Buyers lose time arguing about brand when the missing line is the fault level. A fuse or a breaker has to carry the load and has to open a short circuit at that location without rupturing. Those are two ratings. Tradinor puts both on the RFQ.
Ampere rating is not interrupt rating
The continuous current sets cable size and heat. The interrupt rating, often written as kA, sets whether the device can clear the bolted fault the transformer can deliver. A 10 kA device downstream of a large transformer is a hazard even if the load is only a few amperes. Ask the panel builder for the available fault current at the device, then buy a rating above it.
Let-through energy decides what the contactor and the cable have to survive. A current-limiting fuse can protect a contactor that a slow breaker will not. If the coordination study says so, write the fuse class on the drawing. Do not substitute a different class because the holder looks the same.
- System voltage and whether the circuit is AC or DC. DC interrupt ratings are not the AC figure.
- Available fault current at the device, from the study or the transformer data.
- Load current and the inrush, so the curve can ride through a start.
- Fuse class or breaker frame, curve, and trip unit if it is adjustable.
- What must survive downstream: contactor, drive, or cable withstand.
What the datasheet must show
| Field | Fuse | Breaker |
|---|---|---|
| Continuous current | Ampere rating and class | Frame and trip setting |
| Fault | Interrupt rating at the voltage | Icu or Ics at the voltage |
| Speed | Time-current curve | Thermal and magnetic curve |
| DC use | A DC voltage rating, if you have DC | A DC rating, not an assumption |
What to send and what to check
Send the one-line diagram, the available fault current, and the load type. On the incoming lot, read the interrupt marking on the device, not only the carton. A 6 A fuse in a 100 kA class is a different part from a 6 A fuse in a low interrupt class. Reject substitutions that match the amperes and miss the class. Confirm coordination with the upstream device if the study named a pair.
What to lock on the order
Put the device, the holder, and the backup protection on one line of the purchase order so a storekeeper cannot mix families. A fuse that fits the holder is not necessarily the fuse the coordination study named. Record the class, the current, the voltage, and the interrupt rating as four fields. For a breaker, record the frame, the trip, the poles, and the interrupt rating the same way. If the study names an upstream device, print that pair on the drawing and on the order. Incoming inspection is then a reading of markings, not a guess. When a distributor offers an equivalent, ask for the curve and the interrupt rating at your voltage before you accept it. An equivalent that matches the amperes and misses the class is a different design.
Questions buyers actually ask
Is a higher ampere rating safer?
No. The ampere rating is the current the circuit is allowed to carry. The interrupt rating is the fault current the device can clear. They answer different questions.
Can I replace a fuse with a breaker of the same amperes?
Only after the let-through and the interrupt rating are checked against the downstream equipment withstand. Same amperes is not the same protection.
What does time-delay mean?
A time-delay fuse or a thermal-magnetic breaker rides through a motor start. It must still clear a short circuit. Ask for the curve, not the adjective.