Actually, to be precise, if you use an ideal voltmeter (with very high impedance), you could get almost any reading from one side of the isolated circuit to ground. The other side will be different by the voltage of your isolated circuit. Your point is that there is no appreciable current path, so even tenuous connections to ground, such as a connection through your body, which may be 100,000 to 1,000,000 ohms will drive that side of the circuit to ground. To work properly, you want to guarantee that a dead short from one terminal to ground will cause a safe amount of current to flow. Typically, these isolated circuits will have monitors that fire an alarm if that condition ceases to be true.
Where do I buy one of these “ideal voltmeters”- they sound like a pretty nifty device.
For DC voltages, an electrometer comes pretty close to being ideal.
A DVM with high input impedance (greater than 1 gigaohm) is a must when doing electronics work. But it is a hindrance when doing electrical power work, as the readings often won’t make sense. When measuring making measurements on AC power systems, it is actually better to have a meter with relatively low impedance (around 1 megaohm or so).
Very true. But for completeness sake it should be mentioned that there is coupling capacitance between the primary and secondary, and (depending on the frequency and the quality of the transformer) AC isolation may not be all that great.
Another thing to keep in mind is that, even if you’re “isolated” from earth ground, you can get zapped touching the hot wire if your capacitance to ground is high. If, for example, you touch a hot wire while touching a grounded, metal surface that is painted, you might still receive a shock. While the paint is an electrical insulator, it’s also thin, which means a series capacitor is created. (Your hand is one plate of the capacitor, the metal is the other plate, and the paint is the dielectric). If the paint is very thin, an appreciable amount of 60 Hz current could flow through the capacitor. This same current goes through you.
The transformer’s secondary coil has three wires coming from it: a wire connected to one end of the coil (L1), a wire connected to the other end of the coil (L2), and a wire connected to the center (N). The latter is often called the “center tap.” As mentioned by other posters, there is 240 VAC rms between L1 and L2, 120 VAC rms between L1 and N, and 120 VAC rms between L2 and N. Frequency is 60 Hz.
Theoretically, we do not need to connect any of the secondary’s wires to earth ground. This would be called a “floating” system. Some outlets in your house, for example, would be connected between L1 and N, while others would be connected between L2 and N. Your range would be connected between L1 and L2.
But while this arrangement would work, there is danger lurking in the background: the voltage between any of the wires (L1, L2, or N) and earth ground is not defined. Normally we wouldn’t even care about this, because an assumption could be made that the voltage between any of the wires and earth ground is probably close to zero. And the equivalent series resistance would be very high. So why even care? Who cares what the voltage is, for example, between L1 and earth ground, or between N and earth ground?
Here’s the reason: the transformer does not have perfect electrical isolation between the secondary and the primary. If the secondary of the transformer is allowed to float, there is a chance it could “float up” to the voltage on the primary winding. If this were to happen, you might have a situation where there is 120 VAC rms between L1 and N, but 7,000 V between N and earth ground. :eek: This is dangerous, because you are often touching something that is grounded. In a situation as described, you would not want to be near L1, L2, or N if you were grounded.
In order to prevent this from happening, we connect one of the secondary’s wires to earth ground. Connecting any one of them (L1, L2, or N) to earth ground would work. It’s best, though, if we connect N to earth ground. That way, the highest voltage between any wire and earth ground is 120 VAC rms. (If, say, we were to instead connect L2 to earth ground, the highest voltage between any wire and earth ground would 240 VAC rms. That’s not too bad. But it would be better to have the highest voltage be only 120 VAC between any wire and earth ground.)
I’m leaving out some subtle details. But hopefully everyone gets the gist of it from this explanation.
When doing home electrical work, I’ve found it much better to avoid touching any wire at all (hot or neutral) whenever possible.
I once got a good shock from a telephone extension jack when a call came in.
I believe ring voltage was 90 volts DC
I believe the ring voltage is 90 volts AC
You might be right!
Bell System spec was 88v AC at 20Hz. This was superimposed on the normal 48v DC power that was on the line all the time.
In actual practice, it could be anywhere from about 75v to 100v, depending on the local Central Office. Frequency sometimes varied too, between 20-25Hz. But that was less common.
You’ll get a transient shock, at least, owing to the single-body capacitance of whatever it is you’re touching. How long that shock lasts, and therefore how severe it is, will depend on the size and shape of the conductive surface. In fact, this is basically what happens when you short to ground: The Earth itself has a capacitance.
Wow, Did I ever open up a pool of knowledge. Thanks everybody, especially engineer_comp_geek for the thorough explanation. Because of that I was able to systematically troubleshoot some electrical problems I was having in my home.
I’m not sure this is true. In an isolated ground system, such as is used in theaters and churches for technical power or other systems, power is brought in to the building typically using 4 wires. These 4 wires are the 3 phases and a ground. Not a neutral, but a ground. This ground is the power company’s ground, and completes the circuit for them. Then the 3 phase wires are connected to the primary side of the transformer, while the ground is connected to the ground lug for the transformer. Then, on the secondary side, the power for a 3-phase panel is connected, with typically both a ground and a neutral being connected to the ground lug. This bonds the neutral to the ground in one location for the ‘separately derived’ system as it is termed in the NEC. Not having the ground connection is against code and could be dangerous.
Why is this done? This is done to reduce the possiblity of ground loops introducing hum and buzz into the system. This is also not good in hospitals, as it could corrupt readings from the sensitive equipment that they are using.
Edit: Also, the connection to the cold water pipe or a ground rod is not to provide a safety ground. It is to provide a path for lightning that strikes the building and could cause damage to the equipment connected to the power system.
I am not a licensed electrician, but I have dealt with plenty of isolated power systems for AV.
Whoops, I should have checked the link. We use this all the time in AV to eliminate ground loops, typically using a high quality transformer that won’t saturate at low frequencies. Engineer_comp_geek covered the system I was talking about in his earlier post.
the grounding electrode is a safety ground. it may also be your neutral if your meter is at the same location.
if lightning strikes your house you can likely kiss lots of what is plugged in good bye. if lightning strikes your house metal in it, not just your electrical wiring, can get an induced current in it. even nails in wood can show burn marks. with a lightning strike there is so much current that paths to your equipment through wiring are as good a path as your grounding conductor. there is so much current to dissipate that almost any material is a good conductor path in attempting to get to the earth, lightning had just traveled miles through nonconductive air, most anyhting looks good after that.
I had a friend whose house was struck directly by lightning a few years ago. You could stand in any room and see where the wires went through the walls by the scorch marks showing through the plaster! (They were incredibly lucky that it didn’t start on fire.)
The house had to be completely rewired, the walls replaced, etc. – nearly completely gutted. When looking at the wires after opening the walls, many of them seemed to be missing the copper entirely – the insulation was burst open and scorched, but the metal was just gone. Some of it was found as once-molten splashes on the back of the walls, but much was just gone!
Lightning is really powerful.
Wow! My mother’s house was recently struck by lightning. It’s an old house that was grounded differently than current homes. It was grounded through the water pipe that comes into the house? The breaker panel basically had a mini explosion and caught the back of the house on fire. Fortunately no one was hurt.
OK, now let’s talk GFI - Ground Fault (Circuit) Interrupter.
It’s my understanding that for maximum usefulness they should be the first recep in the series so as to protect all downstream outlets.
My house isn’t. As a matter of fact on this one circuit it isn’t even the first bathroom outlet in the circuit.
Two questions:
- should I look into moving my GFI outlet to the first outlet in the circuit coming from the breaker panel (that outlet is actually located in my basement)?
- Maybe more of a theory question, but what good are they anyway? Isn’t that what the breaker is for?
This particular circuit: breaker panel->downstairs recep->upstairs guest recep->master bath GFI recep then is splits to 1) other master bath recep and 2) downstairs garage recep
There are different types of protection devices.
FUSE:
A fuse is just thin strip of metal that burns out if too much current goes through it. You hardly ever see these any more. Fuses are dirt simple, but you have to replace them if they blow.
BREAKER:
A breaker, like a fuse, opens up the circuit and stops current flow whenever there’s too much current. It essentially does the same thing as a fuse, only if a breaker trips, all you have to do is reset it. You don’t need to replace it.
A breaker only protects you from too much current, typically 15 or 20 amps.
GROUND FAULT CIRCUIT INTERRUPTER (GFCI):
The electricity should be going out one wire and back through the other wire (it’s AC, so which one is going out and which one is coming back is constantly alternating). A GFCI measures the current in each wire to make sure that this is true. If the current in the hot wire is different than the current in the neutral wire, then some of the current has found another path, which is generally a bad thing and the GFCI shuts the circuit off.
Let’s say for example you are in your bathroom, and you touch something that is electrically “hot”, and at the same time you touch a water pipe or you have your hand in the water. Current flows from the “hot” connection, through you, through the water and/or water pipe, and you generally have a bad day. If you manage to get 15 amps of current flowing you’ll trip the breaker, but if there’s less current than that, then the breaker won’t help you at all. You can easily die from currents that are a lot smaller than 15 amps, so this is one of the easier ways for you to end up getting killed by electricity. If you have a GFCI though, it detects the imbalance in current between the hot and neutral and shuts the circuit off, saving you from a possibly fatal experience.
GFCIs trip at 5 mA. In case you are wondering where this number comes from currents as low as 5 mA (0.005 amps) can potentially throw your heart into fibrillation and kill you. These low level shocks aren’t likely to kill you, but you aren’t guaranteed to survive either. The risk of death gets higher as the current goes up.
ARC FAULT CIRCUIT INTERRUPTER (AFCI).
What happens if you’ve got a bad connection in something like an extension cord? It gets hot and starts a fire. A breaker won’t help you because you’ve probably got way less than 15 amps flowing. A GFCI won’t help you either because all of the current from the hot still ends up back in the neutral. But, your house burns down, which most people consider to be a bad thing.
This is where an AFCI comes in. If you’ve got something arcing (like a frayed extension cord) it will shut the circuit off.
Protecting the entire circuit with a GFCI is of course better, but your greatest chance of having a ground fault is in the kitchen or bathroom. You definitely want to get that one unprotected bathroom outlet on a GFCI. The rest isn’t so urgent.
You may want to look into adding AFCIs to your bedroom circuits as well.