Balanced 240V AC mains supply, simply does it

[Dave Hewitt]

Hi
Under fault conditions will both fuses blow at the same time? The commercial units seem to rely on an overload trip on the primary of the transformer.
Dave.

[Reid Malenfant]

To confirm Ali's (Tait) comment, both of the secondary lines must be fused. And, strictly speaking, both being phase lines, ought to be coloured brown.

Agreed, however the fuses will only protect the transformer secondaries, there is a much better way of incorporating vastly more protection

Hi
Under fault conditions will both fuses blow at the same time? The commercial units seem to rely on an overload trip on the primary of the transformer.
Dave.

Not necessarily, though in reality it doesn't matter As long as one blows then there is no circuit fullstop.

Your comments about the circuit breakers are correct, & this can be used to an advantage. I was mulling it over as frankly i'd hate to have the second transformer still kicking out 120V or there abouts if one had shut down. Then i thought about what would be in the circuit & came up with a rather perfect solution. You see if one transformer shuts down by the circuit breaker opening there is still a circuit for the other transformer to put 120V though. This is simply through the secondary of the shut down transformer & back to the working secondary. I don't like this myself so i'll devise a simple system to shut the thing down fully.

What i propose is a simple circuit with a relay that is held open when both secondaries are powered (240V accross both) & drops out with only one working (120V). When this closes it'll deliberately create an earth fault & along with a 30ma 25A RCD will shut the whole thing down. Much like the test button on the RCD itself

The 25A section of the RCD will protect the whole thing from any overload, the 30ma section will protect against either phase output from any earth faults in any equipment connected to the unit itself. The above mentioned relay circuit (which will need a push to break switch inline with the relay contacts to start the whole thing up!) will shut it down if one secondary goes offline due to a primary circuit breaker opening or one of the secondary fuses blowing...


Oh it's all good fun when things like this get you thinking

[Snoopdog]

Fascinating stuff and despite all the caveats about safety etc., I applaud Mark's post, the knowledgable contents of his contributions to this forum and the quality of the accompanying 'artwork.'

[Barry]

I used the term 'fuse' as a general term for circuit protection. Agree with Mark, that under a fault condition one side of the secondary could still be connected and supplying 120v.

A relay fitted in series with both sides of the secondary and operated by an over-current sensing circuit is the best solution.

Again I would advise extreme caution: 120V can be lethal - just remember that the next point of safety might be the RCB in your CU (if you have these fitted) and these usually require an inbalance of 10mA to operate. And they can take 25ms to operate (two and a half cycles at 50Hz). 10mA might still be lethal!

Regards

[Reid Malenfant]

<snip> I used the term 'fuse' as a general term for circuit protection. Agree with Mark, that under a fault condition one side of the secondary could still be connected and supplying 120v.

A relay fitted in series with both sides of the secondary and operated by an over-current sensing circuit is the best solution.

I'm giving this some thought, well i'm having all sorts of thoughts actually You have given me a rather interesting idea that would in effect be better than a relay on the output. How does output shut off in less than 1 micro second (0.000001 seconds or less) sound in the event of a fault?

I'm working on it drinkinone.gif

Before anyone says that's not possible i can tell you it's very possible indeed

[Puffin]

Mark. What are the pictures I can't see in your posts?

[Reid Malenfant]

Mark. What are the pictures I can't see in your posts?

You can't? That's odd, well i'll be sticking some more up inside of half an hour so feel free to take a look & let me know if you still can't see them. They are hosted on Photobucket, i'm not sure if that's the reason but i think everyone else can see them

[Joe]

I can see 'em. Not that I'll be building whatever it is!

[Reid Malenfant]

I can see 'em. Not that I'll be building whatever it is!

Actually that's quite possibly a good idea from what i have just discovered


Well some muppet has possibly messed this up & the muppet isn't me It was mean't to be balanced mains, well i guess it would have been if both transformers actually produced the same voltage! As it is i'm not sure yet if the slight imbalance will force an earth current to flow, this will have to be tested before things proceed any further.

Honestly, never get a man to do a machines job! There is more than likely a turn or two extra on one of the mains primaries or one is a turn or two short, either way it's not balanced perfectly

Ok, here we are with both transformers powered up - looks ok at 235.8V.



Here we have the (admittedly temporary) internal wiring. As you can see the transformers have different ends of the secondaries earthed. If you earth the same ends you'll just get about 118V from live to earth, though if you measured accross from both lives (and they were balanced precisely) you'd read 0V.

Just a few words here,this is why i mentioned you'd need a multimeter to test everything - because of muppets! You have to satisfy yourself that both (in the case of these transformers) red leads are actually connected to the same end of the secondary/s & are "in phase" before connecting them together. It's simple enough to measure between the two output sockets from red lead to red lead, i found 0V in both cases. Someone else might find otherwise - muppet strikes again!







This is the bit that you just don't even contemplate & only discover it when it's already rather too late in the day All may not be lost yet though as i have a cunning plan. Unfortunately it's not something hardly anyone here is likely to be able to copy because it involves some seriously in depth electronics

Voltage output of each transformer rfap.gif






Here incredibly simplified is the basic idea behind not only getting balanced mains but also virtually eliminating any distortion. This would take care of that slight inbalance & get mains distortion down to below 0.1%. It would involve two low voltage but high current power amplifiers & some circuitry to generate an ultra low distortion sign wave at the exact frequency & in phase with the incoming mains. If the output of either of the Live outputs is distorted each amplifier would add a signal to correct the error. The same or similar to the PS Audio Power Plant Premier except with a 6.6KW output




Before going mad though i'll stick a 1KW load on the transformers & see if any earth current flows

[HighFidelityGuy]

This is fascinating stuff!
Thanks Mark.