home electricity question; please help

As I live in Spain, my house is wired for 220v and I have just setup a 110v setup with separate 15amp outlets with ground, which starts from a 1000watt step down transformer off of the 220v setup.

Testing the new outlets with a digital multimeter, I get a readout of 110v, that’s fine.

However, from one blade of the outlet and the ground I get ~200v and from the other blade and the ground i get ~90v.

I don’t know why, but I was expecting to get 110v from one side and half of that from the other.

any comments appreciated.
thanks, bardos.

When you use a transformer, the resulting voltage is only defined between the two blades, not between either blade and the ground. Kind of “random” measurements should be expected.
That is the theory, as long as “ground” means literally that, something connected directly to earth and having nothing to do with your electrical equipment.
Your case may be different, as what you use as ground may well be used to ground the transformer itself and maybe some other equipment as well (this is the case in most home setups, with or without transformers, and it is ok). So anything between 0v and 220v is ok, and you may experience that the numbers change whenever you switch some other device on or off.

As a quick answer, I’d guess that you’ve got an “isolation” transformer. It provides 110 V between its own terminals, but that voltage isn’t tied to the active, neutral or ground on the 220 V system. That shouldn’t really be a problem, except in rather extraordinary circumstances, i.e. if you connect several pieces of 110 V equipment, each with internal faults to the casings.

Other possibilities:

  • The neutral on the 220 V system itself is floating. What are the voltages to ground on the 220 V system?

  • There could be some unusual winding arrangement in the transformer. Can’t think of a good reason why this would be done, but it’s possible.

Other information you could provide:

  • Make and model of the transformer

  • Voltages between each of the 110 V blades and each of the 220 V blades.

>Other possibilities:

>- The neutral on the 220 V system itself is floating. What are the voltages to ground on the 220 V system?

               ~23v and ~200v

>- There could be some unusual winding arrangement in the >transformer. Can’t think of a good reason why this would be >done, but it’s possible.

>Other information you could provide:

>- Make and model of the transformer:

            Seven Star THG-1000 (chinese made)

>- Voltages between each of the 110 V blades and each of the >220 V blades.
110-111v on the 110v system and 215-220v on ther 220v system

thanks, bardos

OK. By googling on the make and model, then looking at the pictures, its pretty clear it’s an isolation transformer.

That means that the voltages you’ve measured between the 110 V blades and ground are not significant. Like MartinL said, its just random.

Provided that you don’t simultaneously use two or more faulty pieces of equipment on the 110 V system, it’s safe.

thanks a lot for your support.


By “not significant”, they mean that those voltages to ground are leakage signals jumping between the transformer primary and secondary. If you hook a light bulb between one of your 110V prongs and a ground connection, the light bulb will not light (and the voltage on that prong will vanish.

USA safety codes say that one side of the 2-prong outlet is supposed to be connected to ground. If you’re finding a 90V leakage signal on the wide prong (the white wire), then your wiring is not standard (in other words, the hot is not hot and the neutral is not neutral, instead you’ve got floating AC.)

The neutral should be grounded (to earth) at the transformer. That’s why it’s called the “grounded” conductor in the NEC. The safety ground, connected to the u-shaped prong in a 120v outlet, is called the “grounding” conductor.

Whatever is happening on the tech side, all is functioning quite well, no blackouts, brown outs, or blown fuses, the transformer has a fuse as well and I now have a functioning dual/voltage system for my house current.

I suspect any vagaries might stem from the fact that both systems are using the same ground, a copper spike driven into the ground; that is both systems are obligated to use the same planet as a ground.

(There does not appear to be a workaround for this except maybe to hitch a ride with a 50 million mile cable onto the Mars orbiter. But then of course it might work out that the cable would not be earthed, right? it’d more likely be marsed)

I am using various American devices in ths house, have been for years, with various and sundry transformers scattered about. Now I have a mains transformer (1000 watt) for the step-down and Amercian outlets throughout the house alongside the Spanish ones. Kind of fail-safe in the sense that you can’t mix up the plugs because one won’t fit into the other.


As others have implied, it is likely that the equivalent source impedance of your measurement is very high.

Digital voltmeters usually have an input impedance of around 10 megaohms. While this is nice in the sense that it approximates an ideal voltmeter in most cases, it is often not good for home electrical use. (The meter’s high input impedance always creates a “voltage divider situation.” When the equivalent source impedance is very high, you can get readings that will lead you to make wrong conclusions.) To fix this problem, stick a 1000 ohm resistor across the meter’s leads when making voltage measurements.

And now some comments & questions for bbeaty and mangeorge: I fully understand why the neutral in my house wiring is tied to earth ground. But let’s say I’m doing something similar to what bardos is doing… let’s say I hook a 1:1 isolation transformer to a 120 VAC wall outlet and I connect a load to the secondary. Are you saying I must ground one of the secondary leads? Why can’t I leave the secondary floating?? It would seem to me that letting the secondary float is a safer than grounding one of the secondary leads.

If I installed a 110 v. system with outlets, etc. in a 220 v. house I would connect one side of the isolation transformer to the neutral wire in the 220 v. house system and carry the green (US color coding) safety ground over to the 110 volt system also. That way if you use, say, an electric drill, there is less danger of an electrical hazard.

I have misgivings about this setup. An isolation transformer is generally intended to drive a single appliance.

When there’s more than one appliance connected to it, there’s a chance of a dangerous fault occuring.

I don’t know (and can’t readily find out) what the Spanish codes say. Input from sailor would be most welcome.

But here’s my point: If a load is operating from a floating secondary of a transformer, then (assuming the transformer is good) there’s very little (if any) risk of shock between the transformer’s secondary and earth ground. In other words, an earth ground won’t make it any safer, and a “ground fault” will not trip the breaker back at the panel.

When a load is operating off the secondary of a transformer, and if the transformer provides excellent isolation, then the only risk of electrocution is between the two secondary leads. Hooking up an earth ground anywhere in the secondary circuit doesn’t buy you any safety points.

Here’s an example: Let’s say I have an isolation transformer and I connect a motor to the secondary. Would connecting one of the secondary leads to earth ground be a good thing? Not from a safety standpoint. In fact, it would make the system less safe. What if I connect the motor’s chassis to earth ground? Would that be a good thing? Not really. You could argue that it would protect against a high leakage condition in the transformer, but that’s about it.

I tend to agree with Desmostylus. I would certainly make sure that the secondary of the transformer has its own fuse or circuit breaker no larger than the cirrent rating of the transformer.

And just for the sake of completeness I would read my house insurance policy for any mention of modifications to the electrical wiring.

All in all, the safest way would be to have a 220-110 volt transormer that could plug into a 220 wall socket and then plug the appliance into the transformer secondary. That way, things are under your direct observation.

BTW: I’d like to add that my previous comments are not applicable to the high voltage step down transformer that’s hanging on the pole next to your house. In that situation, one of the secondary wires must be connected to earth ground. I’m talking about an additional isolation transformer that is installed downstream from the HV transformer, such as a 1:1 isolation transformer operating from 120 VAC.

I just looked at bardos’ transformer. That’s actually an adapter, which should be “plug 'n play”. What others have said about the high impedence of multimeters is true. They’ll measure millivolts with the prods in the air.
With a load connected, you should read;
Wider flat blade (neutral) to u-shaped blade (ground) 0v.
Narrower flat blade (hot) to u-shaped blade (ground) 110v.
Hot to neutral 110v.
If both blades are narrow, neutral (on the female, not the male) will be the one on the left with the ground down. Smiley face position. :slight_smile:
Does this thing have UL approval? :wink:

Well, I’ve put this forward before, to some derision, but there is a good chance that you have reversed polarity somewhere in the circuit. Reading a high potential between neutral and ground on an isolation transformer is a common indicator of this condition. While it is not normally dangerous, it can cause problems with sensitive electronics and can cause your surge protector to blow a fuse. If the polarity is correct, you should have negligible voltage readings from neutral to ground.

We had constant problems overseas with running 110v printers connected to 220v computers. I believe it was stray voltage leaking through the parallel connecting cable and frying motherboards like you wouldn’t believe. Once we corrected the polarity glitches, problem solved.

Oops. That should have read “autotransformer”, not “isolation”. Autotransformers are common, although they shouldn’t be.

What it buys you is a predictable response to ground faults in the wiring system. It means that a breaker or GFI will open if the hot grounds, or has little to no effect if nuetral grounds. With no intentional grounding of the system, there is no response to the first fault and the second fault is now completely unpredictable.

And as pointed out, the isolation of the transformer has to be bullet proof for this non-grounded system to work safely. Anything happens to the transformer and lots of stuff can go wrong.

Don’t get me wrong, if you have a hand drill running dierctly from the secondary of a isolation transformer, you’re OK. But the OP has wiring all over his house and this is not such a good thing, especially if he is using edison base lamp holders.

With all due respect, what in the hell are you talking about?

First of all, there is no “hot” or “neutral” on an isolated secondary. It is a differential and isolated source. Secondly, if either secondary conductor comes in contact with earth ground, the breaker at the box will not trip. Why should it?

If you have an isolated secondary, the only way to trip the breaker back at the box is to short the two conductors together, and it does not matter if one or the other is connected to earth ground.