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 per scenario as needed.

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.)

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.

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 edit panel shows an OCPD Sizing or Switch Sizing section with:

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
Warnings and errors from the sizing run

For switches, the section 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), this section 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 or dismiss them.

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

ekx may support coordination analysis in future versions.

Device catalog and model selection

ekx includes a device catalog for selecting specific manufacturer breaker frames and trip units:

In the edit panel for a protection device, look for the Device Model section.
Select a breaker frame from the catalog.
The system shows compatible trip units for that frame.
Select a trip unit to auto-populate device ratings and settings.

The catalog enforces compatibility constraints - you cannot pair incompatible frames and trip units.

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

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

Configure advanced settings in the edit panel under the Device Settings section. You can also upload a manufacturer datasheet and use AI device settings parsing to auto-populate these values.

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 (current / 80% continuous rating per NEC 240.6)
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