Using Isolation Transformers

Thread Starter

RAMBO999

Joined Feb 26, 2018
244
I have read about the use of an isolation transformer to seperate a device eg: oscilloscope from a power source.

The gist I get is that it is primarily to protect the device from inadvertent grounding of probes and burning out the transistors and other parts in the scope by preventing the grounding reaching the consumer unit. Broadly speaking.

If that's the case isn't there an increased risk of electrocution as a trade off? If for some reason the scope shorts out to ground.

I would appreciate your opinions.
 

MisterBill2

Joined Jan 23, 2018
6,111
The use of an isolation transformer is to separate the device being probed from the mains, NOT the test equipment. Isolating a scope is sometimes done and I do not advise doing it.
AND this is very important!! ANY device with mains voltage applied requires that anybody working on the insides understand how to do it safely!!! This means that you need to understand what you are doing well enough to do it safely.
Use the isolation transformer to isolate the item being scoped, but before connecting any scope leads, use an AC voltmeter with the common connected to the "green wire" safety ground to first verify that the device is isolated.
There is a great deal of information available on how to be safe, and exactly what is hazardous, but there is also some of fear-filled garbage on the dangers of electrical servicing. Separating the two is a challenge on many occasions.
 

Thread Starter

RAMBO999

Joined Feb 26, 2018
244
The use of an isolation transformer is to separate the device being probed from the mains, NOT the test equipment. Isolating a scope is sometimes done and I do not advise doing it.
AND this is very important!! ANY device with mains voltage applied requires that anybody working on the insides understand how to do it safely!!! This means that you need to understand what you are doing well enough to do it safely.
Use the isolation transformer to isolate the item being scoped, but before connecting any scope leads, use an AC voltmeter with the common connected to the "green wire" safety ground to first verify that the device is isolated.
There is a great deal of information available on how to be safe, and exactly what is hazardous, but there is also some of fear-filled garbage on the dangers of electrical servicing. Separating the two is a challenge on many occasions.
So if there is a power board, for example, that needs to be powered up to investigate it the isolation transformer should be between the board and the source. Rather than connecting it directly to the power source as it normally would be in service.
What's the reason for that?
 

Hymie

Joined Mar 30, 2018
819
When using a class I scope, the 0V of the scope measurement circuit is connected to earth. This can cause problems if for some reason the circuit under test cannot be referenced to earth.

I can recall many years ago disconnecting the earth of a class I scope to avoid this problem; the reason being I needed the functions that scope offered and had no other available that could do the job.

Great care should be taken if doing this since should the basic insulation of the scope fail, the scope chassis (and the circuit under test) could become live. Using an isolation transformer to power the scope (rather than disconnecting the scope earth) is probably a preferable/safer option.

When using a class I scope in this mode, the scope chassis will be at the potential of the 0V probe connection of the circuit under test – so care should be taken not to touch the scope chassis if this could be at a hazardous voltage.

Note that many mains isolation transformers providing an isolated secondary mains voltage, nevertheless feed through the real world earth to their output socket which may need disconnecting (otherwise the scope chassis will still be earthed).
 

MisterBill2

Joined Jan 23, 2018
6,111
OF COURSE an isolation transformer needs to provide isolation. Thus before trusting one it is mandatory to verify if it actually does completely isolate the device connected to it. THIS CERTAINLY includes the "Green Wire" ground connection. Certainly one can instead isolate a scope, I have seen it done several times. I have also seen a tech get a rather nasty shock from reaching up and touching a non-insulated part of the scope while working on a device. THAT is why I do not recommend isolating the scope instead of what is being probed. Just because somebody does it does not make it a smart idea.
 

KeithWalker

Joined Jul 10, 2017
918
Do NOT disconnect the scope chassis from mains ground!!!! If you do, you are putting your life at risk if you are making measurements at mains potential.
If you have to measure across a component that is at live mains potential and are not able to isolate it with a transformer, as I mentioned above, use two scope channels, both at the same amplitude setting and both traces centered, with the two probe tips connected to the measuring points. Use "A-B" mode or invert the "B" channel to display the waveform across the component. Before you do this, make sure you are using probes that are rated for the voltages you are measuring.
Regards,
Keith
 

AlbertHall

Joined Jun 4, 2014
9,899
Do NOT disconnect the scope chassis from mains ground!!!! If you do, you are putting your life at risk if you are making measurements at mains potential.
If you have to measure across a component that is at live mains potential and are not able to isolate it with a transformer, as I mentioned above, use two scope channels, both at the same amplitude setting and both traces centered, with the two probe tips connected to the measuring points. Use "A-B" mode or invert the "B" channel to display the waveform across the component. Before you do this, make sure you are using probes that are rated for the voltages you are measuring.
Regards,
Keith
And make sure that the probe earth clips cannot connect to anything.
 

Thread Starter

RAMBO999

Joined Feb 26, 2018
244
Does your phrase "consumer unit" imply you are considering attaching the scope probe directly to 240Vac mains ?
I'm not sure what you are asking. But when I plug anything into a wall socket I am technically attaching it directly to the CU via the cable between the socket and the CU. That's true of everything. My original question is whether it would be better if I plug an isolation transformer into the socket and, if we are talking scope, the scope into the isolator. If you were wondering whether I harboured any plans to hard wire a scope into the CU as if it were a shower, an emmersion heater or a cooker the answer is NO. That would be silly.
 

MisterBill2

Joined Jan 23, 2018
6,111
Thanks for the link. I will read it.
That is exactly what I have been suggesting. But also investigate the grounding connection, since adding a ground to an otherwise isolated device may cause problems.
Also keep in mind that even if a device being checked is adequately isolated, hazardous voltages can still exist within that device. That is a separate concern.
 

MisterBill2

Joined Jan 23, 2018
6,111
Please explain. What problems?
If the hardware, such as the chassis or framework, of the item being measured is grounded then that is an additional connection to the scope or other measuring instruments and the readings may include voltages that are not part of what is intended to measure. So the big hazard is to data accuracy.
 
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