FI is the EICR code that says "I can't determine the correct classification without more work." It's used when access, equipment in service, or ambiguous symptoms prevent the inspector from confirming whether a defect is C1, C2 or C3. This guide explains exactly when FI is the correct call and how to scope the follow-up so it actually gets done.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
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FI is defined as "Further investigation required without delay." The classification indicates the inspector has identified a potential defect or non-compliance but cannot confirm the appropriate classification (C1, C2, or C3) without additional inspection or testing that was not possible during the EICR visit.
FI is not "I think there might be something." FI is "there's evidence of a potential defect that I cannot fully investigate today — and this needs follow-up before a safety conclusion can be drawn." If you cannot articulate WHAT to investigate, FI is the wrong code.
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FI is reserved for scenarios where the inspector cannot make a final classification within the scope of the current visit. Common legitimate uses:
FI is treated as an "unsatisfactory" trigger when the potential issue relates to safety. Practically, this means:
FI is not a "tomorrow problem" escape hatch. If the evidence at the time of inspection supports C1 or C2, use that classification — even if the inspector would like more data. FI is for genuine evidence gaps, not commercial convenience.
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A vague FI ("further investigation recommended on socket circuit") is almost useless — the follow-up electrician has no scope, the responsible person can't commission the work, and the original inspector has provided no value. A good FI specifies four things:
"Consumer unit at meter cupboard shows audible humming under load. No visible damage to enclosure or terminals during visual inspection. Insulation resistance test could not be performed as the property was in occupation and prior notice of isolation was not given. Further investigation required: isolate the installation overnight, perform full insulation resistance test at 500 V, inspect all consumer unit terminals for tightness and signs of overheating, and remove and inspect main switch. Work to be carried out by a competent electrician with EICR experience within 28 days."
Once the further investigation is complete, the inspector (or follow-up electrician) issues a supplementary observation document that converts the FI into its final classification:
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