power transformer fuse

[benidubber]

Hi!
I was looking at various power supply wiring schematics and I saw somebody places a single fuse to open one of the mains AC inputs in case of abnormal consumption,
while somebody else shows it is safer to fuse both the AC inputs coming from the wall.
which is the difference, and why to fuse both?
another question I have is about fusing secondary windings, does somebody recommend to do it?
thanks for answering such a boring questions!!!
bye
sandro

 

[JohnRoberts]

Hi!
I was looking at various power supply wiring schematics and I saw somebody places a single fuse to open one of the mains AC inputs in case of abnormal consumption,
while somebody else shows it is safer to fuse both the AC inputs coming from the wall.
which is the difference, and why to fuse both?

Fusing both leads is protecting against something other than a simple transformer overload. Perhaps protecting against shorted caps on the mains. 

another question I have is about fusing secondary windings, does somebody recommend to do it?
thanks for answering such a boring questions!!!
bye
sandro

UL safety testing will look at probable component faults and sometimes add secondary side fuses to prevent fire or other safety issues from downstream faults.

JR

 

[benidubber]

So I understand is better to use 4 fuses for a center tapped transformer.
2 on the primary and 2 on the secondary ACs.
I'll do this way.
But now I can't understand why usually just 1 fuse is used for diy power supplies (jlm audio one for example).
thanks for the answers!
bye
sandro

 

[fazer]

I thought it depended on the power your on.  We use 240 split to 120 with the neutral tied to ground so the hot leg is fused.  but if you use 240 both sides need fused because they are both live.  Not sure if that helps for your question.

 

[PRR]

> if you use 240 both sides need fused because they are both live.

No. Euro 230V wall outlets are zero V and 230V. One side is nominally "grounded".

But that does not affect _fusing_. Fuses don't see voltage, they see current. If you have a simple overload, a fuse ANYwhere in the circuit will break it.

Normally only one fuse is used.

It is proper to use a "Fused IEC Power Inlet", which has the power cord socket and the fuse in one. These are normally one-fuse.

You don't usually fuse the secondary. If there is only one winding, then a short on the secondary will blow the primary fuse.

An exception is when a secondary supply is sized for much less power than the total demand, or when additional parts need more protection than the power transformer. A two-6550 amplifier may pull 180 Watts at 600V for plates, 20VA for heaters. Fuse the primary for something more than 200 VA, say 2A 120V. A dead-short on the 20VA heater winding could be 200VA, which will not be enough to blow the primary fuse. The heater winding will smoke and ruin a large hunk of iron.

 

[benidubber]

thanks for replies!

You don't usually fuse the secondary. If there is only one winding, then a short on the secondary will blow the primary fuse.

It was my opinion that this is true also for a center tapped secondary windings transformer..
I believed that "as all the current in my circuit comes in to the circuit from live and goes out thru the grounded input, a single fuse on one of them will do the job"
is that wrong?

thanks
sandro

 

[johnR]

On a 230V system where the neutral side is nominally 0V, one of the main functions of a fuse in the live side is to protect against shock if there's a short to the equipment chassis (which has to grounded for this to work).

European safety regs require a fuse on the live input to the equipment, and it must be of a low enough current rating to protect against fire as well as shock. In practice this means that equipment using large toroidal transformers (over about 300VA) often has a high enough turn-on surge to blow the input fuse. In this case it's necessary to either limit the inrush current (eg. with a soft start relay or a  NTC thermistor), or to use a bigger input fuse and add extra fuses to the transformer secondary (both sides if there's a grounded centre tap) to maintain fire protection. Once you get above 500VA or so you will probably need to use the first method, ie. limit the inrush current, to avoid tripping breakers in domestic power systems.

 

[VacuumVoodoo]

On a 230V system where the neutral side is nominally 0V, one of the main functions of a fuse in the live side is to protect against shock if there's a short to the equipment chassis (which has to grounded for this to work).

European safety regs require a fuse on the live input to the equipment, and it must be of a low enough current rating to protect against fire as well as shock. In practice this means that equipment using large toroidal transformers (over about 300VA) often has a high enough turn-on surge to blow the input fuse. In this case it's necessary to either limit the inrush current (eg. with a soft start relay or a  NTC thermistor), or to use a bigger input fuse and add extra fuses to the transformer secondary (both sides if there's a grounded centre tap) to maintain fire protection. Once you get above 500VA or so you will probably need to use the first method, ie. limit the inrush current, to avoid tripping breakers in domestic power systems.

In many EU countries outlets are not keyed so neutral can be any of two current carrying pins. That's why both sides of mains input are fused. Also, we do put fuses on all secondary windings for fire protection, usually rated at ca 1.5 to 2x nominal load current of the winding. This has added advantage of making CE certification testing simpler and easier to pass.

 

[Kingston]

In many EU countries outlets are not keyed so neutral can be any of two current carrying pins. That's why both sides of mains input are fused.

That's still not true. See PRR's post above. Fuse does not care which direction the current comes from. It will still blow all the same.

 

[VacuumVoodoo]

In many EU countries outlets are not keyed so neutral can be any of two current carrying pins. That's why both sides of mains input are fused.

That's still not true. See PRR's post above. Fuse does not care which direction the current comes from. It will still blow all the same.

Well, this is what I have experienced: had a shorted capacitor after the choke in a PI high voltage supply ripple filter. One of the fuses blew on going off standby. Changed fuse, it blew again. Changed fuse, turned the plug in wall outlet 180 degrees. The other fuse blew. Every time it was the fuse in Live leg that blew. Figure out why.
Also, CE norms specify pretty exactly when you need double fuses and when not. One being that having only one fuse where mains cord is permanently installed will leave live wire "Live" even after the fuse blows if it is placed in the neutral wire. Something about protecting the service tech from getting fried if he forgets to unplug the mains cable from the wall outlet.

 

[Kingston]

Something about protecting the service tech from getting fried if he forgets to unplug the mains cable from the wall outlet.

I would want to protect my gear from a service tech careless as this.  

Anyway, looks like I have to read up on those CE standards a bit.

Every time it was the fuse in Live leg that blew. Figure out why.

It just would not matter with a single fuse. A correctly sized fuse will blow nevertheless. It would have protected your amp just the same. Maybe you simply always had a smaller rated fuse in a particular leg by chance. Mains fuse tolerance is somewhere in 10% range which might explain it.

 

[johnR]

That's why both sides of mains input are fused.

Is that a local Swedish regulation?  In many years of servicing gear in the UK I've never seen this on single phase equipment (including export equipment with SEMKO approval, but that was a while ago).

 

[johnR]

In many EU countries outlets are not keyed so neutral can be any of two current carrying pins. That's why both sides of mains input are fused.

That's still not true. See PRR's post above. Fuse does not care which direction the current comes from. It will still blow all the same.

Unless there's only one and it's in the neutral line, in which case it won't blow if there's a short between live and a grounded chassis.

 

[johnR]

Also, we do put fuses on all secondary windings for fire protection, usually rated at ca 1.5 to 2x nominal load current of the winding. This has added advantage of making CE certification testing simpler and easier to pass.

This is not required specifically by regulations AFAIK. If the mains input fuse provides adequate protection against fire hazard that's all that's needed. If the input fuse needs to be oversized to withstand transformer inrush, secondary fuses will probably be required. They would also be needed if there are multiple secondaries that can't be adequately protected with just an input fuse.

 

[JohnRoberts]

I have seen fuses on the secondary side of circuitry required by UL to mitigate some concern they found in their testing. I don't recall the details of this exact design, since it wasn't mine and my only involvement was a run in with UL inspectors where they tried to shut down shipments of units already assembled and sitting in the finished goods warehouse without a correct value internal fuse "sticker".   I successfully argued with their superiors that the human safety risk from a missing internal fuse sticker, for a fuse that consumers don't even know is there, and are instructed not to mess around inside, was too small to warrant their costly remedy.  

It would have cost us more than the profit on those units to tear them down and add the sticker...   I did jack up the production line supervisor who neglected to add the sticker (which was on the BOM). But that's another long story... Just another day in the factory.  

JR