STANDBY GENERATORS
It should be noted that not all engine
alternator combinations are able to achieve a 60 per cent 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.
9.78 Battery Charging Systems Clarification: 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, they are 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 Clarification: 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, we 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 750 L 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.
Comments: For a 2000kVA generating set, the 10 hour day tank rating can have a capacity in the order of 5000 L and to
Dual battery systems – one mounted either side of the base frame.
meet in full the 200 hour requirement this can be in the order of 100,000 L (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 per cent 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.
Derogations: 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.
9.87 Fire Dump Valve/Tank Clarification: 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.
9.88 Exhaust Systems/ Emissions Mitigation Clarification: Incorporating exhaust emission reduction technologies for standby generators is 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 aftertreatment 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 Selective Catalytic Reduction (SCR) unit. IFHE DIGEST 2026
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
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