LSI vs LSIG Circuit Breakers: Coordination, Fault Current & Ontario Code

LSI and LSIG are not breaker sizes. They describe the protection functions inside an electronic trip unit. The distinction matters when coordinating upstream and downstream devices, evaluating available fault current, and applying Ontario's ground-fault protection requirements.

The Four Trip Functions

Electronic trip units measure current through sensors and apply a programmed protection characteristic. The letters identify available functions—not universal setting ranges. Pickup values, delay bands, I²t options, instantaneous override behaviour and short-time withstand capability must be taken from the selected manufacturer's data.

How to Read a Time-Current Curve

A time-current curve (TCC) uses logarithmic axes: current increases to the right and operating time decreases toward the bottom. The plotted band—not a single line—accounts for device tolerances. Loads, transformer inrush, motor-starting curves and conductor or equipment damage limits are normally overlaid with the protective-device bands.

What the curve is telling you

Selective Coordination: Separation to the Available Fault Current

The objective is simple: for a fault on a downstream circuit, the downstream protective device operates while upstream devices remain closed. The proof is more nuanced. Curves should be separated through the range where TCCs are valid, and the result must be checked against the calculated available short-circuit current at the downstream bus.

Where studies commonly go wrong

• Comparing breaker frame or handle ratings instead of exact catalog numbers, trip units and settings.
• Checking only the overload region and ignoring instantaneous operation at the downstream bus fault level.
• Assuming that an adjustable instantaneous function can always be turned off.
• Coordinating G only with another G function while ignoring downstream phase overcurrent devices.
• Adding delay for selectivity without re-running the arc-flash study; longer clearing time can increase incident energy.

Ontario: What Rule 14-102 Actually Requires

Ontario's current Code is the 29th edition/2024 OESC, effective May 1, 2025. OESC Rule 14-102 addresses ground-fault protection on specified solidly grounded circuits. Its two principal current thresholds are:

Rule 14-102 also limits the protection setting and delay: subject to the coordination scheme permitted by the Rule, the maximum setting is 1200 A and the maximum delay is 1 s for ground-fault currents of 3000 A or more. The full Rule, Diagram 3, Appendix B notes and any current ESA bulletin must be reviewed for the actual installation.

Important distinction: the OESC requires a ground-fault protection outcome. It does not say that every qualifying installation must use a breaker whose catalog suffix is “LSIG.” An integral G function, external relay and sensor scheme, or coordinated switchgear arrangement may be used when it satisfies the Code and product requirements.

Ground-fault protection is not a GFCI

The G in LSIG is equipment ground-fault protection at distribution-system current levels. A Class A GFCI is a different protective function intended for personnel protection at milliampere levels. The terms should not be used interchangeably on drawings or specifications.

Available Short-Circuit Current Comes First

Coordination cannot be evaluated without the available short-circuit current (ASCC) at each bus. The utility source, transformer impedance, conductor length and size, motors and generators can all affect the result. The interrupting rating of each overcurrent device and the short-circuit rating of the equipment must be suitable for the fault current at its location.

Start With the Serving Electricity Distributor (LDC)—not an Assumed Fault Level

In Ontario, the formal industry terms are electricity distributor and local distribution company (LDC). “Local hydro company” is common plain-language wording for the same organization. Before completing the model, submit a technical inquiry or service-connection request to the serving LDC. The requested utility value is the available fault current or source impedance at a clearly defined point of supply. SCCR is not the number the utility supplies; it is an equipment rating that the calculated fault current is checked against.

An infinite-bus calculation remains useful as a conservative screening case, especially before utility data are available. It should not automatically become the only source case in the final short-circuit, coordination or arc-flash model. Use the LDC's stated current or impedance, the applicable normal and alternate configurations, and any identified future-maximum condition; retain the infinite-source case only where the study basis requires it.

Information to request from the LDC

• Maximum three-phase symmetrical RMS fault current at the point of supply, with the exact voltage and reference location.
• Maximum single-line-to-ground (SLG) fault current at the same point, where available. It supports equipment ground-fault protection and relay sensitivity/coordination studies; it is not, by itself, a complete grounding-system or conductor-sizing study.
• Minimum fault current when protection sensitivity, ground-fault operation or minimum arcing-current cases are being studied.
• Source X/R ratio or available asymmetrical/peak-duty information, if the utility can provide it. X/R affects DC offset, first-cycle peak current and momentary/close-and-latch duty.
• Utility transformer data—kVA, voltage ratio, connection and nameplate or guaranteed impedance—when the service transformer is utility-owned.
• System configuration and basis: normal/alternate feeds, parallel transformers, open or closed ties and whether the value represents current or planned maximum conditions.
• Project identification: service address, account or service reference, requested service size/voltage, one-line diagram and the point where the data will be applied.

Subject: Fault Current Data Request — [Project / Service Address]

Hello,

For our short-circuit and coordination study, please provide the following at [point of supply / PCC] at [nominal voltage]:

1. Maximum three-phase symmetrical RMS fault current;
2. Maximum single-line-to-ground fault current, if available;
3. Source X/R ratio and minimum fault current, if available;
4. Utility transformer kVA, ratio, connection and impedance, if applicable;
5. Applicable system configuration and present/planned maximum basis.

Please identify the exact reference point and voltage for the supplied values.

Thank you,
[Name / company / contact information]

Worked Fault-Level Examples

The calculation workflow below is adapted from a project fault-level sheet with project identifiers removed. It separates transformer contribution, upstream-source impedance and a simplified rotating-motor contribution instead of treating every bus as an infinite source.

How the finite-source calculation works

Downstream fault current will usually be lower because conductor and system impedance are added. Conversely, rotating-machine contribution and alternate operating configurations may raise the first-cycle duty at some buses. Study the credible source configurations rather than using a single transformer calculation everywhere.

A Defensible Study Workflow

1. Build the one-line and source cases. Confirm utility data, transformer kVA and impedance, conductor lengths, generators, motors and tie states.
2. Calculate ASCC at every relevant bus. Use the appropriate symmetrical and asymmetrical duties for the equipment being checked.
3. Verify protection and equipment ratings. Check interrupting ratings, equipment short-circuit ratings, conductors and any approved series combinations.
4. Enter exact device data. Model catalog number, frame, sensor/rating plug, trip unit, pickup, delay, I²t choice, instantaneous override and ground-fault sensing method.
5. Overlay TCCs and equipment limits. Include loads, motor starting, transformer inrush and damage, conductor damage and the maximum available fault current.
6. Check manufacturer selectivity tables. Use tested data where curves enter the instantaneous or current-limiting region.
7. Re-run arc-flash and document settings. Coordination, equipment protection and incident energy must be evaluated together. Issue a setting schedule and commission the installed scheme.

Technical References

The following primary sources support the Code edition, Ontario thresholds and the engineering principles used above. Manufacturer documents are application guidance; they do not replace the OESC or project-specific engineering.

1. Electrical Safety Authority — Ontario Electrical Safety CodeConfirms the 29th edition/2024 OESC and May 1, 2025 effective date.
2. ESA Bulletin 10-22-5 — Delta-to-wye conversion requirementsOfficial ESA bulletin restating Rule 14-102 thresholds for solidly grounded systems.
3. ESA Bulletin 2-11-27 — Plans and specificationsIdentifies ground-fault protection design information required for plan review where multiple sources exist.
4. Schneider Electric — Electrical Distribution Fundamentals: System ProtectionElectronic trip functions, TCC coordination, short-time delay and instantaneous override.
5. Schneider Electric — Zone-selective interlockingManufacturer guidance for short-time and ground-fault ZSI operation.
6. Eaton — Circuit breaker time-current curvesManufacturer TCC library for exact product data.
7. Eaton — Selective Coordination Breaker ApplicationExplains tested breaker-pair selectivity tables and their relationship to TCCs.
8. IESO — Find Your Local Distribution CompanyDefines local electricity utilities as local distribution companies (LDCs) and identifies the distributor serving each Ontario service area.
9. Toronto Hydro — Service connection requestsOfficial route for new services, upgrades and other connection work; the applicable LDC process varies by service territory.
10. Toronto Hydro — Metering Requirements, 750 V or LessRequires service equipment short-circuit and interrupting ratings to be sufficient for the maximum available fault current.
11. Hydro One — Connection and customer contactsContact route for customers in Hydro One's service territory; large and specialized connections may be routed to dedicated teams.

Frequently Asked Questions

Continue Reading

Short Circuit & Selective Coordination Explained

Arc Flash Hazard Analysis: What Building Owners Need to Know

Transformer Overcurrent Protection: OESC Rule 26-254