When fire departments call for an additional alarm, what does it mean? Who responds?

You may hear it if you own a scanner. Whenever there's a fire, firefighters call for a first alarm, then maybe a second... or even more.

But what does it mean? What does each alarm indicate?

Basically, it's a way for fire departments to call for additional resources at the scene of a fire or other incident — either by calling in more of their own crews, calling in off-duty firefighters or seeking mutual aid from other departments.

"The multiple alarm system is a way of bringing additional resources to an emergency in a way that’s organized, predetermined and scalable to the incident," said Jake Wark, spokesman for the state Fire Marshal's Office. "Most fire departments have what’s known as a run card, which lists the resources that will respond to an incident starting at a first alarm and going through multiple alarms. You can think of it as a batting order in that, at any given time, the incident commander will know which resources will come from where for the next alarm."

All fire departments use the alarm system, although there are slight differences based on department staffing.

How a large department uses alarms

For Framingham, a large departmnent that has more resources than most others, a first alarm means a response involving three engines, a ladder truck, a heavy rescue truck, a shift commander and a rapid intervention team and a total of 19 firefighters, Chief Mike Dutcher said.

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A second alarm would bring in 12 more firefighters — the rest of the on-duty shift — to the scene. After that, the department would bring in mutual aid for each additional alarm, typically three new apparatus that are fully staffed.

Often times, another commanding officer will also come to the scene and help keep everything moving smoothly.

"So, this is really — I'm not sure if it's complicated — but it can be a little convoluted," Dutcher said.

How a mid-sized department uses alarms

In Milford, a smaller department, the idea is the same but the execution is slightly different, Fire Chief Mark Nelson said.

"They'll be similarities, but there will also be differences," he said. "What is a first alarm in Milford may be different than what is a first alarm in Framingham."

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When Milford firefighters arrive at a report of a fire, they bring two engines, a ladder truck, a rescue truck and a command officer truck. Once a fire is confirmed, they will call a first alarm. The department will "recall" off-duty firefighters, which will summon any off-duty firefighters to the scene if they can come. That ends up usually being about a dozen, Nelson said.

They also call for mutual aid, which brings Hopedale and Hopkinton to the scene with Medway providing station coverage.

If the fire goes to a second alarm, Medway responds to the scene, along with Franklin and Bellingham. Upton covers the station.

Each additional alarm will bring in the department that is covering the station and two other additional departments to the scene, Nelson said.

"For each alarm, I'm typically getting three more trucks and 12 more firefighters to the scene," he said. "Sometimes it's not necessarily the size of the fire, (but) when there is extreme cold or extreme heat, we need the manpower just to cycle people through to keep everyone safe."

How a small department uses alarms

In Southborough, all fires pretty much mean there will be mutual aid, Fire Chief Steve Achilles said.

"I know in Southborough, we really count on our mutual aid partners," he said.

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A first alarm means the department will send an engine and ambulance to the scene, with Hopkinton, Ashland and Westborough providing mutual aid. A second alarm will bring in five more pieces of apparatus, along with firefighters from other departments, with that being repeated for each alarm.

"It's a measured, predictable approach," Achilles said. "Before the alarm system, it used to be a bit more erratic. Now, instead of calling the dispatcher and telling them each piece of equipment we need, we just say what alarm it is and they know exactly who to call. It's a really streamlined way to manage things."

Norman Miller can be reached at 508-626-3823 or nmiller@wickedlocal.com. Fo r up-to-date public safety news, follow him on Twitter @Norman_MillerMW or on Facebook at facebook.com/NormanMillerCrime.

This article originally appeared on MetroWest Daily News: West of Boston fire chiefs explain how alarm system works