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brain teaser #10429 06/12/02 05:15 AM
Joined: May 2002
Posts: 68
Eandrew Offline OP
My forman gave me this question . Maybe he just play'm with me but here it goes:

You have 5 wires run from the basement of a building to the roof. Your on the roof. You can go to the basement and come back to the roof only once. How can you use a continuity tester to determine which wire in the basement is the same wire on the roof. In otherwords, how could you ring out all five wires by only going from the roof to the basement and back to the roof.
. -Erik

Work Gear for Electricians and the Trades
Re: brain teaser #10430 06/12/02 06:34 AM
Joined: Oct 2000
Posts: 5,389
sparky Offline
use 5 resistors?

Re: brain teaser #10431 06/12/02 07:48 AM
Joined: Aug 2001
Posts: 7,520
pauluk Offline

It's quite possible to identify the wires with just a continuity tester and one trip.

You'll probably need to go get a pencil & paper to follow along... [Linked Image]

Starting on the roof, link together any two of the five wires. Now select another two of the remaining three wires and link those together. Identify the remaining open wire as #1, then start your journey down to the basement.

Now, at the other end of the cable, start probing with your continuity tester until you find a pair of wires which complete a circuit. Label these wires #2 and #3 (it doesn't matter which way round; we'll sort that out later). Now find the other pair of wires which gives continuity and label those wires #4 and #5. By default, the remaining wire must be #1.

Thus far, we have #1 identified at both ends and the other four wires identified to the level of two pairs.

Still in the basement, link #1 to #2. Link #3 to #4. We now have all five wires daisy-chained up and down the building. Go get a cup of coffee to fortify yourself for the climb back to the top!

Back on the roof, remove both links you made earlier, but remember which wires were paired by the links. Now, out with the tester again and find which wire has a circuit to #1. Identify this wire as #2. The wire which was previously linked to #2 must, logically, be #3.

Only two to go now. With one probe on #3, find which of the two remaining wires has continuity and label it as #4. The last wire must be #5, of course.

This method has identified the wires and proved that all five have continuity from roof to basement.

Re: brain teaser #10432 06/12/02 08:56 PM
Joined: Jan 2002
Posts: 1,457
Electricmanscott Offline
Ouch my brain!

Re: brain teaser #10433 06/12/02 09:24 PM
Joined: Apr 2002
Posts: 257
master66 Offline
Ok, your on the roof.

Go to the basement and get the walkie-talkie from your buddy in the basement leaving him only one.

Go to the roof and sort it out over the walkie-talkies.

Thats only one trip too. HA!

Re: brain teaser #10434 06/13/02 04:40 AM
Joined: Jun 2001
Posts: 29
Resistor Offline
First read them for voltage or (lick the wire, only for expert electricians or bosses). No voltage, check continuity, take your first trip downstairs and hook all wires together and check continuity then you will know. After that check for branch circuits.

[This message has been edited by Resistor (edited 06-13-2002).]

Re: brain teaser #10435 06/15/02 05:31 AM
Joined: Aug 2001
Posts: 7,520
pauluk Offline
Hey, I thought of something like that too! Connect one wire to 120V, another to 277V, etc, then go touch the other end and see how high you jump! [Linked Image]

If you don't mind though, I'll stick with my other method. [Linked Image]

Re: brain teaser #10436 06/15/02 06:19 AM
Joined: Jun 2001
Posts: 29
Resistor Offline
Paul, all he said was check for continuity! As far as I'm concerned I think I did that. Would'nt be the first time I was wrong.

Re: brain teaser #10437 06/15/02 08:08 AM
Joined: Aug 2001
Posts: 7,520
pauluk Offline
Sorry, it was the "lick the wire" part that brought out my sarcasm! [Linked Image]

Yes, I would certainly check for any existing shorts or connections between wires before starting as well. I agree that your suggestion would check the continuity of all five wires, but I took the problem to be much more than that.


How can you use a continuity tester to determine which wire in the basement is the same wire on the roof.

I took this to mean that the problem also entailed identifying the wires at each end.

Isn't this what you meant Erik?

Re: brain teaser #10438 06/15/02 08:27 AM
Joined: May 2002
Posts: 68
Eandrew Offline OP

WOW, that was smart. I had to draw out a picture for that one. But sure enough, that works.

Here was my thinking:

on the roof: tie 3 of the 5 wires together. ground out one wire. leave the last wire open.

down in basement: identify ground wire as having continuity with ground. identify open wire as having no continuity. The other three wires will read continuity between them. tie the identified ground to one of the three remaining unidentified wires. and ground out another of the unidentified ground wires.

Back up on the roof: Identify which of the three wires is open, grounded, and tied to the original grounded wire.

Of course, a walk-talkie might work better.

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