Re: status report, questions, etc.
Subject: Re: status report, questions, etc.
From: "SROYS" <SROYS-at-radiology.ab.umd.edu>
Date: Thu, 16 Nov 1995 14:12:13 EDT
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> 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)