A container seal is a uniquely numbered security device fitted to the door locking bar of a shipping container after it is loaded. Its job is not to stop a determined thief — it is to prove tampering. If the seal number recorded at origin matches the seal on the doors at destination, and the seal is intact, the chain of custody held.
ISO 17712 is the international standard that defines what counts as a high-security seal. C-TPAT and most customs programs require it.
The three ISO 17712 classifications
High security (H)
Bolt seals and equivalent devices that pass the standard’s strength tests for tensile, shear, bending and impact resistance. These are the only seals that satisfy C-TPAT minimum security criteria for containers moving internationally.
Security (S)
Cable seals and similar devices meeting a lower strength threshold. Useful for domestic trailers and lower-risk moves, but not sufficient where a high-security seal is specified.
Indicative (I)
Plastic strip and pull-tight seals. They break easily by design and serve only to show that a door was opened. Appropriate for LTL trailers, cages and internal transfers, not for ocean containers.
What ISO 17712 actually requires
The 2013 revision and later editions added requirements beyond raw strength:
- Mechanical strength testing to defined thresholds for the H classification.
- Tamper evidence. The seal must show visible, non-reversible evidence if it is opened or manipulated.
- Manufacturer security auditing. The producer must control seal blanks, numbering and distribution so seal numbers cannot be duplicated or diverted.
- Unique, permanent marking of the number and the manufacturer identity on the seal body.
That third point is the one importers overlook. A bolt seal that passes a strength test but comes from an unaudited supplier is not a compliant ISO 17712 high-security seal, because the numbering chain is not controlled.
Seal numbers on your paperwork
The seal number must appear consistently across the document set — the bill of lading, the packing list, the equipment interchange paperwork and the customs entry. A mismatch between the seal number on the BOL and the seal on the doors is treated as a security event, and customs may hold and examine the container.
When a container changes hands at a terminal or a yard, the seal condition is recorded on the equipment interchange receipt. That document is your evidence of when a seal was intact and when it was not — which becomes critical if you later file a claim.
Seal inspection: the VVTT method
C-TPAT training uses a four-step check that anyone receiving a container should run before cutting:
- View the seal and locking mechanism for anything unusual.
- Verify the seal number against the shipping documents.
- Tug the seal to confirm it is properly affixed.
- Twist and turn the seal to confirm it does not come apart.
If the seal is broken, missing, or the number does not match, stop. Photograph the doors before opening, notify the shipper and your customs broker, and record the discrepancy. Opening first destroys the evidence and turns a documented security exception into an unprovable over, short and damaged dispute.
Who applies the seal
For a shipper-loaded full container, the shipper applies the seal at the loading point and records the number. For LCL cargo consolidated at a container freight station, the CFS operator seals the box and issues the number on the consolidation documents. Where a container is opened lawfully — by customs for an exam, or at a bonded facility — a new seal is applied and the replacement number is documented, creating a seal history rather than a break in the chain. Go Freight seals and documents every container it loads at its bonded Miami facility as part of its warehousing and cross-dock services, and re-seals after any authorized CFS deconsolidation.
Frequently asked questions
Is an ISO 17712 seal required for C-TPAT?
Yes. C-TPAT minimum security criteria require a high-security seal meeting ISO 17712 on all loaded containers moving in international traffic. Only the high security, or H, classification qualifies.
What is the difference between a bolt seal and a cable seal?
A bolt seal is a solid metal pin and locking body that usually meets the ISO 17712 high security classification. A cable seal uses a steel cable through a locking body and typically meets the lower security classification. Bolt seals are the standard choice for ocean containers.
What should I do if a container arrives with a broken seal?
Do not open the container. Photograph the doors and the seal, verify the number against your documents, and notify the shipper and your customs broker immediately. Opening first destroys the evidence you would need for a claim or a customs explanation.
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