3.1 BS 7671 Part 6 Inspection & Testing Sequence

Introduction to BS 7671 Part 6 Statutory Framework

Part 6 of BS 7671:2018+A2:2022 (The IET Wiring Regulations) outlines the statutory and technical requirements for Inspection and Testing of electrical installations in the United Kingdom. Before any new installation, addition, or alteration is put into service, it must undergo Initial Verification. This process guarantees that the installation strictly complies with the safety standards prescribed by BS 7671 and the Electricity at Work Regulations 1989 (EAWR). The primary objective is to confirm that the design, equipment selection, cable sizing, protective device selection, and physical erection are sound, ensuring robust protection against electric shock, thermal burns, electrical fires, and mechanical breakdown.

Initial Verification vs. Periodic Inspection

It is vital for electrical inspectors to distinguish between Initial Verification and Periodic Inspection, as they serve fundamentally different purposes, possess different legal implications, and mandate different scopes of testing.

Initial Verification is conducted during construction and immediately upon completion of a new electrical installation, partial rewire, or circuit addition. Its purpose is to certify that the brand-new work complies fully with the latest edition of BS 7671. Initial verification demands a 100% thorough visual inspection and complete testing of every single circuit before the supply is energized.

In contrast, a Periodic Inspection is an assessment of an existing, operational electrical installation. Its objective is not to re-certify past design choices against new regulations, but to determine whether the installation remains in a satisfactory condition for continued safe use. A periodic inspection evaluates wear and tear, age-related insulation deterioration, environmental corrosion, mechanical damage, and unauthorized alterations made by occupiers. Unlike initial verification, periodic inspection permits targeted sampling techniques, provided the inspector and duty holder agree on the scope and limitations.

The Comprehensive Visual Inspection

Before picking up any test instrument or applying test voltages, a competent inspector must perform a thorough, methodical visual inspection. BS 7671 mandates that visual inspection must be conducted while the installation is safely isolated from the electrical supply. The inspector checks several critical parameters:

• Conductor Routing and Identification: Cables must be safely installed within prescribed safe zones (e.g., within 150mm of corners or ceilings, or running vertically/horizontally to accessories) or incorporate mechanical protection such as earthed steel conduit. Conductors must be correctly identified by color sleeves or alphanumeric markers (Brown for Line, Blue for Neutral, Green-and-Yellow for Protective Earth).
• Termination and Joint Integrity: All electrical terminations must be mechanically secure, correctly torqued to manufacturer specifications, and free from strain.
• Equipment Selection and Environmental Suitability: Electrical equipment must be appropriate for the external influences it will encounter, such as IP ratings for ingress of water or dust (e.g., IPX4/IPX5 in bathrooms, IP66 outdoors).
• Protective Devices and Enclosures: Verification that basic protection (insulation of live parts, IP2X enclosures, IP4X top surfaces of consumer units) and fault protection (RCDs, MCBs, earthing conductors) are installed correctly.

Only when the visual inspection is 100% complete and satisfactory can instrument testing commence.

The Mandatory BS 7671 Testing Sequence

The sequence of instrument testing prescribed by BS 7671 Part 6 is a strict safety protocol. Performing tests out of order can yield inaccurate or misleading test data and, more critically, expose the testing technician and site personnel to lethal electrical hazards. Dead tests—those conducted with the electrical supply completely isolated and locked off—must be successfully executed before any live tests are attempted.

The mandatory sequence of dead tests is as follows:

1. Continuity of Protective Conductors: Verification of the Circuit Protective Conductor (cpc), main protective bonding conductors, and supplementary bonding conductors.
2. Continuity of Ring Final Circuit Conductors: Verification of end-to-end resistance and correct ring connection for line, neutral, and cpc conductors.
3. Insulation Resistance: Testing conductor insulation integrity at high DC voltages (250V, 500V, or 1000V DC).
4. Protection by SELV, PELV, or Electrical Separation: Verifying isolation of extra-low voltage systems where applicable.
5. Basic Protection by Barriers or Enclosures: Checking site-erected barriers or enclosures.
6. Insulation Resistance of Non-Conducting Floors and Walls: Measuring floor/wall insulation in special installations.
7. Polarity (Dead): Confirming single-pole devices (MCBs, switches) are correctly connected in the line conductor only.
8. Earth Electrode Resistance: Measuring earth rod resistance prior to energization on TT systems.

Following successful dead testing and safe energization, live testing proceeds in the following sequence:

1. Earth Fault Loop Impedance (Zs & Ze): Measuring supply impedance and total loop impedance to confirm automatic disconnection of supply.
2. Prospective Fault Current (PFC/PSCC/PEFC): Determining maximum prospective fault current to verify breaking capacity.
3. Residual Current Device (RCD) & Functional Testing: Testing trip times (0.5x, 1x, 5x IΔn) and verifying mechanical trip buttons, AFDDs, and SPDs.

Safety Rationale and Preparation

The safety rationale for this sequence is clear: verifying continuity of earth conductors first ensures that if an insulation fault occurs during later testing or energization, the earth path is already proven to exist, allowing protective devices to operate. Testing insulation resistance before energization prevents destructive short circuits or earth fires upon power-up.

Prior to testing, the inspector must execute the Safe Isolation Procedure using an Approved Voltage Indicator (AVI) and proving unit compliant with HSE GS38, locking off the main switch and retaining the key. Test meters must comply with BS EN 61557, and test leads must feature fused probes, finger guards, and shrouded 4mm tips.

Certification and Reporting

Upon completing inspection and testing, results must be formally recorded. For initial verification, an Electrical Installation Certificate (EIC) or Minor Electrical Installation Works Certificate (MEIWC) is produced alongside a Schedule of Inspections and Schedule of Test Results. For periodic inspections, an Electrical Installation Condition Report (EICR) is issued. These legal documents assign professional accountability and serve as essential safety records under the Electricity at Work Regulations 1989.