Radial Feeder Protection Coordination Project

This project produces a protection-coordination package for a radial low-voltage feeder. The objective is to select and justify protective-device settings that carry normal load, detect credible faults, coordinate with upstream protection where possible, and expose cases where full selectivity is not technically defensible.

The project is a screening workflow, not a replacement for a full short-circuit, arc-flash, and protection study using approved software and device curves. It is useful because it forces the engineer to connect load current, available fault current, pickup settings, operating modes, device ratings, and validation evidence.

Project Objective

Coordinate protection for an industrial 480 V radial feeder supplied by a 13.8 kV to 480 V transformer. The deliverable must include:

• one-line boundary and source modes;
• transformer full-load current and transformer-limited fault current;
• feeder load and motor-starting screen;
• long-time pickup setting;
• instantaneous or short-time pickup setting;
• selectivity margin against the upstream main breaker;
• alternate-source limitation;
• validation and handover evidence.

System Boundary

The simplified distribution path is:

1. utility source;
2. 13.8 kV to 480 V transformer;
3. 480 V main switchboard breaker;
4. radial feeder breaker;
5. motor-control center and downstream loads.

Use the following data:

Quantity	Value
transformer rating	2000\ \text{kVA}
transformer secondary voltage	480\ \text{V} line-to-line
transformer impedance	5.75\%
radial feeder breaker frame	600\ \text{A}
feeder cable ampacity	500\ \text{A}
maximum measured demand	320\ \text{A}
largest motor-starting current seen by feeder	1200\ \text{A} for 6\ \text{s}
maximum close-in feeder fault current	28\ \text{kA}
minimum far-end feeder fault current, utility source	3.2\ \text{kA}
minimum far-end feeder fault current, generator source	1.6\ \text{kA}
required coordination margin	0.20\ \text{s}

The feeder supplies production loads. A downstream feeder fault should trip the feeder breaker before the upstream main breaker whenever the system mode and equipment capability allow it.

Step 1: Calculate Transformer Full-Load Current

For a three-phase transformer:

Use:

Then:

Engineering Comment

Transformer full-load current is not the feeder pickup setting. It establishes the source equipment scale and helps check whether protective-device ratings and short-circuit assumptions are plausible.

Step 2: Estimate Transformer-Limited Fault Current

Using transformer percent impedance as a first screen:

where:

Therefore:

Use:

Engineering Comment

The actual close-in fault current for the feeder is given as 28\ \text{kA} after source impedance, cable impedance, and study assumptions are included. The transformer screen is still useful because it checks order of magnitude. A breaker with only 18\ \text{kA} interrupting rating would be suspect before detailed coordination begins.

Step 3: Choose Long-Time Pickup

The feeder maximum measured demand is:

A simple load margin is 125 percent:

The feeder cable ampacity is:

Select:

This satisfies:

Engineering Comment

The long-time pickup should not nuisance-trip at normal load, but it must still protect the cable and feeder equipment. A setting above cable ampacity may preserve continuity while quietly removing thermal protection.

Step 4: Screen Instantaneous Pickup

The largest motor-starting current is:

Choose an instantaneous pickup above motor starting:

Check against the minimum far-end fault current in utility-source mode:

Detection ratio:

The setting is above motor starting and below the minimum utility-source fault:

Engineering Comment

The utility-source mode passes the first screen, but the margin is not generous. Device tolerances, CT accuracy, arcing-fault current, temperature, and curve bands can erode the apparent ratio. The detailed study must use the actual trip unit curve and tolerances.

Step 5: Check Alternate-Source Mode

The minimum far-end fault current in generator mode is:

Compare with the selected instantaneous pickup:

Therefore the selected instantaneous element may not detect the weakest generator-fed far-end fault.

Engineering Comment

This is the most important result in the project. A setting that works with utility fault current may fail in generator mode. The solution is not to hide the mode. The design may need source-mode-specific settings, ground-fault protection, differential protection, zone-selective interlocking, generator protection review, or a defined operating restriction.

Step 6: Check Upstream Selectivity Margin

For a downstream feeder fault of:

use the device-curve readings:

Device	Clearing time at 3.2\ \text{kA}
feeder breaker	0.10\ \text{s}
upstream main breaker	0.35\ \text{s}

Coordination margin:

Required margin:

Because:

the feeder and main breaker coordinate at this current.

Engineering Comment

Coordination is current-specific. Passing at 3.2\ \text{kA} does not prove selectivity at all fault currents. The time-current curve must also be checked at higher currents where instantaneous elements, short-time bands, and current-limiting behavior may overlap.

Step 7: Identify High-Current Selectivity Loss

At a close downstream fault, suppose:

If the upstream main breaker instantaneous pickup is active at:

then the upstream main may trip for the downstream feeder fault. Example clearing times are:

Device	Clearing time at 12\ \text{kA}
feeder breaker	0.08\ \text{s}
upstream main instantaneous	0.05\ \text{s}

The upstream main would clear first:

This fails selectivity.

Engineering Comment

The setting package must state the limitation. If service continuity is important, the engineer should evaluate disabling or delaying the upstream instantaneous element, using zone-selective interlocking, applying differential protection, or accepting a defined nonselective region for safety and equipment-duty reasons.

Step 8: Produce the Setting Recommendation

A defensible preliminary recommendation is:

Function	Preliminary setting	Reason
feeder long-time pickup	400\ \text{A}	above load, below cable ampacity
feeder instantaneous pickup	2400\ \text{A}	above motor starting, below utility-source far-end fault
upstream main instantaneous	disable, delay, or raise after study	avoids upstream trip for downstream high-current faults
generator-source operation	separate review required	far-end fault current may be below instantaneous pickup
maintenance mode	reviewed separately	may trade selectivity for lower incident energy

This is not final until manufacturer curves, CT ratios, breaker trip-unit tolerances, cable damage curves, arc-flash analysis, and commissioning tests are reviewed.

Failure Modes and Controls

Failure mode	Effect	Control
pickup below motor starting	nuisance trip during normal starts	compare with starting current and duration
pickup above minimum fault current	fault may not clear	minimum-fault-current check in every source mode
upstream instantaneous too low	loss of selectivity	curve overlay and selective-interlocking review
interrupting rating too low	breaker may fail during fault	maximum fault-current and equipment-duty check
generator mode ignored	weak-source faults not detected	mode-specific protection study
settings changed without record	future study becomes invalid	controlled settings file and relay/breaker report

Validation Evidence

The project handover should include:

1. one-line diagram and approved source modes;
2. transformer, cable, breaker, CT, and trip-unit data sheets;
3. maximum and minimum fault-current study cases;
4. time-current curve plot with feeder and upstream devices;
5. load current, motor-starting current, and inrush basis;
6. interrupting-rating and short-time withstand check;
7. generator-mode or inverter-mode limitation;
8. arc-flash and maintenance-mode interface note;
9. final settings table with revision control;
10. commissioning record showing installed settings match the approved package.

Decision

The utility-source radial feeder setting can proceed to detailed curve validation using:

and:

The package must not claim complete selectivity in every mode. Generator-source operation fails the minimum-fault-current screen, and the upstream main instantaneous element can defeat selectivity for high-current downstream faults. The correct engineering decision is to document those limits, revise upstream settings or protection architecture where required, and release only the source modes that have verified fault detection and coordination evidence.