4.4 Additions & Alterations to an Existing Installation
Key Takeaways
- Regulation 132.16 requires that the rating and condition of existing equipment, including the distributor's, and the earthing and bonding arrangements are adequate for the altered circumstances before any addition or alteration is made
- Neither Reg 132.16 nor Reg 641.5 requires the existing installation to be made fully compliant with the current edition - only defects that detrimentally affect the safety of the new work must be remedied
- Observed shortcomings that are left in place must be recorded on the certificate under "Comments on the existing installation"
- The new work is fully verified as initial verification, while the existing supply is checked by measuring Ze and prospective fault current and confirming main protective bonding
- An EIC is issued for a new circuit or a new consumer unit; a MEIWC is issued for minor work with no new circuit, one certificate per circuit worked on (Reg 644.4.201)
Additions and Alterations
Additions and alterations to existing installations are common work, and the governing regulation is 132.16, not anything in Part 6. It states that no addition or alteration, temporary or permanent, shall be made to an existing installation unless it has been ascertained that the rating and the condition of any existing equipment, including that of the distributor, will be adequate for the altered circumstances, and that the earthing and bonding arrangements are adequate where they are necessary for the protective measure applied to the addition or alteration.
Three obligations follow from it:
- The existing supply and earthing arrangements must be adequate for the new work — the existing installation must safely carry the additional load and fault current.
- The alteration must not impair the safety of the existing installation — you cannot make an existing installation less safe by adding to it.
- The new work is fully verified — it is treated as initial verification for the added or altered part, and Regulation 641.5 requires the assessment of the existing installation that Regulation 132.16 calls for.
This means the inspector must measure the existing supply characteristics (external earth fault loop impedance, Ze, and prospective fault current, PFC) to confirm they can support the new work. The new work itself is then inspected and tested as for any initial verification.
How compliant does the existing installation have to be?
This is the most misunderstood point in the topic, and a reliable exam discriminator. Neither Regulation 132.16 nor Regulation 641.5 requires the existing installation to be brought into full compliance with the current edition of BS 7671 before an addition or alteration is made.
What the regulations require is narrower and more practical:
| Finding in the existing installation | Required action |
|---|---|
| A defect that detrimentally affects the safety of the new work | Must be remedied before the addition or alteration is energised |
| Earthing or bonding inadequate for the protective measure the new work relies on | Must be corrected before the addition or alteration is energised |
| A non-compliance that does not affect the safety of the new work | May remain, but must be recorded on the certificate under "Comments on the existing installation" |
That comments box is not optional decoration. It is the mechanism by which the person ordering the work is told what was found and not put right, and it is what protects the installer who added a socket to a sound circuit in an installation with older shortcomings elsewhere. Leaving it blank when departures were observed is itself a defective certificate.
RCD Requirements for Additions and Alterations
BS 7671 has progressively expanded RCD requirements over successive editions. Key provisions relevant to additions and alterations:
| Provision | Requirement |
|---|---|
| Socket-outlets rated up to 32 A | 30 mA RCD protection required for additional protection (Reg 411.3.3), with a limited exception for a labelled outlet dedicated to a specific item of equipment in non-domestic premises following documented risk assessment |
| Domestic socket-outlets | RCD required regardless of whether portable outdoor equipment may be connected |
| Bathrooms and shower rooms | All circuits require 30 mA RCD protection (Section 701) |
| Outdoor lighting | RCD protection required |
| Cables concealed in walls at a depth of less than 50 mm | 30 mA RCD protection required unless the cable has an earthed metallic covering, or is in earthed conduit, trunking or ducting, or has mechanical protection against penetration by nails and screws |
When adding a circuit or socket to an existing installation, the inspector must verify that the existing arrangement can provide RCD protection. If the existing consumer unit does not have an RCD, the work may require upgrading the protection — the new work cannot simply be added without meeting current BS 7671 requirements.
What Is Tested: New Work vs Existing Supply
The testing is divided between the new work and the existing supply:
New Work (tested as initial verification)
- Dead tests: continuity of protective conductors, insulation resistance, polarity (dead).
- Live tests: Zs at the new accessories and outlets, RCD operation on the new circuit, plus phase sequence and functional testing where applicable.
- Visual inspection: full inspection of the new work.
Existing Supply (measured for adequacy)
- Ze (external earth fault loop impedance) — measured at the origin to confirm the existing earthing arrangement is adequate for the new work.
- PFC (prospective fault current) — measured at the origin to confirm protective devices can handle the fault level.
- Main protective bonding — verified present and adequate.
The existing installation as a whole is not re-verified — only the new work and the supply characteristics that affect it. If the existing installation has defects that affect the safety of the new work, those defects must be addressed or the work cannot proceed safely.
Finishing the Work
Altered work is completed like any other new work: the live tests are followed by phase sequence where the circuits are polyphase (Reg 643.9, section 11.1) and by functional testing of the assemblies, switchgear and controls affected (Reg 643.10, section 11.2). On an alteration these two matter more than usual, because the commonest by-product of board work and rewiring is a transposed line conductor or a switch that no longer controls what its label claims.
Certification for Additions and Alterations
The certification issued depends on the scope of the work (recap from Chapter 3):
- Electrical Installation Certificate (EIC) — issued for a new installation, a new circuit, or a new consumer unit or distribution board. This is the most comprehensive certificate and includes the full schedule of inspections and schedule of test results.
- Minor Electrical Installation Works Certificate (MEIWC) — issued for minor work that does not include a new circuit. Examples: adding a socket to an existing ring final circuit, adding a lighting point to an existing circuit, modifying a circuit's accessories.
If the work involves a new circuit or a new consumer unit, an EIC must be issued — a MEIWC is not appropriate. The choice of certificate is determined by the scope of the work, not by the installer's preference, and a MEIWC must not be used for a new circuit even where that circuit supplies only a single point.
Two procedural requirements are easy marks:
- Regulation 644.4 — the original certificate is issued to the person ordering the work, with a duplicate retained by the person issuing it.
- Regulation 644.4.201 — a separate Minor Electrical Installation Works Certificate is issued for each circuit worked on. One certificate covering four circuits in one visit is not compliant, however convenient it is.
When adding a new socket-outlet to an existing ring final circuit in a domestic premises, what must the inspector verify about the existing supply and earthing arrangements before the new work is put into service?
An electrician is adding a socket-outlet to a sound existing ring final circuit. The installation has no RCD protection on the lighting circuits and some accessories are showing wear, but neither affects the circuit being altered. What does BS 7671 require?
Which certificate must be issued when the work involves installing a new circuit or replacing a consumer unit in an existing installation?