Verifying Earth Fault Loop Impedance on Siemens 3VL MCCB Feeders

1. Commissioning Context

A 300 m UPS feeder originating from a Siemens 3VL moulded-case circuit breaker (MCCB) fitted with an ETU22 electronic trip unit presents a verification challenge that does not arise on a 5 m MCB sub-circuit. The cable impedance alone can shift Zs (earth fault loop impedance) well above the tabulated values in BS 7671, and the protective device is an MCCB with adjustable electronic characteristics, not a fixed-curve MCB. The commissioning engineer must therefore:

• Confirm Zs is low enough to drive the ETU22 into its instantaneous or short-time band within the disconnection time permitted by BS 7671.
• Confirm the prospective fault current (PFC / I_pf) is below the breaker's I_cu rated ultimate breaking capacity.
• Record measured values and the OEM-published trip characteristic used for comparison.

BS 7671 applies to virtually all low-voltage electrical installations, including the larger LV switchboards described here; it is not limited to MCBs in domestic premises. The fact that the standard's Zs tables are written around MCBs is a presentation issue, not an exclusion of MCCBs. The verification principle in Megger's loop testing technical note applies to any protective device: sufficient current must flow during a fault to operate the fuse or circuit breaker within the required disconnection time.

2. MCCB vs MCB: Why the Test Method Differs

The MCB classification (B/C/D) is a shorthand that maps the instantaneous trip multiple to the expected inrush of the connected load. An MCCB has the same physics but exposes the engineer to the actual multiplier dials: Ir (long-time pickup), Isd (short-time pickup and delay), and Ii (instantaneous pickup). The ETU22 is a microprocessor trip unit; its settings, not the BS 7671 MCB tables, are the basis for assessing disconnection time.

3. Siemens 3VL MCCB and ETU22 Trip Unit Architecture

The 3VL range covers three frame sizes:

The ETU22 is a basic electronic trip unit providing three protection functions:

• L (Long-time / overload): Ir adjustable from 0.4 to 1.0 × In; trip class 10 by default.
• S (Short-time / short-circuit): Isd adjustable 1.5 to 10 × Ir with fixed short-time delay t_sd ≈ 0 / 100 / 300 ms.
• I (Instantaneous): Ii fixed at 2 to 15 × In depending on frame.

ETU22 is not an earth-fault trip unit; ETU25, ETU27, ETU45, and ETU76 in the 3VL family add a programmable G (ground-fault) function. At the stated 1 × In setting, the overload band will not trip on a sub-second fault, so the disconnection time is determined by the short-time or instantaneous band of the ETU22. The manufacturer's time-current characteristic (TCC) curve for the specific 3VL breaker and ETU22 dial positions is therefore the only correct reference for assessing disconnection time.

4. Regulatory Framework

BS 7671 Regulation 612.11 requires the measured value of earth fault loop impedance to be compared against the value that will ensure the protective device operates within the disconnection time permitted by Section 41. For MCCBs, this permissible value is calculated from the OEM TCC rather than read directly from Table 41.2/41.3.

5. Earth Fault Loop Impedance: Theory and Components

The earth fault loop for a TN-S or TN-C-S system follows the path:

Source transformer → Phase conductor (R1) → Fault location → CPC / earth conductor (R2) → Source star point

For practical testing the loop is split into two impedances:

• Z_e — external impedance from source transformer star point to installation earth bar; effectively Z_sys at the origin.
• (R1 + R2) — line and protective conductor resistance of the test circuit between the origin and the test point.

So Z_s = Z_e + (R1 + R2) at the test point. The corresponding prospective fault current is:

Ipf = U0 / Zs     (single-phase or three-phase derived to PE)
     = 230 / Zs  for a 230 V TN-S system

The cable contribution over 300 m dominates Zs on long feeders. Using copper at 20 °C (ρ = 0.0172 Ω·mm²/m) and ignoring reactance:

Add Z_e (typical 0.05 to 0.35 Ω for a TN-S system at the secondary of a distribution transformer) to obtain Zs. For a 50 mm² 4-core cable over 300 m with a low-impedance source, Zs commonly lies between 0.30 Ω and 0.60 Ω. I_pf at the load end will then be 230 / 0.45 ≈ 510 A — well within the ETU22 instantaneous band of any 3VL rated 100 A and above.

6. Deriving the Permissible Zs from the ETU22 TCC

Step 1 — Identify the required disconnection time:

• TN final circuit ≤ 32 A: 0.4 s (Reg 411.3.2.1)
• TN distribution circuit: 5 s (Reg 411.3.2.2)
• TT system: 1 s where R_A × I_Δn ≤ 50 V, otherwise 5 s (Reg 411.5)

Step 2 — Read the current required to clear within that time from the ETU22 TCC for the dial settings on site. For a 3VL3 250 A at Ir = 250 A, Isd = 5 × Ir, t_sd = 100 ms, the typical short-time clearing current is around 1250 A for 0.1 s.

Step 3 — Calculate maximum permissible Zs:

Zs(max) = U0 / Iclear
        = 230 / 1250  ≈ 0.184 Ω

Step 4 — Compare with the measured Zs. The measured value must be less than or equal to Zs(max), corrected for conductor temperature. BS 7671 Table 41C / Note 2 in Chapter 41 expects measurement at ambient (typically 20 °C) and either:

• a correction factor for conductor operating temperature (1.2 for 70 °C PVC, 1.28 for 90 °C XLPE), or
• use of an R1 + R2 measured value, which already reflects the cold cable impedance.

7. TN vs TT System Application

If the UPS feeder is part of a TN-S installation (the common arrangement in commercial/industrial LV switchgear), the ETU22 alone can be the protective device. If the system is TT — for example, an isolated generator-fed UPS — an RCCB is mandatory and the MCCB becomes back-up protection only. The MCCB's ETU must still clear the fault, but the 30 mA RCD guarantees touch-voltage compliance.

8. EFLI Test Procedure on a Live MCCB Feeder

The 3VL MCCB is not always dead during commissioning of a parallel sub-system. A controlled live test is permissible provided the methodology below is followed. A typical sequence uses a Megger MFT-1741+ or equivalent 2/3-wire loop tester with PFC calculation.

1. Confirm authority to test with the duty holder; isolate the load-side RCD/RCCB if the test instrument will otherwise trip it. Apply lockout/off to the UPS input isolator; do not rely on the MCCB alone for personnel safety.
2. Verify the test instrument on a known reference or prove unit; record the on-site loop impedance at the origin of the MCCB busbar (Z_ref) and confirm it is within the expected Z_e.
3. Measure R1 + R2 at the far end of the 300 m run (UPS input terminals) using the high-current loop range; high-current injection overcomes any parallel earth return path. Repeat for each phase-to-CPC combination.
4. Capture Z_s at the UPS terminals, then the I_pf the tester calculates. Repeat the test at the MCCB outgoing terminals to obtain a feeder-only Zs delta.
5. Record ambient temperature for the temperature-correction calculation if results will be compared against hot-condition limits.
6. Apply correction (Cmin factor from BS 7671 Table 41C) if the limit was set at conductor operating temperature; do not double-correct.
7. Compare the corrected measured Z_s with Z_s(max) derived from the ETU22 TCC. Document the trip dial positions used for the comparison.
8. Function test the ETU22 with a secondary injection test set (e.g., Megger TPT or OMICRON CMC) to confirm Ir, Isd, and Ii pick-ups against the dials. Many 3VL breakers have a built-in electronic test button that can be used in addition.

9. Documentation and Verification Checklist

Sign the Electrical Installation Certificate (BS 7671 Form 1 for the new work, or Form 2 for additions) and the Minor Works Certificate where appropriate. Attach the TCC sheet, ETU22 dial sheet, and instrument calibration certificate.

10. Common Pitfalls and Field-Proven Caveats

• Reading the wrong TCC: The ETU22 TCC depends on the frame and the Isd / t_sd dial. An ETU22 fitted to a 3VL3 250 A has a different short-time characteristic from one fitted to a 3VL1 160 A. Always request the curve for the breaker serial number.
• Forgetting the 5 s rule: A 300 m UPS feeder is a distribution circuit, not a final circuit; the 5 s disconnection time of Reg 411.3.2.2 applies, but only if there is an RCCB downstream that handles the touch-voltage limit. The MCCB must clear within 5 s on its own if there is no RCD.
• Confusing MCB Type C with MCCB adjustable Isd: Type C MCBs typically trip at 5–10 × In in the instantaneous band. An ETU22 with Isd = 5 × Ir and t_sd = 100 ms is functionally similar but is not labelled Type C. Treat the ETU22 TCC as authoritative.
• Live injection on a UPS rectifier: Many UPS front-end rectifiers have active PFC and IGBT input bridges that will see the loop tester's injection as a fault. Either isolate the UPS before the test (preferred) or consult the UPS manufacturer for immunity tests.
• Phase-rotation and N-E voltage: A 300 m run can develop a non-trivial N-E voltage under load that biases the loop test. Measure under no-load if the loop test reading is unexpectedly high.