The Electrical Contractor Network

ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Books, Tools and Test Equipment for Electrical and Construction Trades

Register Now!

Register Now!

We want your input!


2017 NEC and Related
2017 NEC
Now Available!

Recent Posts
Forum Software Upgrade coming soon...
by Ruben Rocha
01/18/17 02:37 PM
Re: Fabricating Guards out of Lexan?
by frank
01/17/17 04:11 PM
Would ELV branch circuits be a good idea?
by gfretwell
01/17/17 01:26 PM
Why cables look like they do
by Texas_Ranger
01/17/17 12:33 PM
100% breakers?
by Texas_Ranger
01/16/17 06:13 AM
New in the Gallery:
Desk-mounted "power-board"
Top Posters (30 Days)
HotLine1 21
Webmaster 20
gfretwell 13
Trumpy 12
ghost307 12
Who's Online
2 registered (Ruben Rocha, Webmaster), 0 Guests and 208 Spiders online.
Key: Admin, Global Mod, Mod
Topic Options
Rate This Topic
#150636 - 07/19/05 04:50 PM High Voltage Probe Specs.
BigJohn Offline

Registered: 03/06/04
Posts: 352
Loc: Boston, MA
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
#150637 - 07/19/05 04:57 PM Re: High Voltage Probe Specs.
mxslick Offline

Registered: 10/24/04
Posts: 785
Loc: Atomic City, ID USA

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?
Stupid should be painful.

#150638 - 07/19/05 06:35 PM Re: High Voltage Probe Specs.
renosteinke Offline
Cat Servant

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
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.


ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals