Fire Department Connection Pressure
Usually, inside the building, there are two pumps involved: the main firefighting pump and the jockey pump. Both pumps use pressure switches, but only the jockey pump has the simple turn on and turn off pressure switch.
The in-building fire pump has the same turn on and turn off switches, but also has a timer. The pump turns on when the pressure in the sprinkler system gets too low, but the pump stays on for a length of time after the pressure gets back to normal.
That means that the question you asked is really a two-part question.
Question Part 1 - "How much pressure do we need to take over control of the fire water in the building?"
This is from "Fire Protection Specialists" at:
Examples of fire pump settings:
i. Pump: 1000 gpm, 100 psi pump with churn pressure of 115 psi
ii. Suction supply: 50 psi from city - minimum static; 60 psi from city - maximum static
iii. Jockey pump stop = 115 psi + 50 psi = 165 psi [11.376 bars]
iv. Jockey pump start = 165 psi - 10 psi = 155 psi [10.687 bars]
v. Fire pump stop = 115 psi + 50 psi = 165 psi [11.376 bars]
vi. Fire pump start = 155 psi - 5 psi = 150 psi [10.342 bars]
vii. Fire pump maximum churn = 115 psi + 60 psi = 175 psi [12.066 bars]
In a typical installation, the main fire pump will turn off once the pump controller has detected a pressure of over 165 psi [11.376 bars], continuously, for the entire "Run Timer Period".
By the way, the term "Churn" can be translated as "agitating the water in the pump", or trying to push the water along into the sprinkler system.
So the sprinkler system isn't damaged when the pump is running, while at the same time no sprinkler head is open to relieve the pressure, the maximum pressure that the pump is capable of providing is usually 175 psi [12.066 bars].
That is all the pressure that most building sprinkler systems are rated to take.
Question Part 2 - "Once We have Exceeded the 'Turn Off Pressure' for the Fire Pump, How Long do We have to Wait?"
The "Run Period Timer" is an adjustment inside the main fire pump controller. At the time of commissioning of the pump on site, the timer is set to avoid cycling the pump on and off. Some manufacturer's Run Period Timers can be set 0-60 minutes.
For the pump controllers I've seen, the Run Period Timers have been set to 20 minutes or 30 minutes.
Why is There a Run Period Timer?
One of the reasons for the timer is to protect the motor.
Over time, electric fire pumps will overheat if they cycle on and off too often, and a diesel fire pump will have real problems if it has to keep turning on and off. When there's a fire, unneeded cycling of the pump motor becomes a real issue.
This cycling is caused by the capability of the pump versus the size of the sprinkler head: the pump has huge capability compared to the tiny opening of a single sprinkler head.
When 3 to 5 sprinkler heads activate in a large sprinkler system, the pressure in the system may take a minute or two to get low enough to turn on the fire pump. Then again, the pump is huge. The pump is capable of pushing up the pressure to its turn off pressure in well less than a minute.
Without a time delay, the pump would turn on every time the sprinkler heads have lowered the pressure, and off every time the pressure is pumped up.
This would happen over and over again. The cycling would burn out motors. During a fire, no one wants a pump motor to burn out.
How Much Pressure is Needed to Override the Fire Pump? And How Long does this Take?
Every building is different; every fire is different.
When water is actually flowing from the fire truck, through fire hoses to the Fire Department Connection on the side of the building, through the internal plumbing and check valve inside the building, there is a pressure drop caused by the internal plumbing resistance.
The pressure drop from the quantity of water flowing, combined with the hose/plumbing resistance can be anywhere from 1-pound psi, to lots of psi pressure drop. The true pressure drop will be different in every situation.
The pressure reading point used to turn on and off the pump isn't at the truck, it is somewhere near the combining point of the water from the fire truck, and the fire pump. It's equivalent to the confluence of two rivers. This water combining point is after the fire pump.
Once the pump finally stops, its water is no longer combining with the fire truck's water, and the pressure at that point drops immediately.
Unless the fire truck, by itself, can provide enough water to keep the pressure above the turn-on pressure for the pump, the pump will turn on again and stay on throughout its time-out period. Even though the fire pump controls are fixed, the measured pressure changes are unknown.
There is a bypass pipe around the fire pump. This is provided in case the fire pump fails. At the very least, the bypass provides city water pressure to push water into the system.
OK. The bypass only helps provide water on the lower floors of the building, but if the fire department supplements this water with its higher pressure from the fire truck, water will reach the higher floor sprinklers.
What can be Known
In a fire, as long as the fire pump is working, and its water supply is sufficient, the fire pump will push water into the sprinkler system. Once the water pressure stays above the "turn-off" pressure long enough, the pump will turn off.
When a fire truck pushes water into the Fire Department Connection, the fire truck is supplementing the quantity of water from the pump.
Then again, if the fire pump fails, the water provided through the Fire Department Connection still provides help fighting fires.
Even if the firefighters have managed to turn off a working fire pump with the water supplemented through the Fire Department Connection, the fire pump is still standing-by, ready to push water into the sprinkler system.
Disconnect the fire truck, and the pump is still ready.