Site Transformer/Step Down Transformer?

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TornadoTed

Well-known member
Joined
Feb 2, 2006
Messages
239
Location
Mid Wales
I have quite a bit of US 110v gear on loan as a friend mainly mixes and rarely records these days. I never have enough step down transformers so I did a bit of research and read that a site transformer is the same thing.

I brought a couple of site transformers and some US 6 way power strips. I cut off the US plug and put one of the big yellow power tool connectors on the 6 way strip. I wired the earth to the earth, black to the pin marked 'L' and the white to the remaining pin. Anyway I thought I would check before possibly damaging his gear. I measured a power strip ran from a proper step down transformer and I get 0v on the neutral and 118v on the positive pin. I then measured the site transformer and I got 58v on the neutral and 58v on the positive.

Is there a way to wire the site transformer that will work right or have I wasted my money?
 
that sounds kinda like an 'equitech'

http://www.equitech.com/articles/power.html

any decent gear should have power transformer inside or wallwart.
as long as chassis is grounded you should be good
 
Was the other probe on the "ground" or "third pin" contact?  And when you say "positive" you mean "live" as opposed to "neutral"?
Shab is correct.  Your "site transformer is in effect a "balanced power" transformer.  You will be OK with everything except very old tube equipment like 50's era combos and the like.
Mike
 
Get some advice from a local sparkie, it's pretty clear that you don't know what you are doing and there are safety issues involved with mains wiring.
 
Yes I put the one probe of my meter in the round earth hole and tested the other 2 holes with the other probe.

I guess I am safe enough using US gear that just has the 2 pins and no earth pin, is that correct?

I read online but can't find the link now that it is dangerous to use a surge protected power strip with a step down transformer, any truth in that and why would that be the case?
 
Have you got an earth connected to the outlet?

Mains wiring can be different in other countries, which is where the potential for hazard comes in.
For the MEN system, the Neutral connection is connected directly to earth at the neutral bar in the metering box. When you put the mains through a transformer you isolate this connection so your outputs are floating, which is what you see. You still have 110V between active and neutral, it's just that the neutral is no longer bonded to the earth connection.
 
On the safety note, I believe you need an rcd after the balancing transformer to make this safe...

If the techno-geeks are to be believed, you should find your kit runs quieter (lower noise floor...) with balanced power... For much the same reasons as signal... Surges etc should be equal and opposite and so cancel through the kit's mains transformer... Ymmv...
 
Moses said:
On the safety note, I believe you need an rcd after the balancing transformer to make this safe...

If the techno-geeks are to be believed, you should find your kit runs quieter (lower noise floor...) with balanced power... For much the same reasons as signal... Surges etc should be equal and opposite and so cancel through the kit's mains transformer... Ymmv...

The RCD will only be effective if you bond the 'neutral' output from the transformer to earth. It detects imbalances between active and neutral, the imbalance caused when some of the return current flows to earth instead of the neutral. However the return current won't flow to earth if there is no path back to the transformer 'neutral'.
 
"Site transformers" are UN-known in the USA (can't even be shipped to the USA). So you should discount anything from anybody in the USA (even me).

The actual specs for site transformers sold in England (or Mid Wales) are very obscure. Is the 110V "centre-point-earthed" or "floating"? BS7671 seems to require centre-point-earth; a bloke who opened his site-transformer (why?) could not confirm that.

So is it PELV or SELV? If you do not understand that question, you maybe should not be re-wiring specialty electric supply devices to unapproved purposes.

The customary US power is 120V with one side grounded at the fusebox. The White wire shows low voltage (<3V) and low resistance <1 ohm) to the green wire.

This is *NOT* the same as 110V earthed-center. As you found, you have 55V from either White or Black to green.

In fact this is not illegal in the US. And 99% of US gear will tolerate 120V center-tap. There's audiophiles who do this. There's also (on this forum) a report of a mishap where a small fault in such a system BLEW-UP thousands of dollars of gear. Power schemes where neither current-carrying conductor is "near" the earth/ground conductor are so rare (world-wide) that equipment is not tested for faults which would only appear in such unusual systems.

I think you need a REAL electrician versed in the depths of BS7671. This is not a situation to rely on idle net-chatter.

Or---
http://www.airlinktransformers.com/american-transformers.asp
http://www.airlinktransformers.com/transformer/us10002-american-120volt-transformer.asp
US1000/2 1kVA American transformer 230V to 2 x 120V (3 pin US plug outlets) British built £64.00
This can be used with standard US 6 way power strips (if you didn't cut-off the US plugs) to power a couple of guitar amps or a whole lot of small stuff.

I _REALLY_ think you want an "American Transformer" plus some power strips. It is made for the purpose. It should be cheaper in a thin tin case than the drop-proof housing required on a site transformer. (Though I don't see a big difference in price.... maybe low demand?)

You STILL have the issue that much US gear, especially older pre-world-market gear, is wound for 60Hz power. A 50Hz power transformer must be a bit larger. Some 60Hz gear does not have enough safety-factor to survive 50Hz for long. (Also any very old clock or record-player will run 50/60th speed.)

For US readers:

"site transformer" means "Construction Site". Power tools used in mud. These are dangerous on US 120V power, more so on 230V power. BS7671 is similar to our NEC Code and specifies special wiring and supply to such situations. The average builder does not need the details: in the UK the tools, cables, and transformers are made to meet the requirements.



> to make this safe...

The kit as sold is "safe" for the intended purpose. I believe it is safe in a dry environment.... except that the US-market loads are not certain to be expecting a center-tap supply (even if Equi=Tech sells such gear).

> RCD will only be effective if

If the 120V winding is truly floating, the RCD can never trip. If it is centre-tapped, then neither current-carrying conductor is "neutral" and bonding one to earth makes a dead-short.
 
PRR said:
If the 120V winding is truly floating, the RCD can never trip. If it is centre-tapped, then neither current-carrying conductor is "neutral" and bonding one to earth makes a dead-short.

No, the secondary winding is floating. You can short one leg of it to earth. If the input neutral is correctly wired then one leg of the primary is already shorted to earth.

 
And here was me thinking that it would be fairly simple!!! I guess I will stick with my current set up and send the site transformer back and get something intended for purpose.
 
About "RCD" and "GFCI" protection units. They work by comparing the current in the Hot wire to the current returning in the Neutral wire. If some of the returning current decides to take a shortcut though your body, then the two currents won't be equal and the device will trip. No Safety Ground is needed for it to work. In the US a "GFCI" may be used on a older two wire circuit to allow using 3 pin plugs.
 
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