13.3 Phase Sequence, Voltage Drop & Additional Functional Testing

13.3 Phase Sequence, Voltage Drop & Additional Functional Testing

Core Testing Principle: Initial verification and periodic inspection extend beyond dead continuity and earth loop impedance to encompass operational integrity. Under BS 7671, three distinct verifications confirm overall system functionality: Regulation 643.9 requires maintained phase sequence at relevant polyphase points;643.11 addresses voltage-drop verification where appropriate; and 643.10 requires functional testing of applicable assemblies, controls and interlocks.

Phase Sequence Testing on Polyphase Systems (Regulation 643.9)

In three-phase electrical distribution systems (400 V line-to-line, 230 V line-to-neutral), three sinusoidal voltage waveforms are generated with an electrical phase displacement of 120° (2π / 3 radians):

  BALANCED THREE-PHASE SUPPLY: SAME FREQUENCY,120° DISPLACEMENT
  With L1 chosen as reference:
  vL1=Vpk sin(ωt)     vL2=Vpk sin(ωt−120°)     vL3=Vpk sin(ωt−240°)
  Maintaining L1/L2/L3 identification preserves this sequence.
  A motor’s required physical rotation also depends on its terminals
  and the viewing direction specified by its manufacturer.

The Standard Harmonized Sequence

Under BS 7671, the harmonized phase conductors are designated:

• Line 1 ($L_1$): Brown
• Line 2 ($L_2$): Black
• Line 3 ($L_3$): Grey
• Neutral ($N$): Blue

For the stated sequence, the corresponding waveform peaks occur120° apart in the order L1, L2, L3. This alone does not define clockwise shaft rotation for every machine or viewing direction.

Why Reversed Rotation Matters

Interchanging any two lines reverses the rotating field of a directly supplied three-phase induction motor. The driven equipment may then operate incorrectly: a pump may lose performance, a fan may deliver inadequate airflow, or machinery may move in an unintended direction. The actual consequences depend on the design; do not assume a universal impeller failure or time to seizure.

Verification and Commissioning

1. Maintain identification: Inspect the L1/L2/L3 connections and identification at relevant points. This can verify phase sequence without unnecessary live testing.
2. Live indication where needed: Use a suitable phase-sequence instrument complying with BS EN 61557-7 or providing equivalent performance and safety. Connect the specified leads to the identified lines under a justified safe procedure; do not infer conductor identity from arbitrary lead colours alone.
3. Interpret the indication: Compare the displayed sequence with the instrument instructions and intended circuit arrangement. A clockwise instrument indication is not proof of every connected motor’s required shaft direction.
4. Check the equipment: Follow manufacturer commissioning instructions for a safe rotation check, including the specified viewing direction and controls. Correct a transposition only after safe isolation, then repeat the affected checks.

Verification of Voltage Drop (BS 7671 Regulation 643.11 & Appendix 4)

Every electrical cable possesses internal resistance and reactance. When load current circulates through circuit conductors, an electrical potential difference develops along the length of the run, reducing the voltage delivered to the terminal equipment.

Why Voltage Drop Matters

Low utilisation voltage can reduce motor starting torque, affect running performance and cause lighting or electronic equipment to malfunction. For the usual fixed-frequency induction-motor approximation, starting torque varies with voltage squared:0.9²=0.81, a19% reduction. Whether that prevents starting depends on the motor and load; it is not an automatic catastrophic outcome.

Table 4 Ab Voltage-Drop Guidance

Regulation 525.1 requires the equipment requirements to be considered. Table 4 Ab in Appendix 4 gives informative values from the installation origin to a load point:

These are design guidance, not statutory voltage thresholds. Check the actual equipment tolerance and supply voltage: subtracting 11.5 V from nominal 230 V does not guarantee218.5 V at the load, since the voltage at the origin can already be below 230 V. Private-transformer guidance likewise does not guarantee satisfactory equipment operation.

Methods of Voltage Drop Verification (Regulation 643.11)

Where voltage drop needs verification,643.11 permits calculation or measurement. Use the appropriate method for the circuit and equipment:

Method 1: The Design Calculation Method

In initial verification, because full operational design load is rarely connected at the time of testing, voltage drop can be assessed by calculation using the tabulated millivolt-per-amp-per-metre $(mV/A/m)$ values from BS 7671 Appendix 4:

$V_d = \frac{(mV/A/m) \times I_b \times L}{1000}$

Where:

• $(mV/A/m)$: Tabulated voltage drop figure for the cable type, cross-sectional area, and installation method (from BS 7671 Tables 4D1B to 4J4B).
• $I_b$: Circuit design current in amperes (NOT the protective device rating $I_n$!).
• $L$: Single-way route length of the circuit in metres.
• $1000$: Divisor converting millivolts to volts.

Worked Example: A single-phase radial circuit wired in $2.5\text{ mm}^2$ flat twin-and-earth cable with 70°C thermoplastic insulation ($(mV/A/m) = 18\text{ mV/A/m}$) supplies a 16 A design load ($I_b = 16\text{ A}$) over a run length of 32 metres: $V_d = \frac{18 \times 16 \times 32}{1000} = \frac{9216}{1000} = \mathbf{9.22\text{ V}}$ For a public 230 V power circuit, the5%guidance value is 11.5 V. The calculated 9.22 V is within that value. Confirm the selected cable data/temperature basis and actual equipment voltage requirement; this calculation alone does not establish complete circuit compliance.

Method 2: The Direct Measurement / Impedance Method

1. Direct Operational Measurement: Measuring the line-to-neutral voltage at the distribution board origin ($V_1$) and simultaneously measuring terminal voltage at the furthest load ($V_2$) while the circuit is operating under full steady-state design load ($I_b$). The difference represents the actual operating voltage drop ($\Delta V = V_1 - V_2$).
2. Line-Neutral Impedance Derivation: Measuring the dead continuity resistance of the line conductor plus the neutral conductor $(R_1 + R_n)$ to the furthest point using a low-resistance ohmmeter. For a predominantly resistive single-phase circuit, adjust the resistance to the relevant operating temperature before using: $V_d = I_b \times (R_1 + R_n)$ This approximation needs refinement where reactance and load power factor are significant. A cold continuity value used without the necessary temperature assessment can understate operating voltage drop.

Functional Testing of Assemblies & Controls (Regulation 643.10)

Under BS 7671 Regulation 643.10, assemblies such as switchgear and controlgear assemblies, drives, controls, and interlocks must be subjected to functional testing to verify that they are correctly mounted, configured, and operate in accordance with relevant equipment standards and design specifications.

                         THE SCOPE OF FUNCTIONAL TESTING
  ┌────────────────────────────────────────────────────────────────────────┐
  │ 1. SWITCHGEAR & ISOLATORS                                              │
  │    • Positive mechanical break, smooth handle action, flag indicators   │
  │    • Verify provided interlocks against the actual assembly design      │
  ├────────────────────────────────────────────────────────────────────────┤
  │ 2. EMERGENCY STOPPING CONTROLS (BS EN ISO 13850 / Reg 537.3.3)         │
  │    • Actuation initiates the specified safe stop category       │
  │    • Latching mechanism holds in OFF position; NO AUTOMATIC RESTART!    │
  ├────────────────────────────────────────────────────────────────────────┤
  │ 3. AUTOMATION & CONTACTORS                                             │
  │    • Motor starters, hold-in coils, thermal overloads, interlocks      │
  │    • Time clocks, photocells (dusk-to-dawn), PIR occupancy sensors     │
  ├────────────────────────────────────────────────────────────────────────┤
  │ 4. INTEGRAL PROTECTIVE DEVICE TEST CONTROLS                            │
  │    • RCD / RCBO push-to-test buttons ('T')                             │
  │    • AFDD manual test facility where provided; follow maker instructions   │
  └────────────────────────────────────────────────────────────────────────┘

1. Switchgear, Disconnectors & Interlocks

• Switching and isolation function: Operate the applicable devices to verify smooth operation, required pole disconnection and correct position indication for their intended function. Check device suitability and manufacturer instructions; do not assume every switch performs isolation.
• Enclosure interlocks: Where provided, verify the intended door/switch interlock and any controlled override against the assembly instructions. Industrial assemblies do not all use one identical door mechanism. Apply the required barriers and safe-access procedure.

2. Emergency Stopping Controls (BS EN ISO 13850 & Regulation 537.3.3)

Test the installed emergency-stopping system safely against the machine’s intended design and instructions, controlling the hazards created by the test:

[!CRITICAL]

Mandatory Emergency Stop Operational Rules

1. Specified safe stop: Category 0 removes power immediately; Category 1 first performs a controlled stop and then removes power. A Category 1 system must not be judged defective merely because it maintains power during controlled deceleration.
2. Maintained stop command: Check the actuator’s latching and deliberate manual-reset function in accordance with its design. The stopping command remains effective until properly reset.
3. No automatic restart: Resetting must only permit a subsequent deliberate restart; it must not start the machine. Verify the intended restart controls and safe conditions. An emergency stop is not safe electrical isolation for maintenance.