In the world of electrical engineering, the success of a project is often determined before the first switch is flipped. Proper preparation, inspection, and handling of cables prevent costly failures down the line. Following our previous Q&A sessions, here are the next 10 essential insights focusing on pre-installation checks, site requirements, and tool safety.
11. What checks are required before laying a cable?
Before any cable hits the trench or tray, ensure these four boxes are ticked:
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Support Structures: All brackets and supports must be installed completely and have intact paint/coatings.
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Specifications: Verify that the cable model, voltage rating, and specifications strictly match the design documents.
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Insulation Integrity:
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General insulation must be sound.
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If there is doubt regarding the seal of oil-paper cables, perform a moisture test.
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Direct-buried and submarine cables must pass DC withstand voltage tests.
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Oil samples from oil-filled cables must be tested and qualified.
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Oil Pressure: For oil-filled cables, the oil pressure should preferably not be lower than 1.47 MPa.
12. How do you quickly check if oil-paper insulation paper is damp?
If you suspect moisture in the insulation paper, use one of these simple field tests:
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Ignition Test: Ignite a small piece of the insulation paper. If you hear a "hissing" sound or see white foam/bubbles, moisture is present.
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Hot Oil Test: Immerse the paper in cable oil heated to approximately 150°C. Absence of "hissing" or white foam indicates the paper is dry.
13. What information belongs on a cable tag, and what are the requirements?
Tags are vital for future maintenance. They must include:
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Design Number of the cable line
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Cable Model
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Specifications
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Start and End Points
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Sequential Numbers (Required for cables in parallel circuits)
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Requirement: The text must be clear, legible, and resistant to fading or peeling.
14. Where should location markers for direct-buried cables be placed?
To ensure accurate locating in the future, place markers at:
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Both ends of the cable.
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Every 50–100 meters along straight sections.
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At all cable joints.
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At bending points where the direction changes.
15. How do you remove moisture from insulating tapes (e.g., Yellow Silk Tape, Fiberglass Tape) before use?
Moisture is the enemy of insulation. Use one of these two methods:
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Constant Temperature Drying: Roll the tape into small rolls (25–30mm diameter). Bake in a constant temperature oven at 110–120°C for 4–5 hours. Cool in a dry environment and store in a sealed container.
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Oil Immersion Method: Place the tape rolls into cable oil heated to 120–130°C, keeping them at least 30mm off the bottom of the pot. Remove only after the oil surface stops producing foam/bubbles. Store submerged in sealed containers filled with cable oil.
16. What is the purpose of the cable outer sheath (Jacket)?
The outer sheath acts as the first line of defense. Its job is to protect the inner sheath from mechanical damage and chemical corrosion, while significantly enhancing the cable's overall mechanical strength.
17. What are the requirements for outdoor cable trenches?
Outdoor trenches face environmental exposure. Ensure:
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The top of the trench is slightly elevated above the surrounding ground level.
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It is covered with concrete slabs/lids.
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Cables are laid flat on supports/rollers.
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There is an effective drainage system to prevent water accumulation.
18. What is the purpose of the cable inner sheath?
The inner sheath (often Lead or Aluminum) serves as a moisture barrier. It prevents the insulation layer from contacting water, air, or other contaminants, stopping the insulation from becoming damp. It also provides a secondary layer of mechanical protection for the core insulation.
19. What safety precautions apply when using side-cutting pliers (Wire Cutters)?
Improper tool use leads to injuries and damaged equipment:
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Check Insulation: Always inspect the insulating handles for cracks or damage before use.
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Don't Overload: Never cut wires thicker than the plier's specified capacity.
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No Misuse: Do not use pliers as a hammer; this will damage the tool and compromise its insulation.
20. What is Dielectric Strength (Insulation Strength)?
Dielectric Strength is the maximum electric field intensity that an insulating material can withstand before it breaks down (arcs or punctures). Once this limit is reached, the material loses its insulating properties and becomes conductive.
Pro Tip for Site Managers:
Always keep a logbook for your DC withstand voltage tests and insulation resistance measurements. In the event of a future fault, this historical data is invaluable for determining whether a failure was due to installation error or external damage.
Which of these checks do you find is most often overlooked on job sites? Let us know in the comments!
