Why 'It Worked Before' Does Not Mean the Installation Was Safe

Functioning and Safe Are Not the Same Thing

An electrical installation that is working — that powers lights and appliances without tripping or causing visible problems — is demonstrating that it can deliver current to loads under normal conditions. It says nothing about whether it will behave safely when conditions deviate from normal.

A fault in an electrical installation typically involves some combination of overcurrent, insulation failure, inadequate earthing, or inadequate protective device response. Many of these faults are latent — they exist in the installation but only become dangerous when a particular set of circumstances arises.

The Case of Deteriorating Insulation

Insulation resistance is a good example. The insulation around electrical conductors degrades over time — affected by heat, moisture, mechanical stress, and simply the passage of time. This degradation is gradual. The insulation does not suddenly fail at a particular moment; it slowly becomes less effective at keeping conductors isolated from each other and from earth.

An installation with significantly degraded insulation may work perfectly normally for extended periods. Current flows along the intended path; appliances operate; the fuse board sits quietly. Then one day — perhaps when there is a spike in humidity, or a cable is disturbed during maintenance, or an additional load is added — the degraded insulation fails at a critical point. An earth fault occurs. If adequate protective devices are in place, the circuit trips. If they are not, the fault can sustain itself long enough to cause a fire or a shock.

The fact that the insulation had not yet failed when the previous tenants were in the property is not evidence that it was safe. It is evidence that the fault had not yet been triggered.

Earthing: The Safety Net That Works Invisibly

Earthing is another area where 'working' and 'safe' diverge. The earthing system in a property is not something that plays a role in normal operation. Lights work, sockets deliver power, and the installation functions whether the earthing is adequate or not — because earthing only comes into play when something goes wrong.

When a fault occurs in an appliance — a live conductor touching the metal casing, for example — the earthing system provides a low-resistance path for that fault current to flow. This causes the protective device (fuse or circuit breaker) to trip and isolates the fault before anyone touching the appliance receives a shock.

If the earthing is inadequate, the fault current does not flow freely enough to trip the protective device. The appliance case remains live. The next person to touch it while standing on a conductive surface receives a shock. And yet, up until that moment, everything had been 'working' perfectly.

Why Standards Change Over Time

Another layer of the 'it worked before' issue is that the regulations that define what safe means are updated over time. BS7671 — the wiring regulations — is revised periodically to reflect new understanding of risks, new technology, and accumulated evidence about what types of installation cause harm.

An installation that was installed correctly to the 1976 edition of the wiring regulations was safe by the standards of 1976. The regulations at the time may not have required RCD protection on socket circuits, or may have permitted certain types of wiring in locations that are now considered higher risk. Over time, the understanding of what constitutes adequate protection has evolved.

An EICR assesses the installation against current standards, noting where it falls short. A C3 observation often represents the gap between what was acceptable when the installation was built and what the current regulations would require. This does not mean the installation was badly done at the time — it means the standard has moved.

The Purpose of an EICR

The EICR process exists precisely because the human senses cannot detect most electrical faults. You cannot see degraded insulation inside a wall. You cannot feel inadequate earthing. You cannot tell whether a consumer unit would trip fast enough in the event of a fault simply by looking at it.

Testing instruments can measure these things. An insulation resistance test can quantify how much insulation has degraded. A loop impedance test can verify that protective devices will respond correctly and quickly enough. An earth continuity test confirms that protective conductors are intact and connected.

The fact that previous tenants experienced no problems is, at best, reassuring. It is not the same as confirmation of safety. The EICR provides the confirmation.