2 wire vs 4 wire smokes

[Mr Spock]

I'm sure this question has been asked before, but I can't seem to find it in a search. The inclusion of single digit numbers make a search difficult.

What are the differences in choosing 2 wire vs. 4 wire smoke detectors? Issues such as cable routing (home runs vs. daisy chaining), reversing and supervision relays, EOL resistors, difficulties and ease, etc..

Assume a HAI panel, 6 smokes as 2 zones (4 upstairs & 2 downstairs). Based on comments on this board I am leaning toward the System Sensor products.

I'll also have 2 CO detectors. If I understand the HAI panel correctly there can be only 4 zones allocated to smokes (Z1-Z4). Not sure if CO detectors count the same as a smoke in this math.

Thanks.

 

[Desert_AIP]

If you are using 2 wire smokes, then you are limited to zones 1-4 on the Omni panel.
You can have multiple zones using 4 wire smokes (I have 10).
I use the 194° heat sensors in the attic as a fire zone in addition to smoke detectors in the living spaces.
Just set the zone type appropriately and use EOL resistors.

You may want to set CO detectors as a gas alarm instead of fire.

 

[Del91574]

2 wire fire alarm is easier to wire and maintain proper supervision. Basically it just combines the power and signal wires, where the 4 wire has them separate. 4 wire typically requires more parts to supervise power, and if the panel doesn't directy offer it, a way to reset them (relay or similar) as well as need a power supervision relay. The 2 wire method requires neither of them.

Wiring, technically speaking, for a fire alarm needs to be a daisy chain, or if home running is done, the # of conductors at each detector needs to be doubled (4 for a 2 wire circuit and 8 for a 4 wire circuit).

There are modules, I know GE used to offer one and System Sensor offers the 2W-MOD2 that allow a 2 wire fire circuit to be "converted" to a 4 wire circuit as far as the panel goes.

CO detectors are their own entity, and how they're wired strictly depends on the units selected, but if possible, I would suggest 1 per zone. If the panel doesn't offer a dedicated CO zone type, if the system is monitored, I would investigate how a potential CS would respond to an alarm, but gas alarm might not be appropriate in all situations.

 

[Ira]

Can you go into a little detail regarding the reset and power supervision relay aspects when using 4 wire smokes, with either a HAI panel (for the OP) or a Elk panel (for me)?

 

[apostolakisl]

4 wire smokes daisy chain together. On the last one in the daisy chain, you use the power leads to close a relay. The two contacts on the relay then attach to the two signal wires with an EOL resistor there as well.

During normal operation, the relay is powered and closed completing the EOL resistor loop on the signal wires. Theoretically this means every smoke is getting power since you did this at the end of the line. If you use the screw down terminals on the smoke detectors, the only way for any of the detectors to not be getting power is if there were an internal failure.

Should there be a loss of power (the wire is damaged), the relay will open, and the signal wires will thus become an open circuit. The panel will now see 13 volts instead of 7 volts (7v is what is sees with an EOL resistor on a closed circuit) on the signal wires. 13v is reported as a "trouble" condition. This will happen the same whether the power wire were damaged as a cut or as a short since either way, the relay at the end of the line will lose power.

Should the signal wire be damaged, the same thing will happen. The circuit will open and again you get "trouble" reported.

If the signal wire is damaged such that it shorts, you will get a false report of fire.

If a detector is triggered (smoke), that detector closes the signal wire circuit shorting it. The panel sees 0v which indicates fire and alarms. The smoke detector once triggered, locks into this mode until you power cycle it. This is why you need to do the smoke detector reset to get it out of alarm state.

They make 4-wire detectors with the EOL relay built in. Some people will use these at every detector, but most only put it as the last one. You could use a separate relay, but it is much cleaner and easier (and usually cheaper) to get one with a built in relay.

 

[Ira]

Thanks. I understand the supervision part of it now.

For the reset function, when used with an Elk panel, I guess that implies that the detectors will all be powered off the Elk's SAUX terminal and resetting will be done using the preset menu item or by triggering a rule that contains the RESET SMOKE POWER action on a THEN statement. Or if the detectors are powered off of a separate power supply, I would need to run that power thru an Elk relay first, and have some way of opening that relay to reset the smokes. Does that sound right?

 

[apostolakisl]

Yes, to use the Elk's built in smoke detector reset you have to power them via Elk's dedicated smoke detector output. Or, as you mentioned, power a relay from that output that holds on the power from something else.

Or you could just go pull the plug manually

 

[Mr Spock]

Lets see if I have this right...

1) The 4 wire smokes require supervision and reversing (reset) relays but the 2 wire smokes do not.

2) Both should be daisy chained with one EOL resistor at the end unit when combining multiple smokes in a zone.

3) Both are able to report a "trouble" condition if the wire is cut (open circuit) or a unit in a zone becomes disconnected.

Not sure I see any advantage to 4 wire smokes other than you can connect more than 4 zones (easily) to an Omni panel. Are 2 wire smokes much more popular? Seems that they should given their easier connectivity.

 

[Ira]

And another question...

Seems like most people, including myself, want detectors with built-in sounders (like the System Sensor 4WTA-B to all go off when any one of them detects smoke. Many of us also want to be able to tell via the automation panel which detector tripped first. Doesn't the System Sensor 4WTAR-B ("R" for relay) get pretty close to allowing for both functions? They could all be wired on the same fire alarm zone with a reversing relay, etc. so that they will all go off in tandem when the first one trips. In addition, the built-in Form C relay in each detector could be connected to it's own automation panel zone so that the panel gets an indication thru the relay which detector caused the alarm.

 

[Mr Spock]

Ira you bring up another question from me. Do both 2 wire and 4 wire smokes activate sounders and indicate by the panel the same?

 

[apostolakisl]

Since 2-wire smokes are powered by the alarm zone, I do believe you are constrained to fewer detectors on one zone.

Elk only puts out 40ma per zone (if memory serves me). Zone 16 might be different (the preferred smoke detector zone), but I don't think so.

Electrically, I am not certain how a 2-wire detector could have a sounder on the unit since it shorts when alarmed. A sounder would have resistance and thus prevent the panel from seeing short. Maybe there is a trick I am not aware of.

As far as the panel is concerned, it sees short when alarmed and responds the same.

I installed 4-wire units in my house and have never played with 2-wire models. The added work of installing the extra 2 wires is insignificant. Elk automatically takes care of the power reset feature. You don't have to have the supervising relay at the end, but I recommend you do (code might say you have to have it). Again, it adds about 30 seconds more work screwing down the extra wires.

As far as using the relay on the 4WTAR-B unit, I do believe the relay closes not when alarmed but when powered up. This is a supervising relay, not an alarming relay.

 

[pete_c]

Thank you Lou for the detailed and concise explaination of the differences between 2 and 4 wire smokes.

 

[Ira]

...
As far as using the relay on the 4WTAR-B unit, I do believe the relay closes not when alarmed but when powered up. This is a supervising relay, not an alarming relay.

According to the installation manual for the System Sensor I3 detectors (2-wire and 4-wire)...

The detector that initiated the alarm condition will have its red LED and Form C relays (if applicable) latched until reset by panel.

 

[Mr Spock]

Electrically, I am not certain how a 2-wire detector could have a sounder on the unit since it shorts when alarmed. A sounder would have resistance and thus prevent the panel from seeing short. Maybe there is a trick I am not aware of.

As far as the panel is concerned, it sees short when alarmed and responds the same.

I did not realize the 2 wire units did not have sounders. Thats a big difference to me, and worth the extra hassle of the relays.

Thanks guys!

 

[apostolakisl]

According to the installation manual for the System Sensor I3 detectors (2-wire and 4-wire)...

OK, that's a little different. Make sure then you don't confuse that relay with the supervising relay that other detectors have. My GE unit relay is for supervising, not alarming. I think the supervising relay is what most people would want since most people are not looking to hook each detector up to both the common smoke detecotr zone and individual zones.