title: ENA Engineering Recommendation P2 - Security of Supply tags: [security-of-supply, DNO, distribution, ENA, technical-standard, network-planning, redundancy, p2, distribution-code] instrument_row: 110 source_file: sources/energy-networks/ena-er-p2.md

ENA ER P2 - Security of Supply

What it is: The GB standard defining how much redundancy Distribution Network Operators (DNOs) must build into their networks. Issued by the Energy Networks Association (ENA). Current version: Issue 8, February 2023 (abbreviated title EREC P2/8, replacing EREC P2/7).

Full document (free PDF): https://dcode.org.uk/assets/uploads/files/ENA_EREC_P2_Issue_8_(2023).pdf

The core problem it addresses

Every section of a distribution network serves some level of demand. If a circuit fails, how much of that demand must be restorable, and how quickly? Without a national standard, each DNO would make that judgement independently, leading to inconsistent investment levels and unequal consumer outcomes across regions.

EREC P2 solves this by classifying all distribution network demand into six supply classes (A through F) based on the size of the Group Demand served. For each class it sets binding minimum requirements: what fraction of demand survives an outage, and within what time. These requirements drive DNO capital investment decisions across the entire distribution network.

The supply class structure and security levels

The six classes scale from local low-voltage supplies up to bulk transmission infeed points.

Class A - up to 1 MW: After a circuit fault, demand must be restored within repair time. No second-outage requirement. Covers small rural or LV networks where a single transformer may serve all demand.

Class B - over 1 MW and up to 12 MW: After a circuit fault, demand minus 1 MW must be restored within 3 hours; full demand within repair time. No second-outage requirement. Network must normally be supplied from at least two normally closed circuits or one circuit with automatic/supervisory switching to alternatives. A minor relaxation (Group Demand minus 1.2 MW, rather than minus 1 MW) is permitted for circuits under 1 km in specific circumstances.

Class C - over 12 MW and up to 60 MW: After a circuit fault, the smaller of (Group Demand minus 12 MW) or two-thirds of Group Demand must be restored within 15 minutes; full demand within 3 hours. No second-outage requirement.

Class D - over 60 MW and up to 300 MW: After a circuit fault, all but up to 20 MW (automatically disconnected) must be restored immediately (within 60 seconds); full demand within 3 hours. After a second outage, for demands over 100 MW: the smaller of (Group Demand minus 100 MW) or one-third of Group Demand must be restored within 3 hours; full demand within time to restore the arranged outage.

Class E - over 300 MW and up to 1500 MW: After a circuit fault, full Group Demand must be restored immediately. After a second outage, all consumers must be supplied at two-thirds of Group Demand immediately; full demand within time to restore the arranged outage. Covers bulk distribution infeed substations at 132 kV.

Class F - over 1500 MW: In accordance with the relevant transmission company licence security standard. Applies to systems regarded as part of the interconnected Supergrid.

"Immediately" throughout means within 60 seconds.

What counts as meeting the standard

DNOs can meet the P2 standard using circuits, non-circuit means, or a combination:

• Circuit capacity: the rating of circuits remaining after the outage
• Transfer capacity: capacity that can be switched in from adjacent networks within the required time
• Distributed generation (DG), demand side response (DSR), or electricity storage: these can contribute to the security calculation, but a single point of failure in any of these must not cause a bigger outage impact than losing one circuit

The key flexibility introduced in Issue 7 (2019) is that DSR activation counts as equivalent to restoring demand. A DNO can therefore meet the P2 standard partly by having firm DSR contracts - customers who will reduce load on instruction - rather than building additional circuit capacity.

Why this matters for DNO investment

P2 directly drives distribution network capital expenditure:

• Radial vs. ring networks: Class C and above require networks that can restore demand within 15-30 minutes without manual intervention. This requires ring main or dual-circuit topology, not radial feeds. A DNO serving a new demand cluster that pushes a Group Demand from Class B to Class C must reinforce the network to meet the tighter standard.

• Reinforcement trigger: When demand growth, new generation latent demand, or load redistribution pushes a group demand across a class threshold, P2 triggers a reinforcement obligation. This is the mechanism through which demand growth (e.g. EV charging, heat pumps, data centres) translates into distribution investment.

• Non-network alternatives: Since Issue 7, DNOs can avoid building new circuits by contracting DSR or DG to provide security contributions. This is the regulatory basis for flexibility procurement by DNOs. EREP 130 gives the methodology for quantifying how much a DSR contract is worth in P2 terms.

• RIIO-ED price controls: Ofgem sets DNO revenues under RIIO-ED. P2 defines the minimum security standard DNOs must achieve, which determines the floor on network investment. Ofgem can grant derogations where a lower security level is justified by cost-benefit analysis.

Group Demand: what it measures

Group Demand is not the same as metered demand at a substation. It is the DNO's estimate of the maximum demand that would appear if all local generation stopped, all DSR was activated, and all demand-suppressing factors were removed. It equals Measured Demand plus Latent Demand.

This matters because a substation supplying 20 MW of Measured Demand may have 8 MW of embedded solar generation behind the meter, giving a Group Demand of 28 MW - placing it in Class C rather than Class B and triggering different security requirements.

Relationship to other instruments

Common misconceptions

"P2 specifies N-1 or N-2 redundancy." P2 specifies outcomes (demand to be restored, within timeframes), not the engineering means. A DNO can meet Class D requirements with N-2 circuit topology, or with N-1 circuits plus DSR, or other combinations validated by EREP 130.

"P2/7 is the current version." P2/7 (Issue 7, 2019) was the previous version. The current version is P2/8 (Issue 8, February 2023), though the change from Issue 7 was minor: a Table 1 amendment for specific Class B supplies.

"P2 applies to individual customer connections." It does not. P2 governs network-level planning. The security of an individual customer's connection is agreed bilaterally with the DNO.