the gap and produce a hot spot where the gap is. A thermal camera may then be used to locate the hot spot under the floor and let you know where to drill.

As was previously mentioned, a neon sign transformer may be a good option for doing this.

IanB:

--- Quote from: sinB on July 22, 2019, 10:23:35 am ---A heating cable is damaged somewhere in the floor, causing the fuse to trip. The cable is then hooked up to the Burn Down Transformer, and a thermal camera is used to observe the exact position where the heating cable gets cold. The heating cable can then be repaired at that exact point.
--- End quote ---

More likely the heating element has gone open circuit and the heating has stopped working.


--- Quote ---Those are expensive, but looks relatively simple to build.
--- End quote ---

Whenever you have "expensive" and "simple" in the same sentence, it is probable that you are missing something important...

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