Go back and read my post. I was explaining the benifits of a grounded system which have hot and nuetral.
Your comment about two conductor shorting being needed to open a breaker only adds weight to my postion. If one of the conductors on your isolated system shorts to earthed metal (like an outlet box) the breaker will not open. You now have a faulted grounded system. Unfortunately, you don’t know which side of the circuit is grounded. You could find yourself touching the wrong side of the circuit when changing the bulb in the lamp, even after you switched it off. You could find the wrong side of the heating element in the toaster was switched off when fishing with your fork to get that damn bagel out. Your appliances have been designed for use on a grounded system and things like computers and coffee makers install over-current and over-temp devices only on what is supposed to be the HOT side of the circuit. Internal faults to ground now have only a 50% chance of caught by the internal protection in your appliances. The risk for fire in these situations is real and significant.
Electrical safety standards require that when using a transformer without a grounded secondary, that an insulation monitoring system is employed to protect against just these hazards. To detect that first fault that a breaker can’t. It is required for a reason: Because having circuits where faults aren’t detected is dangerous.
billy knows his stuff, Crafter_Man. An isolation transformer is guaranteed safe for only one piece of equipment. With more than one, it becomes unpredictable.
I disagree. I don’t think he understands the concept of isolation. If one side of a 120 V isolated secondary “shorts to earth metal,” and if you’re grounded and you touch it, you will not get shocked…
Why are we so sure that the OP’s device is isolated? Does the 220v net where he is use a neutral, and is it at earth potential? I think their system uses a 380/220 wye, or something like that.
Here, in the US, if we have a 220v piece of equipment and want to use a transformer for the control circuit, we ground one leg of the 110v side.
BTW; Most 110v ground fault interruptors I’m aware of work by placing a coil (a current transformer) around both conductors. If a fault occurs an imbalance is detected by that coil, creating a current which trips the device. There is an imbalance because the fault current is not returned through the neutral. Under normal operation the current flow in both conductors is equal and cancels each other.
Right. GFCIs are set to trip if the current differential exceeds 5 milliamps. However, I’d amend part of your wording to read: “Under normal operation the current flow in both conductors is equal and opposite in phase and cancels each other.” A minor, but important nitpick.
Oops, forgot something. Here’s how you tell if it is isolated;
Unplug the transformer and turn the switch to “on”. Be sure it’s “on”.
Get your multimeter and set it to read resistance.
Connect one probe to one of the blades on the 220 plug side, then measure to both slots (not the u ground) on the 110 side.
Do the same using the other blade on the 220 side.
You should now have 4 resistance readings. If one of them is zero then the neutral is carried through. If all 4 are infinity, then it’s isolated.
Ground on the 220 plug to ground on the 110 recptacle should be zero.
Remember, you’ve only got 9a @ 110v. capacity.
First of all, it does not matter if you’re grounded or not. Secondly, all you’re saying is that “if you touch both conductors, you’ll get shocked.” I agree 100%, and nothing I have said contradicts this.
Eh? I don’t think so. How do you figure? I’ve spent a few minutes thinking this over and I don’t see how they can be in phase if they’re travelling in opposite directions.
The currents have to be out of phase and equal in magnitude for their magnetic fields to cancel each other. If they were in phase they would add.
The gfci senses an imbalance when current goes to ground instead of returning through the neutral.
Wait a minute. I’ll be right back.
The currents are in time phase. If the current is increasing in the black wire (US color coding) then it has to be increasing in the white wire at the same time if the circuit is electrically short. I.e. insignificant fraction of a wavelength.
However, since the two currents are going in opposite directions through the coil each one produces a magnetic field and these fields are in phase in time but are in opposite directions in space resulting in no net field in the coil.
If there is an insulation breakdown, say in a drill motor, so that a leakage current flows to the motor case this current returns to the source via the green wire. So the black wire now has the motor current plus (vectorially) the leakage current while the white wire has only the motor current. There is no longer cancellation of the fields, a voltage is generated which trips the ground fault interrupter.
No, the currents are increasing in the same direction. That is, if the sinusoidal current in the black wire is going through zero toward postitive then the current in the white wire is doing the same thing at the same time. As and analog, if the velocity of water at some point in a pipe is increasing, then the velocity at a point downstream has to be increasing at the same time because water is considered to be incompressible. If the pipe is long enough, there will be a delay same as with an electrical circuit.
The phase difference is in the magnetic fields one of which is increasing with its north pole to south pole being one direction while the other is increasing with it north pole to south pole being in the opposite direction because the currents are identical in the two wires in the absense of leakage currents but are in opposite directions. Right hand rule and all that.
I just don’t know how to respond to this one. QED , I know you know this at least as well I do, probably better. Perhaps you can explain it better than I am.
I’m trying to explain why using an ungrounded ditribution is less predictable than a ground referenced one, specifically when it comes to reaction of the protective devices, and increases in shock hazard, due to faults; both initial ground faults and secondary faults.
That’s right. If you loop the conductor through the coil again, so the current is going in the same direction twice (or more), the magnetic fields will add. That’s why it’s called a current transformer (CT).
I’m going to Popeye’s. Anybody want some wings?
You guys are arguing the same thing. You’e saying they have the same phase but in the opposite direction, and QED is saying opposite phase, same direction. It seems like a semantic arguement to me. Clearly, you both understand exactly how they work.
I think what billy is saying is that if one side of the isolation transformer secondary comes into contact with ground, the isolation has effectively been destroyed. In a normal situation, if you are accidentally grounded and contact either secondary terminal, you won’t get shocked, as you stated yourself. However, if while you are doing this, the other secondary terminal should make ground contact, you’ll be a-smokin’.
