Why R-phase heats up when balanced & constant load applied on 3 phases

[naresh321]

Dear Sir,

I have major problem in my site, where 3 phase supply is being used for Chiller. A 4 core 300 sq mm Cu/XLPE/Armoured/PVC cable connection given to Chiller Compressors through 400A isolator.

At Isolator incoming side R-phase cable is getting so hot and melting XLPE insulation and damaging Isolator too. The other 2 phases (Y & B) remains normal without heat up. More over load on three phases is balanced and fixed. When we checked for the over load current with clamp meter the current passing through R-Phase is less than Y & B Phases (Almost equal currents measured R-157A, Y-163A, B-159A).

$ months back we replaced the Isolator with new one and covered the melted XLPE insulation with insulation tape after cleaning the melted insulation. There is no loose connection we tightly connected the lug termination.

Please find the attached photo showing the present condition of R phase cable for your review..

Kindly let me know the possible reasons for above problem.

Thanking you.

Best regards,
Naresh

 

[naresh321]

Dear Sir,

http://files.engineering.com/getfil...65-b6fb-6db1dfdfa942&file=20141001_100525.jpg

Please find the photo attached in the above link.

Thanking you.

Regards,
Naresh

 

[Compositepro]

It sure looks to me like you simply have a high resistance connection between the cable and the lug crimped onto it. Once such a connection is overheated, the contact surfaces become oxidized and you must replace the lug and carefully clean the cable end or replace the cable.

 

[ScottyUK]

I agree - you've cleaned up a badly-crimped lug and connected it back to a new isolator. 160-odd amps shouldn't trouble a cable that size. If that joint has run as hot as it appears to have then the copper will be badly oxidised: you will need to cut back to bright copper and splice in a new conductor section. You might need a barrel-type through-joint in addition to the lug. Heatshrink it rather than taping it. Tape falls off given time.

What type of crimp tool and die set did you use? At 300mm² I would be insisting on a hexagonal crimp, not an indenter type. It's hard to tell in the photo. Has someone actually cut the lugs down too? That is bad practice and if not de-burred properly that might actually explain the bad joint.

Make sure the lug and the die set are from the same manufacturer. One of my contractors learned the hard way that using a cheap lug with an expensive crimp tool results in a bad joint and a grumpy engineer. Luckily we found them before the circuit was energised.

 

[itsmoked]

Notice how the cable damage recedes as you get farther from the crimp? That points back to the 'hot spot'.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Classic poor connection heating. The breaker or switch that the cable is connected to may also have internal heat damage. Don't waste time trying to clean the cable. Cut it back to where the copper is clean and shiny, or replace the entire cable.
You may want to measure the voltage drop across each pole of the switch or breaker under load. You should see much less than a volt. If the affected pole has a significantly higher voltage drop than the other two poles, that is a strong indicator of internal heat damage and if so, the device should be changed out also. You meter probes should NOT be on the terminals. The probes should be on the straps coming out of the device so you are only measuring internal voltage drop, and not a possible extra voltage drop across the terminals.
Here are some actual values that I measured in 2008 across a switch with two poles in bad condition and one pole just days away from catastrophic failure. The condition was discovered by walking past the switch and feeling the heat radiating from the switch cover.
A phase 0.148 Volts
B phase 0.1 Volts
C phase 1.89 Volts (failing)


Bill
--------------------
"Why not the best?"
Jimmy Carter