Power Cable Insulation Rating Question 1

[swz202]

We have a 600V ungrounded delta system. Is power cable with 600V insulation rating enough for this appilcation? My thought is, under a phase-ground fault condition, the other two conductors will subject to overvoltage.....

 

[alehman]

If the system is 600V phase-to-phase and one phase becomes grounded, the other two will be at 600V relative to ground. The maximum voltage phase to ground is 600, regardless of how the system is grounded (unless you have an arcing ground fault, but that's a whole other subject).

 

[tinfoil]

If you are spec'ing rather than using up old stock, I would go to 1000V rather than 600V cable.

As alehman pointed out, you are going to have your two ungrounded phases up to your ph-ph voltage on a l-gnd fault, and your system may be NOMINALLY 600V, but I suspect that you will often have actual OPERATING voltages 5% higher than your nominal.

Loading up a cable to (and perhaps beyond) 100% of its rating (and L-gnd faults are the most common kind) is not a good engineering practice

 

[tulum]

600v cable is fine for the application.

600V cable is different than MV cable...

In manufacturing all cables you must take into consideration both the mechanical durability of the cable and the insulation/dielectric level required.

However, in 600V cable to achieve the mechanical durability the dielectric is "layed on" so thick that when it is check for insulation at 600V it is actually well above 100%.

It is standard practice to use 600V cable on this type of system.

Regards,
TULUM

 

[swz202]

Got the idea. Thank you all.

 

[swz202]

Another thought on this issue, we'll use 1000 volts rating cables for this application.

Following is from IEEE Red Book:

The selection of the cable insulation (voltage) rating is made on the basis of the phase-to phase voltage of the system in which the cable is to be applied, whether the system is grounded or ungrounded, and the time in which a ground fault on the system is cleared by protective equipment. It is possible to operate cables on ungrounded systems for long periods of time with one phase grounded due to a fault. This results in line-to-line voltage stress
across the insulation of the two ungrounded conductors. Therefore, such cable must have greater insulation thickness than a cable used on a grounded system where it is impossible to impose full line-to-line potential on the other two unfaulted phases for an extended period of
time.
Therefore, 100% insulation level cables are applicable to grounded systems provided the protection devices will clear ground faults within 1 min. On ungrounded systems where the clearing time of the 100% level category cannot be met, and yet there is adequate assurance that the faulted section will be cleared within 1 h, 133% insulation level cables are required. On systems where the time required to de-energize a grounded section is indeÞnite, a 173% insulation level is used.