intermittent short?

Reply

  #1  
Old 11-19-13, 10:36 AM
Member
Thread Starter
Join Date: Nov 2013
Location: USA
Posts: 4
intermittent short?

I recently purchased a home that was upgraded from fuses to breakers. The new box uses a 60 amp breaker to feed an old sub panel (fuses) in the garage that supplies power to an addition and the kitchen. The new breaker will intermittently trip and will often reset after I let the circuit "rest" for a period of time.
1. The electrician who installed the panel has replaced the breaker, just to rule out a defective breaker.
2. The short does not exist when the electrician is here, making the cause difficult to isolate. He said the problem is likely in the line that connects the new box to the old box.
3. The circuit (when working) pulls 11 amps with a full load, including an electric heater, computer, TV, lights, etc.
4. The fuses in the sub panel have not blown, only the new breaker.
5. The symptoms are the same with the mains pulled at the old box, which reinforces the electricians suspicion.

With the new breaker turned off and the mains pulled at the old box , I checked for continuity with these results:
1. No continuity between the two legs at the breaker.
2. No continuity between one leg and ground.
3. very little continuity between the other leg and ground (could this leg be the problem? Why would there be even the slightest continuity?).

The line is not accessible in the attic due to the way the old owner added on to the house and I want to avoid the expense of running 100 feet of new line if it is not necessary.
1. If there was a direct short, then why only "sometimes"?
2. If the wire is expanding due to heat from the load to create the short, then why would it trip the breaker with the mains at the old box pulled 2 hours after resetting the breaker?

I appreciate your insight.
 
Sponsored Links
  #2  
Old 11-19-13, 10:45 AM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 14,365
Despite you 11 amp measurement I suspect an overload, not a short. Is this just the 15 amp breaker tripping?
 
  #3  
Old 11-19-13, 10:49 AM
Member
Thread Starter
Join Date: Nov 2013
Location: USA
Posts: 4
No, It is the 60 amp breaker feeding the sub panel.
 
  #4  
Old 11-19-13, 10:58 AM
Member
Thread Starter
Join Date: Nov 2013
Location: USA
Posts: 4
My first thought was that an appliance was causing an overload, So I pulled the main fuses at the sub panel to eliminate any possibility of load and the breaker still trips after 20 minutes to as long as 2 days.
 
  #5  
Old 11-19-13, 02:25 PM
CasualJoe's Avatar
Member
Join Date: Jan 2010
Location: USA
Posts: 9,221
The line is not accessible in the attic due to the way the old owner added on to the house and I want to avoid the expense of running 100 feet of new line if it is not necessary.
Is the feeder in conduit or is it a cable? If a cable. what type is it? How many conductors does it have?
 
  #6  
Old 11-19-13, 02:37 PM
canuk's Avatar
Member
Join Date: Sep 2013
Posts: 293
I'm thinking the same as pcboss --- an overload rather than a short.

Perhaps one conductor in the feed between the main & sub-panel could be loose and arcing.
 
  #7  
Old 11-19-13, 03:04 PM
Member
Thread Starter
Join Date: Nov 2013
Location: USA
Posts: 4
It is cable with 2 conductors and a ground. It looks like 4 ga. aluminum.Appears quite old (possibly late 50's as the home was built then). Connections are tight. One night I had some friends over and told them to watch the sub panel for arcing as well as other places such as the attic that I thought might be the problem and there was none that they could see when I tried to reset the breaker. Problem is that with one section of the attic being not accessible, I cannot determine if a splice exists in that line. Could arcing occur if there is no possibility for load? The mains on the sub are removed, so load past the box cannot occur.
 
  #8  
Old 11-19-13, 05:26 PM
CasualJoe's Avatar
Member
Join Date: Jan 2010
Location: USA
Posts: 9,221
It is cable with 2 conductors and a ground. It looks like 4 ga. aluminum.Appears quite old (possibly late 50's as the home was built then).
If it's from the '50s it isn't aluminum, but might be tinned copper. If it's aluminum it's not from the '50s. Assuming this is old SEU cable, what is the voltage rating on the sheathing, if you can read it? If it's the old 300 volt SEU cable you will show continuity both between insulated conductors and between insulated conductor and bare neutral conductor. Normally this is just leakage through the insulation. The newer service entrance cables are 600 volt rated and won't show the leakage. My opinion is the feeder needs to be replaced with either an SER 4-conductor cable or conduit with individual conductors.
 
  #9  
Old 11-20-13, 05:25 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: NJ - USA
Posts: 43,512
With a standard ohmmeter you should not see any resistance between wires in a cable that short.

You've removed the loads at the sub-panel yet the breaker in the main panel that supples power to the sub-panel keeps tripping. That is dangerous. That means that something is creating enough of a short to trip that breaker. That cable needs to be replaced. Whatever is causing the eventual resistance that causes that circuit to trip could get hot enough to start a fire.
 
Reply

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


Thread Tools
Search this Thread
Display Modes
'