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The Security and Quality of Supply Standard (SQSS)
The SQSS is one of the 11 GB industry codes governing the electricity system. It is administered by NESO and sets the minimum security standards that Transmission Licensees must use when planning and operating the national electricity transmission system. It determines how much redundancy must be built into the system and how the system must be operated to maintain security of supply.
Legal foundation: Transmission Licence conditions D3, E16; ESO Licence conditions E3, E7.
Current version: Version 2.11, effective 13 March 2026 (134 pages).
Source file: sqss.md
What the SQSS does
The Grid Code tells Users what their equipment must do. The CUSC tells them what they pay. The SQSS tells the system planners and operators how much network capacity to build and how to operate it safely. It answers the question: what happens when things go wrong?
The standard sets deterministic security criteria -- rules that say "if this fault happens, these consequences are unacceptable." System planners must invest in enough transmission capacity to meet these rules. System operators must operate the system in real time to comply with them.
Structure at a glance
The SQSS covers three parts of the transmission system, each with separate onshore and offshore criteria:
| Part of system | Planning criteria | Operational criteria | Voltage limits |
|---|---|---|---|
| Generation connections | Section 2 (onshore), Section 7 (offshore) | -- | -- |
| Demand connections | Section 3 (onshore), Section 8 (offshore) | -- | -- |
| MITS (backbone) | Section 4 | Section 5 (onshore), Section 9 (offshore) | Section 6 (onshore), Section 10 (offshore) |
Plus Section 1 (Introduction), Section 11 (Definitions), and Appendices A-J.
How the security classes work
The most important table in the SQSS is Table 3.1, which defines six security classes for demand connections based on the size of the demand group at each Grid Supply Point:
| Class | Group Demand | What must happen after a fault |
|---|---|---|
| A | 0-1 MW | Supply restored in repair time (hours to days) |
| B | >1-12 MW | Supply restored within 3 hours |
| C | >12-60 MW | Most supply restored within 15 minutes; up to 12MW can be lost |
| D | >60-300 MW | Supply maintained immediately; up to 20MW auto-disconnection permitted |
| E | >300-1500 MW | Supply maintained immediately (up to 60MW for 60 seconds) |
| F | >1500 MW | Full supply maintained immediately under all conditions |
The bigger the demand group, the more redundancy must be built into the connection. This drives the number and configuration of circuits at each Grid Supply Point.
Generation connection rules
For generation connections (Section 2), the key rules are:
- A single transmission circuit fault must cause no loss of power infeed (2.6.1)
- A single generation circuit or busbar fault must not cause losses exceeding the infrequent infeed loss risk (currently 1320MW in England and Wales, 1000MW in Scotland) (2.6.3)
- Overhead line connections are limited to 5km for large power stations (>=2000GWh annual output) and 20km for others (2.7)
MITS design criteria
The Main Interconnected Transmission System (Section 4) must be designed so that under both Security and Economy background conditions, no single fault causes overloading, voltage violations, or instability. The two background conditions use different generation dispatch assumptions (Appendices C and E) to test the network under realistic operating scenarios.
Operational criteria
In real-time operations (Section 5), the ISOP (NESO) must ensure that the system can withstand single secured events without unacceptable consequences. The key limit is that a double circuit or busbar fault must not cause more than 1500MW of supply loss (5.3.3).
Voltage limits
The SQSS sets voltage limits in six tables for different timescales and conditions. The key distinction is between planning timescales (tighter limits, used for investment) and operational timescales (wider limits, used in real time):
- 400kV (>300kV): Planning pre-fault +/-2.5%; Operational pre-fault +/-5%
- 275kV (200-300kV): Planning pre-fault +/-5%; Operational pre-fault -5%/+9%
- 132kV: Planning pre-fault +/-5%; Operational pre-fault -5%/+10%
Voltage step changes at transmission-distribution interfaces are generally limited to +/-3% for routine switching, relaxed to +/-6% for faults and infrequent events.
National security provisions
Version 2.11 includes provisions (1.24) allowing the Secretary of State to direct the ISOP under ESO Licence condition B4 where there is a national security risk. The ISOP must comply with such directions even if this means departing from normal SQSS requirements.
Relationship to other instruments
- Grid Code -- additional quality of supply criteria; Emergency Manual Demand Disconnection provisions
- CUSC -- TNUoS methodology references SQSS planning standards; SQSS criteria drive the need for transmission investment that feeds through to charges
- STC -- SO-TO interface; offshore reactive power requirements (Section K)
- Transmission Licence -- D3 and E16 are the legal basis for TO compliance
- ESO Licence -- E3 and E7 are the legal basis for ISOP compliance
Governance
The SQSS is governed by the SQSS Panel, comprising NESO and Transmission Owner representatives, with Ofgem as Authority. Modifications are numbered GSR-xxx. The Governance Framework is in Appendix J.
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Last updated: 2026-04-05