How to do the SSEN fast-track impedance test

Why SSEN ask for it

An SGI-3 application is the largest of the G99 Small Generation Installation procedures — aggregate generation approaching 60 A per phase, export-limited to 32 A — so before approving one the DNO must be satisfied the local network can absorb the export. The phase-to-neutral loop impedance at the premises is a direct measure of how electrically strong that supply is: the lower the impedance, the smaller the voltage rise the exported current causes on the network.

Every other GB DNO runs that assessment itself, from its own network records. SSEN instead accept a measurement taken at the premises by the installer, notified on their form FO-NET-CAB-008 under their procedure PR-NET-CAB-004 — and an SGI-3 application carrying a qualifying measurement takes the fast track instead of waiting on a full design study. The form is optional: without it the application still goes in, and SSEN assess it the slow way.

None of this touches the national framework. The measurements are SSEN’s own ask, the threshold is SSEN’s own figure, and neither changes which G98/G99 documents a job needs — CLRD’s routing never reads them.

The 180 mΩ threshold

PR-NET-CAB-004 (§6) permits connection under the fast track where the phase-to-neutral loop impedance is below 180 mΩ. At or above it, the application goes the normal SGI-3 route and SSEN run a full design study — CLRD tells you which side a reading falls as you type it, and either way the job proceeds: the verdict is information, never a block.

SSEN’s own form prints the limit as “180 mΩ or less” while their procedure says below 180 — the two disagree at exactly 180.0. CLRD takes the procedure’s stricter reading, and a result that close to the line is worth re-testing anyway: lead placement and contact resistance move a milliohm-scale reading more than you’d think.

Step 1 — Check the job qualifies

• The DNO is SSEN — either licence area: Southern Electric Power Distribution (central southern England) or Scottish Hydro Electric Power Distribution (the north of Scotland).
• The route is G99 SGI-3 — the measurements ride the pre-install application (the G99 A1-2), which is the thing being fast-tracked.

In CLRD you don’t need to work this out: the SSEN Fast Track panel appears on the Pre-Install Application card exactly when the job qualifies. No panel, no test to take.

Step 2 — What you need

• A calibrated loop impedance meter — calibration is SSEN’s explicit requirement, so have the certificate current. Null (zero) the test leads before measuring: against a 180 mΩ threshold, lead resistance is a material fraction of the reading.
• A camera or phone — the form needs two photographs (Step 4), one of them taken mid-measurement.
• A competent electrical contractor — the procedure prescribes who takes the readings as well as how. The sequence below is a summary of PR-NET-CAB-004 §5, not a substitute for it, and safe isolation practice applies throughout.

Step 3 — Take the measurements

SSEN prescribe the method exactly, because a reading taken any other way is wrong in a way nobody can spot afterwards:

1. Switch the consumer unit off at its main switch.
2. Temporarily disconnect the earthing conductor, so parallel metallic paths (bonded pipework and the like) can’t sit alongside the supply and distort the reading.
3. Test at the incoming isolator of the consumer unit: measure the phase-to-neutral loop impedance, then the phase-to-earth. Both are required on every supply — on a TN-C-S (PME) supply the two come out near-identical, and SSEN still want both stated.
4. Record the voltage the tester displays during the test — the measured figure, not the 230 V nominal. It is a phase-to-neutral reading, so it is around 230 V on a three-phase supply too, never the 400 V line-to-line figure. The statutory range is 216.2–253 V (230 V +10% / −6%).
5. Record the two prospective currents if the meter reports them — prospective short circuit current and prospective earth fault current. Many loop testers display them beside the impedance; some don’t, and SSEN accept the cells blank (their form asks for them “if available on loop meter”).
6. Photograph the meter mid-measurement, with the phase-to-neutral figure legible on the display (Step 4).
7. Restore the earthing conductor and re-energise.

Never remove the SSEN cut-out fuse to take any of these readings — it is SSEN’s sealed equipment, and the incoming-isolator method exists precisely so their fuse stays in place. And keep the declared figures honest to the display: SSEN may re-measure, and a phase-to-neutral reading more than 5% off what was declared may be treated as a breach of the procedure (§7).

Step 4 — The two photographs

• The loop impedance meter, mid-measurement — the tester’s display showing the phase-to-neutral reading, taken during the test itself. This is SSEN’s corroboration of the figure you declare, so the number must be legible and must match it.
• The supply termination — the DNO’s supply terminates at the cut-out, so this is a photograph of the service head with its fuse carrier and the meter tails in frame. In CLRD it is the job’s own Cut-out Photo slot on the Pre-Install Application card: one photograph, asked for once, serving the document pack, the submission emails and this form together.

Step 5 — Enter it in CLRD

Open the SSEN Fast Track panel on the Pre-Install Application card and record the readings, whether the meter reported the prospective currents, and the meter photograph; add the cut-out photograph in its own section of the same card. The fast-track verdict shows beside Save as you type the phase-to-neutral figure.

CLRD holds the form out of the application until every reading and both photographs are recorded, because an incomplete FO-NET-CAB-008 voids the application — SSEN allow 7 working days to supply what’s missing before they take the measurements themselves and recharge the cost (§5). Complete, the filled form files with the G99 A1-2 automatically — there is nothing else to attach.