Balanced Power Supply

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Ali Tait
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#1 Balanced Power Supply

Post by Ali Tait »

I have gotten hold of a 3kva toroid with 120-0-120 secondaries and fancy giving a balanced supply a go. This was sold to a chap on another forum by a hi fi company, he was told it was a prototype, and the proper version was silly money, about three grand I think.

Anyway, he was sold it with no protection at all, no fuses, no MCB, no RCBO or anything at all - for which they should be lined up against the wall and shot IMHO - he's not of a technical bent, so didn't know any better.

I'd like to do this properly, so to that end have bought a suitable enclosure. I'm also going to buy a 16A MCB for the primary, and a double pole 16A RCBO for the balanced secondaries. He mentioned it can trip the supply on switch on, so I have bought an SL 32 surge limiter rated at 1R 30A, and I'll mount a double socket on the box too, to plug everything in to.

I'll wire the two secondaries through the RCBO, and run the CT to the mains earth.

The question is, have I missed anything? I assume the RCBO will work just the same as in a conventional circuit, and all will be hunky dory and safe?
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#2

Post by Andrew »

A diagram might help us, Ali. I think the thing to bear in mind, as you know, is you now have two lives with respect to earth. Don't you guys do this sort of thing in the power industry? I mean its not conceptually a million miles from protecting a 3 phase set up, in safety terms is it?

Surge protection is a good idea, definitely might be a fuse blower as that big iron core charges up.

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Nick
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#3

Post by Nick »

I have a DC blocker board somewhere about you can have to throw in the primary as well if you like Ali.
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#4

Post by jack »

Remember that MCBs etc are sold at various surge ratings to prevent switch-on trips. Most domestic ones are "C" rated. Motor start ones (what you need) are "D" rated. You may have to get them from your local electrical factors (the sheds normally only stock domestic "C" rated circuit breakers)...

hth
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Ali Tait
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#5

Post by Ali Tait »

nickds1 wrote:Remember that MCBs etc are sold at various surge ratings to prevent switch-on trips. Most domestic ones are "C" rated. Motor start ones (what you need) are "D" rated. You may have to get them from your local electrical factors (the sheds normally only stock domestic "C" rated circuit breakers)...

hth
Aye, I looked at that, but looking at the 3kva balanced trannie on the Airlink website, they use "C" rated breakers.
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#6

Post by Ali Tait »

Nick wrote:I have a DC blocker board somewhere about you can have to throw in the primary as well if you like Ali.
That would be great thanks Nick, I was going to get one of those IEC sockets with built in blocker.
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#7

Post by Ali Tait »

Andrew wrote:A diagram might help us, Ali. I think the thing to bear in mind, as you know, is you now have two lives with respect to earth. Don't you guys do this sort of thing in the power industry? I mean its not conceptually a million miles from protecting a 3 phase set up, in safety terms is it?

Surge protection is a good idea, definitely might be a fuse blower as that big iron core charges up.

Andrew
It's the balanced bit throwing me a little, not sure if the RCBO will work correctly.

Here's the sort of thing I'm thinking about-

Image
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#8

Post by Mike H »

I made the mistake of Googling! :D

But yes seems has to be a 2-pole cut-out. Some think a 240V device is OK others think must be for 110V specifically.
 
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#9

Post by Ali Tait »

You get rcbo's that range from 110 to 240v, so I guess they will be fine.
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Mike H
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#10

Post by Mike H »

Aside from that, looks OK to me. :thumbleft:

As though I know anything about it :wink:
 
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#11

Post by Andrew »

Ali Tait wrote:
Andrew wrote:A diagram might help us, Ali. I think the thing to bear in mind, as you know, is you now have two lives with respect to earth. Don't you guys do this sort of thing in the power industry? I mean its not conceptually a million miles from protecting a 3 phase set up, in safety terms is it?

Surge protection is a good idea, definitely might be a fuse blower as that big iron core charges up.

Andrew
It's the balanced bit throwing me a little, not sure if the RCBO will work correctly.

Here's the sort of thing I'm thinking about-

Image
Those RCBO are just two coils (solenoids?) one pulls and one push at a switch, I think, so they look for current not voltage so it doens't matter what the relative potential is, .i.e single ended or balanced does it?

Do I have that right?
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#12

Post by Mike H »

I think so too, L & N currents must be equal, and none to earth.

Most of the Google searched arguments were about whether the RCBO disconnects both L & N, but I think they normally do don't they?

I mean on ours if you connect N to E it trips out (guess how I know :lol: ), so it must disconnect N as well as L.
 
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#13

Post by Ali Tait »

Andrew, yes I believe so, but wasn't sure, hence the post. I think as long as they will see a difference in each leg if there is a fault on one, and trip, I'll be happy.

Mike, from what I've read, both legs need to trip as you could have a fault one one leg, and the fault current could circulate via the other leg, hence the need for both to trip.
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#14

Post by Andrew »

Not sure if its both or just Live, tbh, but both seems safer.

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#15

Post by Ali Tait »

Aye, my thoughts too. Ok, doesn't seem like I have made any glaring errors, so I'll go ahead and order the bits. Thanks all.
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