overheating Circuit breaker

[lazorko]

I could use some advice:

A church had a 100A single phase 120V/240V 20/24 circuit subpanel supplying lighting loads in the sanctuary. Complaints were that several of the breakers were getting very hot, accompanied by burning odor. Each of (3) 20A breakers served (2) ceiling fixtures lamped witth (9) 100W incandescent lamps. We calculated the load on each branch to be 1800W or 15A - within the 80% continuous loading on a 20A branch; measurements revealed that actual loads ranged from 16A - 17A on the three breakers. Neutral currents for each circuit were identical to currents on the hots.

We found that the panel bus insulation had begun to burn under the three breakers and replaced the panel with a new copper bus 125A panel and wired (6) new switches to control the lights (they had previously been controlled by flipping the breakers).

The three circuit breakers are still getting hot, getting noticably warm within several minutes of loading. The heating starts at the point of contact between the breakers and the panel bus - not at the screw terminal or #12 THHN.

We advised replacing the incandescent lamps with comparable compact flourescents to reduce the loads, but are concerned that this is occuring in the first place. Why is the panel getting so hot? Would the 1A (on the load drawing 17A) difference between permitted continuous loading of a 20A circuit and the actual continuous load make the differrence between a safe installation and a fire hazard? What about the circuits loaded to only 16A, 80% of the circuit rating?

We would appreciate your thoughts and advice.

 

[jro]

Re: overheating Circuit breaker

Have you tried balancing out the load across the two buss-bars, somtimes that can cause heating of the breakers and buss-bars.

 

[don_resqcapt19]

Re: overheating Circuit breaker

Are these breakers "plug-in" or "bolt-on"? "Plug-in" breakers have more contact resistance at the bus connection than "bolt-on" breakers. How hot is hot? The UL standard for small circuit breakers permits a 50?C rise at the breaker line and load terminals for a breaker loaded to 100% of the breaker rating. This for a breaker in open air at an ambient of 40?C. You can view this information on page 3 of this Square D document.
Don

 

[lazorko]

Re: overheating Circuit breaker

Thanks for the reply-
The plug-in breakers occupy three consecutive positions and are at top (closest to main lugs) of panel - about as balanced as they'll be. While we didn't measure, the temperature rise may now be within the allowed 50C rise limit. The fear is knowing the damage that occured to the previous panel (WELL over 50C rise, to be sure!), and our goal of preventing a recurrance. However the old panel had a solid aluminum bus which may have created increased contact heating.

 

[bennie]

Re: overheating Circuit breaker

Is this panel in a wire closet or in an area with no air movement?

 

[don_resqcapt19]

Re: overheating Circuit breaker

I would look at spacing these breakers away from each other to let the heat more easily dissipate.
Don

 

[hurk27]

Re: overheating Circuit breaker

Just one question I noticed that you didnt mention if you replaced the breakers after replaceing the panel. the contacts would of been damaged from the old panel? or did you change the manufacture all together? sounds like a ITE panel.

 

[karl riley]

Re: overheating Circuit breaker

When circuit breakers are routinely used as light switches, would not the contacts wear prematurely? Could this be involved?
Karl

 

[templdl]

Re: overheating Circuit breaker

The standard UL489 breaker test for breakers up to 100a to 6000 full load and 4000 no load at 6 per minute and must still pass calibration tests.
I would consider some other cause of heating. However, once heating occurs that has been caused by wire terminations or stubs for example, that heating can cause the contact spring tension to fail which often results in poor contact preasure which then results in the contacts themselves to heat in fail.

 

[alan mcneil]

Re: overheating Circuit breaker

What is the manufacturers name on the breaker, and did you change the breakers as you did the panel?
Are you still getting a burning smell from the panel .

 

[rrrusty]

Re: overheating Circuit breaker

You said that there are three c/b feeding the lites; It sounds to me that until you split the load up to 4 c/b you will still have a balance problem per phase; How much is your total load? Try installing a panel with bolt-on c/b and see if that helps; Did you check the connections of the feeders?

 

[pierre]

Re: overheating Circuit breaker

Lazorko

If you do find a solution, please be sure to post it, I am very curious to see what might of caused this.

Pierre

 

[jro]

Re: overheating Circuit breaker

Lazorko when you say balanced as can be, what do you mean, can you give amp loads for each phase, and have you found the problem, please post.

 

[lazorko]

Re: overheating Circuit breaker

Thanks for all the input!
About the loads: each #12 THHN branch on 20A CB feeds two fixtures containing nine lamp holders each - building maint. asserted that they recently relamped with 75W long life incandescents - ceiling is 20' up, so we took them at their word - but current measurements show a range of 15.6A - 17A on the three branches, so I suspect that they have a few 100W lamps up there, too.

Neutral currents for each branch exactly equal the hot current -so there is probably no ground fault.

The old panel was Crouse-Hines. The new panel is a GE - and of course, we dumped all the old breakers in the trash - all of the breakers in the vicinity of the burnout showed at least some visible signs of heating.

The feeders are (and were before) cool to the touch; the heat generated at the buss probably dissipated through breakers down the line before reaching the lugs.

Balancing three 1920W loads on a single phase service would necessarily mean putting two loads on one leg; but as I mentioned, the feeders weren't heating - just the buss in the area of breaker contact. I would expect a bus tang to be able to deliver at least a 20A continuous load without heating.
Last Sunday, the church had service with lights on for 10 hours - no heating--yet. I suspect:
- some 75W lamps are actually 100W. In fact, I wonder how loose manufactures are with power ratings on lamps. We still can't explain the high 17A reading unless some of the lamps are 150W - maint. doesn't purchase greater than 100W lamps.
-the aluminum bus may have formed an oxide coating more rapidly with even a slight increase in operating temperatures. Over time, the resulting increased contact resistance resulted in ever greater heating. Hopefully, copper will hold up better.
-the panel is in a utillity closet and is outside the A/C cooling zone so the panel would have been less able to stay cool. There are no easy fixes for that!
-use as switching may have degraded electrical contact in the C/B; switches are now installed , so that will no longer be a problem.
- we are recommending a gradual relamping to compact florescents to reduce power draw. The church uses a lot of high - tech A/V, radio communication and IR remotes, so we would want to make sure the florescents don't interfere with that equipment.