Just my opinion, but I think the Fire Department Connection serves at a secondary supply. The Fire Department will not rely on the pump, and will always hook up to the FDC anyway.
NFPA guidance provides a minimum reasonable standard of protection. If the prescribed maintenance is followed, then historically there is a very low failure rate. Many owners do provide a backup pump if they feel that the cost is justified.
There is no way to protect against ALL risk (if 2 pumps are better... why not 3... or an on-site water supply, or 2 on-site supplies...)
If the codes are too cost prohibitive, they wont be followed.
It is a long answer.
Firstly, this is why we evaluate the reliability as well as the capacity of the public water supply when testing it. Sometimes having to add a tank and a pump to deliver the proper water. We add a safety factor that is appropriate for the demand and the supply. For example, if I know that in Ontario, California there is a huge water demand during some months due to growing strawberries, my flow test in June is going to be high compared to February when the strawberries are almost ready. If there is a golf course nearby, a flow test in the late afternoon will be much higher than a flow test in the early morning.
Secondly, most fire sprinkler systems are not Life Safety systems. They are for property protection. Unless it is a residential system, or allowing occupants time to escape (e.g. multistory and hospital), the sprinklers are protecting the building and its contents. Occupants should have left long ago. My rule of thumb has been that if you're using quick response, it may be life safety. If you're using standard response, you're protecting the property for the insurance company. Incidentally, insurance companies love to add extra fire pumps when the value gets high enough. Even redundant pumps and tanks.
James nailed it. The FDC serves as the "backup". You will see two pumps provided in hi-rise buildings a lot of times, when the height of the building exceeds the pumping capacity of the responding fire department. For example, locally that height is 250'. We are required to provide two fire pumps in parallel when buildings are over 250'. One fire pump is the main, with the other being secondary.
And to add to that, in almost all scenarios a bypass is installed around the pump that will allow city pressure to be supplied to the system(s). In pretty much all scenarios, other than the higher floors of hi-rises, the city water does provide some benefit. I don't remember the exact stat, but I believe almost all activations of fire sprinkler systems during a fire event involve only 1 to 2 heads. We actually had a small wet system on test overnight one time, with the valve shut off, and 1 head activated at a fire created by construction debris and it extinguished the fire with only test pressure and water.
The fire sprinkler system is a critical component in life safety of a building. IBC provides many exceptions to when a building is "fully sprinklered". Reductions in rated separations, reductions in fire hydrant flow demands, increased egress travel distances and increased building heights and areas are allowed due to this capability. These exceptions are allowed with the exception that, the sprinkler system will suppress the fire to a point in which that occupants can safely evacuate the building and the the fire will be controlled until the fire department arrives to fully extinguish it.
The only time a fire pump is needed to supply the flow and pressure demands for the fire suppression systems is when there is an inadequate water supply pressures based on the system demands. If the supply cannot meet the demand, a booster pump is required. Conversely, if the water supply can meet the pressure and flow requirements, a fire pump is not needed.
If the water supply can meet the building system or fire flow demands there is no need to provide additional costs of equipment, infrastructure, power, etc. On top of the initial cost of everything involved with a fire pump and its components are all of the residual costs. Fire pumps and their appurtenances must be maintained and tested regularly which is not done very well in some cases. Why would an owner want to pay large amounts of money for something that that is not required, and have to hire additional maintenance personnel to take care of it as well.
It is not the intent of NFPA to provide additional items within system that could fail or need repair placing the fire suppression systems out of service. That is why NFPA prefers that systems are designed as simple as possible to maintain reliability in the event of a fire.
Lastly remember when a fire department rolls trucks to a scene as the existing fire suppression system is operating to "control/suppress" the fire, they will connect the the local hydrants and to the fire department connection on the building. The FDC will be pressurized by the fire department's pumper truck to boost the sprinkler and or standpipe system until the fire department has fully extinguished the fire. If initial hydraulic calculations prove that the all system demands can be met without the use of a fire pump, then one is not required.
Lots of great answers here. The short answer is that the local fire department is effectively the backup to the stationary fire pump.
Having worn several hats in my fire career from professional firefighter, to loss control, to design engineer, its interesting that "some" very large insurers consider local fire departments as tertiary response, and instead require for a full secondary fire pump to achieve true high-protected risk status.
Jesse raises a good point. In some regions of the world two fire pumps are considered critical especially for high risk environments with significant values at risk to cover impairments.
In many cases the Fire Department will not enter without effective fire control and will adopt defensive firefighting tactics. They cannot be relied upon to fulfill the role of a back up.
Many insurers will not take the risk on a several hundred million dollar plant or more in Europe for example without two fire pumps. I usually see primary electric plus diesel as standby both rated to meet required demands.
As indicated by Graham and others, some other countries have other requirements and some insurance companies ask for more (based on insured values, not life safety).
In the UK, for example, you have 3 fire pumps each capable to supply 50% of the demand (i.e. one out of service and you still have 2 x 50 % = 100%)
In France, you have one main fire pump (100%) and a smaller electric pump rated at 250 gpm @ 90 psi with a small dedicated water tank for 30 min supply. This is based on the fact that most fires are controlled with less than 5 sprinklers in operation during less than 30 min.
Many insurance companies require 2 separate fire pumps / tanks, based either on the TIV (Total Insured Values) or MFL/PML (Maximum Foreseeable Loss / Probable Maximum Loss).
FDC as a back up is fine, but only when you have adequate Fire Department with a reasonable response time and enough water to supply the fire trucks, which is not the case everywhere in Europe.
So yes, it would be a good practice to have 2 separate fire pumps for life safety, but could be cost prohibitive for a small office building or small retail center.
NFPA is for minimum requirements, thus one pump, but does not prevent you to install 2 (or more... for nuclear power plants, for example) if you think it makes sense
referring nfpa 20; if I install a diesel engine driven fire pump then does it also requires an electric motor driven fire pump as primary?
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