On Thursday, February 15, 2018 at 2:00:21 PM UTC-5, James Wilkinson Sword wrote:
On Thu, 15 Feb 2018 01:54:27 -0000, Ralph Mowery wrote:

In article , says...

Being in the UK I'm not sure how this works in the US. But before those namby pamby circuit breakers, you just had -110V, 0V, +110V. You could take 110V from either of the 0V and 110V wires, or 220V from the two 110V wires. Do the circuit breakers think there's an earth fault if you try to run a 220V device off two 110V wires?
I thought an American circuit was paired - i.e. the two 110V lines were linked and run off a dual breaker which would accept a 220V device?




I think you have it correct. In the US what we have comming into the
house are 3 (or 4) wires. At the pole is a transformer that is center
tapped. across the full winding is 240 volts. From the center wire
(called the neutral) to either of the other wires is half that or 120
volts. So we can have either 120 volt or 240 volt devices. There is on
the newer items a safety ground that connects to the neutral at the
breaker box.



A sneaker way to run the 240 volt devices (you did not hear it from me)
is to find two recepticals on the different legs . YOu then run a wire
from each of the hot wires to the 240 volt device. It helps for safety
if you also run a ground wire. You do not need the neutrals if only a
240 volt device is hooked up.
If you do for some reason need a 120 volt device also, you can run the
neutral wires. As they are connected together at the breaker box, you
can use either one or both of them. Just be warned that if a single
breaker trips , the other side will still be 120 volts to ground and
neutral.

Don't you get a problem with earth leakage detection on the circuit breakers?
In the UK this is quite simple, the breaker measures the current through 240v and through 0V, and if they differ, some must have escaped to ground and it cuts the power.
Now how would this work in the USA, where you can have current flowing between -110V and +110V, or from one of the 110V wires to 0V? You'd have to have a very clever mechanism that monitored THREE currents and compared them all somehow.


Why do you think it's so hard to measure current in 3 wires, versus 2? It's not.