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

• 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.

•  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

• 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 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.

• 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.

3. Transformer selection and earthing arrangements

• Key issues: High losses, unsuitable or missing earthing and poor segregation of HV and LV Systems

In addition, there is a lack of transformer protection systems i.e. restricted earth fault protection and/or over-temperature protection. The integrity of the transformer star-point earthing (grounding) a transformer is essential for ensuring safety by limiting touch voltages during faults, providing a return path for fault current, allowing protection devices to operate, stabilising system voltage during unbalanced conditions or surge events, and reducing electromagnetic interference in communication/control cables. In addition, poor levels on transformer protection are evident in many cases.

Transformer selection must consider technical performance and clinical impact. In addition, the Transformer is the handshake between the HV & LV systems. Therefore, HV/LV Protection systems are required which should include restricted earth fault (REF) protection which should be arranged as shown in Figure 2. Note: HV dc system used for tripping circuit.

Where hermetically sealed transformers are specified and high-pressure relays are fitted, then these also should trip the HV breaker using the HV dc system for the tripping circuit.

Liquid filled transformers (ON & KNAN typically) should also have high temperature (HT) monitoring and high temperature trip (HTT) protection units fitted to protect these valuable assets.

In the case of transformer high temperature protection, these should trip the LV breaker and not the HV breaker as high temperature is load driven with typical arrangement as seen in Figure 3.

• Recommendation: Use Tier 2 or Eco Design transformers. Employ TN-S earthing for essential supplies. Ensure high integrity start point earthing installed. Ensure appropriated HV/LV inter-tripping & transformer protection system systems are in place to meet Electricity at Work Regulations 1989.

4. Main LV intake switchboard

• Key issues: Frequent failure points include lack of Form 4 or 5 internal separation and outdated protection interfaces.

There is a large legacy of LV switchboards which are non IP2X especially on cable termination chambers where exposed Live copper conductors are present. In addition, incorrect protection on main incomers such as unrestricted earth fault protection where an earth fault in the system incorrectly trips the main incoming breaker.

• Recommendation: To enable safe working on main LV switchboards, they should be designed for Form 4 (Type 6 Minimum) ingress protection. Protection must allow for upstream HV discrimination. Replace legacy systems lacking arc containment or proper segregation.

5. HV/LV protection and inter-tripping systems

Breakers, especially air circuit breakers (ACBs) and moulded case circuit breakers (MCCBs) are dispatched from the factory with their protection units set to a minimum as factory default settings. Unless adjusted during commissioning, nuisances tripping can occur. DC tripping/closing batteries are common failure points.

•  Recommendation: Use 24/30V dual rectifier battery systems with BMS monitoring. Implement local and remote alarm annunciation. Test battery banks quarterly. Note: there should be separate HV & LV tripping battery systems typically 24volts and the 110volt VT supply should not be used for emergency tripping.

6. Emergency standby generators

The hospital emergency standby generator forms the key secondary power source in the increasingly common case of interruptions on the DNO local area network. However, in many cases they either inadvertently start, fail to start and/or fail during a short period of operation.

• Key issues: Inadvertent operation or failure to operate for many reasons, such as:

a) Lack of solid star point earthing leading to equipment failures during testing/operation.

b) Starter battery failure.

c) Failure of fuel lines and/or fuel contamination.

d) Failure of phase failure relay and/or incorrect selection-commissioning.

e) Lack of/or Inadequate testing regimes.

f) Failure Automatic Transfer Switches (ATYS).

g) Inadvertent operation of protective devices.

h) Lack of load bank testing points.

i) Lack of resilience/redundancy should the generator fail for any reason.

Other issues include incorrect earthing, poor LV switchgear control interface. One of the most common causes of power interruptions is related to malfunction of the conventional reliance on outdated phase-failure relays. Note. This is variable voltage only with no time domain selection.

Modern practice includes using digital generator controllers with graphical HMI, event logging, and BMS integration.

• Recommendation: Confirm G99 compliance if operating in long-time operation, ensure correct rating, specify digital (phase failure) controllers with HMI, and test under load conditions. If using traditional phase failure relays, ensure they are voltage/time variable.

7. Automatic Transfer Switches (ATYS) — Key considerations in hospital power resilience

Automatic Transfer Switches (ATYS) are a critical component in virtually all hospital secondary backup systems, enabling seamless transfer between normal and emergency power supplies. However, their reliability is only guaranteed when properly designed, configured, and tested in alignment with site-specific conditions.

• Key issues: Factory default settings applied leading to nuisance inadvertent operation — particularly if voltage sensing thresholds set too sensitive.

These parameters must be adjusted to reflect the actual site voltage characteristics, accounting for expected tolerances and the behaviour of the local distribution network.

Ultimately, correct ATYS configuration is not a one-size-fits-all approach. It requires coordination with electrical design engineers and commissioning specialists to ensure the system supports safe, reliable, and resilient healthcare operations.

•  Recommendations: The switch’s timing logic is equally important. Both transfer and retransfer delays must be programmed with suitable time lags to accommodate voltage sags, transients, or momentary outages. This prevents nuisance switching and ensures system stability during supply restoration. We suggest 3-5 seconds typically.

8. Uninterruptible power supplies (UPS)

UPS systems provide critical backup power to Group 2 Medical Areas and key services such as IT, medical records, and security systems. Their reliability directly impacts patient safety and operational continuity.

• Key issues:
• No Wraparound Bypass: Limits ability to maintain systems without disruption.
• Battery Failures: Aged, warped, or leaking batteries; missing terminal insulators.
• Hazardous gas: A rotten egg smell may indicate Hydrogen sulfide (H₂S) release— highly toxic and flammable.
• Poor wiring: Misconfigurations reduce safety and performance.
• Lack of fundamental means of isolation of both UPS and its battery

A zero-phase shift transformer is required to maintain a stable neutral-earth reference. It is best installed on the UPS output, though the input or bypass may be suitable in some designs so consult the UPS manufacturer.

• Recommendations:

1. Use dual battery systems for resilience.

2. Add thermal monitoring for large installations.

3. Test inverter performance annually.

4. Provide isolation at both UPS and battery ends.

5. Include remote battery trip controls outside the UPS 
 room.

6. Fit fire/smoke dampers on UPS room ventilation.

7. Relay alarms to staff base and estates via BMS.

9. Medical IT systems and N+1 redundancy

Medical IT systems provide essential electrical protection in Group 2 medical areas (e.g. theatres, HTU, SCBU), preventing equipment failure during earth faults and protecting patients from leakage currents. These systems use isolation transformers (typically 4—10 kVA, single-phase) without centre-tap earthing.

• Key issues:
• Absence in some Group 2 areas.
• No N+1 resilience.
• Shared earth paths.
• Lack of alarm integration or visibility.
• Recommendation: Adopt modular N+1 IPS/UPS designs, verify transformer impedance, and link alarms to BMS for remote visibility to clinical and Estates staff.

10. Harmonics, power quality and energy monitoring

Undetected issues like harmonic resonance and THD can pose serious risks. All new connections must now comply with engineering recommendations (EREC) G5 1920. The ENA guides stipulate THD limits: Voltage ≤5% (LV), ≤3% (HV); Current emissions must comply with planning levels.

This waveform capture displays both the voltage and current waveform which indicates a significant presence of 5th & 7th Harmonic distortion typically found on electrical systems with significant element of frequency-controlled motors.

• The introduction of harmonic rich loads, for example IT loads which generate Triplen (150hz) current, the resultant distortion can be demonstrated with the 500amp 50HZ load and the subsequent distortion by adding 100amps of 3rd Harmonic current as seen Recommendation: Install PQ analysers on transformer/ generator incomers with intermediate level analysers on sub-main feeders. Trend harmonic distortion and power factor and use dashboards for threshold monitoring and appropriate pre-settings to detect transient conditions.

11. Power Factor correction

Power Factor (PF) correction units were an integral 
part of every low voltage electrical infrastructure due 
to the inherent poor power factor on historic 
inductive loads. However, since the introduction of frequency-controlled drives which operate at near unity power.

A typical example of a current waveform capture with frequency drive connected (Figure 5, top graph) and with the PF isolated (Figure 5, bottom graph). Note: Elimination of zero crossing and much improved waveform.

In worse case scenarios, resonances can occur between the frequency drives and the PF equipment, leading to catastrophic failure of the capacitor banks, sometimes leading to fire.

• Recommendation: Switch PF equipment off/on and check PF values at transformer main incomers. If no improvement or change in PF, isolate PF system and record in logbook.

12. Electrical risers

Many NHS Hospitals, especially city-based sites, where spatial constraints lead to multi-story buildings and electrical risers form the main method of distribution power.

• Key issues: Overloading (1-phase), water ingress 
from adjacent rising water pipework, and more 
serious, the loss of the busbar Neutral connection. If 
this occurs, over voltages will appear on every distribution board with serious consequences of burnt-out electrical equipment. From experience, this normally presents an over voltage on the 1-phase distribution boards in the order of 380 volts depending on the connected load.
• Recommendation: Consider installation of ‘over voltage’ detection devices on new riser busbars and on old ones if isolated for maintenance purpose. Ensure load is well balanced. For resilience, consider dual supplies to each riser via ASCO, Atys and/or simple interconnectors between risers.

13. Commissioning and independent validation

• Key issues: Lack of independent testing and just as important is lack of commissioning. These can compromise patient safety.
• Recommendation: Engage third-party agents or Authorising Engineers to witness tests and maintain comprehensive commissioning records. All variable parameters should be recorded on commissioning records.

14. ‘As built’ record drawings and documentation

Accurate ‘As-built’ electrical drawings are essential for meeting legal obligations under the Electricity at Work Regulations (EAWR). Regulation 4(2) requires systems to be maintained to prevent danger, while Regulation 29 mandates keeping records to prove compliance.

Without accurate drawings, it is impossible to:

• Identify circuits reliably during fixed wire testing.
• Isolate circuits safely.
• Confirm compliance with BS 7671.
• Track system changes effectively.

Their absence increases the risk of error, safety hazards, and non-compliance.

• Recommendation: Contact switchboard manufacturers with works order and serial numbers to obtain ‘as-fitted’ drawings. Establish a structured drawing management system to support ongoing compliance and safety.

15. National defects

From time to time, design defects are uncovered on electrical equipment and in some cases, inefficiencies in design can sometimes lead to operator errors with the undesirable outcome of an incident in which equipment suffers damage or, in worse cases, injuries or death can occur.

Whilst manufacturers should report any defects to their equipment, the end user often raises alerts. The Electricity Networks Association (ENA) operates the NEEDers system, which publishes known defects on its portal.

These alerts are circulated via NHS England (formerly NHS estates). Hospital estates teams can access these alerts through the NHS Collaboration Hub which each Trust should have access to.

16. Condition reports

Considering the age of the electrical infrastructure in many NHS hospital and the changing characteristics of electrical loads, there are several recommendations.

• Recommendation: 5-year risk-based condition reporting complete with budget costs and priority schedule should be completed to include all the elements noted with specific attention on supply capacities, resilience including HV/LV electrical intakes, standby generators, UPS systems, Medical IT Systems, electrical risers, and a sample of final distribution boards.