Protection devices

Protection devices (circuit breakers, switches, and fuses) protect conductors and equipment from overcurrent conditions and provide safe isolation/switching points.

Device types

HV breakers (high voltage):

Medium voltage applications: 2.4kV to 38kV
Vacuum or SF6 gas interruption
Substation and industrial distribution
High interrupting capacity: 500 MVA to 5000 MVA

LV breakers (low voltage):

Building distribution: 120V to 600V
Thermal-magnetic or electronic trip
Molded case or insulated case breakers
Interrupting capacity: 10kA to 200kA

Fuses:

Current-limiting protection
Must be replaced after operation
Higher interrupting capacity than breakers
Lower cost than circuit breakers

Switches:

Single-throw (disconnect switch): Closed or Open
Double-throw (load-break switch): Throw A, Throw B, or Open
Used for feeder/source transfer and isolation
Carry a rated current and short-circuit interrupting rating

Choose device type based on voltage level and application.

Creating protection devices

Drag HV Breaker, LV Breaker, Switch, or Fuse from toolbar.
Drop on canvas.
Connect line side (top handle) to source.
Connect load side (bottom handle) to protected equipment.
Configure ratings in edit panel.

Switch toolbar behavior:

Toolbar has one Switch item.
In edit panel, set Switch Type to single-throw or double-throw.
Set Operating State in the edit panel, or right-click the device and choose Open, Close, Throw A or Throw B. In a scenario the state applies to that scenario only.

Rated voltage and current

Rated voltage:

Maximum system voltage the device can interrupt
Must meet or exceed system voltage
Example: 480V breaker on 480V system

Rated current:

Continuous current capacity
From NEC sizing or load calculation
Standard sizes per NEC 240.6: 15A, 20A, 30A, 40A, 50A, 60A, 70A, 80A, 90A, 100A, 125A, 150A, 175A, 200A, 225A, etc.

See OCPD sizing for sizing guidance.)

Main breaker auto-sizing

When a breaker or fuse protects a downstream bus, ekx sizes it from aggregate downstream demand on that bus instead of the first single load found downstream.

The edit panel explains the selected size using:

aggregate downstream demand
continuous-duty/nameplate requirement
optional main-breaker sizing margin
protected bus rating
protected conductor ampacity when a constrained conductor is present

If the protected bus is in Auto mode, ekx can propose a bus upsize so the main breaker does not exceed the bus rating. If the protected bus is Manual or Locked, the bus rating is treated as a hard limit and any blocked margin or sizing conflict is shown in the sizing result.

For main and aggregate breakers, ekx also verifies the source-side protected conductor path. If one source-side cable is found, its effective ampacity caps the breaker. One exception: when the feeder serves a motor and other loads, NEC 430.63 can require a device above the conductor that NEC 430.24 sized for it. That conductor is protected under Article 430 (NEC 240.4(G)), so ekx sizes the breaker to the 430.63 minimum, leaves the conductor at its 430.24 size, and reports the conductor as protected under the motor feeder rules. A transformer secondary conductor keeps its cap. If the breaker is directly connected to one source or transformer, ekx sizes without a conductor cap. A simple one-in/one-out upstream bus is treated as a pass-through proof point, so ekx keeps the nearest breaker-side cable as the protected conductor and continues toward the source boundary. If the source side is ambiguous, has a branched upstream bus, or has multiple possible source paths, the breaker upsize is blocked until the topology is clarified.

Linked main/aggregate breaker, protected bus, and protected-conductor suggestions are applied as one coordinated group. ekx recomputes the current server-side sizing plan before writing and refuses the whole group if any required member is stale, missing, manual/locked, catalog-invalid, or topology-invalid.

Interrupting capacity

Interrupting capacity (AIC or kAIC) is the maximum fault current the device can safely interrupt:

Low interrupting capacity:

10kA: Residential, light commercial
14kA: Commercial, far from utility
18kA: Most commercial buildings

Medium interrupting capacity:

22kA: Industrial, near transformers
25kA: Heavy commercial
42kA: Large industrial

High interrupting capacity:

65kA: Service entrance, near utility
100kA: Utility-grade equipment
200kA: Special applications

Interrupting capacity must exceed available fault current at the installation point. Use short-circuit analysis to determine required AIC rating.

For switches, Rated Short-Circuit (kA) uses the same interrupting-capacity concept and must exceed available fault current at the switch location.

Switch operating states and power flow

Switch state is respected in topology and analysis:

Single-throw (`disconnect_switch`)
Closed: power passes between top and bottom.
Open: path is blocked.
Double-throw (`load_break_switch`)
Throw A: power passes top to bottom throw A.
Throw B: power passes top to bottom throw B.
Open: both throws are blocked.

Only the active throw path is energized. Inactive/open throws do not carry current in results.

Breakers and fuses follow the same rule: an open breaker or a pulled fuse carries no current, so the buses beyond it lose supply in every study. On the canvas and in the PDF export an open device is drawn with its load-side lead broken and labelled OPEN.

For an automatic transfer switch, connect the normal supply to Throw A, the emergency supply to Throw B, and the common terminal to the load. Right-click the switch and choose Rotate 180° so its throws face the supplies. The selected throw's supply then feeds the load in power flow, short circuit, and arc flash, and arc flash credits only the protective device on that supply.

Fuse classes

Class J:

Current-limiting
600V rated
High interrupting capacity (200kA)
Rejection-type (prevents wrong fuse installation)

Class L:

Large ampere ratings (601A to 6000A)
Current-limiting
Bolt-in mounting

Class RK1/RK5:

General purpose
Current-limiting
Various amp ratings

Class CC:

Branch circuit protection
600V, 0-30A
Space-saving

Class T:

Very high interrupting capacity (200kA)
Compact size
Fast-acting

Select fuse class based on application and required interrupting capacity.

Circuit breaker types

Molded case circuit breakers (MCCB):

15A to 2500A frame sizes
Thermal-magnetic trip
Fixed or adjustable trip settings
Common for building distribution

Insulated case circuit breakers (ICCB):

800A to 5000A
Electronic trip units
Drawout or fixed mounting
Main switchboard applications

Low-voltage power circuit breakers (LVPCB):

800A to 5000A continuous
Microprocessor-based protection
Communications and metering
Highest selectivity and coordination

Number of poles

Single-pole:

120V circuits
One hot conductor
Lighting and receptacles

Two-pole:

240V single-phase
Two hot conductors
Residential and commercial loads

Three-pole:

208V, 480V, 600V three-phase
Three hot conductors
Motors, transformers, three-phase loads

Poles must match circuit configuration.

Insulating medium

Air magnetic breakers:

Standard low-voltage breakers
Arc extinction in air

Vacuum breakers:

Medium voltage (5kV to 38kV)
Arc extinction in vacuum bottle
Long maintenance intervals

SF6 gas breakers:

High voltage transmission
Sulfur hexafluoride gas insulation
Excellent arc quenching

Oil breakers:

Older medium/high voltage designs
Rare in new installations

Device mechanism

Thermal-magnetic:

Bimetallic strip for overload (thermal)
Electromagnetic coil for short-circuit (magnetic)
Simple and reliable
Fixed time-current characteristics

Electronic trip:

Microprocessor-based protection
Programmable settings
Ground fault protection available
Communication capabilities

Hydraulic-magnetic:

Hydraulic time delay for overload
Magnetic instantaneous for short-circuit
Used in molded case breakers

Protection device sizing

Protection devices protect conductors and equipment from:

Overload:

Excess current below short-circuit levels
Thermal protection
Long time delay (seconds to minutes)

Short-circuit:

Fault currents (thousands of amperes)
Magnetic or instantaneous trip
Fast operation (<0.1 seconds)

Ground fault:

Unbalanced current indicating ground fault
Separate ground fault relay or integral GFCI
Protects against shock and fire hazards

OCPD sizing breakdown in the edit panel

When auto-sizing has run for a breaker, fuse, or switch, the Sizing tab of the edit panel shows:

Selected OCPD size and required amps
Cable ampacity cap context (when available)
Details expansion with calculation steps, NEC references, and feeder-demand breakdown for main breakers
Medium-voltage breaker minimum frame context when a breaker OCPD above 1 kV is raised by the configured floor
Warnings and errors from the sizing run

For switches, the tab can also show a Sizing Margin control. This uses the project switch sizing margin by default, can be overridden per switch, and re-runs switch sizing when saved.

For motor-fed breakers and fuses, the details include motor FLC multiplier context (for example Tight/Standard/Maximum mode). For motor-fed switches, the details use the switch current-rating basis of 115% of motor FLC before any switch sizing margin.

If no persisted OCPD result exists yet (for example first render or unsaved temporary components), the sizing breakdown is hidden.

Pending suggestions and linked cable changes

Protection devices can also show pending sizing suggestions in green on the canvas and in the edit panel.

These green values are proposals, not saved changes. You can review them and accept them or keep the current value.

In some cases, a breaker or fuse suggestion is linked to a cable suggestion on the same branch. If the breaker recommendation was calculated against a proposed conductor change, accepting the protection-device suggestion may also accept the dependent cable suggestion.

That linked acceptance is how ekx keeps the reviewed sizing set consistent. NEC affects how the breaker is calculated, but NEC does not require the UI to silently approve both items together.

See OCPD sizing for NEC sizing requirements and Auto-sizing for the current review workflow.

Coordination and selectivity

Protection devices must coordinate to isolate only the faulted section:

Selective coordination:

Upstream device remains closed when downstream device operates
Minimizes outage area
Required by NEC for critical loads (healthcare, fire pumps, etc.)

Time-current coordination:

Plot device curves on log-log paper
Verify upstream device is above (slower than) downstream
Maintain separation ratio

The Protection view can plot TCC curves and screen nominal curve pairs for crossings or tight spacing. After you run a short-circuit study, select a device and open Latest study to see its evaluated operating time at each reachable fault location. The tab loads up to 50 results at a time with Previous and Next controls, uses the current attributed at that device's cable terminal or transformer winding, distinguishes operating time from clearing time, and calls out unavailable current attribution. A device after converging sources is evaluated once using its combined physical current. If the study uses an attribution contract this app does not support, EKX keeps the ordinary short-circuit results but does not calculate protection operating times from those currents.

> Important: If EKX cannot uniquely prove the current through a device, Latest study shows that current as unavailable instead of guessing. This feature does not apply a full tolerance analysis or replace a professional coordination study.

Device catalog and model selection

Use Device Catalog to inspect manufacturer devices, imported settings, source evidence, frame-to-trip-package compatibility, and the full TCC. Add or correct records through the EKX catalogue import skill. To use a catalogue record in a project:

Add and connect a fuse, LV breaker or HV breaker on the project SLD.
Select the component and open the Equipment tab in the edit panel.
For an LV breaker, pick the part in Frame. For an HV breaker, pick the interrupting part in

Breaker Model.

Pick the trip unit in Trip Unit Model, the fuse in Fuse Model, or an HV breaker's relay

in Protective Relay Model. A breaker with a built-in thermal-magnetic trip shows it as Built in and skips this step. Until a trip unit is chosen, the settings area reads "Select a trip unit to view its settings."

For an LV breaker or fuse, pick the curve in Trip curve. A trip unit imported from an ETAP

library, such as a Siemens 3VA electronic trip unit, has no Trip curve pick: it starts on its slowest dial settings, and you set its dials in the Protection view.

Each pick saves at once, and Cmd+Z (Ctrl+Z) undoes it in one step. When you pick a part that has only one trip curve, or a default one, ekx applies that curve for you. If replacing the device needs your confirmation, click Apply — replaces the assigned device. An Apply trip curve button appears only when a curve is shown but not yet applied; it reads Reapply trip curve when the device settings or catalogue curve changed since it was applied, and Try again after a failed apply.

The supported imported-profile arrangements are:

Fuse model and reviewed fuse profile
Breaker frame with a compatible fixed-setting electronic trip unit (an ETAP-imported trip unit

is set on its dials instead)

Breaker with an integrated thermal-magnetic trip
Breaker frame with a compatible separate thermal-magnetic trip unit

The edit panel shows the curve and a read-only Settings list, not a TCC. Use View curve in Device Catalog to inspect the source curves and full configuration. Use Go to Protection View below the settings to open the project device in the Protection view, where you edit its settings and see its TCC and latest study results.

> Important: Applying a profile changes the protection-device configuration only. It does not > create or connect components and does not run a short-circuit study.

Incomplete profiles remain visible with the reason they are not ready, but cannot be applied. Separate trip packages require an explicit compatibility relationship with the selected breaker frame. If a saved profile later changes or becomes unavailable, the project characteristic is marked Needs reapply or Needs replacement until you reapply it or pick another curve; ekx does not substitute a different curve at the same rating.

Advanced device settings

Protection devices support detailed settings beyond basic ratings:

Electronic trip unit (ETU) settings:

Long-time pickup and delay
Short-time pickup and delay
Instantaneous pickup
Ground fault pickup and delay

A trip unit imported from an ETAP library shows the real trip unit's dials, with their names and positions, and its curve is drawn from those dials. A 0-second short-time band trips at the breaker's own clearing time. These trip units have no ground fault or instantaneous off, because the source library does not offer them. A Siemens 3VA ETU 350 follows Siemens' published behaviour: its long-time delay is timed at 6 x Ir and its short-time pickup and delay follow Ir, so lowering Ir moves the whole curve.

Relay overcurrent settings:

Time-overcurrent curves
Pickup multiplier
Time dial

Overload relay settings:

Thermal overload class
Trip current setting

Fuse settings:

Fuse class and type
Speed characteristic

The edit panel lists these settings read-only under Settings. Edit them in the Protection view; Go to Protection View opens the device there. For reusable manufacturer data, add a sourced Device Catalog record and assign it to the project device.

Protection device annotations

After power flow analysis, protection devices (including switches) show:

Rated loading percentage (current / device rated current)
Derated loading percentage for LV breakers: continuous-load current counts at 125% on an 80%-rated breaker, other current at 100%
Color-coded by loading: green (≤90%), yellow (>90% to 100%), red (>100%)

See Interpreting results for details on dual loading metrics.

OCPD sizing - NEC sizing requirements for protection devices
Short-circuit analysis - Determining required interrupting capacity
Cables - Protecting conductors with appropriate OCPD
Interpreting results - Reading protection device annotations