Specifying
Shield termination: the requirement neither callout covers
The connector specification tells a supplier what the connector is. The cable specification tells it what the cable is. Neither says how the shield gets from one to the other — and that joint is what determines whether the shielding you paid for does anything. It has to be specified on its own, and on a surprising number of drawings it simply is not.
Why it falls between the two
A MIL-DTL-38999 callout selects series, class, shell size and insert arrangement. An M27500 designator selects the shield style, material and coverage inside the cable. Between them sits a physical operation — taking a braid and connecting it to a shell or a ground lead — that neither document governs.
When it is left off, a supplier will still build something, because it cannot build anything otherwise. What it builds is whatever is normal in that shop, and “normal” ranges across methods whose electrical behavior differs substantially.
The methods, and what each trades
| Method | How it works | Trade-off |
|---|---|---|
| Pigtail to a ground lead | Braid gathered and run as a wire to a contact or ground point | Simple, repairable, inspectable. The gathered length is a wire, and behaves like one |
| Solder sleeve | Heat-shrinkable sleeve with a solder preform that bonds a ground lead to the braid and seals the joint | Repeatable and sealed; still a pigtail electrically; heat-sensitive to install |
| Band strap to a backshell | Braid folded over the backshell and clamped by a band | Continuous contact around the circumference; needs the right backshell and a calibrated tool |
| Conductive backshell with ferrule or sock | Shield captured mechanically into the accessory and thence to the shell | Continuous path to the connector shell; adds cost, size and part count |
The distinction that matters is not the hardware but whether the connection is circumferential or gathered to a point. A braid terminated all the way round keeps the shield continuous with the connector shell. A braid gathered into a pigtail introduces a length of conductor, and a length of conductor has inductance — whose impedance rises with frequency. So a pigtail that is perfectly adequate for a low frequency shield becomes progressively less effective as frequency climbs, which is why the same harness can pass a continuity check and still fail an EMC test.
The practical consequence for a drawing: if the pigtail method is acceptable, say so and state a maximum gathered length. If it is not, say that instead. “Terminate the shield” leaves the decision with whoever is holding the braid.
The solder sleeve specification, and a citation trap
Solder sleeves have their own specification. It covers “environment resistant, heat-shrinkable, solder-type shield terminations” which “may be used on data-bus, RF, and shielded cables in applications where the operating temperature does not exceed 150 deg C”.
Read that temperature ceiling against the cable you chose. A 200°C-capable wire terminated with a 150°C-rated sleeve is a harness limited by its terminations, and the limit will not be apparent from the cable part number.
Now the trap, and it is the reason to check every legacy drawing. That specification was issued as MIL-S-83519A, dated 21 June 1984, and is superseded — the live document is SAE-AS83519. Custody moved from the Department of Defense to SAE, as it did across much of this family: the connector specification most people still call MIL-C-38999 is MIL-DTL-38999 today.
A drawing citing a superseded number is not automatically wrong — a contract may deliberately freeze a configuration — but it is ambiguous, and nobody downstream can tell which was meant. Either cite the live document, or state in the notes that the superseded revision is intended and why. This is the third form of the same discipline on this site, alongside AS6081's removal of its own test methods and revision drift in the connector and cable specifications.
What to put on the drawing
- The termination method, named — not “terminate shield”.
- Where the shield lands: which contact, which backshell, or the connector shell.
- If pigtails are permitted, the maximum gathered length.
- Whether shields are terminated at one end or both, and which end — this is a design decision with consequences and it should not be inferred.
- The hardware specification and its current designation, with the revision.
- The temperature rating required of the termination, checked against the cable's.
- The workmanship standard for the joint — IPC/WHMA-A-620 covers shield termination among the criteria an assembly is inspected against.
- How it will be verified: continuity is not shielding effectiveness. See cable assembly testing for what a test specification should say.
Point four is worth dwelling on. Terminating a shield at both ends can create a path for current to flow along the shield; terminating at one end avoids that but leaves the far end unreferenced. Which is right depends on the system, and it is an engineering decision that belongs on the drawing rather than in a supplier's habit.
This is a buyer's orientation, not a substitute for the specifications. Confirm current designations and revisions against the published documents.
Last reviewed 2026-09-20