Value chain · CLM-1211
In plain terms
Britain's main transmission grid runs at 400 kV and 275 kV, and parts of the 132 kV network in Scotland. On the 400 kV network the voltage normally stays within 5 per cent of its nominal value and never beyond 10 per cent outside abnormal conditions. The rules read do not say how the connection voltage for a given plant is chosen.
The claim as recorded
The GB Security and Quality of Supply Standard defines the Main Interconnected Transmission System as "all the 400kV and 275kV elements of the onshore transmission system and, in Scotland, the 132kV elements of the onshore transmission system operated in parallel with the supergrid". The Grid Code (ECC.6.1.4.1) states that above 300 kV "The minimum voltage is -10% and the maximum voltage is +10% unless abnormal conditions prevail", with voltage normally within 5 per cent of nominal and "voltages between +5% and +10% will not last longer than 15 minutes unless abnormal conditions prevail"; from 110 kV up to 300 kV it normally stays within 10 per cent of nominal. Neither text read states what sets the voltage a given plant connects at.
Evidence behind this: T1, the SQSS definitions and the Grid Code Connection Conditions read directly.
Scope and limits
The MITS definition excludes generation circuits and transformer connections to lower voltages, so it names the backbone voltages, not the voltage of any plant's own connection. The plus-or-minus signs are read from the text layer, not a render. DC-connected Power Park Modules have their own voltage tables (ECC.6.1.4.2). Revision 41 and v2.12 text; later issues should be checked before publication.
What it assumes
No assumptions are recorded against this claim.
Where it comes from
PDF 65 / printed 65, verbatim: "This comprises all the 400kV and 275kV elements of the onshore transmission system and, in Scotland, the 132kV elements of the onshore transmission system operated in parallel with the supergrid". SRC-452 PDF 202, printed ECC p.10 of 143, verbatim: "operating at nominal voltages of greater than 300kV"; "The minimum voltage is -10% and the maximum voltage is +10% unless abnormal conditions prevail"; "voltages between +5% and +10% will not last longer than 15 minutes unless abnormal conditions prevail". The "within 5%" and "within the limits 10%" figures are paraphrased: the text layer drops the plus-or-minus sign before them.
Basis: SQSS Version 2.12 (29 May 2026); Grid Code Issue 6 Revision 41, ECC.6.1.4.1
Cite this
energyaperture.com/c/clm-1211
CLM-1211, Energy Aperture, as of 2026-06-05.
Connected records
Other published records linked to this one. A list, not a judgement.
- Also from SRC-238
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- Also from SRC-452
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