The cable burnt out by super heating caused by:
-insulation temperature was more elevated than permissible for a long period .The cable was wrong elected-less conductor cross section than it is required.
-contact failure -between conductor and lug, between lug and terminal [motor or breaker/switch].
Or:
-insulation failure due to a super voltage [produced by a switch disconnecting-mainly if it is a vacuum breaker].
Good possibilities above. Some other thoughts:
Vibration.
Voltage stress concentration as lead passes by a corner
Rodents?
Bad connection. Possibly there was a splice in the lead cable from motor shop or cable manufactuerer
gremlins: thread237-241996
What voltage? Describe location of failure. Preferably post some pictures.
Motor cables are protected thermally by the overloads in the motor starter (or by the VFD or soft start if applicable). Check to ensure that the overloads are not set higher than the motor FLA. I would also check the cable size as noted above.
Add: Water in the conduit, cable insulation not rated for immersion, i.e. XHH rating, but not XHHW. (been there, done that, got the T-shirt and the hat)
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376
One other check is the ambient temperature and the cables rated ambient temperature. If the ambient temperature is higher than the cable rating, the insulation can quickly become ineffective.
A bad (high resistance) connection will develop a lot of heat (I2R) which will be conducted by the conductors. This heat and resulting temperature rise may cause cable failure, it may cause a short as the plastic insulation softens and the conductor pushes through the insulation to contact a grounded surface or the cable may be overheated to the point of failure. The heat mat also conduct to the device terminals and cause failure. Another possibility is improper entry into a ferrous enclosure. All phases must enter a ferrous enclosure through the same opening. I remember an installation where A phase and half of B phase (two parallel conductors per phase) entered one opening and half of B phase and C phase entered through another opening. The heat developed in the conduit and in the enclosure was conducted to the conductors. The heat was then conducted to the disconnect terminals and a 400 amp disconnect switch was destroyed by heat corrosion. The customer told me that the switch had been repaired or replaced three times due to heat corrosion failures. The load on the 400 amp switch peaked at about 200 Amps. If the installation is properly sized then a loose or bad connection is by far the most probable cause. It the installation is substandard, then the cause may be any of the suggestions by other posters.
Bill
--------------------
"Why not the best?"
Jimmy Carter
