17 Watts for each smoke alarm!
Discussion
Aico have been very helpful and got back to me quickly.
The have send me a pdf which explains that the straight Volts x Current measurement does not work as smoke Alarms are capacitive loads.
This means V & I are out of phase where I lags V
So the actual Power = V(instantaneous.) x I(instantaneous.) which is lower because they are not at the peak levels at the same time.
And to measure it you need a special meter which takes account of the Power Factor (the delay between V & I).
My only worry now is that my electricity meter is not smart enough to take account of power factor.
My next step to make sure is to shut off everything except the smoke alarms and see what my old fashioned spinning wheel electricity meter is recording.
The have send me a pdf which explains that the straight Volts x Current measurement does not work as smoke Alarms are capacitive loads.
This means V & I are out of phase where I lags V
So the actual Power = V(instantaneous.) x I(instantaneous.) which is lower because they are not at the peak levels at the same time.
And to measure it you need a special meter which takes account of the Power Factor (the delay between V & I).
My only worry now is that my electricity meter is not smart enough to take account of power factor.
My next step to make sure is to shut off everything except the smoke alarms and see what my old fashioned spinning wheel electricity meter is recording.
Spunagain said:
And to measure it you need a special meter which takes account of the Power Factor (the delay between V & I).
My only worry now is that my electricity meter is not smart enough to take account of power factor.
What, Like a smart meter? PH irony. Why on earth have you got 10 smoke alarms? How big is the loft (or house) FFS?!My only worry now is that my electricity meter is not smart enough to take account of power factor.
A quick google suggests modern digital meters do sample the voltage and current instantaneously to get the correct power numbers.
It is not clear to me how the old fashioned motors behave with a reactive load.
I had to have a mains powered smoke alarm fitted in every room and hall and landing for building regs as I have had a loft conversion done. It was either that or fit fire doors in every room.
It is not clear to me how the old fashioned motors behave with a reactive load.
I had to have a mains powered smoke alarm fitted in every room and hall and landing for building regs as I have had a loft conversion done. It was either that or fit fire doors in every room.
I would say it's not worth worrying about the power factor. The big consumers of electricity in a house (ovens, heaters, newish PCs etc) will all have PFs approaching 1.
A clamp-type meter won't be that accurate especially at low current levels.
Don't see an issue with 10 smoke alarms, don't forget there are different types (heat, ionising etc)
A clamp-type meter won't be that accurate especially at low current levels.
Don't see an issue with 10 smoke alarms, don't forget there are different types (heat, ionising etc)
Just done a test for you (same make of smoke alarm). We have 4 (with a Co alarm on that circuit-same make) and with only those on they are taking 4W in total. I now know the battery back up in my mirror clocks have failed & have taken the opportunity to synchronise all clocks in the house 
ETA, we have 4 including the CO, 3 smokes.
ETA, we have 4 including the CO, 3 smokes.
Edited by Dave_ST220 on Thursday 1st June 11:33
Dave_ST220 said:
Just done a test for you (same make of smoke alarm). We have 4 (with a Co alarm on that circuit-same make) and with only those on they are taking 4W in total. I now know the battery back up in my mirror clocks have failed & have taken the opportunity to synchronise all clocks in the house
ETA, we have 4 including the CO, 3 smokes.
Cheers Dave that tallies with the quoted 0.8 watt allowing for a little production variation.ETA, we have 4 including the CO, 3 smokes.
Is yours an old mechanical meter?
I always thought supply meters measured real power. I have done a quick search and this link appears to confirm that is the case with old, rotary, meters as well.
http://www.theiet.org/forums/forum/messageview.cfm...
http://www.theiet.org/forums/forum/messageview.cfm...
eliot said:
I'll lift the leg into the breaker leading to my 5 aico's and alarm panel and get a current reading for you.
Just measured with the DVM inline like yours - .29A @ 241v for 6 units - 11.64w each.My subpanel had its own wattmeter and flicking the smoke breaker on and off barely registers - i think that understands power factor because you can cycle round the display to show the pf.
Setup:
https://flic.kr/p/V8EgLj
edit: i have 6 units, not 5
Edited by eliot on Thursday 1st June 20:50
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