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Re: status report, questions, etc.

> I'm not trying to argue, just to understand...
> The CT has 400 turns, which would, I assume, give a 400:1 ratio.
> Why don't I just get a run away current if I use an ammeter?  What's 
> limiting the current?  Mark Barton suggests a burden load, which 
> makes sense as an answer to me.  I'll try things out, but I hope to 
> understand things a little better, so the I can keep the smoke inside 
> the meter. :-)

Good question, Chip.  I've given a little thought to the matter myself, but 
I don't know enough about transformers and electromagnetics yet to 
give an answer other than "That's how the current transformers that I 
have work".  Also, the books that I have that have anything to say about 
current transformers refer to transforming the current, alas without 
going into any of the technical details.

The only thing that I can think of that might apply (back from my EE 
courses many moons ago) is that a source can be characterized by it's 
short-circuit current and open-circuit voltage.  Given a CT's fairly low 
short-circuit current (.25A or so) and the high open-circuit voltage, that 
would indicate that a CT has a fairly high characteristic resistance, 
which would limit the current and prevent the smoke from being 
released from your low-resistance ammeter.

NOTE AGAIN: in addition to the voltage across an open CT possibly 
being high enough to be dangerous to touch, the refence to CT's that I 
was reading last night also mentioned that the voltages might get high 
enough to compromise the insulation on the CT wiring


Steven Roys (sroys-at-radiology.ab.umd.edu)