How do you choose a security seal for cargo, a container or a wagon?
You choose a security seal on two criteria: its strength class and its locking mechanism. The international standard ISO 17712:2013 sorts seals into three classes — “I” (indicative), “S” (security) and “H” (high security) — and for international container shipping, customs programmes such as C-TPAT require class “H” specifically. An indicative seal only shows that someone opened a load, whereas a high-security bolt or cable seal also physically resists forced entry — you can only remove it with a bolt cutter. Railway wagons, meanwhile, use dedicated locking-sealing devices approved by the railway. Below is how to avoid both overpaying and picking a seal that’s too weak.
A seal is the last link in a chain of cargo protection: correct securing holds the load in transit, while the seal records that no one gained access to it. Let’s take it in order.
The three seal classes under ISO 17712
Your main reference when choosing is the ISO 17712:2013 standard. It sets uniform procedures for classifying mechanical seals for freight containers and, by strength (barrier capacity), splits them into three classes:
| Class | Purpose | What it gives you |
|---|---|---|
| I — Indicative | Records that a load was opened | Comes off by hand or a light tool; shows someone touched the seal |
| S — Security | Mid-level protection | Stronger than indicative; needs a tool to remove |
| H — High Security | International freight, customs | Bolt and cable seals; removed only with a bolt cutter; required by C-TPAT |
One key nuance: class “H” can’t be self-declared. An independent laboratory accredited to ISO/IEC 17025 confirms the strength. So a “high-security” seal without a certificate is just marketing. If you export the cargo through customs, take a certified class “H” seal.
The main seal types and where to use them
Within the classes, seals differ by the design of their locking mechanism. Here are the main types that cover most tasks:
| Seal type | Design | Typical use |
|---|---|---|
| Bolt seal | Steel pin with a locking head | Doors of containers, vans, covered wagons; class H |
| Cable seal | Steel cable with a clamping lock; splays when cut | Grain hoppers, tanks, odd-shaped lugs; class H |
| Plastic indicative | Polymer strap/tongue with a snap lock | Internal logistics, cash-in-transit, meters; class I |
| Lead seal + sealing wire | Galvanised wire and a lead/plastic seal under a press | Wagons, valves, meters, multi-point protection |
| Barrier container seal | Reinforced metal, ISO 17712 | International containers; barrier to entry and tampering |
Railway seals: why they’re a special case
For rail, an ordinary seal isn’t enough. Here you use locking-sealing devices (ZPU) that both lock and seal the wagon at once, and the railway administration must approve them. Without that approval, a device can’t run on the network.
Take an example from our catalogue — the ZPU “Universal-1M”: a “C-power” class device with a nominal breaking force of up to 18.8 kN (1,800 kgf). It locks and simultaneously seals covered and refrigerated wagons, tanks, grain carriers, hoppers, and car-carrier wagons and containers with a lug-hole diameter from 5.2 mm. Its cable is 500 mm long and 5.2 mm thick, it withstands −60 °C to +55 °C, and each unit carries a unique 7-digit number. VAS “Latvijas Dzelzceļš” officially approves it (document DT-6.4.1/205-2023, dated 14.03.2023).
For grain hoppers, a handy option is the GUARD 1.5 mm / 500 mm cable seal, with a tensile strength of 750 lbf (3.3 kN): when cut, the cable splays, so tampering shows at a glance and no one can reinsert the seal. A laser etches a 7-digit serial number onto each one, and LDZ AS approves it for rolling stock. And the cheapest option for multi-point protection is the Ø16 mm lead seal (Pb 99.9%) paired with galvanised sealing wire under a press: it’s crimped with sealing pliers and works from −40 °C to +70 °C — a classic for wagons, valves and meters.
The practical takeaway: choosing a railway seal starts not with price but with the question “is this device approved for this wagon type and this railway?” Everything else is secondary.
What to check before ordering seals
- Level of protection. Decide the class: I, S or H. For customs and export, only certified H to ISO 17712.
- Lock type for your point. Doors — a bolt seal; odd lugs and hoppers — a cable seal; a wagon — a ZPU.
- Railway approval. For wagons, make sure the ZPU is approved by the relevant railway.
- Numbering and marking. Sequential numbers, a barcode or a logo mean traceability and protection against swapping.
- Material and diameter. Check the pin/cable diameter and the lug hole so the seal physically fits.
A seal records that cargo stays untouched, but the securing keeps it safe in transit. When a load is both properly secured — as we covered in how bad securing can void your insurance payout — and correctly sealed, you protect both sides: against shifting and against unauthorised access.
What’s in the catalogue. Indicative and metal seals, ISO 17712 barrier container seals, sealing wire and railway ZPUs — in the Security Seals section, with separate railway seals and metal seals.
FAQ: choosing security seals
How do you choose a security seal for cargo?
Decide two things: the protection class under ISO 17712 (I, S or H) and the lock type for your sealing point. Internal logistics suits an indicative seal, valuable cargo a security seal, and international container shipping and customs need high security, class H. For wagons, use locking-sealing devices approved by the railway.
What is ISO 17712 and the classes I, S, H?
ISO 17712:2013 is the international standard for classifying mechanical seals for freight containers. It splits seals into three strength classes: “I” — indicative (they show opening), “S” — security (mid-level), “H” — high security (maximum barrier strength). An independent laboratory accredited to ISO/IEC 17025 confirms class H.
How does a high-security seal differ from an indicative one?
An indicative seal (class I) only records that a load was opened and comes off by hand or with a light tool. A high-security seal (class H) is a steel bolt or cable seal that physically resists forced entry, and you can remove it only with a bolt cutter. Class H is exactly what customs programmes such as C-TPAT require.
Which seals do railway wagons need?
Wagons use locking-sealing devices (ZPU) that lock and seal the wagon at once, and the railway administration must approve them. One example is the ZPU “Universal-1M”, approved by Latvijas Dzelzceļš for covered wagons, tanks, grain carriers and hoppers. Sealing wire with lead seals is also widely used.
Do seals need certification?
For class H, yes — it’s mandatory. An independent laboratory accredited to ISO/IEC 17025 must confirm a high-security seal’s strength. Without that testing, a “high security” claim carries no weight. Railway ZPUs additionally need approval from the relevant railway for specific wagon types.
Why do seals carry a number and a barcode?
Every security seal has a unique sequential number, and often a barcode or logo. That provides traceability: you record the number on the shipping documents, and no one can swap in another seal with the same number. On receipt, staff check the number on the seal against the number on the waybill.
Author: LPX Trade Expert Editorial Team — a supplier of seals and cargo securing equipment. This guide is based on the ISO 17712 standard and railway-administration requirements.
Last updated: 30 August 2026.
Sources: CBP: ISO 17712 High Security Seals (classes I/S/H, C-TPAT requirements); What is ISO 17712 (classification and testing); ISO 17712:2013 (classification of mechanical seals); ISO/IEC 17025 (accreditation of testing laboratories).
This article is for information only. Seal requirements depend on the cargo type, the route and the rules of the specific railway or customs authority. Seals and ZPUs are in the catalogue at lpxtrade.lv.