Isolation Transformer

A fellow who was a laundry equipment mechanic gave me a 20 amp 120 V 1:1 transformer. I suppose it was to isolate a circuit for safety reasons. I asked a competent electrician about it and he said it offered no safety protection and would be dangerous to use but he would like to have it. Until I was told that it offered no protection I had assumed that it provided protection when working on a bench that was connected to earth ground.

If it has the primary & secondary windings isolated (will have if truly an isolation transformer), then it does offer some protection. Using it will mean that neither output wire will have voltage relative to earth ground, so contact with either one should not run current thru your body to ground. HOWEVER, contacting both output wires simultaneously will still apply 120v to your body. I use one when working on some old radios & TVs that operate directly off the line with no input transformer, since without the transformer, once the plastic/wood case & knobs are removed, the metal chassis is now connected to either neutral or hot of the power line, depending on which way you orient the plug into the receptacle.

Well, your "competent electrician" is mistaken. The purpose of an isolation transform is to isolate power from ground, making it impossible to create a ground fault. Pretty much all medical equipment is isolated to protect patients from electrocution. Yes, there's still 120 volts out, but the output is floating from ground. Grab both output leads and you'll get a shock; but you can grab either lead and ground and you won't feel anything. Stepup/stepdown transformers in consumer electronics also serve as isolation transformers, even though their primary purpose is to supply different voltages to the device.

Moonlite, with all due respect to electrician you spoke to, its my professional electrical engineers opinion what he said "isn't necessarily always exactly true" (it depends on how you wire and use it, see below).

Heres the real deal: If its a true ISOLATION Transformer NOT an autotransformer, it has no wired physical connection between the Input and Output, the Primary over to Secondary energy transfer takes place by electromagnetic induction.

THEREFORE if you're using it and come in contact with one of its outputs while your bare feet or other hand or whatever is on wet ground or a grounded metal work bench etc YOU "more then likely" DONT GET ANY BAD SHOCK (notwithstanding capacitive or inductive low energy transfer). The isolation x formers voltage is ONLY with respect to the other transformer terminal NOT earth ground or grounded metal objects (unless its bonded to the other isolation transformer terminal which usually its not). HOWEVER if you were using the utility (NON Isolated source) its Neutral is tied to mother earth and the equipment grounding conductor (third green safety ground wire) so touching one of those wires while barefotted on wet ground or touching a grounded metal work bench can give a bad shock.

TO MY OLD TIRED ENGINEERS BRAIN, THAT DOES AFFORD SOME DEGREE OF PROTECTION when used properly. That's one reason for SAFETY (among others) they design make and use isolation transformers in certain situations.

HOWEVER there is a degree of truth to what the electrician said. If using the utility, the grounding electrode conductor is wired to the outer metallic conductive case/frame/shell of a tool or appliance and can have contact with a metal work bench etc. THEREFORE if there's a hot to case short, the grounding conductor carries fault current back to the panel so the breaker trips. With a 2 terminal isolation transformer there's normally no equipment grounding conductor arrangement.

THIS IS THE SAME THEORY I TRIED TO EXPLAIN LAST WEEK (but unfortunately most non electricians failed to understand OSHA and the NEC rules) WHY OSHA AND THE NEC don't earth ground portable generators on job sites (the OSHA article says it can create an electrocution hazard) that serve plug and cord connected tools via onboard receptacles. If you did earth ground the generator and on wet ground you ACCIDENTALLY COME IN CONTACT WITH A HOT WIRE YOU GET SHOCKED, buttttttttt if there was no reference to mother earth (by having NO earth ground rod) if you touch a hot wire you most LIKELY DO NOT. That's why instructions that come with generators etc say ground per the NEC because the NEC and OSHA cover all the different situations when you earth ground and when you do not. ALSO lay gents do not understand the difference between Neutral BONDING and Neutral GROUNDING and EARTH GROUNDING and what's a GROUNDING ELECTRODE. Hey that's NOT a bad thing about non sparkies, it can take years of training and experience to get it and some never do lol

NO WARRANTY no way I can cover all the bases and every conceivable situation nor explain in a paragraph what takes whole books so this is ONLY a short approximated abbreviated NOT 100% PERFECT ALL CORRECT answer, I just do the best I can in a short time to help and try to save a life, SO THERE TAKE OR LEAVE IT or do what Billy Bob and his brother in law Bubba tell you, its your life. Other electricians or non electicians may have different opinions, to each their own, this is mine FWIW which is nothing, same as what was paid for lol

John T

Um forgive me if im wrong but i dont think they would have made a device like that if it wansnt any good. But he wants it should have told you something LOL. The o yea that thing is junk but ill take it off your hands

(quoted from post at 06:03:23 07/07/16) A fellow who was a laundry equipment mechanic gave me a 20 amp 120 V 1:1 transformer. I suppose it was to isolate a circuit for safety reasons. I asked a competent electrician about it and he said it offered no safety protection and would be dangerous to use but he would like to have it. Until I was told that it offered no protection I had assumed that it provided protection when working on a bench that was connected to earth ground.

Click to expand...

Isolation transformers can be used for many, many reasons.

Here in an Electrical Engineering lab, we may use one to isolate a measurement device like an oscilloscope from Earth ground so that we can use the scope to measure between two different voltages in a circuit without grounding either one.

We may also use them to isolate test equipment that has enough leakage current (by design) to trip a GFCI (Ground Fault Circuit Interrupter) from a GFCI-equipped outlet, so that we can use that piece of test equipment.

In a hospital, an isolation transformer may be used to isolate patient-connected electronics from those same leakage currents. One electronic device may have a safe leakage current level, but connect a bunch of them to the same patient, and it could stack up.

In all of those cases, there is no blanket statement that can say that they have created a condition that has more or less "protection" than you can get from a good old wall outlet.

In every circumstance, the level of protection depends on whether a person can become the preferred path of current from a conductor at one voltage to a conductor at another voltage.

In short, protection is only provided by the operator knowing what potentials he or she is putting across their body at all times.

If somebody gave me an isolation transformer, the first thing I would do is measure the resistance between all of the output terminals to all of the input terminals, to see if ground has been jumpered across the transformer. I would also measure the resistance between all of the input terminals and each other, to see if either of the "neutrals" are bonded to "ground".

With that knowledge, I would then use accordingly. To use accordingly, I would need to know if surfaces that I could touch while using the isolation transformer are grounded or not. I would also need to know if any equipment being connected to the isolation transformer is a "Class I" (needs an Earth ground to be safe) or a "Class II" (has double or reinforced insulation, so doesn't need Earth ground to be safe.)

If you don't know how to use that knowledge, then an isolation transformer indeed offers you no protection.

When people talk about electricity, there are so many blanket statements thrown around. For example. "Voltage doesn't kill. Current kills." Most of these statements are half-truths at best. Trust none of them Every situation is unique.

Add to the above...after I measured the resistance between all of those terminals...I would still plug it in and measure the voltages between all of those terminals. A capacitor between any of those terminals would measure open at DC, but then would look more like a short at AC.

If you don't have the internal schematic of the transformer and know its history (i.e. has somebody been in there), all bets are off.

You would really have to measure to find out what is what.

The simplest and probably the most effective way is to have shop outlets protected by a GFCI.

To your statement "When people talk about electricity, there are so many blanket statements thrown around. For example. "Voltage doesn't kill. Current kills." Most of these statements are half-truths at best. Trust none of them Every situation is unique. "

I say AMEN BROTHER I've been here over 15 years trying my best to help and Ive heard them all lol. Electrical or legal questions draw more responses them most other questions, lots of opinions out there, some right, some may get you killed.

Best wishes, good post you made.

John T BSEE, JD, retired Electrical Engineer

I'm GUESSING you've never worked with a TV with a "hot chassis"?

(quoted from post at 16:00:06 07/07/16) I'm GUESSING you've never worked with a TV with a "hot chassis"?

Click to expand...

nd never needed to float an oscilloscope to observe waveform between two elevated potential sources, such as phase to phase in a 3 phase delta.

Motor oil questions draw a lot of responses too !!!!!

But the good thing about those is if you use the "wrong" brand it's not going to kill you ! And your motor or engine won't much care either.

Best used to isolate electrical noise from communication and data systems.

The transformer likely served one of two purposes. The first would be to electrically isolate sensitive equipment, such as older electronic devices. A transformer would cut down on a lot of line noise, acts as a big choke coil to attenuate high frequency noise.

The other application was in Hospitals. An isolation transformer would be used with an un-grounded secondary, the current coming out the transformer on one wire would have to equal the current going into the transformer on the other wire. That way none would go through the patient. For your workshop, a modern GFCI receptacle will work better.

Old tube TV used metal frame as common. If an outlet wasn't wire to code working on them would be shocking. An isolation transformed was used. Scopes also used frame as common.

My Rv generator is like an isolation transformer, no connection to ground.

My dad went to a trade school in the early 40 to be an elecrican. He wired his dad's house before electricity made it to the country. Back then wasn't a third wire. Common was the earths ground and was called ground.

Dad told me a single hot wire could go to well pump. Connect the ground, today's common, to the well pipe which makes a good ground.

(quoted from post at 18:41:30 07/07/16) Old tube TV used metal frame as common. If an outlet wasn't wire to code working on them would be shocking. An isolation transformed was used. Scopes also used frame as common.

My Rv generator is like an isolation transformer, no connection to ground.

My dad went to a trade school in the early 40 to be an elecrican. He wired his dad's house before electricity made it to the country. Back then wasn't a third wire. Common was the earths ground and was called ground.

Dad told me a single hot wire could go to well pump. Connect the ground, today's common, to the well pipe which makes a good ground.

Click to expand...

Good ground or not, running neutral current through the ground system ain't too bright.

(quoted from post at 13:33:54 07/07/16)

(quoted from post at 16:00:06 07/07/16) I'm GUESSING you've never worked with a TV with a "hot chassis"?

Click to expand...

nd never needed to float an oscilloscope to observe waveform between two elevated potential sources, such as phase to phase in a 3 phase delta.

Click to expand...

No floating is required anymore. O'scopes now have very good differential probes. I learned about floating scopes from a now-retired co-worker who designed TV flyback transformers back before I was even born. He always had a three prong to two prong " cheater" or isolation transformer in his toolkit for such things. Me? I either use differential probes, or two grounded probes and waveform math...or a newfangled battery powered scopemeter.

Isolation transformers are also not the best method of isolating noise from one circuit to another in all cases. Parasitic capacitance between windings will couple higher frequency noise right across. They may or may not be good noise filters. Depends on the spectral content of the noise and the magnetizing inductance, parasitic inductance and parasitic capacitance of the transformer...among other parameters.

Labs that do conducted emissions measurements and conducted immunity tests use a LISN or Line Impedance Stabilization Network to filter out noise from the device under test. There is usually an isolation transformer inline also to keep the phase to ground leakage current from the LISN's oversized "Y" (phase to ground) capacitors off of the building mains.

Jaden's point about GFCI is well taken. If you connect an isolation transformer between yourself and a GFCI....you have now possibly defeated the GFCI's protection for all circuits connected to the secondary of the transformer.

Hey Good Neighbor George, you state "My Rv generator is like an isolation transformer, no connection to ground."

It would be a problem when driving to "earth ground" an RV Genset now wouldn't it lol

Of course, as I'm sure you're aware, but maybe we can help Billy Bob, the RV Genset most likely has a BONDED Neutral (not FLOATED like some brands of small portable generators) and the Neutral is connected to a "GROUNDING ELECTRODE" which in this case happens to be the RV's iron frame. In that situation the Genset is wired and configured as a "SEPARATELY DERIVED SOURCE" and when you transfer switch it with/to the Utility, you switch HOT AND NEUTRAL.

CLEAR AS MUD IM SURE LOL

Hope you make it to Elnora this September with Wilson so we can visit.

John T

(quoted from post at 22:07:58 07/07/16) Hey Good Neighbor George, you state "My Rv generator is like an isolation transformer, no connection to ground."

It would be a problem when driving to "earth ground" an RV Genset now wouldn't it lol

Of course, as I'm sure you're aware, but maybe we can help Billy Bob, the RV Genset most likely has a BONDED Neutral (not FLOATED like some brands of small portable generators) and the Neutral is connected to a "GROUNDING ELECTRODE" which in this case happens to be the RV's iron frame. In that situation the Genset is wired and configured as a "SEPARATELY DERIVED SOURCE" and when you transfer switch it with/to the Utility, you switch HOT AND NEUTRAL.

CLEAR AS MUD IM SURE LOL

Hope you make it to Elnora this September with Wilson so we can visit.

John T

Click to expand...

most likely" that is why these post get so ???????????????????????