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Cable Insulation Level Selection
3

Cable Insulation Level Selection

Cable Insulation Level Selection

(OP)
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?  

RE: Cable Insulation Level Selection

600V.

RE: Cable Insulation Level Selection

(OP)
David,

In the Standard ICEA S-95-658-1999 "Standard for Nonshielded Power Cables Rated 2000 V or less for the Distribution of Electrical Energy" Section 3.3 it defines the 173% insulation level for cables applied on a system where the time required to deenergize a grounded section is more than one hour.  It states that the phase to phase voltage should be multiplied by 1.73 to determine the voltage rating which for a 480 volt system would be 831 volts.  This requirement is also described in IEEE 141-1993 in the Chapter on "Cable Systems" Section 12.4.1. Does this not apply to 480 volt High Resistance Grounded Systems?

RE: Cable Insulation Level Selection

As far as I know there is no NEC requirement for insulation more than 600V for 480V systems, even ungrounded.  I can't recall ever seeing anything other than 100% insulation (and always only implied, never stated) for 600V insulation.  above 600V the next step is the 2kV class unless you are looking at very specialized conductor insulation.

RE: Cable Insulation Level Selection

(OP)
David,

The NEC doesn't adrress a lot of issues in industrial applications.  I am concerned about the inadequate voltage rating for the power cables used for this type of system when a section is grounded and the two remaining phases have voltages exceeding 800 volts.  I think this is a safety issue for people working on these systems.  I recently refused to put my PE stamp on electrical drawings because of this issue and removed the high resistance ground from the incoming service in order to approve the drawings.  Maybe I went overboard.  But I think the NEC should address the issue.

Regards,
Tim O'Hearn

RE: Cable Insulation Level Selection

Maybe I am missing something here.  During a single-phase ground fault on a balanced HRG system, a neutral shift exists due to the voltage drop across the neutral resistor.  The voltage-to-ground of the 2 remaining unfaulted phases to ground is in theory up to the line-to-line voltage (i.e. 480 V line-to-ground vs. 277 V line-to-ground during normal operation).

RE: Cable Insulation Level Selection

According to IEC Standards, for 400 V, for instance the cable will be 0.6/1 kV [something like above ICEA standard]. Usually the insulation thickness of European cable is less than for US cable [of the same voltage level].

RE: Cable Insulation Level Selection

Tim, 600 V insulation is adequate for 480V systems.  Why would a cable rated for up to 600 volts not be suitable for 480 L-L and 277 L-G?  Even during a fault, this insulation is sufficient as the L-L and L-G are still only 480 volts.

Read NEC 110.4 - "The rating of electrical equipment shall not be less than the nominal voltage to which it is connected".  Thus, your cables rated at 600 volts are sufficient for a 480V system.   

------------------------------------------------------------------------
If it is broken, fix it.  If it isn't broken, I'll soon fix that.

RE: Cable Insulation Level Selection

TurbineGen

Unless the 480 V HRG network allows a ground fault to stay more than one hour we donot need 173% insulation.(We need 133% if it is less than one hour) During a ground fault it is true that the line to ground voltages of the healthy phases will be equal to line to line voltages. We should remember that these are steady state voltages.

But this 173% insulation is required to look after cable from transient overvoltages till the time the the ground fault is located and isolated.

As 7anoter4 pointed out this question does not arise in LV IEC cables because we use 0.6/ 1 kV cables for LV systems.

The similar thing applies for HV IEC cables. Depending on the time duration for ground fault clearence, we should specify whether the cable belongs to Category A, B or C.

 

RE: Cable Insulation Level Selection

0.6/1kV is (U0/U) U0 = The rated voltage between conductors and earth metallic screen;
U = The rated voltage between conductors, for which the cable designed.
The 1.73 formula is legit on ungrounded systems. During a fault voltage spikes  are amplified by short duration capacitance to ground. In that case on 480 volt systems I use 7anoter4's rating of 0.6/1kV cable.

 I would venture to guess that HRG would alleviate the need for that however.

 I have no references however. Being a marine world we just kind of wing it.  

RE: Cable Insulation Level Selection

(OP)
In IEEE 141-1993 Chapter 7 "Grounding" Section 7.2 "System Grounding" it describes the following:

"High-resistance grounding will limit to a moderate value the transient overvoltages created by an inductive reactance connection from one phase to ground or from an intermittent-contact phase-to-ground short circuit.  It will not avoid the sustained 73% overvoltage on two phases during the presence of a ground fault on the third phase."

I beleive this is the basis of the requirement for 173% insulation rating if the ground fault on a high-resistance grounded system will remain for more than 1 hour.

RE: Cable Insulation Level Selection

Speaking strictly from experience, I've NEVER seen anything other than 600 V cable for any 480 V system, including old 480 V ungrounded delta systems.  And I've seen quite a few.  

I don't know what to make of the ICEA or IEEE 141 statements, but most 480 V ground faults on HRG systems will exist for MUCH longer than one hour.  More like days in many cases.

Transient overvoltages due to repetitive arcing fault re-strikes should be much lower on a high-resistance grounded system as opposed to an ungrounded system.  That is the whole point of the HRG system.   

RE: Cable Insulation Level Selection

Normal line-to-ground voltage on cable:  277V
Line-to-ground voltage during grd fault:  1.73*277 = 480
Required cable rating = max line-to-ground voltage times 1.73 = 1.1*277*480 = 527V  assuming a 10% overvoltage <600V.
  

RE: Cable Insulation Level Selection

(OP)
jghrist,

You neglected the phase relationship between the voltages.  In addition this is an unbalanced ground fault on a three phase system which need to be handled by symmetrical components.

RE: Cable Insulation Level Selection

If cable insulation is sufficient for the voltage of the enclosed conductor to ground, the insulation of two cables will under all conditions be sufficient for the voltage between two phases.  Your 480V system, not matter how grounded, will not see significantly more that 480V between conductors regardless of fault type.  You can move the whole voltage triangle around relative to ground, and you can collapse the triangle, but you can't materially expand the triangle.  Other than an arcing ground fault on an ungrounded (capacitively grounded) system, you can't get to 600V to ground on a 480V system.  The arcing ground fault can cause severe overvoltages that are extinguished when something flashes over and no cable system is intended to protect against those overvoltages.

Why do you continue to want to fight years of accepted practice?  600V insulation is suitable for all 480V systems, no further testings/ratings needed.

RE: Cable Insulation Level Selection

I like dpc have never seen other than 600 volt insulated conductors used on 480 volts systems here in the US.  I also agree that for an ungrounded system it is likely that the ground fault will exist for days if not weeks before it is corrected.

RE: Cable Insulation Level Selection

(OP)
davidbeach,

I don't agree with your statement that this is the accepted practice.  You seem to ignore what is in the IEEE Recommended Practices and ICEA industry recognized standards which are, in my opinion, the accepted practice.

There is also a publication by GE on electrical system grounding written in December 1972 with gives the same recommendations.  I have also discussed this with a power cable manufacturer which also agreed with the need to specify 173% insulation if a high-resistance ground system is utilized on a low-voltage power distribution system.

resqcapt19,

Just because you never seen other than 600 volt insulated conductors used on a 480 volt system doesn't justify not specifying it when required for a high-restance grounded system 480 volt system.



 

RE: Cable Insulation Level Selection

timohearn,

My friend, you are swimming upstream against the collective knowledge of engineers with about 100 years of combined experience in the field right here.  I can sympathize with trying to justify your earlier decision, but I don't think you are going to make much headway.  When you've dug yourself into a hole, the first rule is to quit digging.

I've looked at the Red Book comments, and I think it was really written with respect to medium-voltage cable, although I admit it doesn't say so explicitly.  

But just because something is written in a "Recommended Practice" doesn't make it accepted practice anyway.  

600 V insulation fully complies with the NEC as far as I can see and that is all I have ever seen used in the past 37 years.  Trying to justify something in excess of that based on consensus standards and recommended practices is a tough road, unless you are buying the cable.
 
 

RE: Cable Insulation Level Selection

In what way is 600V insulation not in compliance with the 173% requirement?

RE: Cable Insulation Level Selection

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

RE: Cable Insulation Level Selection

Quote:

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.
 

RE: Cable Insulation Level Selection

Quote (jghrist):

I see no basis for requiring the insulation to be rated for 173% of phase-to-phase voltage.
Nor do I.

RE: Cable Insulation Level Selection

(OP)
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
 

RE: Cable Insulation Level Selection

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].
 

RE: Cable Insulation Level Selection

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.   

RE: Cable Insulation Level Selection

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.





    

 

RE: Cable Insulation Level Selection

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?

RE: Cable Insulation Level Selection

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.
 

RE: Cable Insulation Level Selection

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
 
 

RE: Cable Insulation Level Selection

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.

RE: Cable Insulation Level Selection

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.

RE: Cable Insulation Level Selection

Quote:

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?
 

RE: Cable Insulation Level Selection

jghrist

173% line to line.

RE: Cable Insulation Level Selection

(OP)
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.

RE: Cable Insulation Level Selection

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] .

RE: Cable Insulation Level Selection

Thanks again timohearn

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