Douglas Krantz - Technical Writer - Describing How It Works

Can I Use Extra End of Line Resistors in a NAC Circuit?

By Douglas Krantz | Descriptions

This circuit is not properly supervised.

Can I Use Extra End of Line Resistors in a NAC Circuit?

Can I Use Extra End of Line Resistors in a NAC Circuit?

Greetings Douglas,

I have an installation where we are doing multiple EOL Resistors on a single circuit. Please see attached drawing and let me know what you think.

Thank You, RM

A fire alarm system is a life safety system; it detects fire and sounds the alarm so people can take action. If some people don't hear this life saving alarm, those people won't take the necessary action to save their lives.

Once in a while, though, something goes wrong with the fire alarm system. That is the reason that fire alarm systems self-supervise themselves; they self-supervise themselves to be able to call the attention of the building owner when the system isn't fully working.

Technical Issues

Your drawing shows a conventional Notification Appliance Circuit, or NAC circuit.

When sounding the alarm, the panel can probably handle the current used by five horn/strobes. The panel, though, is supposed to detect when a horn/strobe is no longer able to get this alarm current.

Yes, it happens a lot. Wires are often cut, wires come loose from their connections, just to keep from being annoyed, sometimes people disconnect their fire horns.. I have had to fix hundreds of circuits that are no longer working because a wire has come loose, or broken. If the panel hadn't detected the problem first, most of the no-longer-working circuits would not have been fixed before a fire occurred.

Continuity Test - Too Many End Of Line Resistors

When the panel isn't sounding the alarm, the panel is supervising the wires of the horn/strobe NAC circuit. The panel supervises the wires by checking their continuity.

The circuit is supposed to be wire supervised, but there are multiple End of Line Resistors. This means the wires are not supervised.

What you are showing is a conventional horn/strobe circuit. While being able to carry power to the horn/strobes, the NAC circuit isn't supervised. The panel won't detect a break in most of the wires.

A properly installed circuit has all the positive wires in a series single-wire path, at the far end there is an end of line resistor in the path, and then the path continues back to the panel through all the negative wires in a series path.

That way, all of the NAC wires are in a series.

For a properly wire supervised Notification Appliance Circuit, the panel is performing a continuity check.

A different way of describing it is the electrons go out of the panel on one screw terminal, through one series string of wires, through the end of line resistor, and back to the panel on the other series string of wires.

In a properly wired NAC circuit, if any wire breaks or comes loose, the current stops, and the panel shows trouble on the circuit.

When there's a trouble where a wire breaks or comes loose from a screw terminal, the pathway is no longer complete. Because the pathway is no longer complete, the continuity-checking current stops, the panel shows a trouble, and the owner of the building calls for service.

This circuit is not properly supervised.

On the circuit you show, there are a total of ten wires. If one of the two wires connected to the panel itself comes loose, all current stops. Then the panel will show a trouble.

However, if any of the other eight wires comes loose, one or two of the remaining end of line resistors will still allow current to pass through. Because the remaining end of line resistors still pass the current through, the current doesn't stop, and the panel doesn't show a trouble.

Even though one or more horn/strobes won't be able to sound the alarm, no one will find out to fix the trouble before a fire occurs.

Bad juju.

Continuity Test - Unsupervised T-Taps

Hold on a second, can two of the end of line resistors be removed to solve the problems?

Removing two end of line resistors won't solve the supervision problem. Some wires can come undone, and the panel still won't detect a trouble.

Listed for Use

According to the National Fire Protection Association (NFPA), all equipment used in a fire alarm system must be "Listed for Use" by a nationally known, third-party testing laboratory like UL, ULC, FM, CE, CCC, etc. The reason for this Listed for Use requirement is so everyone will know that the equipment will actually work as advertised.

The wiring for the equipment is shown in the Installation Manual for the panel. Inside the manual are diagrams showing exactly the wiring method used by the testing laboratories when they performed their test.

Any other method of wiring, including any added T-taps or end of line resistors that isn't shown on the diagram has not been tested. No one will know if the system works at all, or if the system works unreliably, or if the system seems to work reliably, but underperforms.

Because the diagram that you sent me shows added T-taps and extra end of line resistors, the panel can't properly supervise the wiring. In other words, the panel is not Listed for Use with the wiring layout as shown in the diagram you sent.

Obtain and Read the Installation Manual

No two manufacturers make their fire alarm systems the same way. As a matter of fact, most fire alarm manufacturers don't even make different models of fire alarm panels the same. Because there're differences between models of fire alarm systems, they all have their own Installation Manuals.

Read and study the Installation Manuals; following the Installation Manuals makes for a better Installation, and also saves a lot of time in fixing mistakes that could have been prevented.

An Installation Manual came inside the box with the panel. If that isn't available, an Installation Manual can be downloaded from the web.

As an alternative, contacting the technical support team for the manufacturer will help in obtaining a manual. Make sure to have the exact model number of the panel before calling technical support.

Douglas Krantz
Life Safety
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