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C1: danger present, and reverse polarity at the originWhat makes something a C1?

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C1 is not only about the consumer unit

Access to live parts anywhere gets the same treatment.

Damaged accessories and broken faceplates where conductors can be reached
Junction boxes with damaged or missing covers
Missing grommets or stuffing glands where fingers can get in
Insulation degraded to the point that live parts are exposed to touch
Any metalwork of the installation that becomes live under a fault
IP ratings not met lands at C1 or C2 depending on what is actually reachable behind the opening.

Reverse polarity at the origin — what actually happens

Line and neutral transposed at the meter tails. Found with a voltage indicator: nothing between line and CPC, 230 V between neutral and earth. That is the giveaway.

C1. Stop. You do not carry on testing the rest of the installation
StepDetail
1Stop testing. There is no value in results taken downstream of this
2Isolate and lock off the main switch
3Contact the DNO — their cut-out, their fuse, their seal
4Contact the meter operator as well — the tails are at the meter, which is their equipment
5Record who you spoke to, when, and what was agreed. Follow the call with an email
6Report it as a C1 observation. The report is unsatisfactory
Reverse polarity at the origin line and neutral transposed at the meter tails meter their equipment sealed main switch L N the tails, crossed ! brown lands on N blue lands on L C1 — stop testing What the voltage indicator says normal transposed line — neutral 230 V 230 V line — CPC 230 V nothing neutral — earth nothing 230 V the same either way — which is why this test alone will not find it You cannot pull their fuse or break their seal — the DNO and the meter operator both have to attend.
Line to neutral still reads 230 V, so that test on its own tells you nothing. The pair that gives it away is line to CPC reading nothing while neutral to earth reads 230 V. That is a C1: stop testing, isolate and lock off, and call both the DNO and the meter operator — the tails are at the meter, which is theirs. Results taken downstream of this are worthless.
✋You cannot pull their fuse or break their seal. Both the DNO and the meter operator have to authorise work on their own equipment. Doing it yourself is an offence, not a shortcut.
You are not expected to stand guard overnight. If they cannot attend until the following day, record it — who, when, what was said — put it in writing, and make sure the duty holder knows in plain terms what the risk is. The paper trail is your protection, because you are the one who started the report.

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