RF Immunity in Smoke Detectors

[Eyeball Kid]

I have a customer who is an amateur radio operator and is complaining
that whenever he is transmitting he trips the smoke detector we
installed (System Sensor 2012HA). He is asking for a smoke that is
better designed for RF Immunity.

Almost all the motions I have seen have some spec listing RFI (typicall
30v/m 10-1000Mhz) but I have not found any such spec on any of the
smokes I have looked at. (System Sensor, GE-Interlogix, DSC)

Anyone have any suggestions for a smoke detector in a RF rich environment?

Anyone done an install at a radio station (the transmitter not the
studio) and what did you use?

 

[SeanS]

Use shielded cable between the Smoke(s) and the control... In most cases,
the wiring acts as an antenna feeding RF into the detection device or the
control (sometimes BOTH)............ Regards, Russ

Russ has some good starting advice. As an amateur radio operator Im
curious as to what band he is transmitting on when this occurs.

 

[Nick Markowitz]

I also avoid smokes with heats on them unless there the new style thermals
as they often picked upRF and static easy .Had a case were kids were running
on a rug picking up static on them selves then reaching up and tuching the
metal heat on the smoke .
Im curious he is having problem as well I maintain a 5000 watt am radio
station WAVL910 with system sensor smokes no problem and amatuer radio is
limited to 2000 watts.

 

[Nick Markowitz]

My smokes are in the 2 directional transmitter huts themselves geting the
full hit.
I even have the ademco 5800 wireless contacts running in the huts back to
transmitter building and no problems

I think theres something else going on.

 

[SeanS]

Probably HF, 10 (28MHZ) & 15 (21MHZ) meters were the hardest to keep out of
tvs, cheap phones, stereos etc.. One big problem some Hams create for
themselves when it comes to RFI, mounting HF antennas in their (This is done
because of CC&R's that restrict outside antennas) attics... There's nothing
like radiating a 100+ watt 28MHZ signal 6ft away from all of your Low & High
voltage wiring...

I was thinking HF and 10 & 15 was probably it as well.

 

[Petem]

As an Ham radio operator and an alarm technician I have seen this occurred
many time

in the time of the 1500 and such line of dsc, putting a capacitor (10 uF)
across the zone lead at the detector(in parallel of the end of line
resistor) would cure the trouble...

now I don't know what of panel you have but this could do the trick...

one other time the capacitor trick could not work so we coiled the wiring on
a ferrite toroid...

hope this help

 

[Petem]

Morgan,you have some problem communicating with most of the world,its normal
that you just cant understand whets that hobby is all about...

;-)

 

[Eyeball Kid]

I'm still waiting to hear from the customer in person (the probelm was
forwarded to me from our service dispatchers) one of the questions I had
for him was what freq is he using to make it trip consistently and how
much power is he puttinginto the antenna and where the antenna is
located in relation to the smoke (and it's wiring). I'd like to arrange
a site visit to see for myself.

Rewring an existing system with shielded wire seems a little impractical
and my first thought was to try an eliminate the problem by adding a
combination of caps and or chokes at both ends of the wire (at the panel
and at the smoke)

One of the questions I want to ask the customer is when he manages to
trip the system does the sounder in the smoke go off or just just the
siren (in which case he is just tripping the zone on the panel (I
suspect it's an Ademco Vista but I haven't pulled the file)

Years ago I had a problem with a customer/ham who was able to arm and
diarm his system by keying his mike. It was an old DSC CP-10 style
panel (arming/diarming by mom contact in this case a DCU-20 Keypad) and
I solved that one by taking the excess wire at the panel end of the
arming input and wrapping it around the shaft of my instrument
screwdriver to form a poor man's inductor.

Hopefuly this one is as easy.

It strikes me as curious that RFI is a commom spec on motions but
unheard of on smokes....

 

[Petem]

One thing I can tell you is that,when come a time,that you hear in the
background noise stack between 2 large power transmission from somewhere in
Arizona or Utah,a faint signal,coming from south Africa,transmitted from a
low power transmitter (we also do that..) and you are able to make the QSO
and send your QSL card on the next day..you just cant understand the feeling
of achievement you have...all the hard work for building this highly
sensitive receiver,and the time to build the Low power transmitter,and of
course,learning of that damn Morse code...followed by installing that
tower(and having your pal next door asking: when will the alien will answer
you back?) tuning this invert v antenna to have a 1:1 swr ( I know I will
achieve it one day,not just in my dream),all this work just to exchange a "
HI! what's your call letter again? "(not really what I would have sent but I
left this part in plain English not in hammish)..

when this come to you,you now understand what's the hobby of ham
radio,before that,you are just simple nerds trying to fiddle some wire an
tube,trying to make something out of this mess....

 

[Eyeball Kid]

Just a follow up for anyone who was curious. I visited the premises and
found the customer was able to trip the zone the smoke detector was on
when tranmistting on either 3.8 MHz or 21 MHz (running about 700 watts).
Interestingly when he did this the siren on the system sounded but not
the piezo on the smoke detector (leading me to believe he is not
tripping the detector just the zone)

I pulled the detector off the mount and found the customer had installed
a snap on ferrite around the wires. Went to the panel and found he had
also put a ferrite around the telephone line (about the only wiring that
was accessible without opening the panel). I moved that ferrite on to
the pair running into the zone being tripped and we ran a test.

No more problems at 21 MHz but he was still able to trip it at 3.8MHz.
So I took my mother's approach to cooking ("If a little makes it good -
a lot will make it better") and installed a 2nd ferrite that I had
happened to bring along. Voila - no more problem - I told him to spend
the weekend trying to make it trip and went home. Almost a week later
and I haven't heard from him.

So the lesson learned is - RFI Problems are easily solved with Ferrites!


The ferrites he had are from DX Engineering
(http://www.dxengineering.com/TechArticles.asp?ID={54C2338D-3ED0-48C3-946F-F343A5C7555D}