Does anyone still use the isolating transformer, during thier normal daily work, for safety reasons?.
What does this device have over an RCD unit?.
Your input please-
Have you ever heard of a 3phase(Star-Delta)
type?.

[This message has been edited by Trumpy (edited 03-04-2003).]

We used to use Isolating transformers, when we tested Armatures for shorted commutators and to sort the coil leads when we were rewinding them, Sure beat an RCD, you only got a shock if you touched both leads, can't say the same about an RCD. The thing I can't understand is that you were only allowed to use one appliance at a time with an isolation transformer, But a generator with floating windings acts the same as an isolation transformer, and it has no such restriction.

I use one regularly on the test bench when working on old AC/DC "live-chassis" radios and TVs.

It's almost essential for such work, as even if you make sure that the chassis connects to the supply neutral, you still don't want to connect grounded test equipment on to it and have parallel neutral currents causing problems.

I have a 1:1 isolation tranformer on the bench at home. However it puts out 125-127 volts when plugged across 120 volts.

Is this a normal manufacturing tolerance for these things? I thought they were supposed to pump out the same amount of voltage they were plugged in to?

I was looking at an iso transf. in another catalog and it cited the output voltage as 120 +/- 5 percent...so I'm starting to wonder...

Dapo,
Welcome to ECN, mate!!, you have the honour of being the first Electrician from Australia, to join us!.
Greetings, from this side of the Tasman.
Hope you find ECN, to your liking,
if you have any questions at all about Electricity or what's connected to it, please, just ask away, there is more than enough collective experience, here, to solve the toughest of dilemma's.

[This message has been edited by Trumpy (edited 03-05-2003).]

[This message has been edited by Trumpy (edited 03-05-2003).]

Sven — It may be closer to 120V with rated load, which is usually the case for most all transformers. On larger [premises-wiring-type] transformers, the label is stamped with percent or per-unit impedance, which generally cooresponds the change in secondary voltage for no-laod to full-load.

Bjarney,

The spouse suggested that too. So I plugged in a 60-watt bulb (and then a 100) and measured the voltage across that.

Ditto the 35-watt tube radio I was working on...after it was properly working. It still read 125-127 volts when the entire setup was plugged into a 120 volt outlet (this was actual voltage at the receptacle).

I don't want to spend 50$ on a new isolation transformer if it's going to do the same thing, since the catalog listing for the device I was considering did say +/- 5% tolerance.

The primary and secondary aren't shorted to each other (I did a continuity test) so that's a good thing.

Wonder if I'm just better off plugging a lightbulb in series with the primary to drop the incoming voltage a bit...and save my money for a Variac

Welcome Dapo! I went throught the threads rather quickly last night and didn't notice that you'd only just joined from Australia. Nice to have a new member from another country. C'mon in.... The water's fine.

Sven,
What's the VA rating on your isolation xfmr? If the xfmr is rated fairly high (maybe 2000VA or something in that range) then I wouldn't expect the secondary voltage to drop that much with just a 100W load.

Paul,

It's a 150 watt transformer -- big enough for small tube table radios that usually use around 30 to 40 watts of power.

I guess a 100 watt bulb was really going near the limit of the thing...since it did get slightly warm.

Sven — If it can be done safely, you may be able to reverse connections to the primary and secondary windings, hopefully dropping secondary voltage a little. “On paper” anyway, in the US, utilities and appliance manufacturers generally agree that 127V is the upper limit for 120V circuits.

I have a 5 KVA 200-240 VAC 1Ø 2 Wire Primary × 120/240 VAC 1Ø 3 Wire Secondary Isolating Transformer with Faraday Shielding and Grounded Core, which is used for my Computer Equipment in the "PC Room".

The Transformer feeds an 8/16 space Square D NQOD Sub Panel, which contains IG kit and all that good stuff.

Have a few other Isolating Transformers which were parts of a 4 wire Wye Isolation setup. Three (3) separate transformers, which are also 5 KVA and shielded, but are straight 120 in, 120 out.

Use here and there to isolate a piece of equipment for testing purposes.

Scott35 S.E.T.

Hmm.. Sounds as though the secondary on your "1:1" xfmr actually has a few more turns than the primary. I'm surprised that the secondary voltage doesn't drop that much with a 100W load if it's only rated 150VA though. The regulation on smaller xfmrs is generally not that close.

.

Bjarney, thanks for the suggestion! Why didn't I think of that?

Yes it can be done safely. The top of the tin can can be opened up (it's held by two sheet metal screws) giving access to the sockets and the flex. All I have to do is switch and re-solder the leads to the corresponding points, so it should be fine. Dropping 120 to 115 isn't bad at all...


Paul, I can live with a few extra turns...well...I guess you get what you pay for (US$20) but at least I can open the can it's in.

This is the device in question:



[This message has been edited by SvenNYC (edited 03-11-2003).]