Where defect tables break
A defect table is not tested in the meeting where it is approved. It is tested at the inspection table, at the end of a long day, when one unit has a loose seam, another has a scuff under the printing, and somebody has to decide whether either is worth stopping a shipment for. A table that only says "critical, major, minor" without examples of each class on this specific product leaves that decision to whoever is holding the clipboard. The result is inconsistency between inspectors, and a supplier who learns to negotiate the category rather than fix the cause.
The table below is written for one product family, a rechargeable hand warmer with a heating element, a control board, an NTC thermistor, an OTP thermal protector and a battery compartment. Categories only become usable once the examples are that specific.
Three classes, and one class that is not negotiable
Critical, major and minor are only useful if each carries a different consequence. A critical defect is one that can cause harm, and it is not averaged against a good batch: it stops the lot. A major defect makes the unit unfit for sale or for the use the buyer paid for, and it is decided against a stated limit. A minor defect is a visible or measurable deviation that does not affect function, and it is typically accepted within a limit or handled with a price adjustment.
The reason to separate the safety class in writing is that safety defects are usually invisible in a sample of twenty units. A missing OTP protector, an NTC positioned against the shell instead of the heater, or a battery compartment without its insulation will pass a functional check and a visual check. The only reliable control is to list each safety characteristic, test it on every unit in the sample, and give it a veto.
Grading table
| Class | Definition for this product | Examples | Disposition |
|---|---|---|---|
| Critical | Can cause harm, fire or a regulatory breach in normal use or foreseeable misuse | Missing or bypassed OTP; NTC detached from the heating element; exposed conductor at the port; battery compartment insulation absent; charger negotiation that drives the cell beyond its rating | Lot rejected, no negotiation on a percentage, root cause required before re-inspection |
| Major | Unit cannot perform its advertised function, or fails a declared performance characteristic | One heating side dead on a dual-side unit; a heat level that does not engage; runtime far outside the declared band at a stated level; charging that stops before the indicator completes; water ingress at the seam | Lot held, sorted or reworked, re-inspected against the same sample plan |
| Minor | Cosmetic or dimensional deviation with no effect on function or safety | Printing offset, scuff inside the seam shadow, button wobble, moulding flash at the parting line, accessory bag wrinkled | Accepted within a stated limit, or credit by agreement before shipment |
Write the safety list one line at a time
A generic critical category invites interpretation. Enumerate instead, and make each line a test with a pass criterion rather than a description. The list is short enough to keep on one page:
- OTP thermal protector present, correctly mounted and electrically in series with the heater.
- NTC thermistor bonded to the heating element, reading within the tolerance band at ambient.
- No path from the heating element or the cell terminals to the shell surface that a finger can reach.
- Battery compartment lined, with cell orientation keyed and no sharp edge against the wrap.
- Charge current limited by the control board regardless of the charger connected.
- Air gap between heater and shell consistent with the validated build, with no adhesive bridging.
- Every unit in the sample passes the above, not a sub-sample.
Item seven is the part suppliers push back on, and it is the part worth insisting on. A safety characteristic sampled at a low acceptance number is not being controlled.
Sampling is a separate decision from grading
Grading answers "how bad is this unit". Sampling answers "how many units do I look at, and how many failures make the lot fail". Mixing them produces the familiar argument in which a critical defect is treated as one item in an acceptance table. Keep the two on separate pages: the grading table defines the classes and their consequences, while the sampling plan states the inspection level, the sample size and the acceptance and rejection numbers for major and minor classes.
For the critical class the sampling plan should not contain an acceptance number at all. Any occurrence in the sample means the lot is held, and the discussion moves to containment of the shipped quantity rather than to the percentage.
What grading does not decide
Three boundaries matter. First, grading is not sign-off: a unit is graded against a golden sample that was approved earlier, and if no sample was signed the inspector has no reference and grading becomes personal taste. Second, grading is not a root-cause process. It records what was found and what happened to the lot; why the OTP was missing, and what changes in the line to prevent it, needs a separate corrective-action record with an owner and a date.
Third, grading does not settle commercial liability. A rejected lot still has to travel, be stored, be reworked or be destroyed, and the cost of that belongs in the purchase order. Writing "supplier bears all costs" is not a term anyone can execute. Write who arranges the rework, where it happens, who pays the freight, and what evidence closes the case.
Fitting the table to the product
Two product decisions change the table before it is signed. If the unit is dual-side, each heated face needs its own functional check, because a single dead face is a major defect that a single-face test will miss; the dual-side heating area comparison explains why the two faces can drift apart. If the unit declares a temperature, the check needs a stated point and a stated stabilisation time, and a uniformity map resolves the argument about which spot on the face counts.
For units shipped with cells, the battery documentation is part of the shipment rather than the product, and a missing test summary is a documentation non-conformity rather than a product defect. Treat it on its own page with its own owner, and keep the UN 38.3 summary checks beside the inspection file so the two records cannot diverge.
Shipping the inspection sheet
- [ ] Each class has at least six examples drawn from this product, not from a generic template.
- [ ] Every safety characteristic is a pass/fail test with a stated method.
- [ ] The critical class carries a veto and no acceptance number.
- [ ] Major and minor limits are numeric and agreed in writing.
- [ ] The inspection sheet names the sample and the revision it was compared against.
- [ ] The report has a signature block for inspector and supplier representative.
- [ ] A photo log references the sample number, not just the date.
Next step
If you are rewriting an inspection sheet for an existing order, send the current version for a line-by-line gap check, or start from this table through contact.
Written for the Emberly heating-element programme. Specifications are illustrative; confirm every figure against a supplier test record.