Insulation Damage

[osotogari]

I am looking for some clear(ish) guidance on acceptable levels of insulation damage on a few types of conductor; XHHW-2, THHN-2 and 2kV PV Wire. I inspect electrical systems and come across varying levels of conductor insulation damage. This is everything from very minor cosmetic nicks all the way to exposed conductor. The area I am continually having issue with is understanding the middle ground and where visible damage is too much. I have see conductor max out an insulation resistance test, yet have serious visible damage albeit without exposing metal.

I find myself in a position of horse trading acceptable levels of damage with electricians and default to a 'no damage is acceptable' position. I am an Owner's Rep, and need to provide quality, but I don't want to be punitive and unreasonable.

My company specifies XHHW-2 frequently. An electrician pointed out that it has a 'soft' insulation that shows minor impacts readily and then prefer alternatives such as THHN-2.

Recently when discussing problem conductors, an electrician pointed me to UL44. There are several relevant sections, but for do not answer the question fully to my understanding.
UL44.
14.1The insulation shall not have any defects visible with normal or corrected vision without magnification.(I take this to mean from the manufacture)
14.2 Any repair or joint in the insulation shall be made in a workmanlike manner resulting in all parts affected in the process complying with the same electrical tests as the remainder of the insulation. The thickness of insulation at the repaired part or joint shall comply with the requirements in 15.1.1.
15.1.1 states that there are average and minimum insulation thicknesses per Tables 15.1-15.6

So I have a couple questions:
What are Standards speak to insulation damage?
What damage is acceptable if any?
If a repair is acceptable, how many repairs to a single conductor are acceptable?
Scenario1: Conductor is pulled, IRT is performed and the value exceeds the minimum or maxes the tester. Damage beyond reasonable cosmetic is noted. Is the conductor acceptable?
Scenario2: Conductor is pulled, IRT is performed and the value exceeds the minimum or maxes the tester. Damage beyond reasonable cosmetic is noted, but the minimum insulation value of Table 15.3 is present. Is the conductor acceptable?

Many thanks.

 

[sameguy]

https://www.3m.com/3M/en_US/electrical-construction-maintenance-us/resources/vinyl-selection-guide/

Start here, as a rep. you need to get guidance from your client unless you are hired to make that decision.

 

[osotogari]

https://www.3m.com/3M/en_US/electrical-construction-maintenance-us/resources/vinyl-selection-guide/

Start here, as a rep. you need to get guidance from your client unless you are hired to make that decision.

Thanks sameguy. When I say owners rep, I work for the owner. I will get a look at that 3M guide, but the question about determination of a good conductor remains.
regards,
Brandon

 

[paulengr]

Insulation resistance doesn’t show a big fnumber change unless the surface is moist or contaminated. Cracks will often show up as stair steps in the polarization. So if you run the insulation resistance all the way to ten minutes for a PI test while graphing it you will see stair steps instead of a smooth curve and quite often a very high PI like 6-10. At least that’s how we detect crazing in motors.

 

[Jraef]

XHHW is a superior insulation to THHN if there is any sort of PWM power running through it, as might be the case in a PV system. THHN uses PVC insulation that might contain microscopic bubbles in it, which become weak points for corona discharge that can take place as a result of the PWM and reflected wave phenomena. XHHW, because it is essentially a heat-shrink tube put over the conductor, does not have this issue. Hence the 2kV rating, as opposed to 600V for THHN. So don't let that electrician sway you against XHHW.

Along the same lines though, compromise of the insulation thickness can also be a problem for corona discharge issues, so should not be ignored. Heat shrink repair sleeves are what people use in the MV/HV world, that would work for LV systems as well.

 

[synchro]

Also XHHW has a somewhat larger insulation thickness than THHN, particularly in the smaller wire sizes. Therefore, if anything, a given amount of missing insulation thickness should have a smaller impact on voltage breakdown levels on XHHW vs. that with THHN.

 

[paulengr]

XHHW is a superior insulation to THHN if there is any sort of PWM power running through it, as might be the case in a PV system. THHN uses PVC insulation that might contain microscopic bubbles in it, which become weak points for corona discharge that can take place as a result of the PWM and reflected wave phenomena. XHHW, because it is essentially a heat-shrink tube put over the conductor, does not have this issue. Hence the 2kV rating, as opposed to 600V for THHN. So don't let that electrician sway you against XHHW.

Along the same lines though, compromise of the insulation thickness can also be a problem for corona discharge issues, so should not be ignored. Heat shrink repair sleeves are what people use in the MV/HV world, that would work for LV systems as well.

Again so if the motor arcs at 1750 V (about the highest rating available) and THHN is tested at 2200 V and doesn’t arc until 2850
V, while XHHW holds out up to 3200 V, it doesn’t matter. The motor arcs first and acts like a surge arresters, “protecting” the cable!

ALL polymers have voids. It’s inherent in the process. In fact the whole reason that DC hi potting has been frowned upon is specifically because of problems with electrical treeing (air voids turning into tracking damage) when service aged XLPE cable is hi potted. Just google “partial discharge” and say IEEE standard 400 (the hi pot standard). There are tons of pictures and examples of this, particularly with XLPE. XHHW is just a type of XLPE and that’s what the X stands for.

In the mean time XHHW has been improved (TR-XLPE) producing XHHW-2 with 90 C rating while THHN has also been improved to THHN-2 but I have no data on their breakdown voltages.

The service rating is irrelevant. Six pulse VFDs take advantage of the fact that “600
V” continuous rated cable is surge rated much higher.

 

[GoldDigger]

Again so if the motor arcs at 1750 V (about the highest rating available) and THHN is tested at 2200 V and doesn’t arc until 2850
V, while XHHW holds out up to 3200 V, it doesn’t matter. The motor arcs first and acts like a surge arresters, “protecting” the cable!

One of the characteristics of transmission line behavior for the high frequency components of fast step waveforms such as those present at VFD outputs, and to a lesser extent in other ECM applications, is that the voltage at points along the transmission line can easily approach twice the nominal pulse voltage produced at the VFD output and seen at the motor.
In that circumstance the motor winding breakdown will not necessarily be the upper limit of the voltage on the wire at intermediate points.

On a long VFD to motor run, wire insulation breakdown can appear as multiple localized failure points at half-wave spacing along the run.

 

[paulengr]

One of the characteristics of transmission line behavior for the high frequency components of fast step waveforms such as those present at VFD outputs, and to a lesser extent in other ECM applications, is that the voltage at points along the transmission line can easily approach twice the nominal pulse voltage produced at the VFD output and seen at the motor.
In that circumstance the motor winding breakdown will not necessarily be the upper limit of the voltage on the wire at intermediate points.

On a long VFD to motor run, wire insulation breakdown can appear as multiple localized failure points at half-wave spacing along the run.

This is basic transmission line theory. We have the normal forward wave. It travels in the cable unchanged. At the motor terminals the motor impedance is very different from the normal roughly 100 ohm characteristic cable impedance. This causes a reflection. Part of the signal is absorbed by the motor while the rest is reflected back towards the VFD. It bounces and returns again. Each time some energy returns in a reflected waves, some passing to the motor. Eventually it dies down in a few milliseconds. While passing through the cable the pulse is unchains

Below 50 feet voltage seen at the motor is the same...electrically “short”. From 50-250 feet it grows to 2x FLA. Above 250
Feet another pulse starts before the first one has totally cleared. This allows pulse heights to double again to 4X at around 500-600 feet.

But the first point on the reflection is at the motor. Burn spots are just where the reflected and.normal waves meet. It can’t exceed motor terminal.voltage. The motor sees the peak.

That’s not to say that the motor doesn’t get replaced and by the time anyone checks the cable is in a sad shape. The peaks are the same as the motor though not less.

 

[GoldDigger]

You are absolutely right that if the motor terminals are nearly a current zero (high impedance compared to the transmission line impedance) the voltage will be a maximum, approaching twice the nominal voltage in the absence of source end reflections complicating things.
Thanks for the correction.

Sent from my Pixel 4a using Tapatalk