Paper-Insulated Lead-Covered Cable

Measure the thing the whole design exists to control before you design anything.

1. Cut three identical strips of kraft paper about 150 × 25 mm.
2. Leave one at room humidity, dry the second in an oven at 105 °C for two hours, and lightly mist the third.
3. Clamp each between two flat metal electrodes 50 mm apart and measure insulation resistance with a 1000 V megohmmeter.
4. Record all three, then re-measure the oven-dried strip after it has sat in the room for an hour.

The three readings will span several orders of magnitude, and the dried strip will start falling within minutes of leaving the oven. That last observation is the important one. Paper is hygroscopic — it does not merely get wet, it actively pulls water out of the air and holds it. An insulation system built on paper is therefore not finished when the paper is dry; it is finished when the paper can never meet air again.

A solid copper rod would be cheaper to make and worse in every way that matters.

1. Take nineteen lengths of 1.5 mm bare copper wire and lay them up as a rope: one core, six around it, twelve around those.
2. Twist each layer in the opposite direction to the one beneath, with a lay length of roughly twelve times the finished diameter.
3. Measure the finished diameter with a micrometer and compare it with a solid rod of the same copper cross-section.
4. Bend both around a 200 mm former and compare the force needed, and whether either takes a permanent set.

The stranded conductor bends easily and springs back; the solid rod of equal copper area fights you and stays bent. Stranding buys flexibility, which matters because a cable must be coiled onto a drum, hauled round bends in a duct and pulled into a trench without cracking the insulation wrapped on top of it. Opposing lay directions stop the rope untwisting itself when tension comes off.

The gaps in the wrapping look like defects. They are the reason the cable can bend.

1. Slit your kraft paper into tapes about 12 mm wide.
2. Wind the first tape helically along the conductor, butted edge to edge with a deliberate gap of roughly 1 mm — do not overlap.
3. Wind each subsequent layer in the opposite direction, and rotate the starting point so no two butt gaps ever line up radially.
4. Build up eight to ten layers, then bend the assembly around a 300 mm former and unwrap it to inspect.

The tapes will have slid over one another at the bend rather than tearing. A continuous overlapped wrap would be a rigid tube; it would split on the outside of every bend and buckle on the inside. Butt-gapped tapes let the wall shear internally like a deck of cards.

The staggering rule is what makes the gaps safe. A gap is a weak spot only if it sits directly above another gap, because that would give the electric field a clear radial run. Rotate every layer and the weak spots never queue up, so the field always has to cross solid paper somewhere.

This step is the entire invention. Everything before it is preparation and everything after it is packaging.

1. Put the taped assembly in a vacuum chamber and pump down, warming it to about 100 °C with a heat gun on low or a warming plate.
2. Hold the vacuum for at least an hour. Watch the gauge: it will keep drifting as water leaves the paper.
3. With the vacuum still applied, admit warm mineral oil until the assembly is completely submerged.
4. Release the vacuum to atmosphere and let the oil be pushed into the paper. Leave it for an hour, then remove and drain.
5. Re-measure insulation resistance and compare it with the dry-paper figure from step 1.

Order matters absolutely: vacuum first, then oil, then release. If you flood with oil at atmospheric pressure the oil seals the outside and traps air inside the wall. Those trapped bubbles are the failure mechanism of the whole technology — gas has a much lower breakdown strength than oil, so the field concentrates in the void, ionises the gas, and produces a tiny sustained discharge that slowly carbonises a track through the paper. It is called partial discharge, it is invisible, it takes years, and it always wins in the end.

The lead sheath is not armour. It is a hermetic bag that happens to be made of metal.

1. Form a lead tube around the impregnated core — extruded seamlessly in industry, and for bench work wrapped from lead sheet with a soldered longitudinal seam.
2. Work the lead down onto the core so there is no annular air space.
3. Close and solder both ends, leaving no pinholes.
4. Warm the finished length gently and watch the seam under a film of soapy water for escaping bubbles.
5. Weigh it, leave it a week, and weigh it again.

A hermetic sheath neither gains nor loses weight. Weight gain means moisture is getting in; weight loss means oil is getting out, which is just as fatal because the paper it leaves behind is full of gas.

Lead was chosen for a specific and unusual combination: it is impermeable to water vapour, it is soft enough to be extruded continuously in a seamless tube around a moving core, it resists soil corrosion tolerably, and it will take solder. Almost nothing else in 1890 did all four. Its cost is that it is heavy, it creeps under its own weight in vertical runs, and it fatigues and cracks if the cable is allowed to vibrate.

Every buried cable fails at a joint long before it fails in the middle of a run.

1. Wrap the sheathed cable with a bedding of bitumen-soaked tape, then helically applied steel tape armour, then an outer serving of bitumen and jute.
2. Now cut the finished cable in half and joint it back together: strip back each layer in a long stepped taper, reconnect the conductors, rebuild the paper insulation by hand, and enclose the whole joint in a filled housing.
3. Re-test insulation resistance across the joint and compare with the unjointed cable.
4. Leave the jointed sample somewhere damp for a week and test again.

The joint will read lower than the cable, and it will drift with time in a way the cable does not. That is the honest result and it is the whole reason cable jointing was a specialist trade with its own apprenticeship. In the factory the paper is wrapped by machine, dried in a vacuum oven and sealed within hours. In a trench, in the rain, a jointer opens all of that up and rebuilds it by hand.

The stepped taper exists because the electric field crowds wherever the insulation thickness changes abruptly. Spreading the transition over a long cone keeps the field gradient gentle. Cut the layers off square and the joint fails at the step, every time.