A practical, no-nonsense guide to container seals — what they do, which types to use, ISO 17712 requirements, how to apply and document them, and the common mistakes that put shipments at risk.

What is a container seal and why it matters

A container seal is a single-use locking device applied after loading to show whether a container has been opened in transit. It’s the primary physical evidence of tampering. A correctly chosen and properly applied seal reduces theft, prevents contraband insertion, supports customs checks, and simplifies insurance claims.

Types of container seals (and when to use each)

Bolt seals — high-security, export & import

Bolt seals are metal seals with a hardened steel pin and locking barrel. They are widely accepted for international shipments and commonly required by customs because they resist cutting and cannot be resealed undetectably. Use bolt seals for high-value cargo, hazardous materials, and when ISO 17712 compliance is required.

Cable seals — flexible strength and visibility

Cable seals use a steel cable and locking head — ideal when you need flexibility (e.g., around irregular latch arrangements) and strong tamper resistance. Cable seals come in different diameters and grades; choose an ISO-rated version for cross-border shipments.

Plastic tamper-evident seals — fast, inexpensive, visible

Plastic seals are lightweight, low-cost, and give clear tamper evidence. They’re suited for domestic transport, low-value loads, or internal controls. They’re not a substitute for bolt or cable seals on international or high-risk shipments.

Metal strip / wire / strap seals — intermediate security

These seals provide stronger tamper evidence than simple plastics and are often used when you need more durability without the cost of bolt seals. They’re good for regional supply chains and repetitive checks.

Electronic seals (E-seals) — audit trail and remote verification

E-seals integrate RFID or tamper sensors to record events and allow remote or automated verification. They’re valuable for high-value, temperature-sensitive, or high-audit shipments where chain-of-custody logs matter.

Key standards: ISO 17712 and other requirements

ISO 17712 defines mechanical testing and classification for high-security seals used in international freight. If your shipment must meet customs or international security programs, choose seals certified under ISO 17712 (e.g., “High Security”).

Shipping lines and national customs agencies may impose additional rules. Always check the destination country and carrier requirements before selecting a seal.

Where and how to place a container seal

Which door to seal

Standard practice: close the left door first, then the right door (the right door opens first). Apply the seal on the locking rod or hasp of the right door so the seal must be broken to open the container.

Step-by-step: Correct application

1. Confirm the container is properly latched and doors align cleanly.
2. Insert the seal through the designated hole in the lock-rod or hasp.
3. Engage the locking mechanism fully until you hear or feel it click.
4. Record the seal serial number on the Bill of Lading and shipment documents.
5. Photograph the applied seal (optional but recommended for evidence).

Verification at checkpoints

At each control point (yard, port, customs), check the physical condition and confirm the recorded seal number matches documentation. If a seal is broken, record the event, do not accept the container without investigation, and follow your incident SOP.

Documentation & chain of custody — what to log

• Seal serial number (recorded on Bill of Lading & manifest).
• Date/time and location of application.
• Operator name or ID who applied the seal.
• Photographs at application (optional) and at handover points (recommended for high-risk cargo).
• Records of any resealing by customs or authorized inspectors (must be clearly documented).

Choosing the right seal for the job — practical checklist

Best practices that actually reduce risk

1. Standardize seal type by shipment class

Create a policy that maps seal types to shipment categories (e.g., bolt seals for international high-value; cable seals for oversized/lumpy loads; plastic seals for internal transport).

2. Train staff — application and verification

Simple errors cause most problems. Run short hands-on sessions so staff correctly apply seals and record numbers every time.

3. Use serialized seals and digital logging

Buy seals with permanent laser-etched serials or barcodes and log them in your TMS/WMS. Digital logs make audits and claims far faster and more defensible.

4. Photograph and timestamp critical shipments

A quick photo at sealing and handoff is inexpensive insurance; it strengthens investigations and helps resolve discrepancies quickly.

5. Treat resealing with strict controls

If customs or an inspector opens a container, require an official reseal with documented ID. Never accept an unofficial reseal without verification.

Common mistakes that trigger claims and delays

• Using low-security plastic seals for international or high-value cargo.
• Not recording the seal number on transport documents — the most common error.
• Applying seals to the wrong door or placing them where they can be bypassed.
• Failing to train staff — inconsistent application or documentation creates gaps.
• Ignoring environmental exposure — rusted seals give suspicious appearance even when intact.

When a seal is broken — immediate actions

If you find a broken or mismatched seal at destination or in transit:

1. Do not unload immediately. Photograph the container and seal area.
2. Report to the carrier, customs authority, and insurer as required by your SOP.
3. Open only in the presence of authorized personnel and document findings thoroughly.
4. Issue a new, official seal after inspection and clearly log the reseal details.

Seal technology trends to watch

Adoption of e-seals (RFID, tamper sensors) is growing where high auditability and real-time assurance matter. Combined with IoT tracking (temperature, geofence), e-seals support advanced workflows for pharma, cold chain, and high-value logistics.

Quick reference table — which seal to choose

Resources & further reading

• ISO 17712 — Mechanical Seals for Freight Containers (official)
• U.S. Customs & Border Protection — import guidance
• IATA — guidance on secure supply chains

Conclusion — make sealing part of your risk control system

Container seals are not a checkbox: they’re a first line of defense. Selecting the right seal, applying it properly, documenting it reliably, and training staff will cut losses, speed customs clearance, and make claims easier to resolve.

Choose the correct seal for your risk — and then make sure everyone knows how to use it.

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