An unloaded (open wire) auto-transformer will still output the set voltage (intact cutting wire).

"isolation" means that there is no path to earth from the cutting wire. The container containing these items if metal should still be earthed.

 

An unloaded (open wire) auto-transformer will still output the set voltage (intact cutting wire).

"isolation" means that there is no path to earth from the cutting wire. The container containing these items if metal should still be earthed.

Thanks for the help.

I saw an article https://www.electronics-tutorials.ws/transformer/auto-transformer.html
that says: "If the secondary side winding becomes open-circuited, current stops flowing through the primary winding stopping the transformer action resulting in the full primary voltage being applied to the secondary terminals."

This led me to believe with no load on the secondary of the autotransformer, the full voltage on the primary would be present on the secondary....

 

This led me to believe with no load on the secondary of the autotransformer, the full voltage on the primary would be present on the secondary....

You interpreted it wrong. If the secondary opens, you will get the full voltage. That you can easily see if a turn opens. An open load will not affect the set voltage.

Now, the importance of fusing the wiper. Say it's 100 VAC (Not 120 - want to make the numbers easier). So, a 100 V, 10 A variac. This means the winding has to be limited to 10 A. So, that's 10 A at 20 V, 30 V, 40 V etc. So, to protect the wire you have to fuse the wiper at 10 A.

A severely worn brush (beyond the replacement interval) could cause the transformer to open and put full line voltage to the wiper. The fuse should offer some protection.

 

An isolation transformer reduces but does not eliminate shock hazard.

From a safety perspective, every connection to an autotransformer should be considered as going directly to the "hot" wire of the AC line, which means there is lethal shock hazard from any of those connections to earth ground. If you consider the case where the "neutral" (which is connected to earth ground at the distribution panel) wire to the primary goes open but "hot" remains intact, this becomes more obvious, because now all connections are just a pieces of wire wrapped around a chunk of steel and connecting to "hot."

An isolation transformer will not protect from shock hazard between the secondary terminals of the transformer. What it will protect from is shock from either of the secondary terminals to earth ground. In many environments, there are lots of metal things around connected to earth, so line to ground shock risk is generally higher than line to line. (Ground fault circuit interrupters are sort of similar in behavior - if you grab hold of both hot and neutral, you'll be fried. If you grab hot while you are grounded the GFCI will detect that the current in the neutral wire is not equal and opposite to that in the hot wire and interrupt the circuit.)

A step-down isolation transformer on the output of the variable xfmr would be safer, since it also reduces the risk of dangerous shock between the secondary terminals, simply by virtue of lower voltage. Electrical shock doesn't have to reach lethal level to do harm. A long as the current is below the can't-let-go threshold, the natural tendency to jerk the shocked hand back can lead to things tossed across rooms, flesh ripped on sharp edges, coworkers elbowed in the mouth, etc.

The brush of a variable autotransformer is the output "hot" connection, so there is no difference between the brush going open and disconnecting the load. A variable autotransformer is essentially an autotransformer with a tap at every turn - the brush selects the tap in use.
There is no change in output voltage with no load, ignoring the small voltage drop that occurs under load due to winding, brush and fuse resistances. If the brush goes open circuit, the load simply loses power. Transformer action is still maintained and each of those "taps" stays at the same voltage as before.

It is (or was) possible to buy a variable xfmr with isolated windings, but they are rare and expensive.

 

The brush of a variable autotransformer is the output "hot" connection, so there is no difference between the brush going open and disconnecting the load. A variable autotransformer is essentially an autotransformer with a tap at every turn - the brush selects the tap in use.
There is no change in output voltage with no load, ignoring the small voltage drop that occurs under load due to winding, brush and fuse resistances. If the brush goes open circuit, the load simply loses power. Transformer action is still maintained and each of those "taps" stays at the same voltage as before.

This is not the fault I was talking about.

If the brush wears, you get metal to metal contact from the brush holder to the windings. This CAN break the auto-transformer.
The failure mode of full voltage on the wiper should be preventable by fusing the wiper.

 

I agree that if you wear the brush to the degree that the metal at the wire end of the brush contacts the windings you could do damage, but the working brush end is wedge-shaped while the body is roughly square, so you'd have problems long before wearing down to the metal.

I'm not sure I follow your logic with regard to the wiper, full voltage and a fuse. You will only get full line voltage on the wiper if the winding loses connection to "neutral" at any point between the wiper and actual line neutral, while the connection to "hot" remains intact. A suitably rated fuse could blow due to overcurrent in the load if the load normally operates well below line voltage and if its neutral side remains connected to line neutral when the xfmr has lost its neutral. The fuse should be there for overcurrent protection, but using fuses for preventing shock hazard just isn't acceptable. If you presented such a circuit for special approval to UL or CSA standards, making the contention that the fuse was for shock hazard reduction, it would be rejected.

I return to my contention that the safe way to make such a system is to use a step-down transformer after the variable transformer, so that maximum voltage to the hot wire is limited under any condition. If I cobbled together a cutter for my own use, I might take the more risky approach. If someone else might use it in a workplace, there would be a serious liability issue.

 

I return to my contention that the safe way to make such a system is to use a step-down transformer after the variable transformer, so that maximum voltage to the hot wire is limited under any condition

Yes, I agree. That is the way to use it.

 

I did something unusual for my bench system. I came across 3 A Variac in my teens and added an ammeter/voltmeter, binding posts, front panel fuse and an outlet. I probably didn't do ground right, but I was only troubleshooting 2 prong polarized things.

The power switch either bypasses the Variac (isolation at 10 A) or Variac (isolated Variac at 3A). the one thing I miss is a Pilot lamp.

The first use was TV's with series filaments and the second was audio power amplifiers. Only one audio amp had a primary short to chassis.

 

Back in the bad old days, hot-chassis audio equipment was pretty common. To save the cost of a line transformer, one side of the AC line cord was connected directly to the chassis (circuit common) and the other side went to a half-wave rectifier. This was horrendously dangerous, since touching the chassis while connected to earth ground was potentially lethal - it depended on the orientation of the non-polarized plug in the AC receptacle. You used to see people who played electric guitars (strings usually connected to circuit common) very gingerly poking at microphones (shell to common) to see if they'd get a shock: the guitar could be "hot" & the mic at ground, which was not a happy-making arrangement for those given to pressing their lips against the mic.

These days, you can find Chinese-made "corn cob" LED lamps that don't have an overall shell- the surface mount LEDs and their terminals are exposed for you to grab. These would never be approved for sale in most countries. I've also seen lots of things like wall-warts and battery charges where the creepage and clearance requirements for safety are grossly violated. You can actually buy such stuff where the order is "fulfilled" by a company named after a very large river in the Americas.

 

Agree, fuses to prevent shock makes no sense.

Here's a random datasheet showing the wiper is fused. http://www.stacoenergy.com/images/s...rmers-and-test-sets/stacovt/drawing/171-2.pdf I just picked a random product.

it's just counter-intuitive to fuse the wiper. Sencore actually fused both the wiper and the input which, I think, is a good idea.

 

My commercially-built bench variable (line cord, switch, fuse & receptacle from the factory) has a fuse only in the wiper, none for the AC input.

The thing is a transformer, so if you have the wiper at a 25% of line voltage, you are "entitled" to output current of four times the line current. The wiper won't like that if the current exceeds the name-plate rating. Neither will the winding.
One of my variables has a bit of a rough patch on one turn of the winding. I used to use it a lot in development of SMPS, so peak currents at turn-on could run to many tens of amps. Even with isolation transformers, an active power factor stage that charges a thousand or two microfarads to about 400 volts is something that you must poke very carefully (and this is where imperfect high frequency isolation makes scoping signals a nightmare). Never again. Too old, too cranky and broken eyeballs - actually semi-repaired eyes, but they are absolutely fixed focus which make bench work horrible.

 

The thing is a transformer, so if you have the wiper at a 25% of line voltage, you are "entitled" to output current of four times the line current. The wiper won't like that if the current exceeds the name-plate rating. Neither will the winding.

That is a good reason why you are advised to use the variac driving a step down transformer, not the other way around.
The step down transformer is designed for high current out.

 

Is there such a thing as a variable transformer that provides TOTAL isolation?

I'm using a Powerstat 116CU. The data sheets I've found for it all refer to it as a variable transformer - so I can't tell if it has an isolated secondary or not. How can I test to find out? Would it be as simple as opening the secondary and measuring the output voltage?

I can add an isolation transformer. The max voltage I ever need is about 45V. If I place a step-down transformer (120VAC down to 48VAC) in front of the Powerstat unit, would that provide the isolation I need?

Since I'm learning from you experts - could I put a fuse and a MOV (rated about 50V) in the secondary to achieve the same protection?

Thanks,
Balad

There are such things as powerstats with isolated primaries. Or maybe they were variacs. Tose each are trademarked names, there might be others. BUT if you are using a correctly installed GFCI device the shock that you can get is very small and you may not notice it. An isolation transformer in the primary side is also a good choice, probably cheaper than a variac with an isolated primary.