Reversing polarity on a 2-wire smoke detector

At the moment, I have an idea of making a hybrid 2-wire and 4-wire system. I have one smoke detector loop that can power 2 and 4-wire detectors, however 4-wire detectors would require a second zone. Zone 8 is a power-limited 2-wire smoke detector loop, and to use 4-wire smoke detectors, power is drawn from zone 8, and another zone is used for the alarm contacts. I’m thinking that I could mix 2 and 4-wire detectors because the 2-wire detectors can be placed on zone 8 and 4-wire detectors would draw power from zone 8 and close zone 7. I would have another advantage of having two zones for smoke detectors instead of one, and the convenience of saving wire. Here’s the problem:

Let’s say that I have 2-wire System Sensor 2151s (only as an example, any system sensor smoke detector that is not polarity dependent can be used as an example) on zone 8 and 4-wire i3s with sounders drawing power from zone 8, and alarm contacts on zone 7. To make the i3 sound, you need to reverse the polarity of zone 8. What happens if a 2151 goes into alarm and the polarity reverses? Will it still be latched in alarm or will reversing the polarity reset it? Bonus question: can you synchronize the polarity reversal module with System Sensor’s sync protocol?

I’ve helped with a few installations utilizing i3s with the internal sounders. If they were not used in an i4 system, the i3 series RRS-MOD was used. This is the module that was designed and used to reverse the polarity for proper sounder operation. They can be used with either 2 or 4 wire detectors, but not both at the same time. I wouldn’t try reversing polarity of a detector that wasn’t designed for that type of application.

I would recommend using both zones 7 and 8 to keep the 2 and 4 wire detectors on a separate loop. Proper supervision for a 4 wire detector is rather different than its 2 wire cousin. Connection is pretty simple, you have color coded wires and an installation manual that can easily be found online.

For your bonus question, we haven’t done that. The module is designed to activate on either a coded or steady signal, depending on the position of the switch. This is to prevent sounder activation upon a burglar alarm. Is this a hobby system or one to be used for life safety?

This is a hobby system. Also, I can only power smoke detectors from zone 8, because zone 7 doesn’t have the current that even a single detector requires.

You can try whatever you want. But I think if you do what you are saying (using zone 8 for power for the 4 wire devices, and the same zone as a zone input for the 2 wire devices) you will lose the listing for the panel, even if it does work. Also, depending on the panel, zone 8 might need to be programmed for 2 wire smokes OR 4 wire power. Some panels this is in the keypad programming, others it’s a jumper.

I will throw this out there… I once found a 4 wire smoke wired up to a 2 wire zone in the field. The + and - power for the smoke was wired directly to the zone, there were no wires on the alarm contacts of the smoke. The smoke detector would go into alarm and trip the panel, basically acting as a 2 wire device. So I think in your situation, if the 4 wire smoke did activate and trip zone 7, it may also trip zone 8. But you won’t know that until you tried it.

While I have seen conventional zones that mix 2-wire smoke detectors, 4-wire smoke detectors, mechanical heat detectors, and pull stations, and when tested, they seem to work, it is not recommended, and goes against most manufacturers’ instructions and probably several code sections.

The issue is that 2-wire smoke detectors operate on a different principle from other initiating devices. Pull stations, 4-wire smoke detectors, and mechanical heat detectors are all basically a normally-open switch. When they go into alarm, they short one terminal to the other, and the panel reads this short on the IDC as an alarm. Some older panels only work with this type of initiating device and cannot be used with 2-wire smoke detectors.

2-wire smokes are similar in principle, in that they place a load, or almost short the two IDC wires when they are in alarm. The different is due to the fact that 2-wire smoke detectors also draw 24 volts DC power from the two IDC wires. If they were to completely short the two terminals, they would bring the voltage of the circuit down to 0 volts and lose power. So a dead short on an IDC from a pull station, heat detector, or 4-wire smoke detector will cut power to all 2-wire smoke detectors on the circuit.

This is not a huge deal, but it can be an issue in the case that there are multiple devices in alarm on a circuit. Say, for instance, that a two wire smoke detector goes into alarm, triggering an alarm at the panel. The LEDs on this detector will light red, as will any remote annunciator LED or integrated relay contacts. If a pull station on the same zone is pulled, the detector and annunciator’s LEDs will go out and any relay will reset. Also, some panels have IDCs that must be configured as either 2-wire smoke detector or pull station/heat detector/etc, if they receive an alarm from the wrong type of device on that circuit, it may damage the panel.

If you do mix them, don’t wire them as Lambda said he found, that is definitely the wrong way to do it. Connect the + and - of the 2-wire smoke detectors to + and - of the IDC. When you add a 4-wire smoke detector to the circuit, connect the two alarm contacts to the IDC wires, polarity doesn’t matter. Connect the + and - power terminals to + and - resettable 24V power from the panel, not an IDC.

As far as reversing polarity goes on this mixed zone, you can reverse polarity on the IDC wires and it will trigger the sounders on only the 2-wire smoke detectors. The 4-wire smoke detectors won’t know the difference though. To trigger the sounders on the 4-wire smoke detectors, you must reverse polarity on the 24v resettable power going to them. Sound confusing? Stick to one type of detector per zone, I promise it will make your life easier.

Thank you for your information. It really helps. But I still have one remaining question that I’m unsure about. If a 2 wire smoke detector doesn’t have a sounder and it is in alarm, and then the polarity reverses to activate the sounders on the same circuit, would the alarmed detector reset?

Not a problem. I think in that case, that detector will not reset. It may be worth testing this one though. I have not encountered this specifically, but I have seen 2-wire smoke detectors work fine with the power reversed. In fact, in one case, we had one accidentally wired to a NAC, and it was running on the negative 24 volts supervision power. When tested, it turned red, but we never received an alarm at the panel! Took a while to sort that one out.

Do be aware that in any production system, you are required to supervise the 24v constant power going to 4-wire smoke detectors. If you don’t do this, the panel will never know if the power wire gets cut and some 4-wire detectors are not getting power. The standard way to do this is to attach a relay powered by 24 volts to the power circuit after the last 4-wire detector. The normally open (that is, open when the relay is not energized) and common contacts should be wired in series with the end of line resistor on the IDC. This way, if the relay loses power, it will break the circuit’s path to the resistor, and the panel will show a trouble.

This also means that when troubleshooting a trouble on a 4-wire smoke detector zone, the first thing to check is if the relay is getting 24 volts power and closed. If it is, the break is somewhere on the IDC. If it is not, the break is on the power circuit.