Fuse calculations for balanced power?

[Jim R.]

I have a couple of amp projects about to get underway that will be wired for balanced power as I'm now using an equitech balanced power system.  Basically what I'm doing here is to wire both hot and neutral as hot lines with their own fuses and switch poles, the question is how does one calculate the fuse rating for this scenario where each incoming lead has half the voltage -- 60-0-60?

If anybody has done this and knows the answer, please let me know -- my gut tells me that I should use fuses of the same rating, or maybe just slightly lower.  It's just that some of the fuses are fairly low to begin with and the next lower fraction may be too low.

-- Jim

[Grainger49]

Jim,

IIRC, the VA product of the primary is equal to the VA product of the secondary.  This is regardless of the grounding technique.  If you have 1.5kVA primary the secondary can produce 1.5kVA from end to end, with the center grounded.  The current is through the whole secondary winding and is the same as the primary.

So I would imagine that each leg would be fused the same... if the primary is 120V then the secondary leads both should be fused the same as the primary hot leg.

Re-reading your post you seem to be asking about the amplifier.  I think this is the same situation.  If the hot lead has 1A fusing the "neutral" lead should have 1A fusing.

[4krow]

   Very interesting question Jim. Grainger, because each leg is sharing voltage, if you will, wouldn't the current be half for each leg? So then, a 1 amp. draw on a SINGLE ended, would be equal to 1/2 amp. draw on a BALANCED circuit?

[Grainger49]

If I have the concept of balanced power it uses a 60V/0V/60V output.  The ground reference is in the center of the winding rather than on the Neutral end.  For one half of the cycle the current is plus to ground in one half, ground to the opposite leg in the other half.  Otherwise the voltage would be zero from end to end if they both fed into the ground reference at the same time.  Imagine two batteries with the negative end tied together. 

Am I getting this right?

[galyons]

The primary rating for common in-line fuses is in amps. A fuse is a current interrupter.  250 volts seems to be the most common voltage for standard in-line fuses. Use to be more 125v, but I think the global distribution has contributed to making 250v more common.  Someone more expert will have to explain the amperage effect of the balanced legs.  But it is the amperage, not voltage that is rated.

What is the benefit of running the line 60-0-60?

Cheers,
Geary

[Paul Birkeland]

Just like the voltage sinewave that we see with AC power, there is also a current sinewave that is shifted relative to the phase of the voltage wave.

I would treat the entire secondary as just a single winding, with one fuse that you would use if the center tap was not groudned.

-PB

[Grainger49]

What PB said, that is what I said.

[Jim R.]

I think I tend to agree with PB and Grainger, but something still tells me that something may not be totally correct about it.

Anyway, Geary and others who are interested in a great article on balanced power in audio from Mix magazine should go to:

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

A far better explanation and theoretical background than I could ever manage.  Note the many references to class A tube audio amplifiers thoughout the text.  This is what we Bottleheads live on and thus this articale should be on everybody's reading list.  It also addresses a lot of the concerns of digital power supplies and digital gear.  It's not overly technical and engineers should have no problems reading betweeen the lines, but very accessible to others as well.

-- Jim

[4krow]

  You may have noticed that fusing a 220V circuit is half of the fusing requirement of 120V. 220V circuits balanced voltage circuits, whereas 120V circuits are single ended.

  The advantage of balanced power is mostly for the common mode rejection of noise.

[Paul Birkeland]

  You may have noticed that fusing a 220V circuit is half of the fusing requirement of 120V. 220V circuits balanced voltage circuits, whereas 120V circuits are single ended.

I've run into plenty of unbalanced 220/240V systems, and several balanced 120V systems.

In a typical US home, yes, the 220V feed is balanced, and the 120V feeds are not.

[4krow]

  Now you've sparked my interest, I was unaware that 220V was produced any other way than as a balanced circuit. Could you explain in what case(s) an unbalanced 220V system is used? Thanks.

[rif]

I thought using 2 fuses was dangerous, at least for the usual non balanced wiring - you wouldn't put a fuse on the neutral and hot lines. In your balanced case, seems like the same danger exists. If only one fuse blows, you'll still have a live 60Vac running in the chassis.

[Jim R.]

Only if you don't switch both power lines, which is why it is preferrable to use a double pole power switch., and then it follows that you should use one fuse per line in case of another line to ground short.

-- Jim

[Paul Birkeland]

  Now you've sparked my interest, I was unaware that 220V was produced any other way than as a balanced circuit. Could you explain in what case(s) an unbalanced 220V system is used? Thanks.

The average step-up transformer is unbalanced.  All step-up autoformers are unbalanced.  I believe many, many 220V generators across the Atlantic are single-phase 220V and unbalanced.

[Paul Birkeland]

I thought using 2 fuses was dangerous, at least for the usual non balanced wiring - you wouldn't put a fuse on the neutral and hot lines. In your balanced case, seems like the same danger exists. If only one fuse blows, you'll still have a live 60Vac running in the chassis.

Yes, 2 fuses normally is a horrible idea, and really a fuse in either leg is roughly equivalent.

While you would have 60V AC coming off half the secondary winding with one fuse popped, this is 60V relative to earth, but nothing relative to the primary of the power transformer of anything plugged into the transformer output.