Douglas Krantz - Technical Writer - Describing How It Works

Why Won't the Strobes Work?

The strobes won't work because of compatibility issues, or AC Booster, signal power expander, booster power supply issues, or voltage drop issues

Greetings Douglas,

I have a Silent Knight 5208 fire alarm control panel. When there's an alarm, all the horns sound off, but none of the strobes flash.

Can you help me or give me suggestions on how to fix this? I've never run into this problem before.

I would like to have at least some ideas of what to look for.

Thank you, G D

There are several possible causes for the problems.
  • Compatibility Issues
  • Signal Power Expander or Booster Power Supply Programming
  • Polarity Issues
  • Voltage Drop Issues


The requirement that all strobes in an area flash at the same time requires that the manufacturers all come up with a method of synchronizing the strobes. This synchronization is to prevent epileptic seizures from stopping some people from escaping fires.

Liability, though, if something goes wrong, is an issue. No manufacturer wants to be accused of providing a system that doesn't work when someone attaches another manufacturer's system to theirs, so all manufacturer's systems are proprietary - not compatible (meaning "It Won't Work") with any other manufacturer's system.

Many times, however, a manufacturer will allow their equipment to be used with several types of synchronization, so long as the installer of a fire alarm system is consistent; all of the horns and strobes are using the same synchronization system.

Make sure that the horns and strobes all use the same synchronization system (Wheelock, EST, Gentex, Etc.) and that the Silent Knight 5208 Notification Appliance Circuit (NAC) is programmed to be compatible with the horns and strobes that are attached.

Signal Power Expander (SPX) or Booster Power Supply (BPS)

If there's a Signal Power Expander (SPX) or Booster Power Supply (BPS) between the Silent Knight 5208 panel and the horns and strobes, make sure the SPX or BPS is programmed to be used with the particular horns and strobes. Often times the SPX or BPS won't pass on the signals from the Silent Knight 5208 to the horns and strobes.

Also, make sure the programming on the 5208 is set up for the Power Expander that's used. Follow the instructions in the installation manual for the 5208. Sometimes there are some extra steps to programming, so always read the installation manual. If you don't have one, get one.

Polarity Issues

If the strobes were installed and an ohmmeter was used to make sure of the proper polarity while the system was not in alarm, the wires should have been installed so the wire that measured negative goes on the positive terminal of the strobe, and the wire that measured positive on the negative terminal of the strobe. Remember, when an alarm sounds, the Notification Appliance Circuit will reverse polarity, and then the positive wire will now be on the positive terminal and the negative wire will be on the negative terminal.

Many times when someone else had replaced a horn or strobe, I have had to reverse the wires to get the strobe to work.

Voltage Drop Issues

This probably isn't an issue, but if the wire is too small, if the wire is too long, if there are too many devices on the circuit, some of the devices won't work. The Signal Power Expander or Booster Power Supply won't show a problem because that is not measured by the NAC Circuit Output on the control panel or the power supply.

To check, go to the last device, where the end of line resistor is located, and measure the voltage while the system is in alarm. The voltage should be at least 23 volts. If the voltage is lower than that, something needs to be changed.

Yes, I know. The datasheet on the strobes show that they are good down to 16 volts. The strobes, though, have to work even after 24 hours of electrical blackout. The batteries at this time are down to 20 volts, which gives you about 4 volts to lose (voltage drop) in the wire of the circuit in the building. A properly engineered system will take that into account.

Douglas Krantz
Life Safety
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