Strobes start off in sync...

[FA Girl]

During a fire drill we had the other day, we had nearly all of our buildings have strobes that were off sync. We just had inspections done in July and they were all in sync then. At the last building we happened to be in an area with multiple strobes and noticed they were in sync. That is until about 10 minutes into the drill when they slowly started to become off sync. It started with just a slight difference in flashing and then went to full on out of sync. Has anyone heard of this before? I have asked several FA guys and none of them have heard of this...

 

[gadfly56]

During a fire drill we had the other day, we had nearly all of our buildings have strobes that were off sync. We just had inspections done in July and they were all in sync then. At the last building we happened to be in an area with multiple strobes and noticed they were in sync. That is until about 10 minutes into the drill when they slowly started to become off sync. It started with just a slight difference in flashing and then went to full on out of sync. Has anyone heard of this before? I have asked several FA guys and none of them have heard of this...

In large buildings, strobes are usually powered by notification appliance booster panels (NAC boosters). You may have more than one per floor, each typically runs four circuits. The head end fire alarm panel will typically send out the trigger and master pulse for the remote NAC boosters. If the distance is long enough, you may have a prorogation delay between the booster panels. Starting out, this is too small to notice, but if you let the strobes run long enough then the boosters could get out of sync. However, all of the circuits on a single NAC booster should be in sync with each other. If ALL the strobes are drifting with respect to each other, not just GROUPS of strobes, then I don't know what the problem is, unless someone set the appliances up as free running and didn't use the panel sync to drive the strobes. In that case, they'd probably get out of sync much more quickly than 10 minutes.

 

[FA Girl]

Thank you so much. It makes sense and I appreciate your time!

 

[jusme123]

The code requires all strobes that are in one viewing to be synchronized. However, there are exceptions to this requirement. For example, NFPA 72-2010, Section 18.5.2.6 states that the “strobe synchronization requirements of... [chapter 18] shall not apply where the visible notification appliances located inside the building are viewed from outside of the building.”

The annex clarifies this exception, stating: “It is not the intent to establish viewing and synchronization requirements for viewing locations outdoors. As an example, there is no need for floor No. 1 to be synchronized with floor No. 2 if there is no visible coupling as in an atrium.”

The purpose of synchronization is to ensure that the fire alarm system visible signals do not cause a photosensitive epileptic to initiate a seizure. The code advises us “that studies have shown that the effect of strobes on photosensitive epilepsy lessens with distance and viewing angle” provided that the composite flash rate is no greater than the flash rate produced by two strobes.

 

[gadfly56]

The code requires all strobes that are in one viewing to be synchronized. However, there are exceptions to this requirement. For example, NFPA 72-2010, Section 18.5.2.6 states that the “strobe synchronization requirements of... [chapter 18] shall not apply where the visible notification appliances located inside the building are viewed from outside of the building.”

The annex clarifies this exception, stating: “It is not the intent to establish viewing and synchronization requirements for viewing locations outdoors. As an example, there is no need for floor No. 1 to be synchronized with floor No. 2 if there is no visible coupling as in an atrium.”

The purpose of synchronization is to ensure that the fire alarm system visible signals do not cause a photosensitive epileptic to initiate a seizure. The code advises us “that studies have shown that the effect of strobes on photosensitive epilepsy lessens with distance and viewing angle” provided that the composite flash rate is no greater than the flash rate produced by two strobes.

Not exactly true. It's more than two appliances in the same field of view, or more than two groups of appliances that are synced separately. So if you were in an atrium space and floors 1, 2, and 3 were synced together, but different from 4, 5, and 6 that were also synced together, you would be in compliance, code wise. The project specs might be stricter.