graphite coating of power cables 4 - Eng-Tips

Graphite is generally used as a conductive coating to enable a sheath test in the field or on the drum, where the cable may not be in contact with an earth medium (eg in conduits, or direct laid in non conductive trenches, or maybe sitting in insulated cable racks in tunnels). It gives it a continuous potential for the sheath test so the whole of the sheath insulation can be tested for damage during installation.
Some graphite will be scraped off during installation, however the user must scrape down into the HDPE or other sheath material to completely remove the conductive graphite. This allows the cable owner to ensure the cable was pulled and laid properly before energising.
Plus it gives the jointers and cable pullers something extra to complain about, when they come home covered in black graphite.
Firstly 144x - sorry, reading back through it I realise that my thoughts were not in line with my typing. I was just trying to make the point that even if you scrape some of the graphite off the cable whilst laying, you will still have quite a bit of graphite embedded in the sheath, making a sheath test still valid and a good indication of damage whilst laying. If you terminate the cable with a gland isolated bushing (for protective purposes), you will physically have to scrape a millimetre or two (depth) of the sheath away (all around the cable) to get rid of the graphite (hence the conducting path), otherwise you will be shorting out the isolation.

The semicon layer that I think paged and vector9 are describing is for stress control and corona minimisation which can be caused by the non circular nature of copper stranded conductors. This is described as the conductor screen, and is usually extruded over the conductors (in extruded cable technology) to provide a circular equipotential surface. The same applies to the insulation screen, where a semicon layer is applied over the bulk insulation (under the earth screen), again to provide a stress free surface. The graphite goes on the outside of the cable, outside the metallic earth screen, hence it is outside the insulation electric field.

Hope this helps

Step-by-step instructions

To make liquid wires, you need to mix graphite with liquid glass. It's actually the graphite that conducts electricity—the liquid glass simply helps keep the graphite powder together in a thick paste.

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You can use the bottle nozzle to apply the "wire."

Attach the longer "+" leg of an LED to your battery holder. You can also use the batteries you make in the other experiment from this set.

Connect the black "-" crocodile clip to your conductive drawing and touch your drawing elsewhere with the bent "-" leg of your LED.

As you can see, your drawing conducts electricity quite well, but the longer and thinner the path connecting the "-" crocodile clip with the "-" LED leg, the worse the connection is. That's because the liquid wire has quite high electrical resistance. Regular metal wires also have electrical resistance, but it's much, much lower. If your wire were made of copper, you'd need a really long piece of it to dim your LED as much as a mere couple of inches of the liquid wire does.

Scientific description

How can a liquid wire conduct electricity?

Every electronic household appliance relies on an electrical system: to work, it needs an electric current flowing through it. For current to be able to flow through such a device, it needs a good conductor – a material that transmits electricity well. These conductors are usually metal wires, as metal wires can easily transfer electrons (tiny negatively-charged particles) from one place to another and circulate electricity through the appliance.

But metals aren’t the only materials that can transmit electricity well! Graphite can also act as a conductor. This experiment revolves around mixing graphite with sodium silicate Na2xSiyO2y+x solution, also known as liquid glass.

For more information, please visit Graphite Wire.

Even a small graphite stripe or pencil mark can conduct electricity, but such “wires” are not thick enough to be good conductors. The liquid glass acts as a glue that both thickens the liquid wire and helps the graphite fragments stick to each other. Touching the diode to this wire closes the electric system, and the diode glows as electric current flows through the circuit.

This mixture can be transferred onto any flat surface, but it must always be continuous – the system should be closed with a single touch of the diode!

How can graphite conduct electricity when it’s not metal?

Graphite is made of carbon atoms, and carbon is a non-metal. So how can it act as a conductor?

Graphite can act as a conductor thanks to its unique layered structure, which gives its electrons the opportunity to move more freely than they otherwise would. This structure is very important – diamonds, which are also made up of carbon atoms, cannot act as conductors because they aren’t constructed this way!

Interestingly, graphite’s conductivity also depends on the direction the electric current is moving: current can flow along graphite layers thousands of times more easily than perpendicularly to them.

What else can we make wires from?

Wires are primarily made of metals due to their superior conductivity. Silver is the best conductor of all the metals, but its high price limits it to select applications within electronics. It and gold are used as contacts in toggle switches and tiny microchip wires. Copper is common in home electrical systems, but its price and mediocre tension resistance can be problematic at larger scales; power lines have wires made of cheaper steel, compromising slightly in terms of heat loss.

But what about liquid wires?

Some electroconductive glues contain silver particles – when the glue dries, the layer becomes conductive. They are used to fix some electric circuits, like car window heaters, where you can't change the wires. Sometimes metal isn't present in the solution in its metallic form, but rather is dispersed as ions. The metal layer is then forced to precipitate from the liquid using chemicals or even lasers. This might seem overly elaborate or exotic, but you hold a device made like this in your hands every day – your smartphone! The antenna in your smartphone was likely made exactly this way: at some point, metal ions were deposited on a plastic component to form a solid metal coating capable of capturing radio waves.

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