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# Household AC power switching question

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#### mightywombat

##### New Member
I have some questions about AC electricity, as it's something I don't deal with very often.

1) When switching an AC power source on or off, is it safer to disconnect both the live wire and the neutral wire, or is it sufficient to just disconnect the live wire to break the circuit.

2) I'm using a relay to disconnect an outlet from the household AC source. How can I determine much voltage and amperage I should expect to pass through the relay if I have a personal computer or its peripherals connected to it?

3) Several people have mentioned that I should use opto-isolation in the event that the relay should fail. How do I implement opto-isolation in the circuit I described? Would it go in a driver circuit for the relay?

Thanks for your help!

1) It is a must that the live (or Line) side of the circuit be switched, but there are several reasons to switch both. AC current is alternating so it isn't polarized, and sometimes lazy or shadetree electricians wire the line and neutral connections backwards. Since the line side is the "live" wire, this can be a dangerous situation because an appliance that is plugged into the wall with the switch off can still be energized if the connections are backwards, creating a dangerous situation if someone has the cover off! Obviously, you should have an appliance unplugged anyway but the point is valid.

2)All of those devices should be rated for maximum power which often includes current draw, but the formula P=ExI can help you figure it out if only power rating is supplied. E=voltage I=current, if you didn't know already. It is also possible to check the rated consumption with an ammeter if you have one.

3)Opto-Isolation for a wall-power circuit? Well I think I'd just be more inclined to use a surge protector or something for a computer but I'll let others comment.

Hey Mightywombat

1) Disconnecting any of the wires will break the circuit. But in some cases, the neutral is not at 0V, and if some of its potential finds a path to ground, it could be nasty. EDIT: Disconnect the live, and keep an eye on the neutral issue providing maximum insulation with the project enclosure. Make sure that the polarity of the mains plug of your project cannot be reversed.

2) This deppends on how many watts the device consumes. For example a 60 watt bulb, running at 120 volts

60 / 120 = 0,5 amp

Check this site Mr. Electricity - How much electricity does my computer use? (Mr Electricity? That man has a problem hehe)

3)

Opto-isolator - Wikipedia, the free encyclopedia

And check the attached schematic. Istead of relay it uses a TRIAC

I'm sure any other member of this forum will correct this or give you more decent information, but it's a start

EDIT: ke5frf hit the button faster than me

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Your dealing with danger here.
Not only your safety, but your insurence policy.
If your house burns down, and it is found to be shoddy wiring by an unqualified/unregistered electrician/handy man, that started the fire, you will get absolutely nothing back from the insurence company.
So make sure you know what your doing..

And just a few pointers...
DO NOT SWITCH THE NEUTRAL.
(sorry for caps, but you need to understand. This is your life we are talking about)
Here in austalia it is illegal and I only guessing here, but i would say it is the same case in other counties.
A bit of background:
In australia the neutral is MEN'd (joined) to the Earthing system in the switchboard before the RCD. This keeps is at earth potential (which is nearly always 0volts).
If you switch the neutral, yes you will break the circuit, but you still have an active that is alive and kicking and the appliance end of the circuit even though your switch is off.
There is an exception with caravans, both conductors are switched, but like you said;
is it sufficient to just disconnect the live wire to break the circuit
yes, only switch active conductors for your situation.

Another pointer..
It is also illegal to use semiconductors and the only form of isolation.
The reason is this.
If a semiconductor fails, It fails in the closed postion, leaving the circuit on.
Which is not good.
Make sure you have a form of machanical isolation (a switch) before the semiconductor.

Im not trying to sway you away from doing this, just make sure everything is safe and legal.
If you feel confident, and get people to check your final approach to the situation, go for it.

3) Several people have mentioned that I should use opto-isolation in the event that the relay should fail. How do I implement opto-isolation in the circuit I described? Would it go in a driver circuit for the relay?
The relay provides isolation without the need for additional opto components.

My research has led me to understand that in the US it is common practice to disconnect both the Line and Neutral leads in case they have been reversed for any reason, be it incompetent electricians or a non-polarized outlet or plug. The design of a light switch - at least in the US - disconnects both lines for these safety purposes. All the new construction and new products in the US have polarized plugs, but older houses, like the one I am in, tend to not have them, and also tend to not have grounded outlets either.

To address your other concern, I intend to either include a commercial power strip overcurrent disconnect circuit in line before the mains power reaches anything else in the device, or chain this device to a commercial power strip when I use it.

1) It's always safer to disconnect both. On a heavily loaded branch circuit the neutral line will actually be carrying a modest mv level line frequency voltage. A fault in some other device on the circuit could cause line voltages to be present on the neutral line as well.

2) Is a little tougher but many multimeter have a current measure mode that will do 10-20 amp for short period of time inline with the meter. Careful construction of the power tap will be needed to avoid fire risk at the tap and make sure you get a good solid connection to the meter. It is however a good idea to rate the relay at the full load power capacity of all the devices connected which is clearly listed on each device. Mind you a PC power supply will have a HUGE inrush current when it's first powered on..

3) Opto isolation is a little more troublesome because relays require a modest amount of current to turn on so you'll need an addtional power supply at the site of the relay unit to provide the power needed by the opto isolator, it can't be derived from the driven signal because it needs to be turned into light and then back, which is horribly ineffcient.. If you're doing that might as well put one on both ends and use a fiber optic cable instead.

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