How to Replace Consumer Units: Part 2

How to Replace Consumer Units: Part 2

Following up on last month’s first instalment, NAPIT’s Paul Chaffers, explains how best to tackle this important job on domestic premises without making it difficult for the installer and the customer.

In the previous edition of PE, we discussed various considerations when replacing a consumer unit (CU) in domestic and similar premises. These included potential pitfalls, the requirements of the Electricity at Work Regulations 1989, the requirements of BS 7671 and industry best practice.

This second article will examine some other issues which need to be considered when replacing a CU.

Pre-work survey

Prior to replacing a CU, the contractor is advised to encourage the customer to have an Electrical Installation Condition Report (EICR) carried out. This is in the interest of both the customer and the installer.

If, for any reason, the customer refuses to have an EICR carried out, then it is recommended that the installer carry out a pre-work survey to find out if there are any immediate or potential dangers or if there is any condition in the existing installation that might cause unwanted tripping of an RCD. The pre-work survey is a precautionary measure in the installer’s own interest.

As part of the pre-work survey, checks need to be made with the customer to see if there are any known defects, faults or damage. The customer must then be advised that power will need to be briefly switched off to allow for the pre-work survey inspection and testing.

Pre-work survey inspection

Safe isolation of the CU is needed to allow for an internal visual inspection. This will reveal the type and condition of the existing wiring system.

Where lighting circuits with no circuit-protective-conductor (cpc) have been discovered, and where the customer will not agree to the necessary remedial work, such circuits cannot be reconnected unless the customer agrees to have all the associated metal accessories (e.g. switches) replaced with plastic alternatives. Although BS 7671 does not specifically require existing circuits to be upgraded to current standards before reconnecting to a replacement CU, any circuits found with defects classed as being immediately or potentially dangerous, or requiring further investigation (C1, C2 or FI), must not be reconnected to the CU. The customer needs to be made aware of this! 

A circuit with an observation which is classed as Improvement recommended (C3) can be reconnected to the new CU but must be noted in the Comments on existing installation section of the Electrical Installation Certificate (EIC), as permitted by Regulation 633.2.

If rubber insulated or lead-sheathed cables are found during the survey, it is strongly recommended not to go ahead with the CU replacement until the cables are changed. An external visual inspection of other accessible accessories should also be made.

Pre-work survey tests

Pre-work survey tests should include as a minimum:

  1. (Ze) external earth fault loop impedance.
  2. (Pfc) prospective fault current.
  3. (Cpc) circuit protective conductor continuity at the end of each final circuit.
  4. End-to-end continuity checks of all conductors of each ring final circuit.
  5. Insulation resistance measurement of the whole installation (test at the CU at the outgoing side of the main switch, between the live conductors connected together and the protective conductors connected to the earthing arrangement).
  6. (Zs) earth fault loop impedance at readily accessible socket-outlets.

If any test conditions are found that result in a C1, C2 or FI classification code, or which cause the unwanted tripping of an RCD, the customer should be informed that remedial work is necessary before the planned CU change can go ahead.

A distress change

A distress change happens when the existing CU has suffered damage. It could be through overheating or from mechanical damage which has left it in a dangerous condition. While this should mean the immediate replacement of the CU, the requirements of BS 7671 still need to be met. The customer needs to be made aware of all the requirements mentioned previously (readers may wish to review the previous article on replacing domestic consumer units).

These requirements may result in remedial works carried out in parallel to the CU change to enable the installation to be left safe and compliant. Such remedial work will obviously have to be paid for and therefore it is advisable that a record of the customer’s agreement to pay for it has been obtained prior to the CU being replaced. This agreement could be in the form of a signed statement or an email.

When carrying out a distress change, and circuits are found to have faults which require time-consuming remedial work, it may be necessary to leave such faulty circuits temporarily disconnected to enable the other healthy circuits to be energised. However, such faulty circuits must be repaired at the earliest opportunity.


We have now concluded this two-part article on the replacement of domestic consumer units.

NAPIT has recently produced a leaflet for customers which has been designed to help them understand the technical requirements involved in replacing a consumer unit.                                                                                 

Advice on additions, alterations and the periodic inspection of metal consumer units was featured in The Competent Person Magazine (Issue 1 2015). The installation of consumer units was featured in The Competent Person Magazine (Issue 1 2017). Copies of these articles are available on the NAPIT website:      

Further guidance on replacing consumer units in domestic and similar premises is given in the Electrical Safety First’s (ESF) Best Practice Guide No. 1, the latest version of which is available to download from:

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