9.1 Continuity Testing Method 1 (R1 + R2)

9.1 Continuity Testing Method 1 (R1 + R2)

Core Testing Principle: Method 1 is the preferred continuity testing technique for radial final circuits during initial verification. By temporarily interconnecting the circuit line conductor ($R_1$) and the circuit protective conductor ($R_2$) at the distribution board, the tester measures the complete round-trip loop resistance at every outlet. This verifies earth continuity, identifies the electrically furthest point of the circuit, and simultaneously confirms correct polarity in a single dead testing procedure.

---

Theoretical Foundation of Method 1

Continuity testing of protective conductors is the first dead test conducted in the prescribed testing sequence mandated by BS 7671 (Regulation 643.2.1) and IET Guidance Note 3. Its primary purpose is to verify protective-conductor electrical continuity by resistance measurement. Inspect the connections and conductor condition as well: a low-current continuity test alone does not prove mechanical security or fault-current withstand.

In Method 1, the circuit line conductor ($R_1$) and the circuit protective conductor ($R_2$) are temporarily bridged at the distribution board. When a low-resistance ohmmeter is applied across the line and earth terminals at an accessory, the test current travels down the line conductor to the board link and returns along the CPC to the meter. The instrument measures the series sum of both conductors:

$(R_1 + R_2) = R_1 + R_2$

  DISTRIBUTION BOARD (DEAD & ISOLATED)                 OUTLET / ACCESSORY
  ┌─────────────────────────────────────┐            ┌───────────────────┐
  │ [Line Conductor Disconnected] ──────┼────────────┼──► [Line Terminal]│
  │                 │                   │  Cable Run │          ▲        │
  │       Temporary Bridging Link       │  (Length L)│          │ Test   │
  │                 ▼                   │            │          ▼ Probes │
  │ [CPC Disconnected from Bar] ────────┼────────────┼──► [Earth Terminal│
  └─────────────────────────────────────┘            └───────────────────┘
                                                                ▲
                                                                │
                                                     [ Low-Resistance Ohmmeter ]
                                                     [   Reads: (R1 + R2)      ]

Why $(R_1 + R_2)$ is Essential

1. Fault Loop Impedance Calculation: The measured $(R_1 + R_2)$ value is added directly to the external earth fault loop impedance ($Z_e$) to calculate the prospective circuit earth fault loop impedance ($Z_s = Z_e + (R_1 + R_2)$) without energising the circuit.
2. Fault Protection Verification: Use the result to assess ADS together with the protective device, earthing arrangement and conductor suitability. For a 230 V TN system, the usual Table 41.1 time is 0.4 s for final circuits rated at most 63 A supplying sockets, or at most 32 A supplying only fixed equipment. Other cases require their applicable BS 7671 time; continuity alone does not prove thermal withstand.
3. Conductor Assessment: Unexpected resistance can indicate a poor connection, damaged or incorrectly sized conductor, or a different cable route. Investigate with inspection and further tests rather than assuming the continuity reading proves every joint sound.

---

Instrument Specification & Calibration (BS EN 61557-4)

Use a suitable protective-conductor continuity tester. A general electronics multimeter may have insufficient test current, resolution or range for this task. Establish clean, stable probe contacts; a 200 mA test is a resistance measurement, not proof that a loose connection can carry fault current safely.

Instrument Performance

BS 7671 Regulation 643.1 permits instruments complying with the relevant BS EN 61557 part or providing equivalent performance and safety. For protective-conductor continuity, BS EN 61557-4 provides the familiar 4–24 V open-circuit and at least 200 mA test parameters. Select a range and resolution suitable for the low values expected; a purpose-made continuity tester commonly resolves 0.01 Ω.

Test Lead Nulling (Zeroing) Procedure

Test leads and probe assemblies possess an intrinsic resistance—typically between 0.05 Ω and 0.20 Ω. If this lead resistance is not removed, it will directly inflate every reading, causing circuits to fail compliance or exceed calculated limits.

                         LEAD NULLING SEQUENCE
  ┌────────────────┐     Firm Probe Contact     ┌────────────────┐
  │ Low-Resistance │ ◄────────────────────────► │ Short Probes   │
  │ Ohmmeter       │   Press "NULL" / "ZERO"    │ Together       │
  └────────────────┘                            └────────────────┘
           │
           ▼
  Display reads: 0.00 Ω (Lead resistance subtracted from internal memory)

1. Insert the standard test leads into the low-resistance ohmmeter terminals.
2. Firmly press the probe tips together (or clip the crocodile clips together on a clean copper surface).
3. Press and hold the NULL (or ZERO) button until the instrument confirms compensation (often indicated by a lead icon or buzzer).
4. Verify that the display reads 0.00 Ω. The instrument will now automatically subtract lead resistance from all subsequent measurements.

---

Step-by-Step Procedure for Method 1

Step 1: Safe Isolation & Proving Dead

Follow the complete safe-isolation procedure in Chapter 2 before accessing conductors. Identify the working boundary and every supply, isolate and secure against reconnection, and provide appropriate warnings. Prove a suitable voltage indicator before and after checking all relevant conductor combinations at the work point.

Opening the board main switch may leave incoming terminals live. Protect against those parts or arrange further isolation; do not treat the entire enclosure as dead merely because outgoing circuits are isolated. Use a suitable proving unit or known source as the approved procedure requires.

Step 2: Distribution Board Preparation & Parallel Earth Paths

Prepare the isolated circuit and account for all possible parallel protective paths:

1. Disconnect and identify the circuit line conductor at the board as required for the test.
2. Where testing a separate CPC individually, disconnect it and arrange the test to exclude parallel connections. A detached earth-bar end does not by itself exclude downstream links through conduit, metal boxes, appliances or other CPCs.
3. Securely link the identified circuit line conductor and CPC at the circuit origin. Use sound, suitably low-resistance connections.

Step 3: Testing at Every Point of Utilisation

With the temporary link secured at the board, take the nulled low-resistance ohmmeter to every accessory on the circuit:

• Socket-Outlets: Test between the Line and Earth pin receptacles at every socket outlet. Always use an approved socket test adapter (breakout plug) to avoid spreading or damaging the brass socket shutter contacts.
• Lighting Points: At luminaires and ceiling roses, test between the switched line terminal ($L$) and the circuit protective conductor terminal ($CPC$).
• Switches: At lighting switches, test between the terminal and earth. The circuit switch must be in the closed (ON) position to complete the loop to the luminaire.
• Fixed Appliances & Isolators: At fused connection units (FCUs), cooker switches, and immersion heater switches, close the switch and measure on the load side between line and earth.

Step 4: Recording the Furthest Point $(R_1 + R_2)$

As you move from accessory to accessory along the circuit run, the measured resistance will increase proportionally with distance from the distribution board:

• Note the resistance measured at every outlet on your test schedule scratchpad.
• Identify the electrically furthest point from the circuit arrangement and satisfactory readings. Investigate an unexpectedly high result rather than assuming that it merely identifies the longest run.
• Record this highest value on the official Schedule of Test Results in the column designated for $(R_1 + R_2)$.

---

Simultaneous Verification of Polarity

Method 1 can also verify radial-circuit polarity under Regulation 643.6 when the known line/CPC link, accessory connections and switching are checked correctly. Remove connected loads where they could confuse the result.

---

Comparison with Calculated Conductor Resistance

Recorded test results must never be accepted blindly. Compare the measured $(R_1 + R_2)$ with the expected resistance for the actual conductor sizes, length and temperature, using the current conductor-resistance tables. Investigate a significant discrepancy.

Tabulated Bare-Copper Conductor Resistances at 20°C

Theoretical Calculation Formula

To calculate the expected resistance of a circuit cable run at 20°C:

$R_{\text{expected}} = \frac{(r_1 + r_2) \times L}{1000}$

Where:

• $(r_1 + r_2)$ = combined tabulated conductor resistance in mΩ/m.
• $L$ = length of the cable run in metres (m).
• Division by 1000 converts milliohms (mΩ) to ohms (Ω).

Worked Example: 2.5 mm² / 1.5 mm² Radial Circuit

A radial socket-outlet circuit wired in standard 70°C thermoplastic (PVC) twin-and-earth cable has a measured cable route length of 28 metres:

• Line conductor: 2.5 mm² ($r_1 = 7.41\ \text{m}\Omega/\text{m}$)
• CPC: 1.5 mm² ($r_2 = 12.10\ \text{m}\Omega/\text{m}$)
• Combined $(r_1 + r_2) = 7.41 + 12.10 = 19.51\ \text{m}\Omega/\text{m}$

$R_{20} = \frac{19.51 \times 28}{1000} = 0.546\ \Omega$

Temperature Adjustment Factor (1.20 Multiplier)

Copper has a positive temperature coefficient of resistance ($\alpha = 0.004 / ^\circ\text{C}$). When tests are conducted at an ambient room temperature of 20°C, the copper is cold. Under full operating load, conductor temperatures may reach 70°C for thermoplastic PVC insulation:

$R_{70} = R_{20} \times [1 + \alpha(70 - 20)] = R_{20} \times [1 + (0.004 \times 50)] = R_{20} \times 1.20$

For our 28-metre circuit: $R_{70} = 0.546\ \Omega \times 1.20 = 0.655\ \Omega$