Power You Can Trust


WB Power Services has a long and proud history of supplying standby generation solutions to the NHS/Healthcare sector. Over many years WB has installed a large number of generating sets, both large and small, in basic containers to complex plant room installations. During this time the business has amassed a significant experience base within its sales, project installation and service teams.

For those outside of the day to day working of this sector, the size and scale of standby generation in a hospital setting is an unknown quantity. For those working from within the sector, the installation of a standby generating set probably isn’t and everyday occurrence, so this paper seeks to, from a “specialist standby diesel generator installers” view, highlight, explain and clarify.

  • The guidance lines up with the way in which diesel generators are designed and built and how this can be at variance with the HTM
  • The guidance contained within HTM-06 can be practically applied
  • Some of the most common apparent ambiguities or variances can readily be overcome

Starting Point

The starting point of any design should be with the relevant standards or guidance documents which in this case of Secondary Power Sources in healthcare settings HTM-06 (2017) and in the case of standby diesel generators it is BS ISO 8528 (updated in 2018).

As with any specification, BS ISO 8528 sets out the basic minimum standards for the equipment (some manufacturers exceed these) and often doesn’t move quickly enough in terms of recognising more cutting-edge product developments. For example, this can be across engine performance or market requirements, and have the tendency to look out of date even when newly published. It is likely to be a similar problem with some aspects of HTM-06, particularly as it is a guidance document and by the nature of the review period, will not take into consideration more recent advances in technology and changes in the law, for example recent changes in emissions requirements.

Why are Standby Generators required?

Hospitals are of course true 24/7 operations, forming a critical part of the local, regional and national infrastructure and varying greatly in terms of physical size, bed capacity, building age, scope of services offered in that location etc. In the UK healthcare sector the Health Technical Memoranda (HTMs) are there to “give comprehensive advice and guidance on the design, installation and operation of specialised building and engineering technology used in the delivery of healthcare”.

The primary reason standby generators are included within a hospital is to provide a Secondary Power Sources (SPS) or simply provide back-up power in the event of a power utility failure. A utility power failure can be anything from just a few seconds to hours or possibly days. Uninterruptible Power Supplies (UPS) are one such SPS, installed mainly to provide power to highly critical equipment for relatively short periods of time typically in the region of 5 – 30 minutes (up to 3 hours in some critical areas) which is more than sufficient to deal with any short term “brown out” or the 10 to 15 seconds required to bring the standby generator(s) online.

It is likely in the case of more recently constructed hospitals, newly constructed wings or recently refurbished areas it is desirable that the electrical load connected to a standby generating set is now the entire hospital / wing. This is due in part to the very diverse nature of the equipment used in all operational areas. When viewed in a more general context the electrical loads can consider to be:

  1. General building load ie lighting and small power
  2.  Mechanical load
  3. Critical area loads (ICU/CCU)
  4. Critical equipment in general ward areas

The HTM speaks clearly about the importance of fully risk assessing and having practical emergency contingency plans inplace that are always available and ready to implement. It also makes clear that design approach adopted “should be mindful of the need to maintain an electrical supply within specific time periods for the safety of patients and staff (Chapter 7)”. Depending on the area and its use these times set out and defined such that supply should restored within time frames of:

  • Greater than 15 s;
  • Less than 15 s, but greater than 5 s;
  • Less than 5 s, but greater than 0.5 s;
  • Less than 0.5 s;
  • No-break.

(Reference IEC 60364-555)
The above times need to be aligned with the distribution strategy (discussed in HTM-06 Chapter 7) and final circuit configurations (discussed HTM-06 Chapter 15).


Generator Sizing

Often one of the biggest challenges for the designer is to arrive at a rating for the generator / SPS as there are many fators to consider not least of which is an allowance for future requirements. HTM06 section 9.18 speaks about “that electrical outages can be very short (less than a few minutes) or for many hours”.” …all generator sets should be designed and rated to provide continuous full load for prolonged periods”.” provision may require a manual or automatic control system with the ability to “load shed” a limited number of the secondary services such as non-essential lighting”. A difficult task indeed and once installed the loading on each generating set should be checked annually to ensure the load remains within the design criteria. The balance of this white paper looks to support to support and inform the design process.

Generator First Step Load Acceptance

Section 9.17 of HTM-06 states “The design strategy and plant sizing should take account of the load to be supplied within 15 s of cold start”.

One of the most common areas of variance encountered when considering sizing / rate the standby generator required comes from the way in which section 9.74 of HTM-06 is drafted and can interpretated. When considering the installation of a new generating set there are of course many things to consider. Some of the critical ones though are the rating of the set (kVA/kW), potential future increases in load requirements, the types of load to be protected and the transient conditions which sit around that.

The HTM speaks about diesel or gas engines being manufactured generally in accordance with BS ISO 3046 which of itself not usually a problem. It then, rightly, moves on to discuss “four categories of load acceptance” set out in the standard for various types of engine operation on the basis of percentage load acceptance for the Class A rating being:

  • Category 1 – 100% load acceptance
  • Category 2 – 80% load acceptance
  • Category 3 – 60% load acceptance
  • Category 4 – 25% load acceptance

Whilst these levels are true specifically in relation to engine performance they do not directly relate to the way in which load acceptance is defined in BS ISO 8528 nor do they clearly define the recovery time required under such load acceptance or transient load conditions.

BS ISO 8528 does not require a generating set to be capable of achieving any specific level of first step load acceptance, as this capability is based on the Break Mean Effective Pressure (BMEP) that any particular engine is able to deliver. First step load acceptance is, in many cases, a key design performance requirement (e.g. pump starting).

Although having been improved and refined over many years, many of the engines (particularly the larger power ranges) used on generating sets in the general commercial market have been in production for many years. Those that are of a newer design typically offer many advantages such as lower fuel consumption and as a result, lower emissions. Those improvements though can come at a cost, and this can be the first step load acceptance capability of the generating set. It is important then to have a clear understanding of what is actually needed and what can be optimally achieved in these areas.

Understanding how the engine and alternator work together under transient conditions

We identified earlier that the BS ISO standard does not require the generating set to achieve any specific level of first step load acceptance, as this is determined by the BMEP that the engine is able to deliver. Hence there is a wide variation between manufacturers and the engines used at various power nodes.

60% first step load acceptance for “any rating” of generating set has for a long time been considered an “industry norm” and is often written into many “standard” consultant specifications. However specific consideration is not given to the actual operating requirements of the infrastructure or being qualified with the essential giving performance classification (G1-G4).

The design of many engines particularly those used on the larger generating sets (+1000kVA) widely used in the healthcare, water treatment or data centres have been around a very long time. Whilst they have been enhanced and improved over time (more electronic engine management, improvements in the combustion process, high pressure fuel injection, improved materials etc) there are few truly new engines. Those that are new typically offer lower fuel consumption, hence lower emissions, and have a more compact footprint i.e. higher power density.

Those improvements can come at a cost, and this can be the first step load acceptance capability of the generating set. Whilst these advancements are accepted within the standard, the “industry norms” often do not keep pace with, nor understand, the changes or embrace the advantages that they bring. This is often the case when we look at the healthcare environment.

Governing Standards / Performance Class ISO 8528-1: 2018 – 8

The standard lays out four performance classes G1-G4 with G4 being the most onerous. In simple terms the performance classes set out the maximum voltage and frequency deviations permitted on the application of a first set load and the time over which the generating set needs to return to a steady state condition and other operational considerations. It is in this area where it is the author’s view that the standard, whilst seeking to provide an all-embracing minimum standard, is failing to keep up with product development.

With the harmonisation of standards across Europe, and to a lesser extent globally, the performance of electrical equipment generally has improved, particularly in relation to the efficient use of energy and reduction of reinjected waveform distorting harmonics prevalent in nonlinear devices. These older devised can be the primary loads of any hospital such as UPS and inverter / soft start drives serving larger mechanical plant.

Class G2:
This applies to generating set applications where its voltage characteristics are very similar to those for the commercial public utility electrical power system with which it operates. When load changes occur, there can be temporary but acceptable deviations of voltage and frequency. (1)

Class G3 speaks more about the “connected equipment making more severe demands on the stability and level of the frequency, voltage and waveform characteristics” (1) and then site examples such as Telecommunications and thyristor-controlled loads. Both rectifier and thyristor-controlled loads can need special consideration with respect to their effect on generator-voltage waveform.” (1)

Class G4 similarly refers to “Data-processing equipment or computer systems” * as its examples.

By inference, the standards suggest that a hospital operates equipment falling into all three categories for example, performance classes G3 or G4 as it uses “telecoms and data processing equipment or computer systems.” Whilst those statements are in essence true, in a vast majority of cases, the type of products referred too i.e. thyristor controlled loads, are in most cases a thing of the past. If they aren’t, they must now meet all of the current harmonic reinjection requirements of a modern world.

In many, if not all cases, a modern well-designed generating set equipped with the appropriate method of excitation can deal with the harmonic impacts of a given load. It should also be noted that all the critical equipment is fed via and protected from mains variation by a UPS package – the same UPS package available in the commercial market and widely used in much less critical applications. The same is also true for the cooling plant – equipment widely used across many different applications exposed to the same mains or generator supplies.

As we highlighted above HTM-06 section 9.74 directly relates to BS ISO 3046 (Engine only). BS EN 8528 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). 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. Only a grossly oversized machine will be able to meet this requirement.

It is key during the design phase that an assessment be made of the actual site specific first step load acceptance requirements. The loadings of various categories of risk areas, any BMS or EMS load management capabilities that might be available etc. These matters should fall within the realms of the projects specification and not be reliant on a ‘general standard specification’. More on this later.

Understanding Generator Power Rating Categories – BS ISO 8528 – 1; 2018 Section 14.3

For commercial reasons, generating set manufacturers use the same engine (with model variations) across a range of power nodes. In addition, a single model of generating set can carry a number of different capacity ratings i.e. 1000kVA ESP, 1000kVA PRP and 700kVA COP; the set rating is dependent on the operational duty / types of loading applied to the generating set, rather than the maximum engine capacity.

Section 14.3 of the ISO standard covers the five different ways in which a generating set can be “rated”. These are COP, PRP, ESP, LTP and DCP. Some of these ratings, for example PRP (Prime) rated sets, have their power ratings set against their ability to deliver into a varying load.

Section 9.73 of HTM-06 states thatEngines should be specified prime-rated. They should be capable of operating at the rated load for a period of 12 consecutive hours inclusive of an overload of 10% for a period not exceeding 1 h, the prescribed maintenance having been carried out. This is known as a Class A rating.”

In the case of a Prime rated set the maximum permitted output of the set should not exceed an average load of 70% (1) of the Prime Rating over a 24 hour period (Load Factor). Within that 24 hour period the generating set is able to deliver a 10% for one hour in 12. They deliver an average load level over a 24-hour period whilst still being able to provide a 10% overload, one hour in twelve.

Section 14.3.3 of the standard describes Prime Power rating as
“…being the maximum power which a generating set is capable of delivering continuously while supplying a variable electrical load when operated for an unlimited number of hours per year under the agreed operating conditions with the maintenance intervals and procedures being carried out as prescribed by the manufacturer.”

It is important to note that the way in which these ratings are allocated will vary from manufacturer to manufacturer, based on engine performance, connected rating of alternator performance and maximum operating ambient temperature.

With this extensive range of generating set capacities and rating options, it can be daunting for the design team to fix the direction of this aspect of the design. For most European markets, the grid is a very stable source of power, and it is very unlikely that power will be off for more than a few hours per year (testing included in this number). This is why, a PRP rated set should be an acceptable option.

When designing a generating set package for any application, it is important to consider likely maximum operating ambient temperatures required as this does play a big part in how the generating sets are designed and rating capacity determined. In a hospital / healthcare setting, the generating set solution must always be rated to supply the peak site load, on the hottest day of the year, unless there is clear alternative direction.

Load Acceptance in the Critical Care Areas

As we identified in an earlier section, the electrical load of a hospital is very diverse in nature being dependant on many factors.
Some of the critical areas are often have an additional layer of protection such as EPS, UPS or power via a battery backed system of varying duration all aimed at providing an uninterrupted supply to the critical area is serves.

An EPS, UPS or DC system only looks for stability of input supply, be it mains or generating set. When returning from battery to either mains or generator, the UPS’s input power requirements slowly ramps up over a period of 5-10 seconds, not in one single “lump” of load. This is an important factor when assessing first step load requirements as none of these load types would be part of the first step load.

Figure 3 – UPS – Main input current characteristic (3)

Central cooling systems have a UPS equivalent, a buffer or reservoir vessel of cooling water which can maintain water temperatures within range for around 30 seconds. This buffer vessel is there to provide time for the generators to start and pick up the cooling / mechanical loads essential for operation of the data centre. Some of the compressors and pumps within this system can be significant in size and can present some quite large load steps to the system. Bringing these back on line can be delayed and staggered; This should be catered for in the design the design process.

Most, if not all modern hospitals have sophisticated building and energy management systems (BMS /EMS). The key points here are that these loads:

  • Can and will be managed on by the BMS/EMS in a controlled way.
  • When these loads are applied the generating set will already be “under load” meaning that the application of the cooling load “isn’t a first step load”.

This means that the generator(s) never get close to seeing anything approaching a 60% load step totally negating the need for this measure and the associated G3 or G4 requirements that accompany them.



Fuel Storage

Fuels and Emissions

This is a subject which weighs heavily on the hospital designer and users minds due to extensive regulation and now potential licencing issues. For some two decades, there has been a drive to reduce CO2 emissions in particular and in the last few years that focus has been extended to Nitrous oxides (NOx) and other smaller particuate matter.

Many of the larger engine / generating set manufacturers are now certifying their engines to use Hydrotreated Vegetable Oil (HVO) which offers a reduction in CO2 of up to 90% (over the life cycle of the fuel which could be as long at 30 years) over conventional BS EN 590 B7 fuel. As with conventional diesel fuel, HVO needs to be looked after during its life cycle with polishing and cleaning systems. There are likely to be some early availability issues, but its adoption is likely to provide a measure of interim
solution. It should though be noted that overall fuel consumption increases by approx. 4% when using HVO v B7 diesel.

Some designers and operators have looked to gas as an alternative as it is cleaner in emissions and particulates than diesel. Having a local source of sufficient capacity, pressure and reliability is often an issue.

Gas, having a lower calorific value than diesel, means the engine swept volume must increase in order to deliver the same power, meaning higher initial capital costs. Starting a gas engine can take much longer and the set is much slower at accepting site loads. Those issues, combined with higher maintenance costs, have ensured gas generation is very rarely adopted.

Many of the larger manufacturers are currently working on a hydrogen fuel solution, running test programmes to assess long term reliability issues. This option looks to offer a promising future but there is some way to go with work still to be done in order to provide a truly “green” source of hydrogen fuel, workable local safe storage and distribution methods.

From a regulator basis, a hospital operations team can confront several different licencing, permitting and regulatory requirements, varying from legal jurisdiction, location to location within a given jurisdiction and size / rating of generator installation etc. Much is covered by the Medium Combustion Plant Directive (MCPD) but other requirements will be locally derived and often dependant on other factors. Due to the relative complexity and local variations available, this article does not seek to cover all aspects just to highlight some key considerations. Some of these are:

  •  Total capacity of installed generating sets.
  •  Thermal input of generating sets (below 3MWth can bring benefits).
  •  NOx emissions.
  • If generators are used for other purposes other than standby i.e. STOR of Grid demand response.

In relation to NOx emissions specifically, it is important to study the information provided by each individual manufacturer. The results provided (as with all of the other elements which are present in the exhaust gas stream) are measured under specific conditions, including engine load and operating temperature, ambient temperature and distance from engine gas flow. All these parameters will vary when at site, hence it is important that each installation is treated and assessed on its own relative merits. The MCPD indicates a requirement to achieve a NOX level better than 190mm/m3 at 100% load and after a max of 20 mins operation.

It is also important to ensure that the units of measurement and remedy are the same. NOx levels are usually given in mg/Nm3 @ 5% O2 at 100% PRP. The established method of NOx reduction is by the introduction of a Selective Catalytic Reduction unit (SCR) into the exhaust gas flow. NOx reduction/ elimination is achieved by injecting ammonia into the gas flow prior to it passing over the catalyst. The use of a precise closed loop ammonia injection system ensures that there is no discharge of unused ammonia. It is important that the exhaust gases (350 – 450C) are at a high enough temperature to ensure the catalysation process can occur; this is achieved by good design and a generating set working at load. As the NOx produced in the engine cylinders directly linked to engine pressure and temperature it should be noted that as the exhaust gases expand and cool then the NOx levels will fall on their own.

There are of course other elements present in the exhaust gases emitted from a diesel engine. These include HC, PM and soot. The amount discharged depends on rating of set, engine type, fuel consumption, quality of fuel used and engine load. Each of these elements can be reduced by fitting a specific reduction unit. There is however a limit to what components can be added whilst staying within the engine’s operating parameters. See WB white paper “Greening Standby Power Generation”.

Starting – Batteries

Section 9.78 of HTM-06 talks about there “usually being two battery-charging systems are supplied with a generating set, a static battery chargers and a belt-driven charge alternator that maintains the battery when the set is running”.
Enhancing generator starting reliability is key to ensuring the performance of any data centre. Fitting dual starting batteries, dual battery chargers and dual starter motors to each set is a quick and cost-effective way of achieving that objective.
Section 9.77 of HTM-06 speaks about the importance of keeping starting batteries in a fully charged state and properly maintained etc. Often the batteries fitted as standard to a generating set are of median quality. Higher quality batteries with a longer operating life should be considered.

Generator Controls

Beginning at section 9.45 MTH-06 talks about the selection of an appropriate generator control panel. Selecting the right generating set control system is key to the overall operation of the system. Firstly, ensure that the selected control system can provide all the necessary communications required for connection to any BMS/ EMS system and site wide protocols.
Generating set control panel selection will also be defined by the tpe of application and operational format too. Different control panels are required for;

  • Single set island mode.
  •  Multi set N+1.
  • Single set operation with short or long term synchronisation to the mains / PES.
  • Multi set operation with short or long term synchronisation to the mains / PES.

Getting the generator package (multi sets) up to speed and on load within the HTM prescribed 15s can be a challenge. The use of dead bus synchronising can see this achieved in around 10 seconds from start signal regardless of the number of sets to be synchronised.

Wet Stacking

Wet Stacking typically occurs when a generator is run at less than 30% of its nominal capacity for extended periods of time. With this level of load the engine doesn’t not achieve or is unable to sustain its optimal operating temperatures needed to fully burn the fuel injected into the engine.
The lack of engine temperature means that the pressure inside the combustion chamber falls below the crankcase pressure and the engine temperature isn’t high enough to ensure the piston rings expand enough to seal the space between the pistons and cylinder walls. The result of this is incomplete combustion of the fuel and a propensity for the engine to draw small amounts of lube oil up from the crankcase which becomes visible as white smoke in the exhaust. A build up of lube oil in the combustion area can cause glazing on the cylinder wall. Additionally, there can be a build up of unburnt fuel or soot in the exhaust line which of itself can also be dangerous.

Many generator applications within the healthcare estate are of a single set type / Island mode operation as per HTM-06 sections 9.22 sand 9.35 supported in Fig 17. Sections 9.27 opens the potential for removing singles points of failure by adding additions generating sets in N+1 or a three set load sharing arrangement. The choice in this risk mitigation route is typically defined by the % of critical load fed by the particular generator package.

Correctly sizing the generator package at the design stage can mitigate the problem of “wet stacking” but a good design strategy will ensure that a permanent load bank, both resistive and inductive, is connected to achieve a full load test on each machine and raise the load to >25-30% at near to unity power factor regardless of the actual load across all machines. Alternatively section 9.39 supports the idea of short or long-term mains synchronising with the mains supply (PES) for the purposes of load testing the generating set and or ensuring operational loading levels of the set are such as not to impact the long term performance and reliability of the set(s) caused by light load running and the consequent “wet stacking issues”. Read WB White paper on “Wet Staking”.

Acknowledgements, Clarification, References and Bibliography.

  1. Some Kohler generating sets offer 75% load factor
  2. Information directly from BE EN ISO 8528
  3. Figure 3 – Information provided by KUP a Kohler Power Systems Company
  4. Thanks to writers of HTM-06 for all direct quotes and references

G R Halliday

Business Consultant

WB Power Services Ltd