isolation

[BCK]

The neutral to ground bond provides a well identified connection between the transformer secondary and ground.

Are the neutral and ground bonded in the secondary of an isolation transformer or are you talking about the bond back at the power source? Doesn't that cause the grounding wire to carry current?

 

[dereckbc]

Are the neutral and ground bonded in the secondary of an isolation transformer or are you talking about the bond back at the power source? Doesn't that cause the grounding wire to carry current?

Yes the Xo of the transformer secondary is bonded to ground. No it will not cause the current to flow on the ground wire. That is the whole purpose of an isolation transformer to form a new N-G bond free of any noise.

Here is the catch. If you power your O-Scope from the isolation tansformer and you equipment under test from a different source, you just created a problem. Power the equipment under test from the isolation transformer or from battery power.

 

[LarryFine]

What about the bonding jumper on the secondary side of the trans?

If you bond to the system/premises EGC/GEC, you have defeated the purpose of an isolated-transformer, which is to minimize the chance of electrocution. Remember the theories that bonding our power system in some ways increases the danger of electrocution?

If you float a transformer secondary, just as with an ungrounded Delta system, the first ground fault creates no fault current. Likewise, the first accidental line-to-earth contact creates no deadly current (beyond capacitive coupling).

As soon as you bond any secondary conductor to an earth electrode, you "lock" that conductor to "0" volts to earth, and every other conductor to the voltage of that winding relative to the bonded conductor. That's why the typical system's voltage to ground equals the voltage to neutral.

Not sure if that is legal without ground fault indicating equipment.

A isolation trans can still 'isolate' even with a grounded secondary.

Legality aside for the moment, I disagree. As I said above, as soon as you bond one conductor to earth, you fix the voltage of all other conductors to earth. Why aren't hospital isolation power systems illegal?

 

[LarryFine]

Why is there a ground terminal on the secondary of many isolation transformers if it is supposed to decouple the two circuits and, if there should be no ground on the secondary, how does this not create a shock hazard?

The ground terminal on an isolation transformer is merely a continuation of the primary's EGC, and should have no connection to the isolated secondary at all.

 

[LarryFine]

Actually, all transformers are isolation transformers, unless you make a connection between the primary and secondary conductors, as you would with buck-boost transformers.

That is not exactly true, if there is an EGC on the primary side bonded to the enclosure and the secondary has a bonding jumper, there is more than magnetic coupling between the systems.

There should not be a secondary bonding jumper. If there is, this is merely an SDS and not an isolated system at all.

Which is a violation unless the proper monitoring is installed (a LIM) and this lab is in a hospital.

Isolation transformers are commonly used on electronics test benches for the exact same reasons: so line-powered equipment can be serviced while energized with minimal risk of electrocution to the technician.

He would have to contact two portions of the power supply, and not make one contact with the power supply and one with the surrounding world at large, in order to be subject to a dangerous potential difference.

 

[LarryFine]

With only one conductor from each system connected together there is no circuit between them so one system can not interfere the other.

No, but any direct connection between the primary and secondary circuits does assure a potentially dangerous potential () between a person and the earth.

 

[LarryFine]

A surge on the primary side in relationship to the common EGC should not effect the secondary side at all.

Bob, I do agree that it shouldn't, but I still see a common conductor to both sides as defeating the term "Isolated".

Exactly! The whole point of isolation transformers is to have no DC conductive pathway between the primary and secondary conductors, just as with cable-TV coax "ground busters" that eliminate video noise caused by ground loops.

Isolation transformers have nothing to do with prevention of the transfer of surges, etc. A surge is merely the modulation of a voltage that a transformer would pass as would an audio tarnsformer.

I can see an Ungrounded Delta to an Ungrounded Delta as fitting the discription as well as the aforementioned Hospital Isolation type of systems . . .

The primary does not need to be ungrounded for the secondary's non-grounding to be effective.

. . . but I can't agree that all transformers (not talking about auto transformers either) are isolation transformers.

Unless there is a bond or other connection between a secondary conductor and a primary, EGC, etc., how can the secondary not be isolated? Isolation means no DC conductive pathway. Of course, there is AC coupling, or there would be no power transferred at all.

 

[LarryFine]

My comments will pertain only to the subject of which the OP is using the transformer for. So let?s untangle some of the confusion. I will start with why isolation transformers are used.

2. To establish a new reference point. Notice I did not say ground reference point however that is the most common method used.

Even though the primary feeder EGC maybe common to the Xo on the secondary does not couple the primary to the secondary together. There is no complete circuit for current to flow.

Sure there is: from the secondary's unbonded conductor (X-1, e.g.), through the technician's body, through whatever earthed surface is nearby, through the primary's GEC and ECG, and into the secondary's bonded conductor (i.e. X-0).

So if there were a disturbance on the primary EGC, it has no effect on what is going on in the secondary because even though there may be a spike or rise in voltage, the new N-G bond reference point just floats up and falls, and whatever equipment is connected to the secondary never sees it.

By tying the secondary's EGC to the primary's EGC, you have (a) assured that the load's chassis is still earthed, and (b) set the potential of the secondary's un-bonded conductor to that of the secondary voltage relative to earth, and thus the grounded surfaces around the technician.

 

[LarryFine]

The neutral to ground bond is broken and the neutral is then isolated. The neutral will then have no reference and can drift. In my mind this is a problem, that is, having an unbonded neutral. The 'grounded' conductor would then not be grounded, am I right?

Correct, but then an accidental contact between a secondary conductor and surrounding grounded surfaces would not endanger the tech.

 

[LarryFine]

The neutral to ground bond provides a well identified connection between the transformer secondary and ground. If this is the _only_ connection between the transformer secondary and ground, then breaking this connection would permit the secondary to drift.

Agreed.

However if the neutral to ground bond in the transformer is the _only_ connection between the transformer secondary and ground, then you cannot have a ground loop problem!

You will only have a ground loop problem if there are _multiple_ connections between the secondary circuit and ground.

Agreed again, and all of this is true with or without an isolation transformer.

The problem that you have here is that your signal measurement equipment has a ground connection on the signal side which is also connected to the electrical system ground. Now you have at least _two_ connections between the electrical system and ground. In theory you could break this loop on the power side by eliminating the ground-neutral bond, and the transformer secondary would remain referenced to your signal side ground.

True, and there is plenty of audio equipment that has either no EGC in the power cord or a 'ground-lift' switch in the input and output jacks.

 

[LarryFine]

Are the neutral and ground bonded in the secondary of an isolation transformer . . . ?

If they are, it's not a true isolation transformer. If you want the secondary bonded to the system EGC, why bother with a transformer? The supplying circuit already has a bonded conductor.

 

[LarryFine]

Yes the Xo of the transformer secondary is bonded to ground. No it will not cause the current to flow on the ground wire.

It will if a person gets caught between an unbonded secondary conductor and any nearby grounded surfaces.

 

[LarryFine]

Obviously, I'm looking at this whole isolation-transformer issue from the protection of personnel side, and not signal-quality, noise, etc. Proper single-point ("star") grounding is a better method of noise control than transformer use is, in my opinion.

 

[roger]

Larry, you can reply to more than one post in a single post, I think George has even shown how to do it in the FAQ's. You should check it out.

Roger

 

[BCK]

Correct, but then an accidental contact between a secondary conductor and surrounding grounded surfaces would not endanger the tech.

In my mind the tech should still get shocked because the unbonded secondary conductors are not at ground potential. However I do understand that they are now unbonded and have no reference.

Let me know if I'm picturing this right. The secondary conductors are 'floating'. The grounded technician grabs either one of the transformer secondary conductors which then also becomes grounded causing no shock. The other secondary conductor is then 120V with respect to the tech, ground, and the secondary conductor that the tech is holding.

Is that the idea? Wouldn't there at least be a short lived shock when he first grabs the conductor?

 

[LarryFine]

Larry, you can reply to more than one post in a single post, I think George has even shown how to do it in the FAQ's. You should check it out.

Yes, Rog, I know, and I've done it plenty of times. I didn't do it here for the sake of clarity of responses to different posters, since there were so many since my first post. We lost power last night. We had the most incredible lightning storm I've ever seen in my life. Six hours non-stop.

Let me know if I'm picturing this right. The secondary conductors are 'floating'. The grounded technician grabs either one of the transformer secondary conductors which then also becomes grounded causing no shock. The other secondary conductor is then 120V with respect to the tech, ground, and the secondary conductor that the tech is holding.

Exactamundo.

Is that the idea? Wouldn't there at least be a short lived shock when he first grabs the conductor?

That's a common perception, like the human body has an energizing current, but again, no, since there is no circuit.

 

[iwire]

If they are, it's not a true isolation transformer. If you want the secondary bonded to the system EGC, why bother with a transformer? The supplying circuit already has a bonded conductor.

Larry even with the bond it is an isolating transformer for the purposes BCK is looking for.

Take a look at Jon's post.

It is not an isolated system for personal protection which will only work under the strict conditions laid out for hospitals. Line monitors etc.

A 'normal' ungrounded system can still kill if you get between a circuit conductor and anything around that is grounded. I believe that is due to capacitance coupling to ground. The larger the ungrounded system the more this could be a problem.

 

[iwire]

No, but any direct connection between the primary and secondary circuits does assure a potentially dangerous potential () between a person and the earth.

Very true.

But irrelevant here as we are not talking about a system for personal protection.

 

[iwire]

Why aren't hospital isolation power systems illegal?

Because they have a very definite purpose and require monitoring equipment that can notify people when it s no longer isolated.

It also requires regular testing.

Larry you have way to many posts to keep up with so I will end for here for now and let Dereck and Jon have a turn.

 

[dereckbc]

This post has gotten too busy and confusing for me.

BCK, me thinks you might be chasing ghost here, and trying to achieve a completely different purpose than what everyone is discussing.

Are you asking this?

When you say isolation transformer, are you referring to one of the SOLA types that you plug into a receptacle, and then plug the oscilloscope into the transformer to isolate the ground on the scope input for bridge type measurements?

If you answer yes, then you are going about it the wrong and very dangerous way. Also if you say yes, throw out everything that has been said here so far because it does not apply. You would violate the scopes warranty, damage it, and likely electrocute the tech’s. You just as well cut the ground pin off the plug cord of the scope, it would work and pose the same dangers as a SOLA isolation transformer.

The right way to approach this is by using one of three methods.

• Differential Measurement System: The most popular and economical solution for floating measurement is the "A minus B" pseudo-differential technique. Most general-purpose dual-trace oscilloscopes have an ADD Mode where the two channels can be electrically subtracted (invert CH 2), giving a display of the difference signal.

• Monolithic Isolation Amplifiers: Connect between the oscilloscope and the circuit-under-test. The signal is coupled across an electro-optical isolation barrier, providing the necessary isolation with superior common mode noise rejection.

• DC Powered Oscilloscope: As the name implies use a dc powered o-scope powered from internal batteries. Recharge when not in use.

Edit:

There is a fourth method. Buy an isolated input scope like a Tektronic THS 700 series and your troubles are history.