Isolation xformer question (again)

Purchased a used iso xformer. Trippe Model IS800. 6.8 amps out, 4 outlets, on/off switch and circuit breaker.

Question: The power IS isolated however all the grounds are tied together. Is this typical? Is my new scope protected? I thought the whole idea of isolation is to get away from earth ground. I can easily disconnect the out put ground from the input ground, but then what? Just let it float. Please educate me. Thank you, Ivan Vegvary BTW, I have 14 days to return if unsatisfied.

That is normal. The inner turn of the secondary of the isolation transformer is usually connect to chassis ground too. The re-establishes the neutral to ground bond that on the primary (input) side could have been lost or has gone Ohmic.

Shaun

** You have been warned over and again NOT to use an isolation transformer with a scope. It is stupidly dangerous !!! ** Not true - one isolates the PSU part of a device from the AC supply so any part of it can be grounded to allow measurements with a GROUNDED scope that are otherwise impossible. ** Kindly educate yourself.

The tranny you have bought is fine just AS IT IS for use with a UN-MODIFIED scope.

.... Phil

Phil, I have been warned not to plug a scope into an iso xformer. I'm not. Circuit under test will be plugged into iso. Scope plugged into wall with proper ground. The education I need is clarification on the common ground. I thought the whole idea of the iso was to keep the two grounds seperate. The currents are seperate but the grounds are the same?

In an industrial environment it is often necessary to run the oscilloscope without protective grounding. If available use an isolation transformer for the scope. Disconnect or isolate the ground pin on the mains plug of the oscilloscope. You cannot run a iron melting blast furnace from an isolation transformer :-) So you have to uncouple the oscilloscope ground from supply net.

Today this is easily done with battery operated scopes, but they were not always available at my times.

w..

This is really bad advice to give to a novice, Helmut. This practice does go on - or did go on in the old days - but it's heavily deprecated and should never be done if it can possibly be avoided in which case only by very experienced techs who know what they're doing. The isolation transformer should supply the device under test - NOT the scope. You do NOT want any ground loops between the two.

Ivan, If I don't get this right I figure Phil will yell at me... so here goes. The idea is that your circuit under test (CUT) will get it's "common ground" when you attach the ground lead from your scope probe to the CUT.

George H.

Probably shouldn't be telling him that. First of all it is not all that com mon, but it does happen. Like when working on big CNC machines for example. However you didn't go into the requirements for this. According to OSHA yo u have to get these things like they use at the bank to fold the line but i nstead of red fabric I think you have to use yellow tape that says "DANGER" on it. They want the operator on insulated floor mats and all that shit, p erhaps even special shoes.

A regular electronics tech, especially a novice, is not (or should not) hav e cause to do any such thing.

Your isolation transformer is correctly constructed and follows the typical convention for these devices. There is isolation between the input line hot and output line hot.

If you were to accidentally disconnect the ground lead and neutral on the output from the bonding point in the chassis you'd then have what you were expecting, which is 100% isolation of hot, neutral and ground from the input side.

Whatever you plug into this modified isolation transformer will now "float".

It can be extremely unsafe, depending on what you're doing. You also need to trust the isolation ability of the transformer and outlet you're using. The tripp-lite one claims to have a ground shield, but they're also known for making really shitty products, so hipot the thing first if you want to play it safe.

Don't float the scope (plug it into the modified isolator). Scopes have cables and knobs you touch, and if they're at some wacked out potential you're going to get shocked. It's easier to keep your hands off the DUT instead when testing or taking readings.

Cutting off grounds and using isolation transformers isn't a death sentence, you just need to be aware of how it works and what you're working with.

There's plenty of hot electrical work that goes on all the time with no problems.

I don't think we should let the OP think that his IT will protect him period. It's still possible to electrocute yourself even when an IT is floating the piece of equipment you're working on. All the IT is doing is breaking the earth reference you normally get with mains voltage; preventing you suffering a shock to earth. But if you accidentally bridge across two points in the circuit you're testing that are at different potentials, you can *still* get fried. So an old rule I was taught from day one (decades ago) that has served me well is when probing high voltage circuits, always keep one hand behind your back so at least you won't get a shock across the chest (the most vulnerable area fatality- wise).

Yeah, by experienced and qualified people who follow laid down procedures and have the relevant safety equipment, not some noob seeking advice on the subject from a bunch of strangers! (no offence to the OP, we were all noobs once).

Agreed. In no way does it protect you from shocks, it only protects you fro m shocks to ground or earth. You get your hands on voltage it could be floa ting up in the sky but you can still die. All it does is to stop a path to earth ground or anything else connected somehow to the mains.

If you want real shock protection put the whole shebang on a GFI (GFCI). It is the only way, and it is not exhaustive. You can STILL get to two circui t points inside the unit that can killya.

There used to be the one hand rule, and in the tube days it REALLY applied. But now, like everything I work on is five or nine volts. I got lazy one t ime last year and got WHAPPED by a tube amp. God damn. Just like the old da ys.

In other words, DO NOT get lulled into a false sense of security here. You can still die. A possibly good example is the safe table saw.

They came out with a table saw with an electronic brake and sensor that wou ld stop the blade immediately if it sensed human skin touching it. Now let people get used to that and then with an old table saw they will cut their arm off.

I think if they outlawed airbags in cars and mandated spikes in the dashboa rd, people would use following distance more than one inch per hundred mile s per hour.

Absolutely. I remember when crash helmets became compulsory for motorbikes here in 1973, the number of serious accidents instantly went through the roof because it gave riders who'd previously been very cautious in their approach to roadcraft the false sense that suddenly they were somehow invincible. And of course we see the same effect at work in countless other areas where governments around the world try to "improve" safety.

Yes.

NO! There are some isolation transformers that do that, they are required for passing outlet-checks of the plug-with-three-lights type, but that is NOT what you want for occasionally working on a hot chassis. You want to get an isolation transformer with, at most, a grounded shield around the secondary windings, but you want those windings NOT bonded to any ground.

This model bonds output-neutral to ground:

This model doesn't:

If, for some reason, you want to ground an odd part of a device on the bench, you can do it, while powering from the second type of isolation transformer. Not with the first.

i find a battery operating scope is much better, even if its on a UPS unplugged.

Jamie

nd"

** There are two classes of AC powered devices, ones with a safety ground a nd "double insulated" ones without. In the latter case, the scope probe gro und clip provides a temporary ground to the device and may be connected to the chassis or other parts of the circuit isolated from the AC supply by a transformer or SMPS.

Ones with a safety ground usually still need the scope probe ground clip co nnected to the chassis to eliminate RF noise and hum voltages spoiling the image on the screen.

An isolation transformer is needed with either class when you need to probe circuits on the AC supply side of a SMPS - but it makes no sense to remove the safety ground from devices that have it. So the transformer used shoul d carry the safety ground through to the outlet.

.... Phil

** That iso-tranny has been modified - from the factory they have the output neutral linked to ground and are fitted with surge protection devices.

You need to check inside and make sure there are no devices fitted from the secondary winding to ground - like varistors or capacitors.

FYI: never use more than one outlet at a time.

.... Phil

Thank you everyone. I now understand.

That sounds cumbersome.

Yes, but there's another hidden danger with the careless use of ITs that comes from over-familiar use I should mention and that is plugging test equipment into one because it makes a convenient "extension lead" if the cable on your say signal generator isn't long enough to make it to the socket. Been there, done that! It's so easy to slip into bad habits.