Outlet Voltage Problem

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rruiz642

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Hello all,

I was hoping someone can help me understand a problem I'm experiencing in some new bldgs. I'm currently in a deployed location (US Military) with input voltage of 230/415V.

Our new generator plant is pushing 240V (phase-to-ground) into our dormatories. Each dorm room has a outlet from Legrand that contains a small transformer converting it to 115/120v on one side and the other 230/240 (legrand Model 6142 80).

While our input voltage is a little high, the output on these devices is reading 146v on the 115/120v side and 270-276v on the 230/240v side. The contractors have stated anything from floating neutrals to lack of load on the dorms. The dorms have not opened yet, but two things: the HVAC (central air) is running on these two story, 100 room bldgs and all outlets (besides this convenience outlet) are running a solid 240-242v (European power).

What I don't understand (besides the unusually high 115/120v side) is the increase in voltage on the Europeans spec side. No doubt I can't allow anyone to use these outlets and can't offer a solution other than disconnecting them. Information is hard to get from out here and my electrical knowledge always needs broadening. The outlet is setup has three connections: Line(115v)--Neutral--Line (230v)

thanks for any thoughts,

Richard
 
Do you know what kind of transformer it is? What kind of meter are you using to test it? Can you meter it under load? Can you meter the circuit before the transformer? you may find different readings with a True RMS meter and/or scope meter if you're not already using one.
 
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The tranformer is listed as a Legrand 7300 90, 230Vac 50/60hz supply, 230V/120Vac 20VA max, meeting BS EN 61558-2-5 stds..whew. We have used varoius fluke meters, including 77 Series III and a 321 (clamp on/volt meter).

We've measured the power at the panel and at the line side of the mini-xfmr. Those voltages were listed in my original email (146/276 respectively). The engineer's suggestion for the higher voltage (no enough load on the circuit) doesn't entirely make sense to me since the max load on the outlet is only 20VA...enough for a razor or so. Perhaps I'm missing something.

Thanks
 
Put a load on it!

Put a load on it!

rruiz642 said:
... The engineer's suggestion for the higher voltage (no enough load on the circuit) doesn't entirely make sense to me since the max load on the outlet is only 20VA...enough for a razor or so. Perhaps I'm missing something.
Click to expand...

(not enough load on the circuit)

I experienced about the same thing, 480V down to 277 on downline Main Air handlers units. This was only the second load on whole service.
This was even on second tier of service for larger air handlers, 3-small Semi trailor size, air handlers, and 300' feet from main. Voltage at distrubrution panel was 494- 490, 286-280 a little less a VSD's. (reading before Breakers where on) Tech cam in and said that all was correct VSD's were at 279 and wasn't concerned, said everything looked good.

As an example, If you consider that alot of engineered things have a safety factor built into there application say 1 to 5, well that 20%.

What desires is 3% at Distribution and 5% and outlet service I think the engineer is well correct all things considered and applied correctly.

Your wires just sitting there purring.
 
Outlet Voltage Problem

All outlets in the room and common areas, besides the bathroom convenience outlet, are a solid 240/242 volts from the transformer. The european side of the convenience outlet (230V) is reading 276. The incoming line-neutral (before the xfmr) is 242v and should just pass-through the device and output 242v. I'm can't explain why the voltage is being stepped up to 270-276.

I can see why, with the higher incoming voltage, the American-side (115/120v) would be a little higher; load or no load, I'm concerned for the folks that plug in their XBOX 360/PS3 gaming systems and smoke them.

Our quick answer is to disconnect the load-side of the breaker and cap off the wires. This will prevent damage to user equipement. Perhaps when the dorm is mostly/fully occupied we can install a few outlets again and measure the voltage. With just 1500 rooms, I am weighing all my options.
 
rruiz642 said:
The incoming line-neutral (before the xfmr) is 242v and should just pass-through the device and output 242v. I'm can't explain why the voltage is being stepped up to 270-276.
Click to expand...
IMO the line side is not being passed through. Probably only the grounded conductor passes through. If the specs you provided are accurate in form, the VA rating on the output being for both voltages would support this conclusion.

I am of the persuasion the elevated voltage readings are from the no load condition. You're only looking at a rated 0.09/0.17A output under load. I suggest you test under load...
 
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Outlet Voltage Problem

Yes, beside spell checking my replies, I'll check under load. One thing I forgot to mention is that the rooms are designed as 2 + 1 (two joined rooms with a shared kitchen/bathroom. In each kitchen is a panel that supplies the two rooms. So out of the 100 room dorm, I have 50 sub-panels in the rooms.

I'm not sure if this affects the feedback I've received so far, but I appreciate all the info I've received.

Richard
US Armed Forces, Deployed
Southwest Asia
 
It took a while, but I found the Legrand online catalog. It's 69MB, unfortunately, but it does have a small amount of information on the 7300 90 shaver socket on page 38.

Attached is an image of the available info, if it helps.
 
These devices are shaving transformers.

In the US, you guys have a GFCI protected outlet near the bathroom sink for the shaver (or hair dryer, or telly, or electric heater) to plug into. In (many parts of) Europe the only sort of socket permitted in a bathroom (*) is a "shaver socket". This is designed expressly to run shavers and nothing more. Inside the shaver socket is an isolating transformer, the secondary of which is floating from everything, and it is rated at a few watts only; the one you've linked to is 20 watts.

This socket is only intended to run a shaver, and they were designed back in the day of a shaver being an electromagnet. Plugging anything else in is asking for trouble. A playstation3 consumes (google) 197 watts, so it will not work so well off a 20VA transformer...

(*) Actually, the definition is more detailed than this, but it'll do for the sake of this discussion.
 
Outlet Voltage Problem

You're spot-on. The outlet is self-resetting, but I have a lot of young folks, who despite being told it (the outlet) can't support small appliances/gaming systems, will plug in their equipment. Unlike other dorms, we weren't planning on providing them a step-down (1500W) transformer as we have done in other older dorms.

My concern, since we are initially moving people in to a 25% capacity of the 100-person dorm, is the first few folks that plug in their gaming systems or TV and potentially damaging a $300-$5000 equipment. As of today I've disconnected the outlets at a few dormatories until the residents are fully moved in; I'll reinstall a few outlets and measure the voltage to see how the outcome with additional occupancy load.
 
Ah - no - a misunderstanding - the little trannies aren't putting out a high voltage because of lack of load on the 240V power system; it's lack of load on the secondary of the shaving transformer. Plug in a little load (say a 7W or 15w bulb) and the secondary volts will drop...
 
dbuckley said:
Plug in a little load (say a 7W or 15w bulb) and the secondary volts will drop...
Click to expand...

Your transformer is acting like a 146V source in series with a 156 ohm resistor. At zero current, the voltage is 146V. At full load (20VA), I'm guessing the voltage is right at 120V. Doing the math, it's the same as a 146V supply in series with 156 ohms.

166.66mA = 20VA / 120V

156 ohms = (146V - 120V) / 166.66mA

I'd guess that the 7W bulb will have about 136V across it:

2057 ohms = 120V^2 / 7W

136V = 2057ohms * (146V / (2057ohms + 156ohms))

If you're brave, I bet the short circuit current is around 936 mA. :D
 
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