Stumped by Phase C overheating

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Bob7135

Member
I have a situation where a customer reported an extremely hot 600 Amp disconnect which feeds a chiller. Opening the disconnect, the wire insulation was burned around 8" up the cables on C Phase. The heat coming from the area around the 2 sets of 300 MCM CU feeding this phase felt like it was coming from a burner. An infra-red spot temp gauge measured 260-280 deg F at the wire termination which was screw lugs, well above the wire rating and ambient in the room. The other phases were close to ambient.

Measuring current and voltage, each phase measured close to 400 Amp, and PH to PH voltages were 208V to 210V. The chiller has electronic controls and had never gone off on over-current that we were aware of.

Upon close inspection, the switch knife blade appeared to be pitted and contact with the switch jaws was in an extremely small area. The only repair part available for this disconnect was the switch jaw.

The switch was disassembled on a shut-down as the switch is fed directly from the utility. New lugs were installed, all current-carrying parts were cleaned and reassembled, and the cables were cut back to reach unburned insulation.

Weeks after the repair, the switch is still overheating on the same phase. The owner believes it may be a utility problem, but I can think of no condition of utility power such as voltage fluctuations or surges that would continuously affect the one phase. I don't think 5th harmonics could be involved either as it is a motor load, therefore I never checked the current with a 5th harmonic Amprobe.

I did not reccommend to the owner that a new disconnect be installed as I don't feel at this point it is a problem with the switch. I may put a recording ammeter on the disconnect for a week just to make sure there isn't any intermittent problems we did not catch the several times we measured the current.

Sorry for the long post, but does anyone have any suggestions? :-?
 

brian john

Senior Member
Location
Leesburg, VA
Perform a fall of Potential test and locate the point of high resistance.

Fall of Potential test (FOP)

With any electrical procedures that involve making measurements with energized conductors care must be taken and proper PPE must be worn

Fall of potential is a measurement of voltage drop (VD), in a circuit that is the voltage lost to heating due to resistance in the circuit. To perform this test one measures voltage from the line side connections to the load side connections, the readings are typically in the millivolts range and require load.

For example with a 3-phase Circuit Breaker that is nuisance tripping, measure current (balance loads are beneficial but not necessary), then measure from the line bus to the load conductors (if bare conductor is exposed), for all three phases. If one phase has a higher that average millivolts reading, then try to isolate this issue. Measure from line bus to the Circuit Breaker bus stab, if all 3-phases have millivolts readings that are the same (or close to the same) measure from the bus stab to the load side conductor termination connector (through the Circuit Breaker), if all these readings are close, next take measurements from the load side termination connector to the load conductor. In this case you can determine if it is a line bus connection issue, bad Circuit Breaker or a load conductor termination issue. Any accessible portion of the device can be tested in this method.
 

beanland

Senior Member
Location
Vancouver, WA
Conducted heat?

Conducted heat?

If the connection or fuse is defective, heat can easily be transfered along the line. Check the voltage drop across the switch-fuse per phase. Proper PPE is a must because there is no protection from utility (priamry fuses!) if arc develops.

Given the safety hazards and possible damage should a fault occur, a new disconnect may be prudent.

Unless there is a ground fault, the phase current in all three legs should be identical. If the utility service is 208Y/120V and the chiller is 208V 3-phase 3-wire, if the chiller has a ground fault, high phase currents could occur. Check.
 

zog

Senior Member
Location
Charlotte, NC
"Upon close inspection, the switch knife blade appeared to be pitted and contact with the switch jaws was in an extremely small area. The only repair part available for this disconnect was the switch jaw. "

Thats your problem, why didnt the knife blade get replaced? Thats an easy part to find. You just didnt look in the right place.

There may also be a pressure adjustment for the contacts depends on the switch type that could be causing your problems, but based on your previous comments it is probally the pitted contact area.
 

charlietuna

Senior Member
first consider the actual full load operating current in relation to the disconnect's rated full load current. if it's close to 80 percent of the switch's rated full load i would highly recommend replacing the disconnect with the next larger size switch(heavy duty and a good brand name). consider what happens when something is overtorqued.... blades and contact surfaces become "misaliegned" and that contact has lost it's full contact surface area. this cannot be seen with the human eye in disconnects. being in the infrared scanning business for 17 years we began to see re-accuring problems in installations by certain contractors in our area. although top of the line, and heavy duty disconnect switches were used, we would finds problems in the "blade portion" of the switches in certain installations. we then talked to the men on these jobs and find out no torque wrenches were used and the same installer was a 240 pound 6'-4" electrician!!! another problem overlooked on large chillers is the length of the feeder cables? they should all be the same length or they will pull an unbalanced current, which can be a problem, especially if the utility compound the problem with low supply voltage. any heavy motor loads-chillers-elevators-pumps and fans should have heavy duty disconnects switches -- the labor is the same --the lifespan can get expensive!
 

zot64

Member
Location
Middle TN
similar problem....

similar problem....

Have a 40 h/p chiller motor running on 200v. Was called to service a "burnt" 3 phase breaker. C phase had melted breaker and wire on line side. Breaker is fed from the bus bar of a larger panel(?)...2/0 wire running 20 ft. to 40 horse motor. Upon new breaker and wire, start the motor and quickly get temp rise from C phase...motor runs short while then trips breaker. Quickie measurements show 80- 82 amps during operation on each phase, 114v across all legs to ground, 199v leg to leg.....wire and connections are good from bus thru breaker to motor...from starter to motor good as well....BTW the motor is brand new, but still made sure the windings were good.
Well what do you guys think?
On the prev. posts there is the explanation of measuring at the bus....what would you see there if one phase is running hot?? Imbalance?
 

zog

Senior Member
Location
Charlotte, NC
zot64 said:
Have a 40 h/p chiller motor running on 200v. Was called to service a "burnt" 3 phase breaker. C phase had melted breaker and wire on line side. Breaker is fed from the bus bar of a larger panel(?)...2/0 wire running 20 ft. to 40 horse motor. Upon new breaker and wire, start the motor and quickly get temp rise from C phase...motor runs short while then trips breaker. Quickie measurements show 80- 82 amps during operation on each phase, 114v across all legs to ground, 199v leg to leg.....wire and connections are good from bus thru breaker to motor...from starter to motor good as well....BTW the motor is brand new, but still made sure the windings were good.
Well what do you guys think?
On the prev. posts there is the explanation of measuring at the bus....what would you see there if one phase is running hot?? Imbalance?

What did you use to test the connections? What values did you get?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
A small sidebar, if you please: after 40+ years in this business, I am still a little amazed at the quality of the answers you get here. You guys that devote time to this site probably never know the appreciation those of us that use your info have for your input.
 

mxslick

Senior Member
Location
SE Idaho
zot64 said:
<snip> C phase had melted breaker and wire on line side. Breaker is fed from the bus bar of a larger panel(?).......wire and connections are good from bus thru breaker to motor...from starter to motor good as well

Were ANY of the connection points previously burned or discolored? Is so, did you replace them or just clean them?

Cleaning will not work in many cases, as you'll still have a high-resistance connection. Result = heat.

Were your voltage readings taken as close to the motor terminals as possible while the motor was running?

Is the motor unusually noisy? (Buzzing or vibrating.)
 

zot64

Member
Location
Middle TN
augie47 said:
A small sidebar, if you please: after 40+ years in this business, I am still a little amazed at the quality of the answers you get here. You guys that devote time to this site probably never know the appreciation those of us that use your info have for your input.

You got an Amen here....
 

zot64

Member
Location
Middle TN
I now realize I high-jacked the thread...my apologies all...noob.

Today I went back and spent some more time on this....First let me re-explain.
Was called on the C phase melt down on the 40 horse chiller motor...they wanted it running right now of course. The C phase wire (2/0 directly from bus of ancient panel fed from 600 amp breaker @ switchgear) had burned and melted the top of the 175 amp breaker(in a box mounted to the side of the ancient panel). Took of the load side leads from the breaker and checked for ground/short...it showed no problems...opened the peckerhead and checked each lead to ground..it had no problems. Checked the three leads going back to the contactor, no problems.
I replaced the breaker with identical...removed line side leads cleaned lugs of all three and replaced the line side C phase conductor that had melted. Hit breaker, hit knife switch, hit start button she ran great...hung out for 45 mins. or so, took amp readings of 85a across the legs..closed up and left.
Next morning client calls...trip breaker. This time the #3 motor lead shows direct short...I say "Replace the motor". $2k later, new motor in place and wired correctly....trip breaker
Now...they have a back up identical set up being fed from the lugs on the top of the contactor on my troubled motor. Runs maybe 40 feet to another contactor, 40 horse chiller pump that is controlled from the cabinet of my motor, has a "normal/by-pass" switch next to the "start-auto/hand" switch. This motor runs fine from the same breaker-knife switch. amp and voltage reading within spec. on that one.
Today I started at the bus with everything un-hooked and connected back each piece and measured all along...bus read properly, leads from bus not connected to breaker read fine. Line side connected, top and bottom of breaker read fine. leads to knife switch read fine torqued them back to spec. bottom side of knife switch ok. readings at top of contactor ok. readings at bottom of contactor ok. Contactor works with out the motor connected and reads fine. leads at the peckerhead read ok. All back together now, torqued, cleaned, inspected.....hit "start"...trip.
I left them on their back-up and sit here puzzeled!

Sorry for the long post but maybe it will ring a bell for someone as to what to do from here.
 

zog

Senior Member
Location
Charlotte, NC
zot64 said:
Took of the load side leads from the breaker and checked for ground/short...it showed no problems...opened the peckerhead and checked each lead to ground..it had no problems. Checked the three leads going back to the contactor, no problems.

I assume you used a megger to do this, what readings did you get to determine there was "No problem"
 

jcassity

Senior Member
Location
24941
bring something from your shop to hook up to the contactor.,, ill assum 120/208 3ph is what your having issues with.

If the breaker does not trip, it sorts your mess and lets you know where not to focus.,, and isolates the problem.
If it does trip,, the so be it.
Either way it sounds like progress to me.


Atleast the breaker is tripping so thats a good thing,, right:)

Circuit isolation and problem identification is easy to do if you just think differntly and tell everyone to stay out of your way.
 

zot64

Member
Location
Middle TN
Contactor had a small 'blister' on the back side from the previous overheating, and was not pulling in all the way on the C phase, causing a 20 amp imbalance. Re-placed with new...works like a champ.

Thanx for the help gent's.
 
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