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WB Power Services has a long history of supplying standby generation solutions to the NHS/Healthcare sector. During this time WB has installed many generating sets and amassed a significant applications experience along the way. Various NHS HTM’s are in place to provide comprehensive advice and guidance on the design, installation and operation of specialised building and engineering technology used in the delivery of healthcare. There are areas of the HTM-06 relating to aspects of a generator installation which can be viewed to be at variance with commercially available products, highlight anomalies between the HTM and BS EN ISO Standards that sometimes do not reflect or consider recent changes legislation or market driven product evolution.

In this white paper we consider some of the most common specification clarifications sought by WB and adopted derogations. For ease of use we have identified each of these by using the HTM 06-01 (2017) reference section.


9.38 Short-term parallel operation – 9.39 Long-term parallel operation


Generating set control panels are available to manage both generator incoming breaker (switchboard) control requirements for short-term or long-term parallel with the DNO Mains supply, offering a range of facilities ranging from soft mains return, “live” load testing to peak lopping and STOR.

Short-term mains synchronising for purposes of soft mains return is a basic standby power application and as such does not require exhaust emissions abatement it being an “Excluded Standby Generator” within the bounds of MCPD regulations.

Longer terms mains synchronising, depending on the operational requirements may be subject to the requirements of the MCPD regulations. (All subject to site specific EA and regional review). Also see 9.88 this paper and also see WB white paper on “Greening in Standby Power Generation”.


9.73 Overload Rating


A Prime Rated Power (PRP – ISO8528) generator will PRP meet the rating as required in HTM06-01. A PRP rated set offers a 10% Overload for 1-hour in every 12-hours as set out in HTM-06. See two WB white papers on Generator Sizing.


9.74 Category of load Acceptance


Generating sets sold within Europe are designed to meet the requirements of BS EN ISO8528 and offer transient performance in compliance with the range G1-G4 (ISO8528) and associated levels of first step load acceptance. G2 performance is aligned with HTM 06- 01 for Category-3 / 60% load acceptance at the PRP rating but frequency recovery will be within 5-seconds (as per G2 ISO8528).

Note: This section should be read in conjunction with HTM section 9.75 as gross oversizing of the generator is necessary to achieve category 2 and 1. Performance at these levels is available typically by utilising a generating set with a larger engine capacity. Minimum operating load levels must be considered.

It should be noted that not all engine alternator combinations are able to achieve a 60% PRP load on first step. Some more modern engines that offer lower levels of emissions often offer lower levels of first step load acceptance at G2 transient load acceptance performance. Active load management via a BMS or EMS often render the need for higher levels of first step load acceptance to be unnecessary. See two WB white papers on Generator Sizing.


9.78 Battery Charging Systems


Most generating sets offer an engine-driven alternator as well as mains-powered battery charging systems. It is possible on many sets to offer dual starting systems (dual chargers, dual battery systems and dual starter motor systems) however, these are not in the HTM guidance rather, an extra feature to provide improved generator starting resilience, mitigating against the risk of discharged/ depleted starting batteries and failed starter motor.

9.80 Battery Types


Wet cell Lead Acid batteries are widely used across the standby generating set market and offer the most cost-effective solution. For critical healthcare applications, WBPS recommend the use of higher specification sealed AGM battery; these offer a more resilient starting battery solution, 9+ year lifetime, superior cranking performance and are maintenance free.


9.81 Fuel Oil main reserve

HTM06-01 prescribes that the volume of diesel fuel oil stored within the day tank and arranged for gravity feed to the engine should be no more than the greater of 750l or the equivalent of 10 hours at full-load (maximum capacity) running of the generator set. In addition to the local store, a fuel oil main reserve for 200hrs full load running for each standby generator set should also be available on-site.


For a 2000kVA generating set the 10-hour day tank rating can have a capacity in the order of 5000l and to meet in full the 200-hour requirement this can be in the order of 100,000l (at full load).

It should be noted that in line with the design criteria set out in BS EN ISO 8528 the average load applied to a PRP or Prime rated generating set over a 24-hour period should not exceed 70% of its prime rated capacity. Generating sets typically up to a rating of 1000 kVA, are available with standard baseframe fuel tanks that can provide between 4 and 8 hours running autonomy (long on some smaller sets). The use of baseframe fuel tanks can often reduce the footprint of the generator installation, the quantity of siteworks and the overall cost of the installation.


It is common for derogations to be applied to the bulk fuel storage tank permitting them to be sized, where space permits, at either 48 or 72 hours running time at full load. The 10-hour capacity day tank should, where possible, be automatically refilled from the bulk tank.


Fire dump valve / tank


Incorporating a Fuel dump facility needs to be carefully considered since the scenario for dumping fuel is typically in a blackout / deenergised state. The risk being mitigated in fuel dumping is primarily to protect lower floor levels from the risk of escaping, potentially burning fuel. As a result, this feature is not prescribed for ground floor or basement-mounted generator installations where gravity flow (without pumping) of the ‘dumped’ fuel is not practical or beneficial. Additionally, the bulk tank will need to be designed, with sufficient additional capacity available at all times, to accommodate fuel dumped from the day tank. We recommend a risk assessment to determine this viability.


Exhaust Systems / Emissions mitigation


Incorporating Exhaust Emission reduction technologies for standby generators is a best practice and frequently a requirement by the authorities on certain sites that are designated as back-up/standby generators.

The application of emissions reduction equipment as suggested by HTM06-01 must be weighed against any impact on Resilience as part of the overall system configuration. The Environmental Agency (EA) has prescribed via the MCPD that for most Standby Generator applications, exhaust emissions after treatment is not a requirement but this is considered on a case-by-case basis by the EA.

The prescriptive requirement by the EA is only aimed at reducing the emissions of Nitrous Oxide (NOx) from the diesel engine with no emphasis made for CO2 or other diesel particulate emissions.

Compliance to MCPD requirements can be achieved by the addition of a SCR unit into the engine exhaust gas stream. The addition of a SCR poses a minimal risk to the resilience of the generator; this risk coming due to blockage of the SCR reactor with soot and other unburnt or partial burn fuel resulting from light load running of the set.  See WB white paper on “Wet Stacking”. Also see WB white paper on “Greening in Standby Power Generation”.

Selective Catalytic Reduction (SCR) unit.

HTM guidance suggests the use of Diesel Particulate Filter (DPF) to help reduce some of the other smaller carbon elements present in the exhaust gas stream. The need for such devices is not based on EA guidelines or requirements for emissions aftertreatment. A decision on the inclusion of such a unit should only be taken once a full assessment of the emissions of the specific engine model has been completed. Many more modern engines come close or exceed all current requirements.

The inclusion of a DPF does though add significant risk to the resilience of the generator. DPF units are prone to carbon-soot build-up which is typically formed by engine operation at low generator load levels. Blockage of the DPF core is proven to be detrimental to the long-term reliability of the diesel engine, the long-term high backpressure imposed on the engine can result in premature engine failure. Periods of off load or light load running should be avoided when a DPF is fitted. Regular testing at high levels of load is essential to ensure the DPC is not blocked. Other exhaust gas treatments are available including a Diesel Oxidation Catalyst (DOC) which can offer reductions up to 60% of CO and 40% of HC. See WB white paper on “Wet Stacking‘. Also see WB white paper on “Greening in Standby Power Generation”.

A full EA assessment should be undertaken prior to adding any new standby generator capacity to site. This assessment should include reviewing the emission performance of the engine of any potential vendors and this information used to inform the need or otherwise for the inclusion of any exhaust gas after treatment equipment may or may not be requited.


16.8 TESTING – Voltage Regulation

The HTM-06 section 16.8 indicates that the generator terminal voltage on starting should not overshoot the nominal terminal voltage by more than 15%, and return to within 3% of the rated voltage within 0.15 s. The generator terminal voltage should not vary by more than 15% following a step load increase from 0% load to 60% load, and then return to within 3% of the rated voltage within 0.5 s.


In this section the HTM sets out levels of transient performance not readily achievable on a generating set unless the generator is significantly oversized for that application. Diesel Generator performance is standardised in accordance with ISO8528. In Part-5 of this standard the transient performance standards are clear set out (Ref: 9.74 comments above). Unless the set is significantly oversized it is unlikely that any vendor will comply with this requirement.  WBPS and Kohler typical offer generating solutions where transient performance better than ISO8528 performance class G2.


G R Halliday

Business Consultant

WB Power Services Ltd




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