New industry Technology regarding to Bussmann fuse, ABB breakers, Amphenol connectors, HPS transformers, etc.
Eaton FAZ Supplementary Protectors: Trip Curves, Current & Pole Selection
Circuit Protection · Selection Guide
A practical guide to matching Eaton FAZ trip characteristics and pole configuration to the circuit, load inrush and required protection scheme.
The FAZ curve letter matters because it determines the magnetic instantaneous-trip range. Select a curve and current rating together with the circuit design, then verify the exact device’s voltage and interrupting ratings.
Quick answer: Compare the load’s inrush current with the curve’s instantaneous-trip range.
At a glance
- Compare the load’s inrush current with the curve’s instantaneous-trip range.
- Use the exact current range and time-current curve for the chosen catalogue number.
- Confirm pole count, voltage rating, interrupting rating and upstream coordination.
- Standard FAZ is a supplementary protector; do not assume it replaces required branch-circuit protection.
In this guide
1. Understand the trip curve before choosing amperage
The FAZ curve letter describes the magnetic instantaneous-trip range as a multiple of rated current (In). The curve must tolerate normal inrush while still providing protection appropriate to the circuit. The source draft’s curve table was incomplete: Eaton’s product aid includes B, C, D, K, S and Z curves, with broader current ranges than the sample values shown in the original draft.
| Curve | Magnetic instantaneous trip range | Current range shown in Eaton product aid | Selection implication |
|---|---|---|---|
| B | 3–5 × In | 1–63 A | For circuits with low inrush; verify downstream equipment and wiring protection. |
| C | 5–10 × In | 0.5–63 A | General-purpose choice for moderate inrush when coordination permits. |
| D | 10–20 × In | 0.5–40 A | Higher inrush loads such as some motors and transformers. |
| K | 8–12 × In | 0.5–63 A | Consider for applications with specific motor/transformer inrush requirements. |
| S | 13–17 × In | 0.5–40 A | High instantaneous threshold; use only where the application and coordination study support it. |
| Z | 2–3 × In | 1–63 A | Sensitive electronic loads; ensure startup current will not cause unwanted operation. |
2. Match the curve to the load’s inrush profile
Use the curve as an engineering selection, not as a shortcut based only on the load label. Record the expected startup current and duration, steady-state current, conductor rating, fault level and the upstream/downstream protection devices. A higher magnetic threshold can reduce nuisance trips, but it can also change fault-clearing behaviour and must be checked against the installation’s protection requirements.
- B curve: low-inrush applications.
- C curve: moderate inrush, often used for mixed loads where the design supports it.
- D curve: higher inrush, commonly encountered with certain motors and transformers.
- K, S and Z: use the exact manufacturer time-current curve and application guidance; do not assume they are interchangeable with B/C/D.
3. Select rated current from the circuit design
The protector rating must be appropriate for the load, wiring and applicable installation rules. Do not simply choose the next current above normal operating current: continuous loading, ambient temperature, enclosure conditions, conductor ampacity, fault current and coordination can all affect the final choice. Check the exact FAZ catalogue number and rating conditions.
4. Confirm poles and common disconnection
FAZ devices are available in multiple pole configurations, including 1P through 4P and selected neutral arrangements. Choose the configuration required by the circuit topology and local code. For a three-phase load, use an appropriately rated multipole/common-disconnect arrangement where required; do not assume three independent single-pole devices provide the same behaviour as a listed multipole device.
5. FAZ is a supplementary protector
Eaton identifies the standard FAZ range as supplementary protectors under UL 1077. They are intended to complement branch-circuit protection within equipment, not automatically replace the installation’s required branch-circuit breaker. Confirm the applicable standard, voltage rating, interrupting rating and upstream protective device. Also distinguish standard FAZ models from FAZ-DC variants; their ratings and applications are not identical.
6. FAZ selection checklist
- Identify the circuit function and load characteristics.
- Estimate steady-state current and inrush magnitude/duration.
- Select a curve using the manufacturer time-current characteristics.
- Choose current rating, poles and neutral arrangement.
- Verify voltage, fault level, interrupting rating and upstream coordination.
- Check whether the required device is standard FAZ or FAZ-DC and verify the full part number.
Frequently asked questions
No. C curve is common for moderate inrush, but the correct curve depends on measured/estimated startup current, fault conditions, wiring protection and coordination.
Not automatically. FAZ is classified as a supplementary protector; follow the applicable code and Eaton documentation for required upstream branch protection.
No. Treat FAZ and FAZ-DC as distinct catalogue variants and check each model’s voltage, current, curve and approvals.
Need help matching a part number?
Share the load type, operating current, startup/inrush profile, circuit voltage, pole requirement, prospective fault current and upstream protection details. These inputs are needed to confirm the right FAZ curve and rating.
Before ordering: confirm the complete catalogue number, regional approvals, electrical ratings and compatible accessories in the latest manufacturer documentation.
Manufacturer references
Use these references for final engineering verification. Data in selection examples can vary by market, release and exact suffix.
New industry Technology regarding to Bussmann fuse, ABB breakers, Amphenol connectors, HPS transformers, etc.
TAG: Eaton FAZ FAZ supplementary protector FAZ trip curves B curve C curve D curve K curve Z curve miniature circuit breaker