Isolation Transformer

[dcbrown73]

Hello all,

I intend to build Bob Heil's Pine Board AM Transmitter.  I also want to (learn to) build my own multi-output bench power supply.   When messing around with AC I obviously want to be safe about it.   I know a lot of isolation transformers are only isolating the hot for noise purposes and that it's not common to find "tech" isolation transformers anymore.

Well, I found what appears to be one on Amazon. (PHC ISO-500 500 Watts AC Isolation Transformer.  As you can see, it lacks the Earth ground pin on the plug. 

I've read warnings from several people that says do not remove the ground pin from an isolation transformer because it's not safe, but I thought for a tech IT, you really didn't want that ground loop potential.  Can anyone explain why they tell you not to do that? I mean, I know it exists for safety reasons, but that flies in the face of a tech version of an IT.  Is there a different more proper way to do it? What would be the difference between breaking the ground inside the device vs removing the ground pin?

Anyhow, the second and main reason for this post was the following questions.

• Since neutral is usually tied to mains ground at the breaker box.  Is it possible that an IT like the one above still has a ground loop potential?  Maybe from the secondary neutral being tied to the primary neutral which is then tied to ground
• While I intent to look inside the IT, how would you physically test ground continuity?  Use a DMM and test IT secondary ground pin to building outlet ground pin (or even neutrals?

Thanks!
Dave

[ArthurDent]

I wouldn't buy that "isolation transformer" because I consider it unsafe. It has a 2-wire plug, so no ground, but the outlets are 2-wire with the ground pin which is useless and misleading because it allows you to plug a grounded plug into the outlet with no safety ground connected. I would only buy a transformer that has a grounded plug, grounded metal case and core, and ground carried to the ground pin on the outlets. Neither of the secondary leads should be connected to ground as this would defeat the purpose of the isolation transformer.

Just because you are using a proper isolation transformer doesn't mean that you can't misuse it and make it dangerous in use.  There are many threads on this site that cover grounding and isolation and how to do it safely. Here is a good video that Dave uploaded.

https://youtu.be/xaELqAo4kkQ

[Jeroentjerad]

Yes. Good explanation....Both options has its advantages and problems. Always think twice when connecting the DUT on a IT, thus without its ground connected to mains earth.
The earth screw on the transformer (galvanic connected to the transformer core) terminal does only protect the transformer itself, and should be connected by the cord to the mains.
An IT gives you never 100 percent safety against all (human) errors but it is a good start.

[bob91343]

My main use for an isolation transformer is to reduce the spark and shock hazards when working on those old AC-DC tube radios and anything else using similar schemes.

For such use, grounding isn't much of an issue.  The main idea is to remove the galvanic connection to the power line, and the transformer does that.  Now I can safely ground my oscilloscope probe lead to the radio chassis.

Of course there are other uses and some of them require careful grounding techniques.  An ammeter and a fuse are good additions to the setup, as well as a variac and voltmeter.

[David Hess]

If it was just an issue of ground being connected on both side of the isolation transformer, then it would be no problem.

The issue is if the isolation transformer connects ground to neutral on the output side defeating the purpose of using an isolation transformer for design and test and isolating the neutral and hot from ground.

[bob91343]

Yes, that's why a two-wire cord on either input or output or both is okay.  A three wire cord is also okay but not both input and output.

Well, change that.  The three wire cord, if it only adds a grounding wire, is okay anywhere.  That way the house conduits and the transformer housing and the load chassis can all be at the same potential, independent of the power line itslf and its current.  The main thing is that there be no continuity between either side of the power line and the chassis.

It's true that the house wiring will connect the ground wire to the neutral in most cases.  But that should cause no harm and, in fact, keep static potential from building.

In order to make it safe, isolation can work but isn't what is necessary.  What is necessary is that the 'hot' side of the line (black wire in most setups) is never connected to the chassis regardless of plug orientation.  But isolation solves another problem, any fault from a poor ground.

I had the latter in the case of my bathroom lights and the ceiling room heater.  The lights were connected from ground/neutral over to one hot side of the line.  The heater was connected from the same ground/neutral to the other side of the power line, so there was 240 Volts from the hot side of the lights to the hot side of the heater.  Somehow there was an open in the ground, so there was 240 Volts across the series combination of lights and heater.  As soon as I switched on the heater, the lights burned out because most of the 240 Volts was across them due to the low resistance of the heater.

This really has nothing to do with isolation but is a heads up that things aren't always what they seem.