Re: 2 wire vs 4 wire Smoke allarms

[robertlbass@xxxxxxxxxxx]

> What's the difference between the relay in a 110VAC smoke alarm and a
> 24VDC smoke alarm?  Nothing...

That is not correct.  110VAC smoke detectors canno9t be supervised by
the alarm control panel.  If they lose primary power the relay will not
fire and the alarm control panel will not know it.  This is a
significant difference.

> There is no "code violation" as long as you maintain a clear physical
> separation between the 110 and the low voltage connections to the
> panel...

Most of the relays (for example, the one that Kidde sells) specifically
state in the manufacturers' instructions that they are not for use with
an alarm control panel.  Code requires that devices connected to a fire
alarm system be "listed for the purpose" by the manufacturer.  Since
such devices are almost invariably not so listed, using them with an
alarm control panel is not code compliant.

> The only thing you might wind up with is an alarm every time
> you have a power failure because in most 110VAC smoke
> alarms the relay activates for a few seconds when power is
> reapplied...

That is wrong.  The more important problem is that during a power
failure the relay is inoperable (it requires both 110VAC and the 9-volt
trigger to function).

> I do agree that the *best* method involves the installation
> of 12 VDC smoke alarms (which means you'll wind up with
> two units next to each other in most applications)...

There are alternatives which are code compliant where there are
existing 110VAC smokes.  One is to simply add a few well-placed system
type (12 Volt) smokes.  Many times over the years when installing
systems in homes with a full compliment of integrated 110VAC smoke
detectors, we would add a system smoke detector on each level.  The
theory behingd this is that either type of detector will wake you up
and save your life but the system detectors can be monitored, perhaps
allowing the fire depoartment to respond before a fire gets out of
hand.

Years ago code was somewhat less than encouraging on this.  Some
inspectors would say, "Sorry, if you want system smokes you have to
install them in every required location."   Others would say that as
long as the home already had all the required smokes they didn't object
to a few "extra" ones.  Most inspectors we worked with took the latter
aproach but there were a few who didn't agree.

In recent revisions to the code the use of "extra" smoke detectors in
this manner is expressly permitted, clearing up questions that some
inspectors used to have.

> On my own system I've run 18AWG to all the 110VAC
> smoke alarm locations, and after the final electrical inspection,
> replaced all of them with two wire smoke alarms connected to
> the alarm panel.  The 110VAC wires were marretted off and stuffed into
> the back of the box.

The previous poster is in Canada.  Pulling a fastr one like this after
the inspection in the USA would be a handy way to get cited, especially
if the fire department later came out on a call and someone decided to
inspect (not all that unusual in many communities).  Since this
newsgroup is frequented by D-I-Yers I suggest you not encourage people
trying to slip code violations past an inspector.

Regards,
Robert L Bass
www.BassBurglarAlarms.com

P.S. -- I'm only spending a little time each evening at the computer
these days so I may or may not reply to further discussion on this
thread.