Non-shielded/Unscreened Power cables - Voltage rating 1.8/3.0 (3.6) kV - Safety issues

[Oblsss]

Dear all,

In the IEC world, every cable designed for a voltage level greater than 1 kV shall have a metallic layer surrounding the core(s), par. 7.3 of IEC 60502-1.
With regard to safety of personnel, the above requirement ensures that the outer sheath of the cable will be safe to touch.

Ηowever, I have come across a number of cables that operate at the above voltage level, or even higher, without any metallic layer (Non-shielded/Unscreened, unarmored etc).

(NEXANS) WINDLINK® MV unscreened 1.8/3 kV (single core)

NSGAFOEU 1,8/3 kV

Non-Shielded type MV-90 Cable (2.4 kV)

From what I understand, the use of the above cables may be allowed depending on the grounding of the electrical system, e.g. in an ungrounded system.
Could you give me examples of systems that allow the use of such cables as well as national or international standards that specify these systems and related equipment, e.g. cables?

My main question concerns the first cable type, see NEXANS WINDLINK_MV cable, which appears to be used in a wind turbine tower. I have serious doubts that the wind turbine tower is an ungrounded system, although I have no actual knowledge of this specific installation.

Any input is welcomed!

Thank you in advance!

George

 

[7anoter4]

According to IEC 60502-1 4. Voltage designations and materials
4.1 Rated voltages
The rated voltages U0/U (Um) of the cables considered in this standard are 0,6/1 (1,2) kV and
1,8/3 (3,6) kV.
NOTE 1 The voltages given above are the correct designations although in some countries other designations are used, e.g. 1,7/3 kV or 1,9/3,3 kV instead of 1,8/3 kV.
Cable 2.4 kV it is not an IEC standard cable but U.S.A.
The U.S.A. standard for safety N.E.C.2017 Art. 310.10 Uses Permitted.
(E) Shielding. Nonshielded, ozone-resistant insulated conductors with a maximum phase-to-phase voltage of 5000 volts shall be permitted in Type MC cables in industrial establishments where the conditions of maintenance and supervision ensure that only qualified persons service the installation. For other establishments, solid dielectric insulated conductors operated above 2000 volts in permanent installations shall have ozone resistant insulation and shall be shielded.

 

[Oblsss]

Hello 7anoter4,

Thank you for the reply!

Regarding type MC cables, in the IEC world, these would probably be considered screened/shielded cables according to par. 9.1 of IEC 60502-1:

9 Metallic screen
9.1 Construction
The metallic screen shall consist of one or more tapes, or a braid, or a concentric layer of wires
or a combination of wires and tape(s).
It may also be a sheath or, in the case of a collective screen, an armour which complies
with 9.2.
When choosing the material of the screen, special consideration shall be given to the
possibility of corrosion, not only for mechanical safety but also for electrical safety.

I have in mind that there are differences between USA & IEC specifications and that is why my main question concerns the NEXANS WINDLINK_MV cable which states IEC 60502-1 as the design standard ...

NEXANS WINDLINK_MV

Any thoughts on this one?

Best Regards,

George

 

[7anoter4]

Without armor MC cable is close to MV cable
However, MV cable nonshielded 2001-5000 V it seems it is forbidden now.
NEC 2011 still allowed use of MV cable non shielded up to 2.4 kV. From 2014 only MC cable is permitted-without shield but armored [more or less as per IEC 60502-1 ART.7.3]

 

[Oblsss]

@ 7anoter4

When voltage values like "2001-5000 V" or "2.4 kV" are mentioned in the NEC, should I consider them as phase-phase or phase to earth voltage values?
I did a quick search and found that, according to NEC,

[highlight #FCE94F]"110.4 Voltages.
Throughout this Code, the voltage considered
shall be that at which the circuit operates. The voltage rating of
electrical equipment shall not be less than the nominal voltage
of a circuit to which it is connected."[/highlight]

Thank you in advance!

George

 

[dpc]

In the US, non-shielded MV cable is still allowed up to 5000 V in industrial facilities. At higher voltages non-shielded conductors can still be used by manufacturers inside manufactured equipment such as switchgear, for motor leads, etc.

 

[Kiribanda]

These unscreened flexible cables are mainly used in situations where flexibilty and smoke free is required.
I have come across several WT installations where,
1) DLO (diesel locomotive cable) 2kV cables running from the nacelle to ground level,
where gen. step-up pad-mounted transformer (690V-34.5kV) is located outside the tower base
2) Draka WindFlex Power 20/35 kV insulated Draka Windflex LSOH cables running from the nacelle to ground level,
where gen. step-up dry type transformer (690V-34.5kV) is located inside the nacelle.
3) IEC rated, EPR insulated, PCP outer sheathed H07RN-F flexible cables from the nacelle to ground level,
where gen. step-up dry type transformer (690V-34.5kV) is located inside the nacelle.
But in all installations, since the metallic screen is not in the main cable, a seperate ground return cable
(typically 4/0AWG) is laid along with these power cables.
Therefore, where flexibility and also low smoke (confinbed spaces like wind towers) is required
these non-screened cables can be sued.

 

[Oblsss]

First of all, thank you both for your answers!

@ Kiribanda

Nevertheless the problem remains, ie the separate ground return cable cannot contain the electric field within the cable core and the cable sheath shall not be safe to touch with the cable energized.
Please recall that I am talking about cables rated at 3 kV phase-phase voltage.

@dpc

You are probably referring to the NEC paragraph below:

(E) Shielding. Nonshielded, ozone-resistant insulated conductors
with a maximum phase-to-phase voltage of 5000 volts shall
be permitted in Type MC cables in industrial establishments
where the conditions of maintenance and supervision ensure
that only qualified persons service the installation. For other
establishments, solid dielectric insulated conductors operated
above 2000 volts in permanent installations shall have ozoneresistant
insulation and shall be shielded.

In the IEC world, type MC cables would probably be considered screened/shielded cables according to par. 9.1 of IEC 60502-1:

9 Metallic screen
9.1 Construction
The metallic screen shall consist of one or more tapes, or a braid, or a concentric layer of wires
or a combination of wires and tape(s).
It may also be a sheath or, in the case of a collective screen, an armour which complies
with 9.2.

Thank you in advance!

George

 

[Kiribanda]

i donot think your concern is valid. The cable is having adequate thickness of insulation
to insulate it for 3kV to ground. Therefore, if the cable is operating within its limits
one should be able to touch it from outside. It is similar to a situation where we can touch
the metallic enclosure of a generator while it is operating at 13.8kV.
The metallic screen is making an equipotential surface for the cable so that the insulation is
not subjected to unbalanced electromagnetic stresses. Also the metallic screen is used for
taking the ground fault current back to the source.

 

[Oblsss]

@ Kiribanda

Thank you for your answer.

Nevertheless, I will have to disagree with you.


[highlight #FCE94F]The metallic screen is making an equipotential surface for the cable so that the insulation is
not subjected to unbalanced electromagnetic stresses[/highlight] and contain the electric field within the cable core.

Please read the following article, especially the "Electrical Stress Explained" part:

Link

As far as the voltage level is concerned, I agree that 1.8 kV to ground (3kV phase to phase) seems like no problem.
However, IEC 60502-1 standard prohibits the construction of cables for a voltage level greater than 1 kV without a metallic layer surrounding the core(s), par. 7.3 of IEC 60502-1.

The DLO cables you mentioned above are designed according to which standard?

Thank you,

George

 

[Kiribanda]

1) The erratic electrical stress to the ground plane shown in the link is true but it is very dominant
at higher voltages. But still one can touch the cable. At 1.8kV the effect is very low. That is why
you can find that screening is available at higher voltages. Also these unscreened cables had been
used for a long time and also continue to be used in the years to come without any trouble.
2) DLO cables are made for CSA C22.2-38, UL File # E90494,ICEA S-95-658/NEMA WC70 etc. etc.
Any data sheet will show the list of compliance stds of DLO cables.