9.2 Continuity Testing Method 2 (R2 Wandering Lead)

9.2 Continuity Testing Method 2 (R2 Wandering Lead)

Core Testing Principle: Method 2 measures protective-path resistance using a long wandering lead and a defined reference point. It measures an individual circuit CPC only when the test arrangement excludes other parallel paths and uses the relevant circuit origin. It is indispensable for verifying structural containment earthing, testing supplementary bonding in special locations, and assessing expansive commercial or industrial installations where cross-linking conductors at the distribution board is impracticable.

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Principles and Operational Theory of Method 2

While Method 1 measures the line and protective conductors in series ($R_1 + R_2$), Method 2 measures a protective path without using the line conductor. The wandering lead itself does not isolate that path from other connected conductors.

One terminal of a low-resistance ohmmeter is connected via a long, flexible extension lead (the "wandering lead") to a verified reference earth point—the origin of the circuit or protective conductor being assessed. If the MET is upstream of the circuit board, the reading also includes that upstream protective path. The other test lead is taken around the installation to probe exposed-conductive-parts, metal accessory faceplates, and equipment enclosures.

  MAIN EARTHING TERMINAL (MET)                             EXPOSED-CONDUCTIVE-PART
  ┌───────────────────────────┐                         ┌──────────────────────────┐
  │ [MET / Earth Bar]         │                         │ [Class I Luminaire /     │
  │        │                  │                         │  Metal Switch Enclosure] │
  └────────┼──────────────────┘                         └────────────▲─────────────┘
           │ Fixed Heavy Clamp                                       │ Mobile Test Probe
           ▼                                                         ▼
  ┌────────────────────────────────────────────────────────────────────────┐
  │ [Trailing Wandering Lead: 30m - 50m Reel]                              │
  │                                                                        │
  │                    [ Low-Resistance Ohmmeter ]                         │
  │                    [ Reads: protective path ]                         │
  └────────────────────────────────────────────────────────────────────────┘

Key Distinction from Method 1

• In Method 1, the test circuit is formed by looping through the circuit line conductor. Therefore, Method 1 cannot be performed on conductors that lack an associated phase conductor—such as supplementary bonding conductors or structural bonding grids.
• In Method 2, the reference path is supplied entirely by the tester's wandering lead. This permits the direct measurement of any protective bonding conductor, earth electrode lead, metallic conduit run, or individual equipment frame regardless of circuit wiring arrangements.

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Instrument Setup & Wandering Lead Resistance Management

Use a suitable low-resistance ohmmeter complying with BS EN 61557-4 or providing equivalent performance and safety, with the protective-conductor test parameters and range discussed in Section 9.1. However, using a long wandering lead introduces a substantial practical challenge: lead resistance.

The Wandering Lead Reel

Wandering leads are typically supplied on extension reels containing 30 metres, 50 metres, or 100 metres of flexible copper cable (usually 1.0 mm² or 1.5 mm² cross-sectional area):

• A 50-metre reel of 1.5 mm² copper cable has an internal resistance of: $R_{\text{lead}} = 50\text{ m} \times 0.0121\ \Omega/\text{m} \approx 0.61\ \Omega$
• If this 0.61 Ω lead resistance is not accounted for, a measured circuit $R_2$ of 0.25 Ω would be displayed as 0.86 Ω—falsely indicating excessive resistance or non-compliance.

Two Methods of Lead Compensation

  METHOD A: ELECTRONIC NULLING (PREFERRED)
  ┌─────────────────┐       Firm Contact       ┌─────────────────┐
  │ Short Wandering │ ◄──────────────────────► │ Short Instrument│ ──► Press NULL
  │ Reel Probe      │                          │ Lead Probe      │     Display: 0.00 Ω
  └─────────────────┘                          └─────────────────┘

  METHOD B: MATHEMATICAL SUBTRACTION
  1. Connect instrument leads across wandering reel: Record R_lead (e.g. 0.61 Ω)
  2. Measure circuit point: Record R_measured (e.g. 0.85 Ω)
  3. Calculate: R2 = R_measured - R_lead = 0.85 Ω - 0.61 Ω = 0.24 Ω

1. Electronic Nulling (Zeroing): Unwind the required length of wandering lead from the reel. Firmly connect the probe at the wandering lead end to the instrument's short lead probe. Press the NULL button on the meter. The instrument stores $R_{\text{lead}}$ in its memory and zeroes the display (0.00 Ω). Subsequent readings compensate for the stored lead resistance; parallel paths and the selected reference point still affect what is measured.
2. Mathematical Subtraction: If the lead resistance exceeds the instrument's maximum nulling capability (often capped at 1.0 Ω to 2.0 Ω), measure the resistance of the wandering lead reel directly. Record $R_{\text{lead}}$ on the test sheet. Subtract $R_{\text{lead}}$ from every raw measurement taken during the inspection ($R_2 = R_{\text{measured}} - R_{\text{lead}}$).

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Step-by-Step Testing Procedure

Step 1: Safe Isolation

Follow the full safe-isolation procedure for the actual work boundary and every supply, including securing isolation and proving dead with a suitable voltage indicator checked before and after use. A main switch can leave incoming terminals live; an annual calibration requirement is not imposed on a simple voltage indicator.

Step 2: Establish the Earth Reference Point

1. Select and identify the test reference: normally the circuit-origin earth terminal for a circuit R2, or the relevant protective-conductor endpoint. Account for any upstream path and exclude parallel connections when verifying an individual conductor.
2. Use a suitable secure clip or connection on an accessible, clean conductive test point. Avoid damage to protective finishes or equipment; follow the equipment and test-accessory instructions.
3. Verify mechanical security—a slipping connection at the MET will invalidate dozens of subsequent readings.

Step 3: Testing Exposed-Conductive-Parts

Take the mobile probe of the low-resistance ohmmeter to every exposed-conductive-part served by the distribution board:

• Class I Metal Luminaires: Touch the metal body, decorative canopy, and earth screw inside the fitting.
• Metal Switches & Sockets: Touch the metal faceplate, fixing screws, and the internal earth terminal of the metallic back box.
• Distribution Equipment: Touch the steel enclosure of sub-distribution boards, motor control centers (MCCs), and isolators.
• Fixed Appliances: Touch the exposed metallic chassis of electric heaters, commercial kitchen equipment, boilers, and pumps.

Step 4: Reading Verification & Recording

• Verify that the reading on the low-resistance ohmmeter is low and stable (typically between 0.05 Ω and 0.50 Ω depending on cable length and cross-sectional area).
• A fluctuating reading indicates a loose termination, corroded joint, or poor probe contact.
• Record the highest verified $R_2$ reading on the test schedule.

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Dedicated Applications of Method 2

Method 2 is not merely an alternative to Method 1; in many electrotechnical environments, it is the only viable testing methodology:

1. Metallic Containment Systems Used as CPC (BS 7671 Reg 543.2.2)

Where steel conduit, steel trunking, or the steel wire armouring (SWA) of multi-core cables functions as the circuit protective conductor, Method 1 is often inadequate. Method 2 allows the inspector to walk the containment run, testing continuity at:

• Every conduit coupling, bend, and inspection elbow.
• Every trunking joint, bridge piece, and lid section.
• Every brass SWA cable gland and earth tag (banjo). This identifies high-resistance joints caused by paint contamination, loose locknuts, or vibrational loosening along the containment run.

2. Supplementary Protective Bonding (Reg 415.2 & Section 701)

In locations of increased shock risk (e.g. swimming pools under Section 702, agricultural premises under Section 705, or medical locations under Section 710), supplementary equipotential bonding connects simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts.

• Because supplementary bonding conductors do not have an associated phase conductor, Method 1 cannot be performed.
• Use a suitable low-resistance measurement between the relevant simultaneously accessible parts. For the general AC supplementary-bonding effectiveness check in 415.2.2: $R \le \frac{50}{I_a}$ Here $I_a$ is $I_{\Delta n}$ for an RCD, or the overcurrent-device current causing operation within 5 s. For a 30 mA RCD, $50/0.03 \approx 1667\ \Omega$. This formula does not replace sound, correctly sized bonding conductors, reliable connections or the applicable special-location requirements. It does not guarantee that touch voltage can never exceed 50 V.

3. High-Bay Luminaires & Remote External Circuits

In high-ceiling industrial warehouses, aircraft hangars, or street lighting columns, distribution boards may be hundreds of metres away, or luminaires may be accessible only via mobile elevating work platforms (MEWPs). Setting up and maintaining temporary bridging links at the distribution board while repeatedly moving between high-level fixtures is impractical and dangerous. A wandering lead allows testing directly at height or at ground-level column doors.

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Determining $(R_1 + R_2)$ from Method 2 Measurements

Under BS 7671, the Schedule of Test Results requires the entry of $(R_1 + R_2)$ or $R_2$ for each circuit. When Method 2 is used, the inspector can determine $(R_1 + R_2)$ by two recognized approaches:

1. Direct Entry of $R_2$: The model forms in Appendix 6 of BS 7671 allow the tester to record the measured $R_2$ value directly in the continuity column, provided the column header is appropriately marked or indicated.
2. Separate Measurement of $R_1$: The wandering lead can be connected to the circuit line conductor at the distribution board (with the circuit isolated). The mobile probe is then applied to the line terminal at the furthest accessory to measure $R_1$. Use the same circuit-origin reference and the same accessory for both measurements, with lead resistance and parallel paths accounted for. The two conductor resistances can then be added: $(R_1 + R_2) = R_1 + R_2$

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Method 1 vs. Method 2: Comprehensive Comparison

[!CRITICAL] The Polarity Danger of Method 2: Because Method 2 measures only the protective conductor to earth, it gives no indication whether the line conductor is connected to the correct terminal or whether single-pole switches are connected in the phase or neutral conductor. For initial verification, complete separate dead polarity checks before energising the outgoing installation. Verify supply polarity separately as applicable; a later live check does not replace the required dead checks.