Okay, let's break down how to determine the clearing time for your 2500A fuse, given a 125,000A fault current, and how it relates to NEC 110.16(B)(3). I need the TCC curve in order to estimate the clearing time. However, I can get us started with what you need to do once you have the curve.

Understanding the Problem and NEC 110.16(B)(3) TCC (Time-Current Characteristic) Curve

This curve is essential. It graphically shows the time it takes for a fuse to clear a fault current of a specific magnitude. You must have the TCC curve for your specific 2500A fuse model to accurately determine the clearing time.

Fault Current

125,000 Amps is a very high fault current.

Fuse Cut-Off

You mention the TCC cuts off at 40,000 Amps. This suggests the curve doesn't extend to currents as high as your available fault current.

NEC 110.16(B)(3) Arc Flash Hazard Warning

This section of the National Electrical Code (NEC) requires equipment to be field-marked with a label containing information about arc flash hazards. Importantly, it directs you to standards and methodologies (like IEEE 1584) to determine the arc flash hazard based on the clearing time of the overcurrent protective device (OCPD), which in this case is your fuse.

Steps to Determine Clearing Time and Compliance


Obtain the Correct TCC Curve

You

absolutely must have the TCC curve for the specific 2500A fuse you are using. The manufacturer's data sheet is the best source. Make sure it applies to the voltage rating of your system.2. Determine Clearing Time from the TCC Curve



If the curve extends to 125,000A

Find 125,000A on the current axis. Trace a vertical line upwards until it intersects the TCC curve. From the point of intersection, trace a horizontal line to the time axis. The value on the time axis is the approximate clearing time of the fuse at that fault current.

If the curve doesn't extend to 125,000A (Cut-Off at 40,000A)

This is a critical issue. You have a couple of options, and neither is ideal:

Consult Fuse Manufacturer

Contact the fuse manufacturer's technical support. They might be able to extrapolate or provide clearing time data even beyond the published curve. Be prepared to provide them with the fuse part number and system voltage.

Use a Conservative Estimate (with caution)

You could conservatively assume the clearing time is less than the time corresponding to the highest current value on the curve (40,000A).

However, this must be done with extreme caution and with the understanding that it might not be accurate, and could lead to underestimating the arc flash hazard. It's far better to get accurate data from the manufacturer. Using the curve you have, you would find the clearing time for 40,000 Amps and use that time.3. Arc Flash Hazard Analysis

Once you have the clearing time (either from the TCC curve directly or from the manufacturer's data), you must perform an arc flash hazard analysis. This analysis uses the clearing time, bolted fault current (125,000 A), system voltage, and other parameters (working distance, electrode configuration, etc.) to calculate:

Incident Energy

The amount of energy a worker could be exposed to during an arc flash. Measured in calories per square centimeter (cal/cm²).

Arc Flash Boundary

The distance from the potential arc source at which the incident energy equals 1.2 cal/cm² (considered the onset of a second-degree burn).

Required Personal Protective Equipment (PPE)

Based on the incident energy, you need to determine the appropriate PPE category (e.g., PPE Level 1, 2, 3, or 4) to protect workers.4.

Labeling (NEC 110.16(B)(3) Compliance)

After the arc flash hazard analysis, you must create and affix a warning label to the equipment. The label must include the following information, as specified by the NEC and the arc flash analysis results: Nominal System Voltage Arc Flash Boundary Incident Energy at a specific working distance or the required PPE category.

Date the analysis was performed.Important Considerations and Cautions



Fuse Coordination

Make sure the 2500A fuse is properly coordinated with upstream and downstream protective devices. A mis-coordinated system can lead to nuisance tripping or, more dangerously, the wrong device clearing a fault, which could increase the arc flash hazard.

Manufacturer's Data is Key

Never rely on generic TCC curves. Always use the data from the fuse manufacturer for the specific fuse model you are using.

Qualified Person

Arc flash hazard analysis and labeling must be performed by a qualified person with expertise in electrical power systems, arc flash calculations, and the NEC. This is usually a professional engineer (PE).

IEEE 1584

The most widely used standard for performing arc flash hazard calculations is IEEE 1584.

Software

There are several software packages available (e.g., EasyPower, ETAP, SKM PowerTools) that can perform arc flash hazard calculations. These tools simplify the process and help ensure accuracy.*

Upgrading Protective Devices: If the Arc Flash incident energy is too high, you may need to consider upgrading your protective devices to ones with a faster clearing time or to Arc Flash relaying.

In summary, to comply with NEC 110.16(B)(3), you need to


Find the exact TCC curve for your 2500A fuse.2. Determine the clearing time for a 125,000A fault current from the TCC curve (or the manufacturer).3. Perform an arc flash hazard analysis using the clearing time and other system parameters.4. Create and affix a warning label to the equipment containing the required information.

Without the TCC curve, I cannot give you a specific clearing time. It's absolutely essential to get that data before proceeding with the arc flash analysis. Flag for review