Repairing a cable, a plug, a contact

Most electrical failures aren't dead components. They're broken wires and dirty contacts.

UNDERSTAND

A circuit only works if it's closed: the current goes out, passes through the load, and comes back. A break anywhere produces exactly the same symptom — nothing works — whether the fault is on the way out or on the way back.

The breaking points are always the same, and this list covers almost everything: where the cable moves, meaning where it leaves a plug or a device; where two metals touch, meaning the connections; where moisture sits.

A degraded contact isn't a clean break: it's resistance appearing. It heats up, oxidizes further, heats up more. A warm contact is a dying contact — the mechanism is detailed in Câbler et protéger une installation.

ACT

Find the fault before you repair anything.

1. Cut the power and check that there's no voltage. Always, and with a tester, never by deduction.
2. Look and bend. A cable breaks near its ends: bend it gently along its whole length; a sheath that's hard, cracked, soft or locally deformed shows you the spot.
3. Test continuity from end to end, wire by wire, with the cable unplugged at both ends — see Le multimètre. Move the cable while you measure: that's how an intermittent fault gives itself away.
4. Check both conductors. The return wire breaks as often as the outgoing one.

Repairing a break.

1. Cut cleanly on both sides of the doubtful area. Don't repair right at the fault: the cable is fatigued for several centimeters around it.
2. Strip without nicking the copper. A nick creates a weak point that will break at the first movement. Rotate the blade around the insulation instead of slicing into it.
3. Stagger the joints. Never join both conductors at the same spot: offsetting them by a few centimeters keeps them from touching if the insulation gives way.
4. Make it mechanically sound first. Twist tightly in the direction of the strands, or better, wrap one wire around the other in tight coils. The joint has to hold on its own when you pull on it, before you insulate it.
5. Solder if you can. Solder brings contact resistance down to almost nothing. Otherwise, a terminal block or a crimped connector does the job.
6. Insulate each conductor separately, then the whole thing. Several crossed turns, overlapping well on each side.
7. Add strain relief. A loop, a knot or a cable tie that takes the pull before it reaches the joint: without it, your repair will break at the same spot.

Cleaning a contact.

1. Scrape off the oxidation with fine sandpaper, a blade or an eraser. A dull contact conducts poorly.
2. Tighten it. Copper creeps under pressure, and a terminal loosens by itself over time.
3. Protect it with a trace of grease once the contact is clean and tight, if the air is damp — see La corrosion des métaux.

ADAPT

The cable cuts in and out. It's almost always a broken strand under the insulation, at a bending point. Cut generously on both sides: a repair that's too short leaves the fatigued part in place.

You have no electrical tape. Heat-shrink tubing is better; failing that, thick adhesive tape, inner-tube rubber cut into a strip and stretched on in tight turns, or dipping the joint in melted plastic.

You have no tools at all. Scrape with whatever you have: a house key, the edge of a coin, a stone, sand on a rag. A loose battery clamp sits on a tapered post: push it all the way down, tap it with a stone, then twist it to lock it. As a last resort, a strip of aluminium (a gum wrapper, a cut-up can) slipped between the clamp and the post takes up the slack long enough to get going. Never bridge the two terminals with anything metal: a car battery can push hundreds of amps and the metal turns red-hot.

You have nothing to solder with. A well-made mechanical joint works for years. What counts is the tightness, the contact surface and protection from moisture, not the solder itself.

The wires aren't the same metal. Copper and aluminum in direct contact corrode — see La corrosion des métaux. Use a connector designed for it, or put a suitable part between them.

It's the cable of a device you want to power some other way. Check voltage, polarity and power before connecting anything. Reversing polarity on direct current destroys things instantly — see Electrical energy in everyday objects and Lire un circuit électrique.

It's on a vehicle. Vehicles are full of cables, flexible and in every size: they're the best source of raw material — see Récupérer sur un véhicule.