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#150636 07/19/05 06:50 PM
Joined: Mar 2004
Posts: 391
BigJohn Offline OP
With many high voltage probes available for use with multimeters, the instructions warn that the probe can only be used on power limited circuits. This meshes with a story I remember hearing about a university student who went to measure voltage on a utility transformer with a "power limited probe", the voltages were within the limits, but the probe exploded anyway.
What's the deal?
It seems like simple Ohm's Law to me: How can identical voltages force different amounts of current through a fixed resistance? And if current flow isn't the culprit, then why does the available power of the circuit make any difference at all?



P.S.: Not sure if this should be here or in "Electrical Theory", so sorry if it's misplaced.

Arc Flash PPE Clothing, LOTO & Insulated Tools
Joined: Oct 2004
Posts: 806

As a guess, I would say the real problem would be from insulation creepage and flashover ratings. To get an idea, take some binoculars and check out the number of "skirts" and overall length of an insulator on your backyard power pole, then compare that to the clearances from the probe tip to your hand! Most probes are severely lacking in that respect.

As for the power-limited requirement, perhaps a power-limited circuit of a given kV doesn't have the "oomph" to create a flashover/puncture insulation like a utility circuit?

Any one from the utility side got any ideas? [Linked Image]

Stupid should be painful.
Joined: Jan 2005
Posts: 5,370
Likes: 1
Cat Servant
Big John, I understand you confusion. Let me try to explain.

Let's first look at the testing methods. When UL tests something, they will often short the wires together, or otherwise over-load it, and NOT allow it to get too hot.
Yet, when they test a "power limiting" transformer (or power supply), when the output is shorted, they REQUIRE it to get real hot.
Why? How can "hotter" be "safer?"

In a word, IMPEDENCE. With AC, wire flowing through one wire affects other wires near it. This often has the result of restricting the electrical flow through that wire. Of course, this added 'resistance' is reflected as heat. Ever wonder why a transformer that has no load on it still remains warm? Impedence is the answer.
If the wires are fine enough, and the coil is tight enough, then a point will be reached where the current available to even a dead short will be limited. This is what "power limiting" means.

One notable seminar instructor has a dramatic way of demonstrating this principle. While I won't go into it here, trust me when I say impedence does exist, and is what makes transformers work.

Ohms law, V=IR is very true, but as stated is incomplete. It does not account for the continuous changing state of an AC system. To do that with math, you have to get into calculus.

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