The seal on a container door is a small piece of metal and plastic that carries a surprising amount of weight. It’s your evidence that the box arrived the way it left, it’s a customs and security requirement, and it’s the first thing anyone looks at when cargo goes missing.
Most seal problems aren’t sophisticated tampering. They’re process failures — nobody recorded the number, nobody checked it at the handoff, or the wrong grade of seal was used in the first place.
What ISO 17712 requires
ISO 17712 is the international standard for mechanical seals on freight containers. It classifies seals by strength and, importantly, sets requirements beyond raw physical resistance:
- Classification by strength. Seals are graded, with “H” (high security) being the class required for international container shipments under most security programs.
- Tamper evidence. The seal must show visible evidence if it’s been defeated and re-applied.
- Manufacturer requirements. The standard covers the seal manufacturer’s own security processes — how blanks are controlled, how numbers are issued, how orders are validated. A seal from an uncontrolled source undermines the whole point.
- Unique identification. Every seal carries a unique number that can be tracked and verified.
CTPAT and comparable supply chain security programs generally expect ISO 17712 high-security seals on international container movements. If you’re in one of those programs, the seal grade isn’t optional.
The main seal types
Bolt seals
A metal pin driven through a locking body, removable only by cutting. These are the standard high-security seal for container doors and what you’ll see on the vast majority of international ocean containers.
Cable seals
A steel cable threaded through the locking mechanism and secured in a one-way barrel. Useful where a bolt seal doesn’t fit — certain trailer configurations, valves, or tanks.
Plastic indicative seals
Pull-tight plastic strips. These are indicative, not high security — they show that someone opened the unit but offer little physical resistance. Appropriate for domestic trailers, tote bins, or as a secondary seal, not as the primary seal on an international container.
Electronic and smart seals
Bolt or cable seals with embedded RFID or cellular reporting that log open events with a timestamp. Higher cost, and genuinely valuable on high-value or high-risk cargo where you want to know when something happened, not just that it did.
What to check, and when
Seal verification is a discipline, not a glance. At every handoff:
- Read the full number and compare it character by character against the documentation — the bill of lading, the delivery order, the interchange receipt.
- Inspect the seal body for cut marks, heat damage, glue, or a mismatched barrel. Defeated seals are often re-attached convincingly enough to pass a casual look.
- Pull-test it. A seal that moves is not a seal.
- Check the door hardware. Hinges and locking bars can be removed without touching the seal at all. Look for fresh tool marks and missing or mismatched rivets.
- Photograph it. A dated photo of the seal number and the door is the single most useful piece of evidence in any subsequent claim.
- Record it on the interchange. The seal number belongs on the equipment interchange receipt at pickup and at delivery.
That last point matters more than most shippers realize. The interchange receipt is the document that establishes container condition at each transfer of custody — our post on interchange receipts and equipment damage claims covers how that record gets used.
When the seal doesn’t match
A broken or mismatched seal at delivery is not something to work around quietly. The correct sequence:
- Do not open the container. Whatever is inside is now evidence.
- Photograph everything — the seal, the doors, the container number, the surrounding area.
- Notify immediately — your carrier, your customs broker, and if the discrepancy suggests tampering, CBP.
- Note it on the delivery receipt before the driver leaves. Exceptions written after the fact carry much less weight.
- Inventory carefully when the container is opened, with a witness present.
There’s a benign explanation a good share of the time — a customs exam. CBP replaces the original seal with its own after an inspection, and the new number won’t match your paperwork. That’s normal, and the exam documentation will confirm it. Our guide to customs exams in Miami explains what those look like.
Seals and drayage in South Florida
A container moving from PortMiami or Port Everglades to a warehouse changes custody at least twice, and each transfer is a point where the seal should be verified and recorded. Carriers that treat this as a checkbox create gaps in the chain of custody that only surface when there’s a claim.
Go Drayage runs company-owned trucks with drivers who record seal numbers at interchange, and our five-acre Miami yard is a 24/7 secured site — details on yard storage. That continuity of custody is one of the practical advantages of an asset-based carrier over a chain of subcontracted trucks.
Frequently asked questions
What is a high-security seal under ISO 17712?
It’s a seal meeting the standard’s “H” classification, which combines physical strength requirements with tamper-evidence and controls on the manufacturer’s own security processes. Bolt seals and cable seals are the common high-security formats; plastic pull-tight seals are indicative only.
Who is responsible for sealing an export container?
The shipper is responsible for applying the seal when the container is loaded and for recording the number on the shipping documents. Once the seal is on, every subsequent party in the chain is responsible for verifying and recording it rather than replacing it.
What should I do if the seal number doesn’t match at delivery?
Don’t open the container. Photograph the seal, the doors and the container number, note the discrepancy on the delivery receipt before the driver leaves, and notify your carrier and customs broker. A mismatch is often the result of a customs exam, but that should be confirmed with documentation rather than assumed.
