Meaning of 133% Cable Insulation Level

[veritas]

The General Cable catalogue specifies cables with a 133% and 100% insulation level. My understanding is that 100% cables are for solidly grounded systems where the neutral voltage does no appreciable rise with ground faults. 133% for high resistance grounded systems where there can be full neutral offset resulting in healthy phase voltages to ground being full line voltage with ground faults. This equates to about a sqrt(3) rise or 1.73.

So where does the 133% comes from?

Does this mean 133% cables have a thicker insulation?

Finally, I am doing some cable reactance calcs using 3 singlephase cables in various installation configurations. I've always assumed that the magnetic permeability of cable insulation material is the same as that of free air. Condcutor shield is made of copper and as far as I am aware there are no ferrous material between conductor centres. Is that a fair assumption?

Thanks.

 

[leafmelly]

It is related to fault time. The insulation level (133% or 100%) mean running for long time under the correspongding votage. For the high resistance grounded systems, if the fault can recover in a short time like 1 hour, 133% insulation level is enough(as proved by experiment). But if the fault last a long time like morte than 1 hours( 8 hours in some country), then 173% insulation is necessary. The difference between 133% and 100% insulation is mainly the thickness of the XLPE material, I think.

 

[veritas]

Ok, so what you are saying is that with increased insulation % higher overvoltages can be tolerated.

What I find interesting though is that there are no 173% cables in the General Cables catalogue. Only 100% and 133% ones. So I'm curious as to what exactly the 100% stands for. How much OV for how much time?

Thanks.

 

[7anoter4]

I agree with leafmelly.
EPRI-EL-5036-V4[extract from ICEA S-68-516/1983] stated 3 insulation levels according to the ground fault clearing time: 100% for less than 1 minute,133% for less than 1 h and 173% for permanent earth fault.
UL 1072/1995 Table 13.1 indicate [for medium voltage cables-5000-35000 V] 100% insulation level for 1 min. maximum 133% for 1 hr. maximum.

 

[cuky2000]

Larger insulation to meet 173% is often achieve with larger rated voltage cable.
See the enclosed table below.

 

[veritas]

7anoter4 - so what you and leafmelly are saying is that the 100%, 133% and 173% are all applicable to a high resistance earthed system. The selection criteria is thus purely based on fault clearance time. My EF clearance times are < 1 min so 100% should be ok. However, I have specified 133% cables so even better. I don't think I would ever need 173% cables unless the system is ungrounded and designed to run that way for an appreciable time for the sake of continuity of supply. I've only ever seen this done in Europe.

Furthermore, if my system is solidly earthed, then I could use 100% cable irrespective of fault clearance time.

Above correct?

cuky2000 - very useful table. Where is it from?

Thanks.

 

[veritas]

cuky2000 - what type of cable insulation is referred to here? XLPE, EPR, etc.? or is there not much difference between them?

 

[cuky2000]

The table is from STANDARD FOR CONCENTRIC NEUTRAL CABLES RATED 5,000 - 46,000 VOLTS Publication # ICEA S-94-649-2000

 

[cuky2000]

The ICEA Standard is applicable to both EPR and XLPE. The SC temperature for both is 250oC. Therefore the insulation performance for EPR & XLPE are close equivalent.