Instrumentation cable: pairs, shields and thermocouple extension

Pairs, and the two shields

Instrument cable is built as twisted pairs, from a single pair to twenty-four or more, in 16 to 24 AWG. Each pair carries one loop signal, and the twist keeps the two conductors in the same noise field so the interference cancels. The pairs are then assembled into a bundle under an overall jacket.

Shielding comes in two layers on the better constructions. Each pair has its own foil shield with a drain wire, and the whole bundle has an overall foil shield with its own drain. That lets a single noisy loop be grounded separately from the group. Cable with only an overall shield is common and cheaper, and the print or a look under the jacket will tell you which one a coil is.

The drain wire is part of the cable

A foil shield on its own cannot be terminated; it is too thin to land on a terminal. The drain wire is the tinned copper conductor laid in contact with the foil that carries the shield to ground. It is usually one size smaller than the signal conductors and it is uninsulated.

At a cut end the drain is the first thing to be damaged. It frays, it gets cut back with the foil, and on a used coil it often ends short of the jacket. That is worth photographing, because a coil with two inches of drain at each end is a coil somebody can terminate, and one with none is a coil that has to be opened up to find it.

Thermocouple extension is a different conductor

Thermocouple extension cable looks like instrument cable and is not. Its conductors are made of the same alloys as the thermocouple itself, so the junctions at the terminals do not create a second thermocouple and shift the reading. Type J, K and T are the common ones, and each has its own colour code and its own polarity.

That means extension cable cannot be substituted with copper, and copper cannot be substituted for it. It also means the pair has a direction: reversing the polarity at one end gives a reading that is wrong by a fixed offset rather than obviously broken, which is the kind of fault that survives commissioning.

Retrofit surplus at a refinery or a mill

Instrument cable is bought for a project and consumed by the loop. A refinery or paper mill retrofit pulls home runs from a marshalling panel to field junction boxes, and the leftover is a set of part reels in a range of pair counts, plus a pile of cut lengths with the shields already dressed.

Lima's refining and heavy equipment work produces this in the same shape as any process plant. The lots that review well are the ones where the pair count and shield arrangement are listed per reel, because those two facts decide what the cable can be used for.

• Pair count and gauge, per reel or per coil
• Individual shields, overall shield, or both
• Drain wire present at both ends and of usable length
• Thermocouple extension identified by type and colour code
• Jacket print, including any tray or burial rating