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scoot_gt 07-17-2007 10:34 AM

240v Question
Hi all -

I have a question for any experts regarding 240v on the US. I know that 240v can be achieved by using both hot/live legs of our typical 120v residential service. My question is does this differ (and if so, how is it different) from the 240v acheived by a step-up voltage converter/transformer?

And if possible, how do either of these differ from a standard UK residential service of 230v?

I'm aware of the frequency differences between the UK and US (50 vs 60Hz) but I'm quite certain this isn't causing my issue.

Some background to my specific issue -
The UK device I'm trying to power has some small solenoid valves as well as some typical microprocessor circuitry. I've tried powering the unit off US 240v (I've installed a 240v outlet off a 240v/40A breaker, using both legs of 'live' and ground to make my 3 wire connection to the UK device - live, neutral, ground). The microprocessor circuitry works fine but the solenoids refuse to open. The device is rated at 230V, 3A. I'm curious as to if I purchase a decent 1000W stepup voltage converter if my results may differ...?

Thanks in advance for any insight!

dmaceld 07-17-2007 11:22 PM

The single biggest difference between US and UK on the 240v supply is the US 240v comes from a center tapped transformer where the center tap is connected to ground. In the UK system one leg of the 240v line goes to ground, so the hot leg is at 240v to ground. In the US system each leg is only 120v to ground.

You can make it work by getting a 120v to 240v transformer, or even a 240v to 240v transformer. You want the 240v output side to have only two leads coming out, not three, and one of those two leads will be grounded.

Be aware that with this setup you have a true 240v line to ground voltage situation, and no separate equipment ground. If you do this be extra safe and use UK type receptacles and plugs so you don't inadvertently connect US 240v equipment to the transformer.

Also make sure you ground the same side of the 240v line that UK does. You may have to open up the equipment to determine which side of the plug goes to ground.

Is there a three prong plug on your UK device? If so, then it will be hot, ground, and equipment ground. There will be no neutral like in the US 240v 3 wire system. If there is an equipment ground in your UK device, then you can wire an EG lead around the transformer coil to the US EG leg.

scoot_gt 07-17-2007 11:53 PM

Thanks for the response dmaceId!

Would a simple step up voltage converter such as this suffice?

The UK device is actually just raw wire with the standard brown, blue, green/yellow used in the UK to denote live, neutral and ground respectively. In my current attempt at connecting to US 240v, brown & blue are tied to 120v legs and green/yellow is tied to ground.

Assuming the product above is sufficient, I should just be able to wire in a 3 prong plug (UK or US connector shouldn't matter), correct?

I didn't quite understand your last advise about EG (Equipment Ground, right?) leads for UK devices with equipment ground. Maybe our terminology of live, neutral, ground is differing? Can you explain further?

dmaceld 07-18-2007 01:52 AM

The transformer should work just fine.

Sorry for the confusion. I just did some checking. Here's the differences. Brown is hot, but it's 240v hot, not 120v hot like our black. Blue is neutral, i.e., it eventually gets connected to the earth just like the white neutral of the US 3 or 4 wire 240v system. Our neutral is a neutral in our 240v wiring. It used to be called ground because it is at ground potential. Their neutral isn't really a neutral until it's back at the transformer and part of the three phase wiring system. The green/yellow would be an equipment ground (they call it protective conductor) which is like the equipment ground of our system. It too gets connected to the earth back at the circuit breaker box. Keep in mind our 240v is usually 4 wires, 120v hot, 120v hot, neutral, and ground (actually equipment ground). 240v is of course the voltage across the two 120v legs and with a balanced 240v load no current flows through the neutral. It's called a neutral because it's half way between the two hot legs, and to differentiate it from the equipment ground conductor.

The UK system is 240v hot, neutral, and equipment ground. All the current goes through the hot and neutral. Back in the circuit breaker box the neutral and equipment ground get connected together, just like our neutral and equipment ground get connected together.

With the way you connected your device to our 240v the blue wire is at 120v with respect to ground. It needs to be at ground potential, the same as the equipment ground.

Still confusing? Think of the UK system as being wired the same as a US 120v 3 wire circuit, except the hot leg is 240v, not 120v. They don't have a second 240v leg, nor do they have 120v in their system.

To figure out which hole of the receptacle the brown wire needs to connect to, check the voltage of the transformer outlet. The hole that is 240v to the equipment ground is the hot wire. The other hole will give you 0v to equipment ground.

scoot_gt 07-18-2007 02:31 AM

Thx for the further explanation!

This is making more sense to me now. And yes, I saw some explanations of UK electric that actually said they have a 3-wire grid similar to the US but with 2 230v legs. Also, their residential only sees 1 leg of the hot.

I guess I will bite the bullet and order one of these up converters and see if my solenoids will work properly then :thumbup: . I will let you know if I was successful!

Just for giggles -
I'm still a little confused about the non-operation but I'm in way over my head for my limited electrical knowledge. Let's assume no Ground connection. I guess it is impossible for current to flow from 120v to 120v (brown to blue wire for my specific UK device)? Must there always be a ground or neutral (at ground level) for current to flow? I thought for a perfectly balanced load on both 120v legs, there would be no current flow on neutral? Of course, with my application, it will be completely unbalanced... maybe this is part of my issue/misunderstanding?

I'm just pondering outloud - I'm ordering a stepup converter regardless :icon_biggrin:

BTW, I tried disconnecting the Ground (green/yellow) wire but had the same results - microprocessor operation ok, no solenoid operation.

dmaceld 07-18-2007 09:32 AM


Originally Posted by scoot_gt (Post 53387)
I guess it is impossible for current to flow from 120v to 120v (brown to blue wire for my specific UK device)? Must there always be a ground or neutral (at ground level) for current to flow?

No. Think of it in terms of +120v on one leg, -120v on the other leg. Current flows from one leg to the other bypassing the neutral in your house wiring. With the UK system it's 240v leg and 0v, or neutral leg. When you connected the system the way you did, the part of the device that would normally be at ground potential was at -120v, so the hot part was still 240v above the ground part. Your device was 'floating' in its own little world. Not a problem unless the grounded portion of your device, if it was accessible, which it probably isn't, got connected to real earth.


Originally Posted by scoot_gt (Post 53387)
BTW, I tried disconnecting the Ground (green/yellow) wire but had the same results - microprocessor operation ok, no solenoid operation.

This makes me think your device is defective, or there's something more to the device you don't have a handle on.

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