Centre Tapped Input to Power Supply

[b7031719]

I have come across a 110V a.c. switched mode power supply that shows Live, Neutral and Earth input terminals and includes fuse and VDR input protection but does not state whether a 55-0-55V a.c. input is acceptable.

In general are centre-tapped supplies a problem for power supply inputs?

 

[VEBill]

...power supply that shows Live, Neutral and Earth input terminals... ...but does not state whether a 55-0-55V a.c. input is acceptable.

Where are you getting 55-0-55 VAC? Is this an isolation transformer with an unnecessary center-tap?

Why not connect the line AC directly to the power supply? Do you need the transformer?

But if you must use the transformer, then let's discuss this option:

First 55VAC - Live
Middle 0VAC - ignore the center-tap (cap and stow)
Other 55VAC - Neutral
Earth Ground - Earth

 

[b7031719]

Where are you getting 55-0-55 VAC? Is this an isolation transformer with an unnecessary center-tap?

Why not connect the line AC directly to the power supply? Do you need the transformer?

But if you must use the transformer, then let's discuss this option:

First 55VAC - Live
Middle 0VAC - ignore the center-tap (cap and stow)
Other 55VAC - Neutral
Earth Ground - Earth

Thanks for the reply.

This is an industrial installation, the supply is coming from a UPS not an isolation transformer. The UPS was specified to be 55-0-55V a.c. and supplies many other loads. Unfortunately there isn't an opportunity to modify for this particular case so I am trying to understand whether it is safe to connect a number of switched mode power supplies to this supply.

 

[VEBill]

and supplies many other loads.

Then you'll need to be super-careful.

For example, if you had connected another load where those other loads actually grounded the UPS center-tap, and then you took the option of connecting the new power supplies with the 2nd 55 VAC output linked to their neutrals, then (depending on plenty of details) you might end up with a strange circuit across the bottom half of your ...0-55VAC output.

For example: 0 VAC center tap, to your original loads, then (perhaps) linked to Earth ground. Then the new Power Supplies neutral (which should not normally be linked to ground, but might in some weird cases), then back to the 2nd 55 VAC output. End result could be a short across the bottom half (0-55VAC) output of the UPS. At the very least, two neutrals with different potentials.

So, probably not advisable. Or at least, you'd need to be super-cautious.

Is the UPS one of those weird industrial ones with the strange output designed to avoid "high voltage" ?

 

[ScottyUK]

I don't think you'll have a problem to be honest. Most SMPS designs couldn't care less whether you observe polarity of the line and neutral, so provided you supply it from the two 'live' conductors it should be fine.

VE1BLL - it would be very unusual to see multiple earths on the neutral / centre-tap. That would be a PEN / PME supply which is prohibited outside of the utility world.

 

[VEBill]

Most SMPS designs couldn't care less whether you observe polarity of the line and neutral...

On that -^ particular point...

...switched mode power supply that...includes fuse...

Presumably the SMPS's input fuse is in the Line side of the Power Supply. Not ideal to reverse the SMPS's input connections thus having internal fuse in the Neutral.

Also, have a look at the very first image on Google Image search of: SMPS input circuit

http://sound.whsites.net/articles/smps-primer.html

Scroll down to "Figure 17 - Typical SMPS Input Circuitry" (sorry, can't link the image here)
I'm not saying it's correct; but there it is. Who knows how many are built this way.

--

For the OP's question: As long as nobody has grounded (or ever will ground) the center tap, then no problem.

 

[ScottyUK]

No way that is legal for use, and any RCD / RCBO / RCCB etc would trip immediately. It has to be a mistake in the drawing.

Fair point about the internal fuse, although it's possible to mitigate the 'problem' with external fuselinks. On the external wiring we would always use double pole breakers or fuse both legs of the supply.

 

[b7031719]

Thanks for the comments. I have been in contact with the manufacturer of the power supply (Relec W Series). It has been confirmed that it is suitable to be operated with this kind of supply however they do offer an optional built-in secondary fuse in the neutral, presumably for this kind of connection.