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My house has 2 year old service, a main 200 amp circuit breaker at the top of the main box. We had a 4 day power interruption and I wanted to hook up a generator to a 220 volt outlet located outside the house.

Before i would do this i would turn the main breaker 200a to off and trip the breakers on the water heater. I was confused to see when I turned the breaker off (lines were down outside) and checked the resistance between the incoming and outgoing connections on the main breaker, when "on" .2ohms but when "off" 3.0 ohms: it seemed like the breaker couldn't possibly isolate the incoming current from the rest of the circuit breakers.

I even took a light and battery from the input side of the breaker to the output side it lit when the circuit breaker was on, and also lit when it was off, but dimmer. I continued to use the generator through extension cords to the appliances i needed.

The power is now back on and the main does shut everything down when tripped, but I need to know why the meter doesn't show infinite resistance when the breaker is open! I can't take the chance of having a generator working and have the main power come on at the same time...

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    \$\begingroup\$ A blank line between paragraphs provides visual relief and improves readability. \$\endgroup\$ Jul 23, 2013 at 20:01
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    \$\begingroup\$ Backfeeding a panel like this is hazardous enough (not code compliance and is likely illegal) that it's really not worth doing at all, except in a dire emergency. If you see this need arising again, this would be a good time to talk to an electrician about installing a real transfer switch. Even though you are certain that you'd never flip the main breaker back on while backfeeding with the generator, you don't know who else might do it (like if you trip and fall in the dark and your wife is trying to get the lights back on). \$\endgroup\$
    – Johnny
    Jul 23, 2013 at 20:54
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    \$\begingroup\$ If you're worried about the generator when power comes back on, I don't envy the poor linemen working to restore your power... \$\endgroup\$
    – user16324
    Jul 23, 2013 at 22:23

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You were measuring the DC resistance of the various loads in your house in series with the secondary of the transformer your house is connected to, via the "neutral" connection, which is not opened by the breaker. It isn't surprising that this amounted to a few ohms. If you really want to make that measurement across the main breaker, you also need to open all of the branch circuit breakers in the panel as well.

BUT (and this is important!), you should not be relying on the main breaker for this kind of isolation. You must use a proper transfer switch, which will completely eliminate the possibility of ever connecting the generator to the external mains.

There are two general types of transfer switches you can buy. The first is a "whole house" transfer switch, which gets installed "upstream" of the main breaker and switches the entire panel between the mains and the backup generator. The other type is essentially another distribution panel dedicated to the generator, and has individual switches for the branch circuits you want to be able to connect to the generator. I use the latter type in my own house.

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You can't check the resistance of a component (the breaker) while its in the circuit. You have no way of knowing what alternate paths the current may find to get back to ground. It has to be removed. Then you can test the resistance between the contact points.

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Generally speaking, yes, they do physically break the connection.

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Notice the moving contact is physically distant from the stationary contact when the switch is flipped? This is how 90% of circuit breakers work.

You might have something else connected, maybe a miswired second circuit breaker or electrical box or faulty old circuit breaker?

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