A container seal is a single-use, numbered device fixed to the door locking gear of a loaded container after stuffing. Its job is not to stop a determined thief: it is to prove whether the doors have been opened since the seal went on. ISO 17712 grades mechanical seals into three classes (I for indicative, S for security, H for high security), and containers moving by sea into the United States are expected to carry an H-class seal, usually a bolt seal. The key takeaway for shippers is that the seal number is evidence: it must be recorded on the shipping instructions, the bill of lading and the carrier's manifest, and checked at every handover. A number that does not match the paperwork is what turns a routine delivery into a hold, an examination or a disputed claim.
This guide covers mechanical and electronic seals on ocean containers. It is different from the customs seal used in road transit under a carnet, which is covered in our guide to the TIR carnet and international road transit under seal.
What ISO 17712 actually covers
ISO 17712, "Freight containers: Mechanical seals", is the international standard for classifying, accepting and withdrawing mechanical seals used on freight containers. As of October 2026, the edition in force that we could find is ISO 17712:2013, which replaced the 2010 version (the standard started life as a Publicly Available Specification, ISO/PAS 17712, in the early 2000s). Check the ISO catalogue for any later revision.
It grades barrier strength through tensile, shear, bending and impact tests run by laboratories accredited to ISO/IEC 17025, requires tamper evidence that an inspector can see in the field, and sets security practices for seal manufacturers, because a strong seal is worthless if duplicates of its number can be bought.
Compliant high-security seals typically carry a unique number (often with a barcode), the manufacturer's name or logo, and an "H" mark for the class. When you buy seals, ask the supplier for the test certificate showing ISO 17712 high-security compliance: CBP's CTPAT criteria name a laboratory testing certificate as the acceptable evidence.
The three seal classes, compared
| Class | Typical forms | What it does | Typical use |
|---|---|---|---|
| I: Indicative | Plastic pull-tight seals, light strap or wire seals | Shows that a door was opened; can be broken by hand | Internal movements, warehouse doors, some domestic trailers |
| S: Security | Heavier metal strap and lighter cable seals | Needs a simple tool to remove; tamper-evident | Lower-risk domestic and regional moves where H is not required |
| H: High security | Bolt seals, heavy cable seals, some barrier seals | Needs bolt cutters or a cable cutter to remove; tamper-evident | Loaded ocean containers, especially to the US; CTPAT and many AEO supply chains |
The class comes from test results, not from the shape of the device. A bolt seal from an unknown maker with no certificate is not an H-class seal just because it looks like one, which is why counterfeit and uncertified seals are a recurring warning in carrier and insurer guidance.
The misconception: a high-security seal does not make the box secure
An H-class bolt seal resists casual removal, but it only protects the locking gear it is fixed to. Containers can be entered through the door hinges or panels, or resealed with a duplicate, leaving the seal apparently untouched. That is why CBP's CTPAT criteria pair seals with container inspection and suggest tamper-resistant hinges or protective plates on the hinges. Treat the seal as one link in a chain: a clean container, controlled loading, a recorded number and a check at every handover.
Is an ISO 17712 seal required, or just recommended?
For sea containers bound for the United States, it is a legal requirement, not only a CTPAT expectation. Under 6 U.S.C. 944(a)(4)(B), added by the SAFE Port Act, all containers in transit to the United States were required to meet the ISO/PAS 17712 sealing standard from 15 October 2008, because the interim final rule the statute called for was not issued by 1 April 2008. CBP applied it to laden containers arriving by vessel; TT Club guidance at the time noted that tanks, open tops and boxes that cannot take such a seal were understood to be outside it. We found no superseding rule as of October 2026, but check with your broker if your equipment is unusual.
For members of CTPAT, CBP's Customs Trade Partnership Against Terrorism programme, the requirement is explicit. The Minimum Security Criteria for U.S. Importers (November 2019 edition) say all CTPAT shipments that can be sealed must be secured immediately after stuffing "by the responsible party (i.e. the shipper or packer acting on the shipper's behalf)" with a high-security seal that meets or exceeds the most current ISO 17712 standard (criterion 6.2). Outside the US, check your own customs administration: Authorised Economic Operator programmes built on the WCO SAFE Framework commonly point to the same standard.
Who applies the seal: shipper, carrier or customs?
Whoever closes the doors after loading should seal them, because nobody else can vouch for what is inside.
- FCL, shipper-loaded: the shipper or its packer applies the seal at the point of stuffing. The carrier often supplies its own numbered seal with the empty container, or the shipper uses seals from its own controlled stock. The bill of lading is then typically claused "shipper's load, stow and count" (SLAC), and US manifests show the shipper's declared description for a sealed container (19 CFR 4.7a(c)(4)(vii)).
- LCL: the consolidator or container freight station that stuffs the box applies the seal, and the consolidator's master bill carries the number.
- Customs and other authorities: if a container is examined at origin, in transit or at destination, the officers break the seal and apply a new one, often their own. CTPAT's guidance on criterion 7.8 notes that once a government breaks a seal, there needs to be a process to record the new seal number, and that it may even be handwritten.
- Carrier or terminal re-seals: a seal found missing or damaged in the carrier's custody is usually replaced and reported as an exception; capture that number too.
CTPAT also expects seal placement to follow a rule: on the secure cam position if the container has one, otherwise at the bottom of the centre-most vertical locking bar of the right-hand door, with a bolt seal's barrel facing upward above the hasp (criterion 6.2 guidance). Low placement matters because higher fixing points are easier to tamper with.
Recording the seal number: SI, bill of lading and manifest
The seal number has to travel through four documents, and each one is a chance to mistype it.
- Shipping instructions. The shipper gives the container number and seal number to the carrier or NVOCC before the SI cut-off. Our shipping instructions template shows where they go.
- Bill of lading. The carrier prints the seal number beside the container number. Whatever type of bill of lading you use, check the draft against your loading record before it is issued: a correction afterwards can mean an amendment fee and, near sailing, a delay.
- Carrier's advance manifest. For US imports, the ocean carrier's cargo declaration must include "the seal numbers for all seals affixed to containers" (19 CFR 4.7a(c)(4)(xiv)), transmitted 24 hours before loading at the foreign port. The importer's own ISF 10+2 filing does not carry the seal number, but it does name the container stuffing location and consolidator, which should agree with the party who sealed the box.
- Delivery documents. The delivery receipt should note the seal number found on arrival.
CTPAT also recommends printing seal numbers electronically on the bill of lading (criterion 7.30), sending them to the consignee before departure (7.29) and photographing the seal in place at stuffing (7.5). All three are cheap and worth doing even outside CTPAT.
Checking a seal: the VVTT method and container inspections
CBP's seal verification process is called VVTT (criterion 6.7):
- View the seal and the container locking mechanisms and make sure they are intact.
- Verify the seal number against the shipment documents.
- Tug on the seal to make sure it is properly fixed.
- Twist and turn a bolt seal to make sure its parts do not unscrew or separate. For a cable seal, the cable should be taut with no slippage in the locking body.
CBP's recommended seal procedures say VVTT should be done with a witness when the seal is applied and again before it is cut at destination.
The inspection that comes before sealing has its own terminology. The current importer criteria require a seven-point inspection of every empty container before stuffing: front wall, left side, right side, floor, ceiling or roof, inside and outside doors (including the locking mechanisms), and outside or undercarriage. Refrigerated containers get an eight-point inspection, adding the fan housing. The 17-point inspection applies to tractors and trailers on land border crossings (seven points on the tractor, ten on the trailer), not to ocean containers. The container number and CSC plate are worth checking at the same time, so the box you inspected is the box on the paperwork.
When the seal number does not match, or the seal is broken
A seal discrepancy is a security question first and a commercial one second. What typically happens:
- At the terminal or in transit: the carrier or terminal records an exception, applies a new seal and notifies the shipper or forwarder. Under CTPAT, a carrier that breaks or replaces a seal must notify the shipper, broker and importer with the replacement number, and the shipper records it in its seal log.
- At customs: a mismatch between the seal on the box and the seal on the manifest is a risk signal. It can lead to a hold or an examination, with the storage, drayage and exam costs usually falling on the importer. CTPAT lists tampering with a container or high-security seal, and an unaccounted new seal, among incidents members should report to CBP (criterion 7.23).
- At delivery: do not cut a seal that is damaged or does not match. Photograph it, note it on the delivery receipt, notify the carrier, your forwarder and your insurer, and keep the seal itself: CTPAT says altered or tampered seals should be retained to aid investigation.
Why the seal decides cargo claims
For a shipper-loaded container, the seal is often the deciding evidence in a shortage claim. If the original seal arrives intact and matches the bill of lading, the usual inference is that the goods were never in the box, or were lost before sealing, which points back at the shipper's own loading. If the seal is broken, missing or replaced without a documented reason, that supports a claim that the loss happened in the carrier's custody. Keep the stuffing photos, seal log and delivery notes together. Our guide on how to file a cargo claim covers notice periods and documents. Damage caused by poor stowage is a different problem, covered in cargo securing inside the container.
Electronic seals, in brief
Electronic seals (e-seals) add a radio-readable identity and an opened or closed status, so gate readers or trackers can report tampering remotely. The ISO 18185 series sets out their communication protocols (ISO's catalogue lists Part 1, from 2007, as confirmed in 2022). They are mostly used on high-value or high-risk lanes, alongside a certified mechanical seal rather than instead of one: if an H-class seal is required, confirm that any electronic device you use also carries ISO 17712 certification.
Seal checklist for shippers
- Buy seals only from suppliers who can show an ISO 17712 high-security test certificate, and keep a copy on file.
- Store seals locked away and log every seal received, issued and used.
- Inspect the empty container (seven points, eight for reefers) and photograph it before stuffing.
- Seal immediately after loading, in the right position, and apply VVTT with a witness.
- Photograph the seal in place with the container number visible.
- Put the exact seal number on the shipping instructions and check it on the draft bill of lading.
- Send the container and seal number to the consignee before departure.
- At destination, apply VVTT before cutting, record the seal on the delivery receipt, and keep any suspect seal.
What to do next
- Check where your seals come from and whether you hold a current ISO 17712 high-security certificate for them.
- Write a one-page seal procedure covering storage, the seal log, placement, VVTT and what to do on a mismatch.
- Agree with your forwarder and trucker who reports a re-seal and its new number, to whom, and how fast.
- Add the seal number to your receiving checklist so the consignee checks it before the doors open.
If you need a forwarder or consolidator that can stuff and seal containers at origin, more than 29,300 logistics companies are searchable by country and service in the CargoLinked directory. You can also post your shipment on the public requests board, which lists freight that shippers have posted for forwarders to quote on directly.
This guide is general information, not legal, customs or security compliance advice. Seal requirements, CTPAT criteria and manifest rules change; confirm the current position with CBP, your customs administration or a licensed customs broker before relying on it.
