relay placement and appropriate usage?

Using Arduino General Electronics
#1

hello -

i have a question regarding the use of 2 relays in my lightweight home automation project.

my current configuration involves this relay simulating a button push on my apartment door intercom (buzzes open the lobby door downstairs). this works beautifully - my phone sends a message to eventghost on my old laptop (dedicated automation server), eventghost then pushes a serial message to arduino, which then turns the relay on and then off immediately, allowing me to gain access to my apartment. the wires connecting the relay, the arduino, and the intercom are all very short.

seeing as how that worked, i wanted to go to the next step and control a power strip from my phone as well. this means connecting another relay to the same arduino. this will require me to lengthen the cables either from the arduino to the relays, or from the relays to the controlled equipment. also, this 2nd relay wouldn't be a quick on/off - it would be held in the ON position for as long as i would want to use appliances on the power strip (crockpot, water boiler, food processor, rice cooker - sometimes a couple at once).

here are my questions:

1, i've used a relay on a gfci outlet before, controlling a crockpot-based yogurt/tempeh fermentation chamber. this worked perfectly. however, could i trust this setup with something like the water kettle? possibly in use while the crockpot or rice cooker is on, as well? my goal is to have my morning phone alarm send an "on" message to the power strip/relay, which will boil the water for me -- preferably without burning the apartment down. tldr, safe?

2, as per my sketch below, where would the best placement be for the relays -- the X or Y position? note that the cable requirements are between ~3 and 5 meters, maybe more. how much of a factor does cable length play with these guys? will interfererence cause my appliances to switch on unexpectedly?

closer to the controlled equipment, or closer to the arduino?

please prompt me to clarify if my communications are too vulgar.

thank you for your time and expertise Nym

#2

You shouldn't have any issues with cable length. You've got low impedance/resistance, low speed (DC on/off) , and (relatively) high current to the relay coils. So basically, any stray electromagnetic energy won't have enough energy to do anything.

There are a couple of good reasons for keeping the power-lines short.

however, could i trust this setup with something like the water kettle? possibly in use while the crockpot or rice cooker is on, as well? my goal is to have my morning phone alarm send an "on" message to the power strip/relay, which will boil the water for me -- preferably without burning the apartment down. tldr, safe?

I'd say it's safe ONLY if it's safe for the power to come-on accidently/automatically for a long period of time with nobody home.

Most appliances can be safely left on unattended, but it might be considered "bad practice" and it probably warns against it in the owner's manual. For example, a coffee pot has a thermal sensor so that it doesn't start a fire when all of the coffee evaporates and it overheats. But, there might be a possibility of starting a fire if there is paper touching the coffee maker and a timer turns it on when nobody is home.

And of course, you need to use heavy gauge wire for the AC, an make sure your relays are a rated for the current, and insulate all of the connections, etc.

#3

great, thanks Doug! i'm guessing that it's better to have the relay closer to the actual device itself than farther away - in my diagram, the relays would go in the Y position.

#4

I agree. The longer run should be the lower voltage/current, so place the relay closer to the appliances. In fact, an AC outlet box/plug with the relay inside it (protect from shorting), would be a good idea. Then you can just plug in ANY appliance there.

#5