What could have caused an EMI Filter Bead shorted by itself?

[jim hardy]

Yes, Dave. All four terminations are shorted.

To Dave's question

Ahhhhh, terminology... One man's short is another man's leakage...

I always use a an analog meter for such tests. That's because on the RX1 ohms range it applies tens of milliamps to the device under test.

Wetness gives a part-scale reading determined by the electrochemistry at the fault.
A hard short circuit that we'd expect to see on those Xrays will read very near zero

A digital meter on a high ohms scale will report a low number for a few hundred ohms of something conductive like carbonized insulation.

I'll bet Dave was thinking the same thing.

Individual tin whiskers will usually get burnt away by the current from an analog multimeter on low ohms scale.

I have seen however a "tin beard" , thousands of whiskers in a clump, growing between two adjacent terminals on a barrier terminal block. It passed enough current to actuate a solenoid valve and trip the power plant .
Tin is not the only metal susceptible

http://nepp.nasa.gov/whisker/reference/tech_papers/2007-brusse-metal-whiskers.pdf

That's why i suggest somehow getting the top off one of the failed ones so you can see inside. A clean interior says "well, it wasn't that."

old jim

 

[Baluncore]

We need some idea of the circuit application / impedance. Does the bead isolate high impedance, low power signals or does it isolate low impedance, higher power supplies? The presence of intact single whiskers in a power supply situation is unlikely, but there may be a surface coating of metal due to vaporisation of many whiskers over time.

An accurate resistance measurement would be most informative.

At some point you need to use destructive testing to further the research. It is almost certain that any whiskers present will be lost if you attempt to physically expose the component's internals for visual examination. That is why in my post #20, I suggested fusing whiskers with a DC supply. By using sequentially larger charged capacitors as an energy source you might get a good estimate the mass of the whisker(s) or short involved.

 

[Clark Magnuson]

That is a 5 mOhm 5 amp single turn balun made of Copper and Ferrite. It it shorted out, that must be an isolation test with an Ohm meter. No debris could represent a short that would shut down that kind of power line. My guess would be contaminated part or board or the part was damaged when soldered, caused a crack. Pieces of Ferrite do not conduct much.