Overcurrent Protection Coordination Methodology — CTI, IDMT Curves, and Selectivity

Protection coordination (also called protection grading or selectivity) ensures that when a fault occurs, only the protective device nearest to the fault operates, isolating the faulted section while leaving the remainder of the system energised. Poor coordination leads to nuisance tripping of upstream devices, cascading outages, and loss of supply to healthy circuits.

This document covers the overcurrent protection coordination methodology implemented in ECalPro, referencing:

• IEC 60255-151:2009 — Measuring relays and protection equipment: Functional requirements for over/under current protection
• IEC 60947-2:2016 — Low-voltage switchgear and controlgear: Circuit-breakers
• BS 7671:2018+A3, Chapter 53 — Switching and isolation
• AS/NZS 3000:2018, Clause 2.5 — Protection against overcurrent
• IEEE C37.112:2018 — Standard for inverse-time characteristics of overcurrent relays

The methodology applies to both relay-based protection (MV and HV systems) and device-based protection (LV systems with MCBs, MCCBs, and fuses). ECalPro generates time-current characteristic (TCC) plots showing the coordination between all devices in the protection chain, from the furthest downstream device to the utility source.

The Current Transformer Interface (CTI) grading margin, more commonly called the grading margin or discrimination time interval, is the minimum time difference between the operating times of two successive overcurrent protection devices at the same fault current. Per IEC 60255-151 Clause 5.3 and standard industry practice, the grading margin must account for:

• Circuit breaker operating time (tripping mechanism + arc extinction): typically 50-80 ms
• Relay timing error (for electromechanical relays: +/- 7.5%; for numerical relays: +/- 5%)
• CT saturation error at high fault currents
• Safety margin for relay reset after fault clearance

The standard grading margins used in practice are:

The grading margin is applied at the maximum fault current at the point of installation of the downstream device. For example, if a downstream relay operates in 0.5 s at 5 kA, the upstream relay must not operate faster than 0.5 + 0.3 = 0.8 s at the same 5 kA.

t_upstream(at I_fault) ≥ t_downstream(at I_fault) + CTI     — (Eq. 1)

Where:
  CTI = 0.3 s for numerical relays
  CTI = 0.4 s for electromechanical relays

ECalPro computes the grading margin at each fault current level across the full range of the TCC and flags any point where the margin is violated.

Inverse Definite Minimum Time (IDMT) overcurrent relays have an operating time that decreases as the fault current increases, following standardised time-current equations. IEC 60255-151 defines four standard curve families:

Standard Inverse (SI):

t = TMS x 0.14 / (I_r^0.02 - 1)     — (Eq. 2)

Very Inverse (VI):

t = TMS x 13.5 / (I_r - 1)     — (Eq. 3)

Extremely Inverse (EI):

t = TMS x 80.0 / (I_r^2 - 1)     — (Eq. 4)

Long-Time Inverse (LTI):

t = TMS x 120.0 / (I_r - 1)     — (Eq. 5)

Where:

The IEEE equivalent curves per IEEE C37.112 use slightly different equations but produce similar shapes. ECalPro supports both IEC and IEEE curve families.

Operating time comparison at TMS = 1.0:

Note the dramatic difference in behaviour: the SI curve is relatively flat across the current range, while the EI curve drops extremely rapidly at high fault currents. This characteristic behaviour is the basis for curve type selection.

The choice of IDMT curve type depends on the ratio between the maximum fault current (I_sc_max) and the minimum fault current (I_sc_min) at the protected zone, and on the characteristics of the downstream protective device.

Guidelines for curve selection:

Key principle: To achieve coordination between two IDMT relays in series, the upstream relay should ideally use the same or a flatter curve type than the downstream relay. Using a steeper curve upstream risks the grading margin being violated at some point in the current range even if it is satisfied at the maximum fault current.

ECalPro's protection calculator allows the user to select any combination of curve types and verifies coordination across the entire fault current range (not just at one point), plotting both curves on the same TCC diagram and highlighting any grading margin violations.

Back-up protection provides a second line of defence if the primary (downstream) protective device fails to operate. Per IEC 60947-2 Clause 2.17 and standard industry practice, back-up protection must clear the fault within a time that prevents:

• Equipment damage beyond the faulted zone
• Conductor thermal damage (adiabatic withstand)
• Risk to personnel from sustained arc flash energy

The "2-second rule" is a widely used guideline: the back-up (upstream) device should clear the maximum fault current within 2 seconds if the primary device fails. This derives from the typical cable adiabatic withstand time at maximum fault current:

t_withstand = (k x A / I_sc)^2     — (Eq. 6)

Where:
  k  = cable adiabatic constant (115 for Cu/PVC, 143 for Cu/XLPE)
  A  = conductor cross-sectional area (mm2)
  I_sc = prospective fault current (A)

For a 95 mm2 copper/XLPE cable at 20 kA fault current:

t_withstand = (143 x 95 / 20,000)^2 = (0.679)^2 = 0.46 s     — (Eq. 7)

In this case, the back-up device must operate in less than 0.46 s, not the generic 2 seconds. The 2-second rule is a simplification that applies to moderate fault levels on adequately sized conductors.

ECalPro computes the actual cable adiabatic withstand time for each circuit and verifies that the back-up protection clears the fault within this time. If the withstand time is less than the back-up relay operating time, the system flags a red FAIL and suggests remedial actions: increase cable size, reduce relay time setting, or add an instantaneous high-set element.

The instantaneous high-set element (sometimes called "50" element per ANSI device numbering) provides near-instantaneous tripping for close-up faults where the fault current exceeds a set threshold. Per IEC 60255-151 Clause 5.5, the high-set pickup should be set above the maximum through-fault current for downstream faults to maintain selectivity.

For low-voltage systems protected by miniature circuit breakers (MCBs) per BS EN 60898-1 or moulded-case circuit breakers (MCCBs) per IEC 60947-2, coordination cannot always be achieved by time-current characteristics alone because both devices may operate in their instantaneous (magnetic) region at high fault currents.

Time-based discrimination works only when the downstream MCB operates in its thermal (overload) region and the upstream device has a slower characteristic. At fault currents above the instantaneous trip threshold (5x In for Type B, 10x In for Type C, 20x In for Type D), both MCBs trip instantaneously and selectivity is lost.

To achieve discrimination at high fault currents, manufacturers provide energy-limiting data (also called let-through energy or I2t data). The principle is:

I2t(let-through, downstream) < I2t(trip-threshold, upstream)     — (Eq. 8)

Where:
  I2t(let-through) = energy let through by the downstream device
                     during fault clearance (A2s)
  I2t(trip-threshold) = energy required to trip the upstream device
                        in its instantaneous region (A2s)

If the downstream device limits the fault energy below the upstream device's trip threshold, the upstream device does not see enough energy to trip, and selectivity is maintained. This is called energy-based discrimination or back-up protection coordination.

Manufacturer selectivity tables are the primary source for MCB-MCB coordination data. These tables specify, for each combination of upstream and downstream device ratings, the maximum fault current up to which full selectivity is guaranteed. For example:

Per IEC 60947-2 Annex A, selectivity can be either:

• Total selectivity: Discrimination is guaranteed up to the rated conditional short-circuit current (Ics) of the downstream device.
• Partial selectivity: Discrimination is guaranteed up to a specified fault current level, above which both devices may trip.

ECalPro's protection calculator includes built-in selectivity tables from major manufacturers and plots the let-through energy curves alongside the time-current characteristics to verify discrimination at all fault current levels.

Fuses inherently provide good time-current discrimination due to their inverse characteristic. For two fuses in series per BS 88-2 (IEC 60269-2), the general rule is:

Ratio rule: I_n(upstream) / I_n(downstream) ≥ 1.6     — (Eq. 9)

This 1.6:1 ratio (some sources use 2:1 for full discrimination) ensures that at any fault current up to the breaking capacity, the downstream fuse clears the fault before the upstream fuse begins to melt. The exact ratio depends on the fuse type and manufacturer; the total pre-arcing I2t of the upstream fuse must exceed the total clearing I2t of the downstream fuse:

I2t(pre-arc, upstream) > I2t(clearing, downstream)     — (Eq. 10)

For fuse-MCB coordination, the fuse operating time at the MCB instantaneous trip current must exceed the MCB total clearing time. Since MCBs clear in less than 10 ms in the magnetic region, coordination is generally achieved if the upstream fuse has a pre-arcing time exceeding 10 ms at the prospective fault current.

ECalPro plots fuse and MCB characteristics on the same TCC diagram, verifying that the upstream fuse pre-arcing curve sits entirely above the downstream MCB total clearing curve across the full fault current range.

The ECalPro protection coordination calculator automates the full coordination study:

1. System Definition: User defines the single-line diagram from source to load, specifying transformers, cables, busbars, and load types.
2. Fault Level Calculation: ECalPro computes the maximum and minimum prospective fault currents at each bus using the IEC 60909-0 methodology.
3. Device Selection: User selects protective devices (relays, MCBs, MCCBs, fuses) for each protection point, with pickup and time settings.
4. TCC Generation: ECalPro plots all device characteristics on a unified time-current diagram, using the standard IEC 60255-151 or IEEE C37.112 equations for relay curves and manufacturer data for MCBs and fuses.
5. Grading Check: At each fault current level, ECalPro verifies that the grading margin (CTI) between successive devices is maintained. Violations are highlighted in red on the TCC plot.
6. Energy Check: For MCB-MCB and fuse-MCB pairs, ECalPro verifies energy-based discrimination using I2t data.
7. Cable Withstand: ECalPro overlays the cable adiabatic withstand curve on the TCC and verifies that all protection devices operate within the cable's thermal limit.
8. Report: Generates a professional coordination study report with TCC plots, grading tables, settings summary, and pass/fail indicators with full clause references.

The report includes all relay settings (pickup, TMS, curve type, instantaneous element settings), the computed grading margins at critical fault current levels, and recommendations for any necessary adjustments to achieve full selectivity.