Cable Insulation Level Selection 3

[timohearn]

For power cables that will be utilized on a 480 V, 3 phase, high resistance grounded system; what should be the selected cable insulation level?

 

[jghrist]

Based on 173% of phase-to-phase voltage it would not be adequate.

But the insulation is not between phases, it is from one phase conductor to ground (assuming the cable is laid in a grounded raceway). You have two thicknesses of insulation between phases.

In a high resistance grounded system, the voltage between the conductor and the grounded raceway can reach 173% of the phase-to-ground voltage, so the insulation is required to be rated for 173% of phase-to-ground voltage. I see no basis for requiring the insulation to be rated for 173% of phase-to-phase voltage.

 

[davidbeach]

I see no basis for requiring the insulation to be rated for 173% of phase-to-phase voltage.

Nor do I.

 

[timohearn]

dpc,

My friend; I am satisfied with the technical decision I made to remove the HRG system due to the insulation levels used in the electrical insulation. Thanks for your comments and I would respectfully agree to disagree with your opinion.

I would recommend that you take a look at ICEA S-95-658-1999 "Standard for Nonshielded Power Cables Rated 2000 V or less for the Distribution of Electrical Energy" . This standard specifies the 100%, 133%, and 173% insulation levels for 600 volt cable.

Again I would like to thank everyone for their comments.

Kind regards

 

[7anoter4]

Manufacturer may produce 600 V 100% insulated or 133% insulated.
For example, Okonite produces 600 V 500 MCM with 65 mils thick XLPE insulation [100%] and also 600 V 500 MCM with 95 mils thickness [133%].
If the ground fault clear time will be more than 1 hour then, for 480 V rated, one has to chose 600 V 133% insulation.
If we compare with IEC 60502-1 0.6/1 kV insulation it seems that IEC states for [400-440 V] 480 MCM [240 sqr.mm] 133% [86.6 mils].
Any way, it will not require use of 2.4 KV insulation thickness [140 mils].

 

[rovineye]

timohearn,
Thanks for citing the IEEE-141 with respect to "It will not avoid the sustained 73% overvoltage on two phases during the presence of a ground fault on the third phase.". That is good to know. We have HRG on our high voltage side but still rate at 173% because the earth fault can be continuous, so it's nice see that so clearly spelled out.

I can tell you, in my industry that for sure it IS standard to rate all power cables at 173% pecent voltage. I think you are correct in your approach. It seems rather unusual that voices on this thread ignore the the clear guidance you cite, without some discussion of the technical reasoning. X# of years of installing or specifying a certain criteria in no way validates the approach when dealing with fault scenarios.

 

[dpc]

If the common practice has been rather successfully applied in thousands of installations over many years, that is a reasonable argument that it is an acceptable practice. Not to mention that it meets the national code that has the force of law in every state. If someone has evidence of a higher level of cable insulation failures in HRG 480 V systems compared with solidly-grounded systems, I have not seen it. More insulation is always "better" in the sense that it decreases the probability of an insulation failure. 600 V cable is also used on 600 V systems, for that matter.

There is a big difference between cable insulation concerns when dealing with low voltage systems as compared with medium-voltage systems.

 

[alehman]

My understanding is that low voltage cable is rated based on phase-to-ground voltage. 600V cable can take 600V phase-to-ground.

Medium voltage cables are rated on phase-to-phase voltage. 15kV cable is good for only 8.7kV phase-to-ground. Medium voltage cable is commonly specified with 133% insulation level, which is said to be suitable for non-solidly grounded systems where ground faults will be of short duration. 173% insulation is recommended by cable manufacturers where ground faults are likely to persist.

Maybe someone can confirm this?

 

[7anoter4]

It seems to me that IEEE-141/1993 is referring only to medium-voltage cable in para. 7.2.5 Quote:
"There are three levels of conductor insulation for MEDIUM-VOLTAGE CABLES: 100, 133, and 173% levels. The solidly grounded system permits the use of 100% insulation level. When the fault on the other system will raise the system voltage above normal during the time of the fault, 133% insulation level should be specified if the fault is cleared within one hour. When the fault will remain on the system for an indefinite time, 173% voltage level insulation
should be used (Bridger 1983 [B7]; NEMA WC5-1992 [B33]; GET-3548 [B35])."
Related to low voltage cables- the former para.- states only 1000 A setting for ground fault protection.

 

[7anoter4]

I think that also low-voltage cable rated voltage is VL-L voltage.
Compared with IEC cable insulation thickness for 0.6/1kV, 600 V cable manufactured as per ICEA / NEMA 600 V is the line-to-line voltage.
For instance: for 500 MCM –XLPE insulated- thickness =65 mils [ICEA] VL-L=600 V
For 480 MCM [240 sqr.mm] IEC thickness=86.6 mils VL-L =1000V

 

[rovineye]

Thanks 7anoter4.

I don't understand why the same voltage rise concern doesn't apply at all voltage levels, but as you cite, it is clear it is not intended to apply at low voltage.

It doesn't change my world however, since the regulatory bodies do require 173% at low voltage, which they define as less than 1000 volts. They require it at high voltage as well (>100 volts). We don't have a medium voltage.

 

[norcalnewb]

I also have never seen anything other than 600V cable used on 480V HRG systems in any industrial application, consisting of refineries, chemical plants, and paper mills. I would think the lost reliability, protection of equipment, and protection of personnel would certainly out way the minimal to non-existent risk of cable insulation failure during a fault on such a system.

 

[jghrist]

It doesn't change my world however, since the regulatory bodies do require 173% at low voltage, which they define as less than 1000 volts.

173% of what? Line-to-ground voltage or line-to-line voltage?

 

[rovineye]

jghrist

173% line to line.

 

[timohearn]

7anoter4

I beleive you might have misinterpreted Section 7.2.5 of IEEE 141-1993. It does not state that only medium voltage cables have these insulation levels. It is merely stating that fact in the context of design considerations for grounding systems. If you review the references that are given at the end of the paragraph; you will note that GET 3548 [B35] is actually entitled "System Grounding for Low-voltage Power Systems" which I have a copy of; and it describes that low voltage power cable also have these insulation levels. This publication also references ICEA S-95-658 which specifies these insulation levels and the associated insualtion thickness for low voltage cable.

 

[7anoter4]

Thank you, timohearn. I agree with you. But still I think that for usual US cable installation more than 600 V for low voltage is exaggerate. In Europe where we don't care so much if the ground fault will be cleared in an hour or more the cable is rated 1 Kv [VL-L] .

 

[rovineye]

Thanks again timohearn