Why Consumer Unit Upgrades Can Reveal Existing Electrical Faults

Why the Upgrade Is Like Shining a Light on Old Wiring

An older consumer unit with rewireable fuses or basic MCBs is, in a sense, tolerant of certain types of wiring degradation. It protects against gross faults — major overloads, solid short circuits — but it will not detect small earth leakage currents or the kind of insulation degradation that develops gradually over decades.

A modern consumer unit fitted with 30mA RCBOs has a fundamentally different sensitivity. It will detect and trip on leakage currents that the old board was simply not capable of noticing. When that sensitive protection is connected to wiring that has been quietly accumulating insulation degradation over the years, the result is tripping — often on multiple circuits, often intermittently, and often for faults that are difficult to locate without proper testing.

The Most Common Problems Revealed

Based on routine experience with older properties across Kent — Victorian terraces, inter-war housing, post-war estates — a number of fault types appear repeatedly when older wiring is connected to modern boards.

• Insulation resistance degradation on older PVC or rubber-insulated circuits, particularly in loft spaces, bathroom areas, or where cables run through exterior walls
• Borrowed neutrals in lighting circuits — where neutral conductors from separate circuits have been joined together, causing RCD tripping when both circuits are used simultaneously
• Earth leakage from appliances — particularly washing machines, dishwashers and tumble dryers with worn motor windings that produce DC leakage current (which requires Type A protection to properly detect)
• Ring main circuits with insulation faults at socket outlets — particularly at older bakelite or early plastic accessories where the wiring connections have moved over time
• Outdoor circuits with cable burial or cable runs in damp areas where moisture ingress has degraded insulation over time

Insulation Resistance: The Key Test

The test that most directly predicts whether a consumer unit upgrade will produce tripping is insulation resistance testing. This is a standard test in an EICR — it applies a test voltage to the conductors of each circuit and measures how well the insulation resists current flow. A healthy circuit will have very high resistance. A circuit with degraded insulation will have measurably lower values.

When insulation resistance testing is carried out before a consumer unit upgrade, the results tell you which circuits are at risk of producing nuisance tripping with the new board, and how seriously. This allows either remediation of the affected circuits before the board is replaced, or at minimum an informed conversation with the landlord about what to expect.

Borrowed Neutrals: A Specific Issue Worth Understanding

Borrowed neutrals — where the neutral conductor of one circuit has been connected to or shared with that of another — are a specific and surprisingly common cause of RCD tripping in older properties. They are a legacy of older wiring practice and extensions to lighting circuits added over time without proper separation of neutrals.

An RCD measures the balance between the live and neutral current on the circuits it protects. When neutrals are shared between circuits, this balance is disrupted when both circuits are in use simultaneously. The RCD sees what looks like a leakage current and trips — even though there is no actual insulation failure or earth fault causing a safety issue.

Tracing and correcting borrowed neutrals requires systematic circuit testing. It is not a problem that resolves itself or that can be addressed by simply fitting a different type of protective device.

Preparing for a Consumer Unit Upgrade

The best preparation for a consumer unit upgrade is an EICR carried out beforehand. This provides insulation resistance values for every circuit, identifies any borrowed neutrals or earthing issues, and gives you a complete picture of the installation's condition before the new board is installed.

Where the EICR identifies circuits with low insulation resistance, the decision about whether to rewire those circuits before or at the same time as the board replacement can be made in advance — as part of a planned scope of works, with a clear cost. This is a far better position than discovering the same problems through tripping after the board has been installed, when the remediation work needs to be arranged reactively and the property may be difficult for tenants to live in in the interim.

For landlords planning a consumer unit upgrade on an older property, particularly one that has not had recent EICR testing, getting the testing done first is the logical sequence.