POWER INFRASTRUCTURE
A practical guide to power-related challenges
With an ageing estate, and the NHS driving toward Net Zero, hospital electrical systems face mounting stress and increasing rates of failure. Drawing on his over 25 years of hands-on experience, Authorising Engineer Eugene Conroy C. Eng, FIHEEM, MIET, MSc, highlights common power-related issues across NHS estates – from design flaws to operational shortfalls. As the NHS advances digitalisation and electrification, resilient and future-ready electrical infrastructure is essential.
This paper serves as a practical aide-memoire for estates managers and designers.
1. DNO supplies and design considerations
n Key issues: Lack of capacity and resilience Many NHS Trusts are unaware of their District Network Operators (DNO) capacity constraints until late in project development, only to discover they are operating above their agreed capacity, incurring untimely and costly enhancement works. Issues include increased capacity for electrification (heat pumps, EV charging), DNOs requiring costly offsite reinforcement, and non-compliance with ENA G81 design standards and ESQCR. There is also an obligation on customers to have an
inter-trip facility on their main HV intake to enable remote tripping of the DNO supply. This is often missed. Note: Dedicated 24v dc tripping circuit required. For
typical arrangement refer to Figure 1, IN UKPN guidance Document EDS-0002 (2018). Note: Where DNO beaker is afforded Tlf or self -powered relays where they have no dc system, the customer dc system (or 110v capacitor driven) inter-posing trip unit shall be used for inter-tripping. Typical arrangement is shown in Figure1. In addition, with the increase in energy saving heat pumps and frequency drives, many customers are now introducing significant harmonic content which is reflected on the DNO network which are exceeding the Energy Network Association Governing Standard (ENA EREC G5/5) limits. n Recommendation: Assess available capacity via early engagement with DNO and secure dual/ resilient supplies where possible. Ensure inter-tripping facility is in place. Install Power Quality (PG) metering on main incomers. Establish clear demarcation lines between DNO/Customer via Site Responsibility Schedules (SRS). Note: each metered supply will have a unique MPAN identification number which is required for any supply related communications.
2. High voltage (HV) network design and validation
n Key issues: Lack of resilience, redundancy and maintainability
Robust HV network design underpins system safety with legacy installations operating at or beyond fault level limits, using cables with insufficient fault-withstand ratings, and lack full protection coordination studies. DNO will advise Prospective Short Circuit Current Level (PSCC -Typically 250MVA = 13kA). Therefore, designers
CB DNO network Customer Customer network
should state their design fault time as either 1-second or 3-seconds as this dictates the size of HV cable required. e.g., for a 13.1Ka fault level, 185mm is required depending on manufacturer. Most DNO’s will adopt the 3-Sec principal as a worse-case scenario. Early warning of HV Insulation failure, smell of ozone = partial discharges = extremely dangerous condition arising. n Recommendation: Validate HV designs using short- circuit analysis and protection grading. Match cable CSA and type to withstand calculated fault energy. Undertake annual ‘Partial Discharge’ tests, maintain as- built records and use validated design software.
DNO dc tripping battery DNO
CB (N/O) auxillary contact
dc shunt trip coil
Inter-posing tripping relay
Customer
dc tripping battery Or
Capacitor trip unit
ATYS switch voltage set at 220v, with FT & RT set to 0.
Figure 1. (UKPN) document No EDS 05- 0002: Emergency trip schematic – switchgear tripping battery.
Customer emergency trip off (EPO) button
January 2026 Health Estate Journal 59
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