Troubleshoot question (Ohm's law?) - Electrician Talk - Professional Electrical Contractors Forum
CLICK HERE AND JOIN OUR COMMUNITY TODAY, IT'S FREE!
Go Back   Electrician Talk - Professional Electrical Contractors Forum > Electrical Forum > General Electrical Discussion


Like Tree9Likes
  • 1 Post By brian john
  • 1 Post By readydave8
  • 3 Post By brian john
  • 2 Post By Bird dog
  • 1 Post By Rora
  • 1 Post By B-Nabs
Reply
 
Thread Tools Display Modes
Old 09-19-2018, 07:07 PM   #1
Senior Member
 
Join Date: Mar 2015
Location: Houston Texas
Posts: 536
Rewards Points: 1,070
Default Troubleshoot question (Ohm's law?)

Had a question about about ohm's law, I think...? So I'm called out to do troubleshoot why some office receptacles have strange voltage. They say the voltage keeps jumping erratically from like 90-115v. I got there during lunch, no one was on their computer, all the computers were shut off. Got out my meter, checked the voltage and everything was 120v, normal... No jumping around or nothing.

Then one of the guys came back from lunch and told me his computer keeps giving him a warning message that there is a voltage issue and then it shuts down... So hmm... I ask him to turn his computer on, and sure enough it does it. Then I put my meter back into the receptacle, ask him to turn on his computer, and sure enough voltage starts jumping around from 90-115v...

I thought most likely there is something wrong with his computer that is causing this? I tried another computer and it did the same thing...

So I ended up going to the panel and started to investigate there first. Sure enough, the breaker was bad. It acted like a good breaker, but when I pulled it off the panel to further inspect it, corrosion started poring out of it, every part of it was corroded, it even started to just chip apart. Anyways, I replaced the breaker, did an amp test to make sure when they are up and running, it's not being overloaded. Everything checked out fine and everyone had 120v and boom, job is done.

I just have a question about that. So the breaker had a high amount of resistance going through it. It was a normal 120v circuit when there was no load on the wire, but whenever I put a load on the wire, the voltage would start jumping around from 90-115v. This has to do with ohm's law? V=I x R? Idk, why does the voltage start going haywire ONLY when there is current applied? Also if V=I x R is true, why does voltage go down when there is resistance. If current lets say = 1, and R (resistance) keeps going up, then according to the equation, voltage would also go up? Hmm... Doesn't seam right, what am I confused on?
JasonCo is offline   Reply With Quote
Join Contractor Talk

Join the #1 Electrician Forum Today - It's Totally Free!

ElectricianTalk.com - Are you a Professional Electrical Contractor? If so we invite you to join our community and see what it has to offer. Our site is specifically designed for you and it's the leading place for electricians to meet online. No homeowners asking DIY questions. Just fellow tradesmen who enjoy talking about their business, their trade, and anything else that comes up. No matter what your specialty is you'll find that ElectricianTalk.com is a great community to join. Best of all it's totally free!

Join ElectricianTalk.com - Click Here JOIN FOR FREE


Warning: The topics covered on this site include activities in which there exists the potential for serious injury or death. ElectricianTalk.com DOES NOT guarantee the accuracy or completeness of any information contained on this site. Always use proper safety precaution and reference reliable outside sources before attempting any construction or remodeling task!

Sponsored Links
Advertisement
 
Old 09-19-2018, 07:45 PM   #2
Russian Bot
 
Helmut's Avatar
 
Join Date: May 2014
Location: THE United States Of America
Posts: 3,434
Rewards Points: 3,749
Default

Quote:
Originally Posted by JasonCo View Post
I just have a question about that. So the breaker had a high amount of resistance going through it. It was a normal 120v circuit when there was no load on the wire, but whenever I put a load on the wire, the voltage would start jumping around from 90-115v. This has to do with ohm's law? V=I x R? Idk, why does the voltage start going haywire ONLY when there is current applied? Also if V=I x R is true, why does voltage go down when there is resistance. If current lets say = 1, and R (resistance) keeps going up, then according to the equation, voltage would also go up? Hmm... Doesn't seam right, what am I confused on?

It's a lose connection. Is your meter a wiggy?
__________________
Alright, Alright, Alright... I'll tell you what to do. Go that way, reeeeeeeeeally fast. If something gets in your way......turn.
Helmut is online now   Reply With Quote
Old 09-19-2018, 07:54 PM   #3
Senior Member
 
Join Date: Mar 2015
Location: Houston Texas
Posts: 536
Rewards Points: 1,070
Default

Quote:
Originally Posted by Helmut View Post
It's a lose connection. Is your meter a wiggy?
I just have a regular Ideal clamp on meter, nothing too special. What do you mean loose connection? The problem was a bad breaker
JasonCo is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 09-19-2018, 08:35 PM   #4
Senior Member
 
brian john's Avatar
 
Join Date: Mar 2007
Location: Gilbert's Corner VA
Posts: 29,857
Rewards Points: 1,486
Default

Quote:
Originally Posted by JasonCo View Post
I just have a regular Ideal clamp on meter, nothing too special. What do you mean loose connection? The problem was a bad breaker
Resistance changes.
The_Modifier likes this.
__________________
Ah, but I was so much older then, I'm younger than that now
brian john is offline   Reply With Quote
Old 09-19-2018, 08:41 PM   #5
neutral member
 
readydave8's Avatar
 
Join Date: Sep 2009
Location: Clarkesville, Georgia
Posts: 1,474
Rewards Points: 564
Default

When does resistance not cause voltage drop? I think maybe never.
brian john likes this.
__________________
Dave Ruth
readydave8 is offline   Reply With Quote
Old 09-19-2018, 08:45 PM   #6
Senior Member
 
brian john's Avatar
 
Join Date: Mar 2007
Location: Gilbert's Corner VA
Posts: 29,857
Rewards Points: 1,486
Default

Quote:
Originally Posted by brian john View Post
Resistance changes.
If you had a fairly decent DMM you could have watched the resistance change by measuring the Voltage Drop across the CB.
The_Modifier, Gnome and 51360 like this.
__________________
Ah, but I was so much older then, I'm younger than that now
brian john is offline   Reply With Quote
Old 09-19-2018, 08:45 PM   #7
Senior Member
 
Dark Knight's Avatar
 
Join Date: Jan 2016
Location: Manitoba, Canada
Posts: 482
Rewards Points: 120
Default

Quote:
Originally Posted by JasonCo View Post

I just have a regular Ideal clamp on meter, nothing too special. What do you mean loose connection? The problem was a bad breaker
The breaker likely went bad BECAUSE of a loose connection.
Dark Knight is offline   Reply With Quote
Old 09-19-2018, 08:46 PM   #8
Senior Member
 
Bird dog's Avatar
 
Join Date: Oct 2015
Location: South East US
Posts: 4,770
Rewards Points: 1,318
Default

Quote:
Originally Posted by JasonCo View Post
I just have a regular Ideal clamp on meter, nothing too special. What do you mean loose connection? The problem was a bad breaker
A loose connection inside the breaker that changes resistace depending on the load. This type of problem is also seen with loose neutral connections.
When the connection has more resistance, there is voltage drop across the connection just like a resistor. So, when the voltage dropped to 90volts, if you put one lead on the busbar & the other lead on the wire terminal of said breaker you would read 30volts (120v-90v=30v). You've just measured the contact resistance of the breaker. You do the same thing on a contactor to see if the contacts are good.
Gnome and Chops146 like this.
__________________
Popcorn munching forum observer
"The supreme art of war is to subdue the enemy without fighting." Sun Tzu
Bird dog is online now   Reply With Quote
Old 09-19-2018, 09:05 PM   #9
Senior Member
 
Join Date: May 2018
Location: florida
Posts: 1,361
Rewards Points: 942
Default

I can not think of the correct way to explain this problem but there is no surprise that the more load you add the lower the voltage will go after the point of resistance
gpop is offline   Reply With Quote
Old 09-19-2018, 09:48 PM   #10
Senior Member
 
Join Date: Mar 2015
Location: Houston Texas
Posts: 536
Rewards Points: 1,070
Default

Ah yeah that makes sense. When you break a neutral completely, you read 120 on the neutral, and 0 on the hot. So if there is a lot of resistance on the neutral, you'd read voltage on both the neutral and the hot, didn't think about that though. Makes sense!

Only thing I don't understand. If your neutral is 100% good, no resistance at all. Your breaker though has a ton of resistance, like in my scenario. V = I x R is Ohm's law. So how is this possible that voltage goes DOWN when you ADD resistance? That would contradict the equation. I do know that the more resistance you have, the less voltage but wtf is up with V = I x R? Makes no sense to me
JasonCo is offline   Reply With Quote
Old 09-19-2018, 09:53 PM   #11
Senior Member
 
Bird dog's Avatar
 
Join Date: Oct 2015
Location: South East US
Posts: 4,770
Rewards Points: 1,318
Default

Draw it out on paper. Btw, the Total Voltage did not drop.
__________________
Popcorn munching forum observer
"The supreme art of war is to subdue the enemy without fighting." Sun Tzu
Bird dog is online now   Reply With Quote
Old 09-19-2018, 09:53 PM   #12
Senior Member
 
Rora's Avatar
 
Join Date: Jan 2017
Location: Canada
Posts: 342
Rewards Points: 680
Default

Kirchhoff's voltage law... voltage drop is proportional to resistance. Without any load, the circuit is not complete (infinite resistance between hot and neutral), therefore you'll see the majority of the voltage drop at the receptacle. Put on a load (PC power supply) and the resistance of the breaker corrosion takes a noticeable portion of the voltage drop.

In other words, no load circuit means ratio of infinite:breaker corrosion in terms of resistance and therefore voltage, so you see all the voltage at the receptacle. Put a load on the circuit and you're talking power supply resistance:breaker corrosion and you'll see more drop in the breaker.
Chops146 likes this.
__________________
Self-confidence isn't 1000V rated.

Last edited by Rora; 09-19-2018 at 09:56 PM.
Rora is offline   Reply With Quote
Old 09-19-2018, 10:37 PM   #13
Senior Member
 
Chops146's Avatar
 
Join Date: Aug 2018
Location: Midwest
Posts: 516
Rewards Points: 1,032
Default

When I read the symptoms, first thing I thought was loose neutral.
Chops146 is offline   Reply With Quote
Old 09-20-2018, 02:19 AM   #14
Senior Member
 
Join Date: Feb 2016
Location: Indiana
Posts: 208
Rewards Points: 416
Default

Dude.. Ohms law says a good breaker is 0 ohms

You wrote, "when I pulled it off the panel to further inspect it, corrosion started poring out of it"

Probably aluminum bus or cheap metal on the panel buss, you need a new panel or panel insert.

Do not ignore this issue -- quote them, it might cost $2k parts + labor, close it or have your boss close it, FIX IT, and be done with it.

Philip

[I love Cricket]

Last edited by philipdybel; 09-20-2018 at 02:28 AM.
philipdybel is offline   Reply With Quote
Old 09-20-2018, 02:58 AM   #15
Watt Pusher
 
B-Nabs's Avatar
 
Join Date: Jun 2014
Location: Port Coquitlam, Canada
Posts: 2,135
Rewards Points: 466
Default

Quote:
Originally Posted by philipdybel View Post
Dude.. Ohms law says a good breaker is 0 ohms

You wrote, "when I pulled it off the panel to further inspect it, corrosion started poring out of it"

Probably aluminum bus or cheap metal on the panel buss, you need a new panel or panel insert.

Do not ignore this issue -- quote them, it might cost $2k parts + labor, close it or have your boss close it, FIX IT, and be done with it.

Philip

[I love Cricket]
"Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points."

https://en.m.wikipedia.org/wiki/Ohm's_law

Pretty sure it doesn't say a word about breakers anywhere.

Sent from my SM-G920W8 using Tapatalk
__________________
Is that a real poncho or is that a Sears poncho?
B-Nabs is online now   Reply With Quote
Old 09-20-2018, 03:35 AM   #16
Senior Member
 
Join Date: Feb 2016
Location: Indiana
Posts: 208
Rewards Points: 416
Default

Bnab, really?

Forget Ohms Law -- what resistance do you expect to see between the line & load terminals of a circuit breaker?

[Remember I Love Cricket]
philipdybel is offline   Reply With Quote
Old 09-20-2018, 04:38 AM   #17
Wire Ninja
 
MDShunk's Avatar
 
Join Date: Jan 2007
Location: Beautiful Cumberland Valley, in PA
Posts: 20,984
Rewards Points: 1,421
Default

Quote:
Originally Posted by philipdybel View Post
Bnab, really?

Forget Ohms Law -- what resistance do you expect to see between the line & load terminals of a circuit breaker?

[Remember I Love Cricket]
Unless you're measuring with a DLRO, by the time you read something on a DMM, the breaker is already trash.

Oh, and if you really loved Cricket, you'd take her camping like some of the other guys.
__________________
One reason not to give DIY advice:
Catch a man a fish and you can sell it to him.
Teach a man to fish and you’ve ruined a good business opportunity.
MDShunk is offline   Reply With Quote
Old 09-20-2018, 09:00 AM   #18
Watt Pusher
 
B-Nabs's Avatar
 
Join Date: Jun 2014
Location: Port Coquitlam, Canada
Posts: 2,135
Rewards Points: 466
Default

Quote:
Originally Posted by philipdybel View Post
Bnab, really?

Forget Ohms Law -- what resistance do you expect to see between the line & load terminals of a circuit breaker?

[Remember I Love Cricket]
I am merely responding to your statement that "Ohms law says a good breaker is 0 ohm". It does not say that. Ohm's Law says that there is a proportional relationship between voltage and current. I'm not taking issue with the idea that there should be negligible resistance between the line and load of a breaker.

Sent from my SM-G920W8 using Tapatalk
HackWork likes this.
__________________
Is that a real poncho or is that a Sears poncho?
B-Nabs is online now   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Troubleshoot question (Odd problem...) JasonCo General Electrical Discussion 17 09-14-2018 04:43 PM
Ohms law Switchgear277 Electrician Apprentice Forum 56 04-14-2018 03:17 PM
In-floor heating thermostat / sensor question Islander General Electrical Discussion 25 02-17-2018 08:10 AM
I would like to ask a real higher-up engineer a question HackWork Milbank Manufacturing 37 02-09-2018 05:20 PM
How many Ohms would I read on a transformer? Basic question JasonCo General Electrical Discussion 33 01-27-2016 04:25 AM


All times are GMT -4. The time now is 04:29 PM.


Powered by vBulletin® Version 3.8.8
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.1
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
User Alert System provided by Advanced User Tagging (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Our Pro Sites Network
ContractorTalk.com | DrywallTalk.com | HVACSite.com | PaintTalk.com | PlumbingZone.com | RoofingTalk.com